CONTENT and DESCRIPTIONS OF ORE DEPOSITS
Porter GeoConsultancy, continued its International Study Tour series of professional development courses during November 2011 by visiting a representative selection of the major gold deposits and ore styles across Australia.
- New South Wales,
Cadia Valley Operations - New South Wales,
Boddington - Western Australia,
Kalgoorlie Superpit - Western Australia,
Sunrise Dam - Western Australia,
Telfer - Western Australia,
The tour commenced during the mid-afternoon of Sunday 13 November 2011, in Sydney, New South Wales, and ended in Perth, Western Australia on the morning of Sunday 20 November.
Participants were able to take any 2 or more days, up to the full tour, as suited their interests or availability, with participants joining and leaving the tour at appropiate locations along the route.
The deposits visited were:
This was a technical tour to precede the major NewGenGold 2011 Conference held in Perth, Western Australia from the evening of 21 to 23 November, 2011.
Geological summaries of the deposits on the itinerary are as follows:
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Cowal - New South Wales .................................... Mon. 14, November, 2011
The Cowal gold operation, which exploits a cluster of structurally controlled, low to intermediate sulphidation epithermal, quartz-carbonate-base metal-gold deposits is located on the western edge of Lake Cowal, ~350 km west of Sydney and ~35 km NNE of West Wyalong in central New South Wales, Australia. The main deposit, which contains the bulk of the known resource, is Endeavour 42 (E42) (#Location: 33° 38' 12"S, 147° 24' 23"E).
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Five main deposits/mineralised blocks have been delineated, spaced at ~1 km intervals within the 7.5 x 2 km, north-south oriented 'Gold Corridor' at Cowal (Henry, McInnes and Tosdall, 2014; Balind et al., 2017). These are, from south to north: E40 where sporadic gold mineralisation is intersected but no significant resource has been delineated; E41, comprising the high grade E41 West and East pods that are ~300 m apart; E42, the current Cowal open pit mine; Regal-Galway, a discontinuous line of steep, high grade lodes connecting the NE margin of E42 with E46 East; and E46, comprising the E46 West Pod and the steeply west-plunging E46 East body.
Prior to 1980, historic prospecting and shallow gold mining activities, followed by modern exploration had taken place in areas of better outcrop to the west of the currently known Cowal deposits. No investigations have been conducted in the immediate area of Lake Cowal due to negligible outcrop and up to 80 m of lacustrine sediment and cyclical flooding. Following their success in discovering the Goonumbla (Northparkes) deposit ~100 km to the NE, Geopeko, the exploration arm of Peko Wallsend, identified the Lake Cowal area as having potential for porphyry copper mineralisation. Geopeko subsequently conducted shallow reconnaissance RAB drilling to bedrock over targets largely selected on the basis of geophysical data. By 1988 when Peko Wallsend was acquired by North Limited, the geology of the Cowal Igneous Complex had been delineated and a number of low grade porphyry copper prospects identified to the south. In addition, an area of ~7.5 x 2 km of >0.1 ppm Au had been outlined along the western margin of Lake Cowal. During the early 1990s, North Ltd outlined the E 41 and E 42 deposits and by 1995 had completed a feasibility study for mining E42. In 2000, North Limited was acquired by Rio Tinto who on-sold the Cowal Project to Homestake Mining. Following a merger in 2001, Homestake was absorbed into Barrick Gold Corporation. Open pit mining commence at E42 in 2005 and processing commenced during the third-quarter on 2006. In 2015, Evolution Mining acquired Barrick Gold's 100% interest in the Cowal Gold operation. Drilling has continued to further evaluate E41 and the other satellite deposits of the Gold Corridor.
The Cowal deposit lies within the Macquarie Arc of the Palaeozoic Lachlan Fold Belt of southeastern Australia. The Macquarie Arc is an Ordovician to Early Silurian intra-oceanic volcanic arc in central and southern New South Wales. It was accreted to coeval Ordovician sedimentary terranes to the west, but was subsequently dismembered during the Middle Silurian to Middle Devonian into several volcanic belts by predominantly east-west extension accompanied by sinistral, arc-parallel strike-slip displacement (Crawford et aI., 2007; Glen et aI., 2007, 2009; Fergusson, 2009). Cowal lies within the southern segment of the most westerly and longest of these belts, the >450 km long, sporadically exposed Junee-Narromine volcanic belt.
The western margin of the Junee-Narromine Belt is marked by the Gilmore Fault of the NNW Gilmore trend in the SW, and by the north-south to NNE Tullamore Fault of the Tullamore trend in the central and northern sections (Glen et aI., 2007). A series of other large, subparallel ductile and brittle fault zones belonging to these two trends also cut and dislocate the belt internally and to the east. Deformation along these fault zones is complex, with reactivation as strike-slip, thrust and normal faults throughout the Palaeozoic (Glen et aI., 2007). The Gilmore and Tullamore trends converge in the southern exposed segment of the Junee-Narromine volcanic belt in the vicinity of the Cowal Igneous Complex (Glen et aI., 2007), host to the Cowal deposit and numerous other epithermal Au and porphyry Cu-Au prospects in the vicinity.
Most volcanic and intrusive rocks of the Macquarie Arc exhibit low to medium K calc-alkalic chemistry, regarded as indicative of subduction-related melts (Crawford et aI., 2007). Locally these rocks have shoshonitic compositions, attributed to an intraoceanic arc setting and derivation of magma from
sources enriched in light rare earth elements (Stern et aI., 1988; Wyborn, 1992). These shoshonitic rocks occur along the intersections of the volcanic belts with NW trending transverse/transfer structures, inferred to be major basement structures obliquely cutting the Macquarie Arc (Glen et aI., 2002).
Gold mineralisation at Cowal is hosted by the ~475 to 460 Ma Ordovician Cowal Volcanic Complex that is exposed within a north-south elongated 40 x 15 km window, surrounded by unconformably overlying Siluro-Devonian sedimentary and volcanic rocks. It comprises a variable mixture of volcaniclastics, coherent extrusive rocks and high-K calc-alkaline to shoshonitic intrusive units (Glen et aI., 2002). The complex is bounded to the west by a broad, highly deformed north-south zone within the Siluro-Devonian sequence, the NNE Booberoi Thrust Zone of the Tullamore trend, whilst the eastern margin is defined by the parallel Marsden thrust. The complex lies along the western edge of the Eastern Subprovince of the Lachlan Orogen, at the southern end of the linear NNW trending Junee-Narromine Volcanic Belt, and is separated from the turbidite dominated Western Subprovince by the NNW-SSE trending Gilmore Fault Zone. The complex is reflected on aeromagnetic images as a 'finger print like', oval shaped magnetic high, one of 16 such igneous complexes in the district (Balind et al., 2017).
The Cowal Volcanic Complex is dominated in its central and northern parts by north-south trending Early Ordovician andesitic to latitic volcanosedimentary successions that have been intruded by gabbro and diorite to syenite intrusive suites. In the south, it comprises large multiphase tonalite to granodiorite intrusions.
The Cowal gold deposits are hosted by a subaqueous Ordovician succession of volcano-sedimentary rocks intruded by diorite to granodiorite stocks and numerous dykes (McInnes et aI., 1998; Crawford et aI., 2007). The volcano-sedimentary successions include fine grained mudstone and sandstone; volcanic sandstone, breccias, and conglomerates; lavas; and shallow-level dyikes and sills. The volcanosedimentary succession represents primary, resedimented and reworked volcanic rocks and sedimentary rocks with complex and rapid lateral and vertical facies changes, although the broad stratigraphy across the district is consistent and traceable (Henry et aI., 2014). In general, within the mineralised section of the Cowal Igneous Complex, the volcanosedimentary rocks dip shallowly to the northwest (Henry et aI., 2014).
The lavas, dykes and shallow intrusions which are interbedded with or have intruded the volcanosedimentary and sedimentary succession are trachyandesitic to andesitic in composition and are variably feldsparphyric. Lavas and shallow intrusions generally have both coherent and autoclastic facies, including peperite and hyaloclastite. The autoclastics are monomictic with jigsaw fit textures, whilst hyaloclastite units are commonly reworked and resedimented. Dykes are generally <1 to 3 m thick with sharp contacts, commonly with fine grained chilled margins, and are interpreted to possibly represent feeders to sills and lavas higher in the stratigraphy (Henry et aI., 2014).
Primary pyroclastic rocks are absent, although many polymictic volcaniclastic facies have abundant clasts of pyroclastic origin. The polymictic volcaniclastic facies all contain a mix of juvenile and nonjuvenile volcanic and sedimentary clasts, and are interpreted to have been deposited by cold, water supported, sedimentary gravity flows (K. Simpson, pers. commun., 2007 reported by Henry et al., 2014). These rocks, and the hyaloclastite, peperite, and laminated mudstone deposits suggest a subaqueous depositional setting for the Cowal district. This is consistent with the recognition of a fossiliferous limestone clast within a polymictic lithic breccia just to the north of the Cowal deposit. The dominance of graded and massive beds and the lack of shallow-water sedimentary structures in any of the clastic volcanic and sedimentary facies indicate deposition below storm wave base, whilst the laminated mudstone units reflect background pelagic sedimentation (Henry et al., 2014).
District Structure and Controls on Mineralisation
The Cowal Volcanic Complex is bounded to the west by the multiply reactivated, NNE trending, 60°W dipping Booberoi Fault which separates the basal conglomerate of the Siluro-Devonian Ootha Group, Manna Conglomerate from the Ordovician rocks of the complex in its structural footwall. Fault rock sericite has yielded a 411±2 Ma age (40Ar/39Ar; Foster et al., 1999; Lyons, 2000), interpreted to be the minimum age of the youngest deformation on the Booberoi fault (Glen et al., 2007). The 15°W dipping and NNE trending Marsden thrust separates the complex from Devonian sandstone and marl to the east. The complex has poorly defined northern and southern margins where they are unconformably overlain by Devonian sedimentary rocks (Balind et al., 2017).
On the basis of seismic profiling and structural analysis the complex has bee interpreted to have been developed over a large domal intrusive at depth with secondary flower structures, north-south faulting and crenulation folding near surface (Glen et aI., 2002). A series of north-south striking prominent arc parallel, and more subtle WNW to NE striking, principal structures have been delineate from aeromagnetic images. These structures have been interpreted to merge and form fault-bounded blocks behaving independantly during deformation. The aeromagnetic data reveals a greater intensity of north-south orientated faults on the western side of the complex. Two of these, the Reflector and Wild Turkey faults, define the eastern and western margins of the ‘Gold Corridor’. Other major parallel faults within the gold corridor (the ROM, Drambuie and Glenfiddich faults and Cowal shear zone) offset both the geology and mineralisation (Balind et al., 2017).
Mapping and drill core logging indicate the north-south and WNW faults are intimately associated with gold mineralisation within the Gold Corridor. The interaction of the north-south faults and NW transfer structures is interpreted to reflect a transient change from east-west to NW-SE compression, facilitating sinistral strike-slip movement on the north-south faults, which in turn created significant normal fault style extension on NW-trending faults. Pre-, syn- and post-mineralisation structures are frequently intruded by basaltic to andesitic dykes (Balind et al., 2017).
In addition to these semi-regional domain bounding structures and deposit scale, grade-controlling north, NW and NE trending faults, the Cowal Igneous Complex is also cut by a poorly defined WNW trending structure, the Marsden lineament, not to be confused with the NNE trending Marsden Thrust (Leslie et al., 2017). This structure broadly juxtaposes volcanic and sedimentary sequences and lesser diorite that host the structurally controlled Cowal epithermal systems to the north, with polyphase, dominantly equigranular granodiorite to monzonite hosts to porphyry systems in the south (Leslie et al., 2017). A number of calc-alkalic porphyry systems are distributed over an area of ~12 x 8 km within the intrusive suites south of this lineament. Outcrop is sparse in this area and targets tested were initially predominantly generated from airborne magnetic geophysical data. These include the Marsden porphyry deposit 16 km SE of E42 which has an indicated and inferred mineral resource of 180 Mt @ 0.38% Cu, 0.20 g/t Au (Leslie et al., 2017). The core of this deposit is characterised by quartz, magnetite, chalcopyrite stockwork veining cutting biotite-magnetite altered quartz monzonite to quartz diorite (Rush, 2013). Similarly, biotite-magnetite altered porphyritic diorite and intermediate to mafic volcanic and volcaniclastic rocks and lesser magmatic breccias, host local stockwork chalcopyrite mineralisation 5 km west of Marsden at the E43 porphyry target. Seven kilometres NE of E43 and 3 km south of E42, at E39, widespread low-grade copper mineralization (e.g., ~0.1 to 0.2% copper) consists of chalcopyrite hosted in quartz-magnetite ±K feldspar stockwork veins with local potassium feldspar alteration halos cutting equigranular granodiorite. Seven kilometres south of E39, at Milly Milly, clotted chalcopyrite replaces mafic phenocrysts overprinted by pervasive sericite alteration. Many of these porphyry systems have not been comprehensively investigated (Leslie et al., 2017).
ENDEAVOUR 42 (E42)
The E42 deposit is covered by 30 m of lake sediments and an underlying Tertiary laterite profile with no outcrop of the host volcanics, apart from some minor gossanous float.
Within the Gold Corridor, drilling has shown the stratigraphy to be divided into two distinct zones separated by the central north-south Glenfiddich fault. To the east, limited information indicates a very steep, westerly dipping sequence that comprises a diorite sill, followed stratigraphically downwards by re-sedimented and coherent trachyandesitic lava, coarse volcaniclastic sedimentary rocks and basal mudstone-dominated sedimentary suite. To the west, between the Glenfiddich and Wild Turkey faults, the volcano-sedimentary succession hosts the mineralisation at Cowal and is known in more detail. In the immediate deposit area, the sequence consistently strikes at ~215° and dips at 40 to 45° NW, but flattens to the south, before steepening to the SW, suggesting it represents the western limb of an anticlinal fold. The succession in the deposit area is as follows (after Miles and Brooker, 1998, Henry et al., 2014; Balind et al., 2017), from oldest to youngest:
• Eastern Volcaniclastic Unit, a >200 m thick package of laminated hematitic mudstones and sandstones that occur to the east of the Wamboyne fault on the eastern margin of the deposit;
• Lower Volcaniclastic Unit, the 'Cowal conglomerate' of Miles and Brooker (1998), an ~100 m thick sequence with massive to graded beds of coarse, well rounded to very angular, clast supported polymict volcanic (andesitic) debris conglomerate, with interbedded laminated siltstone and mudstone, and evidence of mass flow;
• Trachyandesite, the 'Golden Lava' of Miles and Brooker (1998), comprising 60 to 110 m of plagioclase-phyric coherent, autoclastic trachyandesite, interbedded with monomictic sand to breccia with clasts of plagioclase porphyritic fragments. This unit is interpreted to represent a submarine lava with associated hyaloclasite and autobreccia;
• Upper Volcaniclastic Unit, the 'Great Flood Unit' of Miles and Brooker (1998), a 250 m thick unit including a sequence of pebbly sandstone; 72 m of vitric and lithic-rich tuffaceous sandstone with mass flow structures and <3 m thick interbeds of laminated mudstone. Graded polymictic breccias overlie the trachyandesite unit;
• Porphyritic Andesite which is plagioclase-phyric and coherent, with associated monomictic breccia facies.
• Intrusive rocks - the lower sections of this volcano-sedimentary succession is intruded by a thick, locally up to 230 m, holocrystalline to euqigranular to porphyritic diorite sill complex (the 'Muddy Lake Diorite' of Miles and Brooker, 1998) and by multiple generations of dykes, with at least six different compositions. Of these, four are pre-gold, namely those with feldspathic, megacrystic, pyroxene-phyric and mafic compositions, while those that are vesicular and dioritic are post-mineral (Balind et al., 2017). These post-Muddy Lake Diorite dykes were emplaced in active faults which are 0.2 to 20 m thick (averaging 1 to 3 m).
A few drill holes have passed through the Muddy Lake Diorite sill into fine to course volcaniclastic sedimentary rocks that may represent a unit lower in the succession than the Lower Volcaniclastic Unit.
The Muddy Lake Diorite, which is one of the hosts to gold bearing veins, has been dated at 453±3.8 Ma (U-Pb; Perkins et al., 1995) and 456±5 Ma (K-Ar, hornblende; 1995; Bastrakov, 2000). A late stage diorite dyke within the NW-trending Wamboyne fault on the eastern margin of the Cowal pit is regarded as post-mineral only on the basis of its lack of both alteration and observed gold bearing veins (Henry et al., 2014). Dating of this dyke yielded an age of 448±4 Ma (U-Pb LA-ICP-MS from 23 zircons; Strickland 2005). Attempts to refine the date of this same dyke produced a scatter of ages from 447.2±8.4 to 465.5±7.3 Ma, with an inferred concordia age of 454.2±4.7 Ma (SHRIMPRG, 11 zircon cores and rims; Henry et al., 2014) and a weighted mean age of 455.6±4.1 Ma excluding one outlier (207Pb-corrected 206Pb/238U; 12 zircons; Henry et al., 2014). Many of these zircons were xenocrystic and hence the results may be problematic. An additional date of 450.5±1.3 Ma (TIMS 207Pb/235U, monazite; Henry et al., 2014) was regarded to reflect a more reliable crystallisation age of the late postmineral diorite dyke as the monazite did not have problems of inheritance or xenocryst incorporation encountered with the zircon sample. These dates constrain mineralisation to between 453±3.8 and 450.5±1.3 Ma (Henry et al., 2014).
Within the immediate Cowal deposit there are several generations of faults, some of which predate gold mineralisation, whilst others are post-mineral. There are two main sets that display metre scale offsets of bedding, although the direction and magnitudes of any slip component is not known. These structures range in thickness from <1 to ~15 m and contain gouge and breccia. The first and dominant set has a NW strike/dip of 322°/69°NE, while the second NNW-SSE trending set averages 155°/64°SW, with a dihedral angle of ~50°. The intersection of these sets has a near horizontal plunge of 15° and a NW azimuth of 328°. These orientations are interpreted to be consistent with an extensional stress field with a near-horizontal intersection of conjugate normal faults (Henry et al., 2014). This sub horizontal intersection and the fact that the faults cut a laterally extensive, uniformly dipping volcaniclastic sequence is interpreted to suggest the stratigraphy was already tilted shallowly NNW prior to normal fault formation (Henry et al., 2014). Most faults are pre-mineral, although the NW-trending (322°) faults were locally reactivated after gold mineralisation, disrupting the mineralised veins emplaced along them. In addition, two small populations of late-stage post-mineral brittle faults appear to predate significant reactivation of the NW-trending faults. The first of these are the young SE-dipping Nerang set of deposit scale reverse fault, which has cut most faults at the Cowal deposit. The final suite are bedding-parallel, with offsets of millimetre to tens of centimetres, interpreted to be a result of footwall shortening, accompanying thrust motion on the overlying Booberoi fault (Henry et al., 2014).
The following alteration styles are recognised in association with the Cowal gold deposit:
• Pre-mineralisation assemblages which are usually only found distal to mineralisation and occurs as an incipient to strongly developed assemblage that includes quartz, albite, chlorite K feldspar ±epidote ±magnetite ± hematite ±carbonate (Balind et al., 2017). This assemblage is present in the Ordovician magmatic and volcaniclastic rocks as a broad background halo peripheral to structures. It is recognised by the weak to total albitisation of plagioclase and the replacement of mafic minerals by chlorite (McInnes et al., 1998). Within the upper volcaniclastic unit, it is characterised by spotty to pervasive chlorite-calcite ±sericite but is accompanied by epidote, magnetite and albite in the diorite sill (Miles and Brooker, 1998). It is overprinted by all other alteration assemblages and gold-bearing veins and is interpreted as the oldest and most regionally extensive alteration phase at the Cowal deposit. This alteration has been attributed to either emplacement of diorite and granodiorite intrusions in the Cowal Igneous Complex or to low-grade regional metamorphism (Miles and Brooker, 1998).
• Syn-mineralisation alteration found in the deposits and prospects of the gold corridor, occurring as three principal assemblages:
i). Chlorite-carbonate-pyrite ±hematite ±leucoxene ±sericite ±K feldspar, which is texturally destructive, and distinguished by its cross-cutting nature and black chlorite, pyrite, carbonate and fine leucoxene grains. It is interpreted to represent a dynamic alteration phase that promoted demagnetisation of magnetic lithologies and the concurrent development of pyrite. This assemblage occurs in all rock types with the exception of postmineral dykes, and predominantly occurs as patchy to pervasive developments, irregular veinlets and clotty disseminations proximal to gold-bearing veins (Henry et al., 2014; Balind et al., 2017).
ii). K feldspar-quartz is a weakly developed assemblage that is almost exclusively restricted to the trachyandesite (Miles and Brooker, 1998). It is characterised by irregular patches of pink-red K feldspar and grey quartz, with sporadically associated epidote, as well as occurring as rare millimetre-scale halos to gold-bearing veins. This alteration overprints the background pre-mineral assemblage, but its timing has not been constrained relative to the black chlorite-carbonate-pyrite and sericite-silica-carbonate alteration, as it is generally spatially distinct from the latter two.
iii). Sericite-silica-carbonate (ankerite) -pyrite ±leucoxene ±albite ±chlorite ±illite is an overprinting phase associated with main stage mineralisation in all the gold deposits of the gold corridor (Balind et al., 2017; Henry et al., 2014). This alteration is texturally destructive, and is dominated by sericite and silica. It has further destroyed magnetic susceptibility and is characterised by pervasive bleaching. It occurs in all rocks except post-mineral dykes, and is best developed in the Upper Volcaniclastic Unit. Sericite-dominated alteration dominantly occurs as fine-grained, centimetre to metre scale halos to subvertical fault zones, quartz sulphide bearing veins, fault-hosted hydrothermal breccias, and intervals containing abundant ankerite veins that have reopened subvertical faults and veins. These halos typically grade outward into background chlorite-dominated assemblages. In contrast to steep structures, inclined veins have narrower alteration halos. The intensity of alteration decreases with depth where it is often fracture controlled, possibly a reflection of the decrease in permeability of the hosts compared to the Upper Volcaniclastic Unit, where it is more intense and pervasive (Henry et al., 2014). Sericite-silica alteration is also more intense and pervasive within a tabular plagioclase-phyric monzodiorite phase contained within the Muddy Lake Diorite at Cowal (Balind et al., 2017).
There are limited observations that suggest that sericite-quartz-carbonate alteration has overprinted black chlorite-carbonate-pyrite alteration in the upper volcaniclastic unit, although Miles and Brooker (1998) interpreted the opposite paragenetic relationship (Henry et al., 2014).
• Post-mineralisation alteration characterised by an assemblage of carbonate ±chlorite ±hematite which is a late stage overprint manifested as cross-cutting carbonate veins containing chlorite and/or being enclosed by hematite selvedges (Balind et al., 2017).
Mineralisation and Veining
A series of vein types are recognised within the main E42 deposit, divided into pre-, syn- and post-mineral deposition:
• Pre-mineral veining - although multiple populations of barren carbonate-dominated veins span the Cowal deposit, two calcite dominated vein sets predate gold bearing veins and cut all lithologies, except the vesicular and diorite dykes. Both are composed of calcite ±chlorite ±pyrite with variably chloritised margins, and vary from ≤1 up to 8 mm, averaging 1 mm thick, with irregular margins. Both sets have taken advantage of pre-existing faults, fractures, dykes and bedding planes (Henry et al., 2014). They are (after Henry et al., 2014):
i). a generally bedding-parallel set striking at ~209° and dipping 43°NW;
ii). a steep suite striking ~121° and dipping 83°SW that are generally parallel to faults and dykes.
• Syn-mineral veining - According to Balind et al. (2017) there are two main mineralised vein types and a third but less common style, mineralised breccias, within virtually all prospects/deposits of the Gold Corridor:
i). Dilational, parallel-sided 'ladder-style' tensional veins that vary from submillimetre to >10 cm in thick that are invariably composed of quartz-sulphide ±ankerite. Each prospect/deposit has a distinct vein dip direction depending on local variations in stress field, e.g., sheeted veins at E42 are 115°/45°S, whilst 1 km north at E46 East, similar sheeted veins are oriented 055°/60°SE;
The bulk of the gold mineralisation occurs in steep sheeted and shallow tensional and dilational vein sets, associated with coarse framboidal pyrite and base metal sulphides contained within the veins. Gold grades are correlated with higher vein density and the mode of occurence of sulphides in those veins. In addition pyrite, the principal sulphde species are sphalerite, chalcopyrite and lesser galena, tetrahedrite and tellurides.
ii). Shear veins that are carbonate-rich with variable sulphide and irregular vein margins. These veins vary from shallow to steeply dipping, with inconsistent strike directions and thicknesses that range from 1 to >500 mm.
iii). Quartz-sulphide breccia, which can contain bonanza gold grades associated with high sulphide content. Quartz sulphide breccia is considered to be of hydrothermal origin formed by implosive fragmentation within dilational sites along brittle faults. These breccias vary from 50 to over 500 cm in thickness and can pinch and swell along their generally limited lateral extent. Strike varies, although most breccia have a WNW to NW trend dip moderately to steep south or NE.
However, Henry et al. (2014) does not specifically distinguishing the shear veins at Cowal, but subdivides and describes in more detail the syn-mineral dilational-tensional veining, as well as the breccia types, as follows:
i). Steep gold-bearing carbonate-base metal veins, interpreted to be the oldest syn-gold veins at E42. Prior to their development, the previously tilted Ordovician host sequence was subjected to brittle faulting, likely developed under tensional stress directed ~NE-SW, followed by emplacement of dykes along NW-trending faults and along a less dominant E-striking orientation. Two sets these steep veins followed fractures parallel to some of the fault and dyke populations. Although there is considerable variation, two main orientations are distinguished, 331°/73°NE and 069°/76°SE, which represent a near orthogonal relationship. Some of these veins follow the margins of pre-mineral dykes, whilst the abundance and frequency of gold-bearing veining at E42 decreases away from faults and dykes. Similar mineralogies are found in both vein sets comprising calcite-quartz-pyrite ±adularia ±sphalerite ±galena ±chalcopyrite ±covellite ±visible gold with or without late-stage ankerite void space infill. Minor pyrrhotite has been identified in pyrite, as well as rare telluride minerals (hessite, altaite and petzite) associated with pyrite, galena and gold (Bastrakov 2000). Pyrite within the veins has been extensively fractured, with carbonate, sphalerite and galena variably filling those fractures along with gold. Gold is also present as inclusions within pyrite, and is commonly spatially associated with sphalerite. Crosscutting relationships in reactivated veins and fractured pyrite grain textures provide evidence of multiple generations of carbonate-base metal vein opening events. The youngest vein infill is late stage ankerite, commonly filling void space around prismatic quartz crystals (Henry et al., 2014).
Both of these steep carbonate-base metal vein sets can be traced for >2 m with thicknesses generally of 2 to 5 mm to as much as 10 cm in some banded veins. Adjoining vein segments typically overlap at their extremities and are linked by oblique subsidiary veins. Vein walls are planar to irregular. Some of the vein textures at the E42 deposit are regarded as typical of epithermal systems (Dong et al., 1995), including zoned crustiform quartz and feathery quartz. No crosscutting relationships have been observed between these two steeply dipping gold-bearing vein populations (Henry et al., 2014).
ii). Steep fault-hosted breccias, which are located along faults parallel to the steep veins described above. Some breccia cement assemblages are identical to vein fill assemblages, suggesting a link to specific hydrothermal fluid events shared in the formation of both. Matrix in these clast-supported breccias is a minor component, suggesting that fault plane abrasion was not the principal cause of brecciation. Three main fault-hosted breccia stages of formation have been recognised, based on cement mineralogy, which temporally progresses from (after Henry et al., 2014):
Stage 1 - barren quartz → quartz-pyrite-sphalerite (±calcite ±chlorite ±late ankerite) → barren quartz or quartz-ankerite.
Stage 2 - ankerite-sphalerite-pyrite (±chalcopyrite ±galena ±quartz); to
Stage 3 - barren ankerite ±calcite ±chlorite.
Clasts are typically rotated, and are generally angular to subangular with negligible milled clast material in the matrix. They are both monomictic to polymictic, including wall rock, mainly volcaniclastic rocks, as well as fragments of dykes and barren and mineralised veins. Most of these breccia zones follow dyke margins and pre-mineral faults that have been reactivated during gold deposition and subsequent events. They are laterally and vertically discontinuous on the scale of tens of centimetres to a few metres and are unpredictable in their distribution. Stage 1 breccias are steeply dipping and contain feldspar-phyric clasts, interpreted to be fragments of dykes that had been emplaced in older pre-mineral faults. Many of the gold-bearing breccia intervals of this stage are spatially related to the Western and Corringle faults cutting through the central sections of the Cowal/E42 deposit, and are also found directly above the Corringle fault. Each of these faults has undergone post-mineral reactivation and filled with cataclastic rocks, which include mineralised vein clasts. Gold bearing stage 2 breccias are locally reactivated stage 1 breccia faults, and dip steeply to moderately to the east and SE. Stage 3 breccias are not mineralised, and are contain clasts dominated by local volcaniclastic wall rock, some older vein clasts, and rare fragmented quartz-cemented breccias and dykes. They have locally overprinted quartz-cemented stage 1 breccias and commonly have euhedral ankerite crystal terminations in void spaces between clasts. These late-stage ankerite-cemented breccias occur along the NW trend of deposit-scale faults, and dip steeply to the east. Ankerite feeder veins to some of these ankerite-cemented breccia intervals locally crosscut inclined gold-bearing veins, indicating that the ankerite brecciation event was post-gold at E42 (Henry et al., 2014).
iii). Inclined gold-bearing carbonate-base metal veins - The dominant gold-bearing veins of this type at E42 deposit are moderately SW dipping, with a much smaller population developed subparallel to slightly oblique to bedding. Average vein orientations are 101°/42°SW and 185°/39°W, respectively. Most have planar, parallel sides and massive infill. These inclined gold-bearing veins crosscut the older, steeply dipping auriferous vein populations with little to no offset of the latter, but in rare cases where it occurs, that offset is only a few mms and is normal. This offset suggests some increment of slip along the inclined vein either during or subsequent to its formation. The mineralogy of the two vein sets is identical, predominantly comprising quartz-calcite-pyrite ±adularia ±sphalerite ±galena ±chalcopyrite ±visible gold ±late-stage ankerite with minor pyrrhotite, hessite, petzite and altaite (Miles and Brooker, 1998; Bastrakov, 2000). Pyrite within the veins is commonly fractured and is variably enclosed by carbonate, sphalerite and galena, as well as gold. While the bulk of the inclined vein set obliquely crosscut bedding, a small population of roughly bedding-parallel planes of weakness were reopened to host gold-bearing veins. As with the older steep vein set, inclined veins include crustiform quartz crystals, some of which have zonal and feathery textures. Inclined veins can be traced on a metre scale, and vary from <1 to 150 mm, averaging 5 to 10 mm. Veins commonly overlap at their extremities, whilst some are hard-linked to the adjoining vein. A few thicker inclined veins are up to several cms across and have curviplanar traces on the pit wall. The thickness if some of these vary along the length of the vein, but retain a consistent geometry, Vein segments with shallower dip angles generally have slightly thicker widths, and conversely, steeper segments appear somewhat thinner (Henry et al., 2014).
The transition from steep to inclined veining suggests that the local stress axes must have changed from sub-horizontal extension and subvertical compression during the emplacement of the early steep vein fill to near-vertical tension and horizontal extension during late inclined vein fill event.
• Post-mineral veining, which occurs as several generations of veins, dominated by carbonate and chlorite that can be divided, from oldest to youngest (after Henry et al., 2014) into:
i). carbonate cementation of reactivated existing gold-bearing veins;
ii). steep north-south striking carbonate veins;
iii). three mutually crosscutting sets of late carbonate-chlorite ±hematite veins.
ENDEAVOUR 41 (E41)
The E41 gold deposit is a satellite of E42, located ~1 km to its south, on the eastern margin of the Muddy Lake Diorite sill. It has been divided into two higher grade mineralised domains, the E41 West and E41 East pods. The E41 East is lithologically complex, hosted in clastic and intrusive facies, whilst the West Pod mineralisation is primarily within a mafic monzonite intrusion. The pods are ~300 m apart, separated by the NW-trending and E-dipping Cowal fault which includes post-mineralisation displacement (Zukowski et al., 2014).
The volcanosedimentary sequence at E41 East is predominantly shallowly NW-dipping, laminated to locally massive, mudstones, with intercalated >1 to 40 m thick beds of coarse polymictic volcanic breccia and volcanic sandstone. These rocks are intruded by sills and numerous dykes. The volcanic facies distribution in the Cowal district has been interpreted to indicate the sequence at E42 and E46 was deposited proximal to a submarine volcanic centre, whilst at E41, the host rocks were deposited in a quiescent deepwater environment, distal to the main volcanic activity, with episodic mass flows of volcanically derived deposits (Miles and Brooker, 1998; J. Cannell, pers. commun., 2005; K. Simpson and D. Cooke, pers. commun., 2006, reported by Zukowski et al., 2014).
Fifteen intrusives have been identified at E41, divided into (after Zukowski et al., 2014):
Pre-mineral Intrusions - which are predominantly mafic to intermediate in composition, typically clinopyroxene (diopside) and plagioclase bearing, with subordinate hornblende and accessory biotite, apatite, titanite and magnetite. These include the Muddy Lake Diorite sill, which is up to 240 m thick, followed by plagioclase-phyric, trachyte and mafic dykes of dioritic composition, then mafic-monzonite to monzodiorite dykes. The Muddy Lake Diorite is spatially associated with mineralisation at E41 East. It, and the volcanosedimentatry sequence, are cut by the pre-mineral dykes, which are controlled by NE-trending and steeply SE to S dipping faults.
E41 West mineralisation is hosted by a 300 m diameter medium-grained mafic monzonite stock, which has a vertical extent of >500 m, and intrudes both the volcanosedimentary succession and Muddy Lake Diorite sill. This stock is reflected by a circular magnetic low within the highly magnetic diorite sill. The stock is interpreted to have been volatile saturated during crystallisation by the presence of and potentially contributed to miarolitic cavities, and was potentially coeval with the mineralisation. It includes a hybrid zone of black, fine-grained monzodiorite and/or enclaves of a megacrystic plagioclase-phyric rock, suggesting magma mixing. This hybrid zone has a width of >50 m in the centre of the mafic monzonite intrusion, expanding to the south, with a vertical extent of >100 m.
A second, smaller mafic monzonite intrusion is located below the E41 East deposit, and this is also a focus of alteration, as described below.
Syn-mineral Intrusions - which include thin, <2 to 10 cm thick 'dykelets' of quartz-, K feldspar- and hornblende-bearing quartz monzonite and aplite, and thicker pyroxene-phyric dykes, which are distributed throughout E41. Some of the 'dykelets' contain gold-bearing clots of pyrite. Pyroxene-phyric dykes, which locally contains minor gold-bearing pyrite disseminations, are observed to crosscut quartz-pyrite veining and K feldspar-epidote alteration, but are, in turn, cut by carbonate-base metal sulphide veining.
Late-mineral Intrusions - which include a syenite dykelet and a hornblende-phyric quartz diorite dyke which contain minor pyrite disseminations and weakly mineralized, <1 mm thick pyrite veinlets.
Post-mineral Intrusions, which are exclusively mafic in composition, and are plagioclase bearing, including a hornblende-phyric dyke, plagioclase-phyric diorite dykes, and an amygdaloidal dyke, none of which contain gold-bearing veins or syn-mineralization alteration assemblages.
Veining, Mineralisation and Alteration
On the basis of limited structural measurements, there appears to be two principal sets of faults and mineralised veins at E41, an early, steeply dipping, NNE to NE trending set, and subsequent, east striking, shallowly south to SE dipping faults and veins. The former is interpreted to be the shallow expression of a deeper-seated crustal weaknesses localised on the southern edge of the mafic monzonite stock at E41 West. Aplite and quartz monzonite dykelets have been localised by these same structures at E41 East and may reflect another deeper seated intrusion (Zukowski et al., 2014).
The main hypogene gold mineralisation at E41 East is localised along the east-west contact between the volcanosedimentary units and the underlying diorite sill forming a 250 m wide and 200 m thick zone, whilst other smaller domains of mineralisation are randomly distributed throughout the mineralised pod. Supergene gold is developed in the weathering profile above the hypogene zone in the weathering profile which is up to 120 m thick (Zukowski et al., 2014).
At E41 West, there are two major zones and several minor domains of hypogene gold mineralisation defined within the mafic monzonite stock, all of which strike north to NNE and dip ~60°E. Supergene gold mineralisation occurs to within 10 m below the present-day surface in an oxidised profile that is <80 m thick (Zukowski et al., 2014).
The E41 deposit evolved from early, high-temperature porphyry-style veins and alteration to lower-temperature epithermal-style gold mineralisation in the following stages (after Zukowski et al., 2014):
Early-stage veins and alteration, which began with hydrothermal magnetite ±biotite alteration in the mudstones, sandstones and volcanic breccias at E41 East, at locations between the two pods, and to the south of E41 East. This alteration occurred as a stage 1 actinolite-magnetite assemblage within the Muddy Lake Diorite sill at E41 East, whilst two equivalent suites, stage 1A, ≤1 to 30 mm thick magnetite veins and veinlets and subordinate 2 to 5 mm stage 1B quartz-magnetite veins are found in the other rocks. Rare stage 2 andradite-bearing veinlets are found close to the mineralised centre of E41 East, restricted to the volcaniclastic sandstones and breccias.
Stage 1 veining at E41 West occurs as 2 to 10 mm vein-dykes of myrmekitic quartz-K feldspar ±actinolite ±pyrite, whilst stage 2 veining is ≤1 to 5 mm thick quartz-carbonate with illite ±ankerite.
The stage 1 and 2 veins are overprinted by selective, 'patchy', pervasive K feldspar ±albite alteration which is widely distributed throughout the Muddy Lake Diorite at E41 East. The same alteration occurs near the mafic monzonite contact at depth below E41 East and extends laterally from the monzonite contact for at least 400 m eastward. It has also been observed forming a thin aureole around the mafic monzonite, which has internally been overprinted throughout by a selective, pervasive chlorite-albite ±K feldspar assemblage. K feldspar ±albite alteration is associated, but not exclusively, with a distinctive hematite dusting/reddening. At E41 East, this reddening is sporadically distributed, but intensifies around the mafic monzonite intrusion, defining a distinctive alteration halo. Several irregular zones of hematite alteration also characterise early stage veining and alteration in the E41 West mafic monzonite stock.
Main-stage syn-mineral veins and alteration, comprising two vein stages which are associated with hypogene gold mineralisation in both E41 East and West. Gold principally occurs as a refractory phase in pyrite, but is also found as grains of Au-Ag tellurides and as inclusions of free gold in pyrite, sphalerite, and chalcopyrite. The two vein stages have been subdivided as follows:
Stage 3, the first of these, comprises mineralised quartz-pyrite veins and veinlets with minor carbonate and chlorite, subdivided on the basis of alteration halos into stage 3A-a, which is typified by distinctive epidote alteration halos; and stage 3A-b, with epidote-K feldspar selvedges. Both stages 3A-a and 3A-b are associated with steeply dipping, north and NE-striking fractures and are common in E41 East, but are rare in E41 West. Main-stage K feldspar-epidote alteration surrounds the southern portion of the mafic monzonite stock at E41 West, and also at depth within the Muddy Lake diorite sill surrounding the mafic monzonite below E41 East. Both epidote and K feldspar-epidote alteration, although associated with poorly mineralised veins, fringe fractures that were followed by later mineralising fluids.
Stage 3A-c comprises pyrite-quartz veins that have illite-muscovite ±chlorite ±carbonate selvedges. These veins are 5 to 100 mm thick at E41 East and 2 to 10 m at E41 West, and are predominately E trending, dipping shallowly to the S to SE, cutting steeply dipping stage 3A-a and 3A-b veins.
Stages 3A -a to c veins generally carry <1 g/t Au at E41 East and West.
Stage 3B occurs as 2 to 50 mm thick quartz-pyrite-adularia veins at E41 East and are morphologically similar to stage 3A-c with which they have mutual crosscutting and overprinting relationships (Cooke and Bloom, 1990), indicating that they formed essentially at the same time. They assay ~3 g/t Au. This stage is virtually absent at E41 West.
Stage 4 is the second mineralising event at E41, and is characterised by base metal sulphide and telluride-bearing carbonate-base metal sulphide veins. Stage 4A crosscut and/or reopen and infill stage 3 veins at both E41 East and West. They vary from ≤1 to 10 mm thick, and at E41 East may reach 50 mm, and carry abundant carbonate species (calcite, ankerite), quartz, chlorite, illite-muscovite and sulphides ±hematite ±apatite. Sulphides include pyrite, sphalerite, galena, chalcopyrite, tetrahedrite and Ag-Au and Bi tellurides. Gold grains, averaging 40 µm in across occur as inclusions in sphalerite and are associated with Ag tellurides and/or with chalcopyrite crystals. Some strongly mineralized stage 4A veins (e.g., Au >30 g/t) contain apatite crystals enriched in rare earth element (La, Ce, Nd and Sm) that are intergrown with muscovite, chlorite, hematite and calcite.
Stage 4B, which largely absent from E41 West, consists of arsenopyrite-ankerite ±pyrite ±Bi tellurides. This veining largely takes the form of infilling of re-opened veins, particularly those of stage 4A, but is seen to cut preexisting stage 4A sulphide bands and associated illite and carbonate-bearing veins and alteration halos.
Stage 4C veins are typically associated with the highest grade gold mineralisation at E41, and are irregular, multistage veins that are typically brecciated and fault related, occupying steeply dipping faults and veins, occurring as 'quartz-sulphide breccia'. They are reasonably common at E41 East, but are only sporadically developed at E41 West. Stage 4C veins are commonly brecciated, occupying steeply dipping faults and veins. Many have reactivated stage 3A-a and 3A-b veins. NE to east trending, steeply SE to south dipping stage 3 veins are the most likely site for development of stage 4C veins.
Stages 3B to 4C veins average 3 to 5.5 g/t Au with several hundred to >1000 ppm Cu.
Alteration during the Main stage progresses from epidote to epidote-K feldspar with hematite dusting associated with stages 3A-a and 3A-b quartz-pyrite veining respectively, to an illite-muscovite-carbonate ±K feldspar ±chlorite ±pyrite alteration assemblage defining the alteration halos surrounding quartz-pyrite veins of stage 3A-c, to quartz-pyrite-adularia and carbonate-base metal sulphide veins and breccias of stages 4A to C.
Late-stage veins and alteration, characterised by veins containing gypsum, epidote and hematite, with abundant calcite and ankerite. These veins, which constitute stage 5 carbonate-quartz ±hematite/specularite with weak illite halos; stage 6 epidate-carbonate-pyrite; and stage 7 ankerite veins are typically irregular to wispy, while some occur in erratic networks that have completely obscured the original rock textures. Gold is absent from these late stage veins.
Late-stage alteration comprises epidote, hematite-calcite and pervasive fault-related clay and mica-bearing mineral assemblages. Epidote forms as weak replacement of plagioclase crystals and groundmass in some post-mineralization dykes (i.e., late 'diorite' and hornblende-phyric dykes). Hematite-calcite has altered the post-mineralisation amygdaloidal dyke. Ankerite, together with other carbonates occur as a late alteration overprint at E41. Weak selective pervasive to locally pervasive illite-muscovite, chlorite and carbonate alteration is found in all post-mineralisation dykes.
Fluids and Chemistry
Fluid inclusion data and stable isotope analyses are interpreted to provide evidence that the mineralising fluids had a magmatic-hydrothermal component, and that gold precipitated from boiling saline waters with ~9.0 wt.% NaClequiv. at temperatures of ~310°C (Zukowski et al., 2014). Stage 4 illite formed at temperatures below ~280°C. Stage 3 veins are estimated to have formed at a depth of ~1 km below palaeosurface at a hydrostatic pressure of ~90 bars. Stage 3 calcite has δ13Ccalcite and δ18Ocalcite values that range from -5.2 to -4.6 and from 11.6 to 12.1‰, respectively. Calculated fluids for these mineral values at 300°C (δ13Cfluid = -3‰; δ18Ofluid = 6‰) are regarded as consistent with a magmatic-hydrothermal source of carbon and oxygen during stage 3 (Zukowski et al., 2014). Stage 4 is inferred to have had a component of meteoric water, based on δ13Ccarbonate and δ18Ocarbonate values range from -6.9 to -0.5 and from 10.9 to 30.1‰ respectively, corresponding to δ13Cfluid and δ18Ofluid values of -5 and -2‰ at 200 to 250°C.
Early vein stages have δ34Ssulphide values ranging from -4.9 to -0.5‰, whilst values for stage 3 range from –5.2 to +0.8‰ with the most 34S enriched samples deposited distal from the mineralised centre. For stage 4, these values are between –7.5 and 2.5‰. The negative isotopic values are consistent with oxidised, predominantly sulphate, magmatic-hydrothermal fluids (Zukowski et al., 2014). Zonation patterns of sulphur isotopes indicate the most negative δ34S values correspond to gold-enriched domains, and also with areas containing high-temperature, porphyry-style alteration facies. Negative sulphur isotope values are interpreted to define zones of upflow of the mineralising magmatic-hydrothermal fluids.
The paragenetic history ar E41 demonstrates a transition from deep- to shallow-level magmatic-hydrothermal activity, which implies synchronous uplift, erosion and unroofing of the system, and telescoping of mineralisation and alteration patterns from progressively shallower depths. High-temperature assemblages (e.g., actinolite-magnetite, biotite and K feldspar-epidote) imply epithermal mineralisation occurred proximal to a magmatic-hydrothermal centre, possibly overlying a porphyry copper-gold system at depth (Zukowski et al., 2014).
Origin of the Cowal Deposits
In their discussion of the origin of these deposits, Balind et al. (2017) observed that the Cowal gold corridor deposits include ore-bearing hydrothermal breccia with chalcedonic silica, which is consistent with a high crustal level of emplacement. They also noted the deposits of the gold corridor have many characteristics common to orogenic gold systems, including: i). a strike-slip convergent tectonic environment; ii). the occurrence of first, second and third order faults; iii). the presence of lamprophyre dykes; iv). mineralisation and alteration styles seen in orogenic deposits; v). absence of mineral zonation at a deposit scale; and vi). zones of geologically complex rheological contrasts.
However, while no single intrusive body can be directly linked to the gold mineralisation, geochronological data suggests a close temporal and spatial association with intrusive-related porphyry Cu and Cu-Au mineralisation, particularly at E41 (Corbett and Leach, 1998; Zukowski et al., 2014). The occurrence of hematite, magnetite and negative 34S isotope values of pyrite suggest oxidised, magmatic-hydrothermal fluids were involved in the gold corridor mineralisation and the association of a base metal component are all atypical of most orogenic~gold deposits. The presence of anomalous Bi and tellurides also strongly imply a magmatic involvement. Balind et al. (2017) therefore interpret the Cowal gold deposit to be a cluster of structurally controlled, low to intermediate sulphidation epithermal, quartz-carbonate-base metal-gold deposits.
Reserves, Resources and Production
At 31 December, 2003, the pre-mining reserve - resource estimates were:
Proven + probable reserves - 63.6 Mt @ 1.19 g/t Au, for 76 t of contained Au
Mineral resources - 47.53 Mt @ 1.04 g/t Au, for 49 t of contained Au.
Mining commenced in mid 2006. Total production to the end of 2010 was ~34.75 t Au.
At 31 December, 2010, the reserve - resource estimates were (Barrick Gold Annual Report, 2011):
Proven + probable reserves - 72.5 Mt @ 1.07 g/t Au, for 77 t of contained Au,
Mineral resources - 48.3 Mt @ 0.98 g/t Au, for 47 t of contained Au (in addition to reserves).
Production during calender year 2010 totalled 9.27 t of recovered Au.
Remaining Reserves and Resources at 31 December, 2017 (Evolution Mining website, February 2019) were:
Open Pit - at a 0.4 g/t Au cut-off
Proven + Probable Ore Reserves - 116.28 Mt @ 0.81 g/t Au, for 94 t of contained Au;
Mineral Resources (inclusive of reserves)
Measured Resource - 46.64 Mt @ 0.70 g/t Au, for 32.5 t of contained Au;
Indicated Resource - 141.99 Mt @ 0.91 g/t Au, for 130 t of contained Au;
Inferred Resource - 5.27 Mt @ 1.5 g/t Au, for 8 t of contained Au.
TOTAL Resource - 193.9 Mt @ 0.88 g/t Au, for 170 t of contained Au.
Proven + Probable Ore Reserves - 1.48 Mt @ 5.14 g/t Au, for 7.6 t of contained Au - at a 3.4 g/t Au cut-off;
Mineral Resources (inclusive of reserves)
Inferred Resource - 5.9 Mt @ 3.17 g/t Au, for 18.75 t of contained Au - at a 3 g/t Au cut-off.
The remaining resource (Evolution Mining presentation, September 2018) includes:
E42, predominantly open pit - 127.5 t Au;
E41 East and West pods underground resources, ~1 km south of E42 and ~ 300 m apart - 9.17 t and 7.3 t of Au respectively;
E46 underground resource ~ 1 km north of E42 - 5.35 t Au;
Galway-Regal-E46 open pit resource, semi continuous with E42, extending north from its NE margin - 15 t of Au;
GRE46 underground resource, down dip to the east of the open pit resource - 18.8 t Au.
Production during calender year 2018 totalled 8.025 t of recovered Au. Total production to the end of 2018 was ~110 t Au.
Cadia Valley Operations - New South Wales, .................................... Tue. 15 November, 2011
The Cadia Valley Operations (CVO), exploit the Cadia Hill, Cadia Quarry (Cadia Extended), Cadia East, Cadia Far East and Ridgeway porphyry gold-copper deposits which are located ~20 km SSW of Orange in the central tablelands of New South Wales, Australia, ~200 km WNW of Sydney (#Location: Cadia Hill - 33° 27' 28"S, 148° 59' 47"E; Ridgeway - 33° 26' 7"S, 148° 58' 35"E).
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Cadia Hill, Cadia Quarry (Cadia Extended), Cadia East and Cadia Far East are low grade, bulk mining, porphyry style Au-Cu deposits while Ridgeway, 3 km to the north-west of the Cadia Hill open pit and 500 m below surface, comprises a compact body of quartz veins, sheeted and stockwork quartz and quartz-sulphide veins and disseminated mineralisation with relatively higher grade gold and associated copper mineralisation. Together, these deposits, and the associated altered and mineralised envelope define a NW-trending, 7 km long by up to 2 km wide corridor, in which mineralisation has been intersected by drilling to a depth of more than 1900 m on its eastern extremity. Two historically mined skarn altered deposits, Big Cadia and Little Cadia, fall on the NE peripheries of this corridor. The Ridgeway orebody in the NW is interpreted to be at the deepest level in the porphyry system, progressively shallowing through Cadia Quarry, Cadia Hill and Cadia Far East to the shallowest at Cadia East in the SE (Holliday et al., 2002).
Mining and Exploration
Copper and gold were first recognised in the Cadia district in 1851, with sporadic production from several small deposits, the largest being the Iron Duke (or Big Cadia) mine which produced >100 000 t of 5 to 7% Cu from oxidised skarn mineralisation between 1882 and 1917 (see the Cadia Skarns section below for more details). Numerous trenches, limited shallow, mostly exploration shafts of <20 m in depth, and a small open pit were developed over Cadia Hill, targeting both gold and copper along discrete structures, but with no significant production. During the world wars, >1.5 Mt @ ~50% Fe were mined from Iron Duke (Wood and Holliday, 1995).
During the 1950s and 1960s, several major companies explored the district, although no significant work was undertaken over Cadia Hill until the 1970s. Pacific Copper was granted an exploration title over the Cadia Valley area in 1968 and completed extensive drilling programs, mostly over Big and Little Cadia to delineate resources of 30 Mt @ 0.5% Cu, 0.4 g/t Au and 8 Mt @ 0.4% Cu, 0.3 g/t Au respectively. Limited reconnaissance drilling in the vicinity of the historic small open pit on the southeastern margin of Cadia Hill encountered low grade mineralisation, with the best intersection being 97 m @ 0.95 g/t Au, including 34 m @ 1.5 g/t Au. No further work was carried out at Cadia Hill until 1985, when Homestake Mining, in joint venture with Pacific Copper, conducted a regional geochemical sampling program that outlined a cohesive rock and soil anomaly with vales of up to 1.1 ppm Au. Follow-up included RAB and RC percussion holes, whilst a single shallow diamond drill hole 100 m west of the old open cut, intersected 96 m @ 0.6 g/t Au. These values were regarded as disappointing and Homestake withdrew in 1986 (Wood and Holliday, 1995).
Over a four year period from late 1986, BHP Gold Mines, which was seeking additional feedstock for the mill at its small Browns Creek gold mine 17 km to the SE, negotiated with Pacific Copper, by then a Bond Corporation subsidiary, to finally purchase the rights to the title over the Cadia skarn deposits in early 1991. In the interim, Newmont Australia and BHP Gold Mines had merged their interests in 1990 to form Newcrest Mining Limited who became the beneficiary of the acquired title. Although the initial concentration was on the resource at Big Cadia, Newcrest geologists embarked upon a program of relogging past drill core, geological mapping and soil sampling. This led to an understanding that the skarn alteration, volcanic-, and newly mapped monzonite porphyry-hosted mineralisation in old workings were part of a larger, >4 km long hydrothermal system of possible porphyry affiliation. It also delineated a cohesive, 800 x 200 m, +0.4 ppm Au and enveloping +250 ppm Cu C-horizon soil anomaly within a zone of potassic altered host rocks in the Cadia Hill area. Initial drilling in late 1992 encountered similar intersections to those of Pacific Copper and Homestake, but progressively improved, until the sixth and final hole of the planned program, intersected 217 m from 56 m depth @ 1.36 g/t Au, 0.15% Cu. By September 1994, an inferred resource of 230 Mt @ 0.85 g/t Au, 0.16% Cu had been estimated (Wood, 2012).
Conceptual reasoning, and 'wildcat' deep step-out drilling below thick Silurian cover along trend to the SE ensued, encouraged by a local exposure of barren but intensely pyritic phyllic alteration. This progressively led to the discovery of Cadia East and the outer fringe of the deeper, higher grade extension, Cadia Far East, in August 1996 with 112 m at 2.1 g/t Au and 0.56% Cu. A mineral resource of 320 Mt at 0.45 g/t Au and 0.36% Cu was eventually estimated for the shallower Cadia East body of mineralisation, but further drilling and realisation of the extent and grade of the richer Cadia Far East was delayed by the Ridgeway discovery (Wood, 2012). During the same period, further step-out drilling to the NW had encountered the Cadia Quarry mineralisation. Then, augmented by the detection in an induced polarisation survey of what was later realised to be the upper peripheral disseminated pyrite halo to mineralisation, the blind, high grade Ridgeway deposit was intersected. This step out drilling had been painstaking and strongly influenced by alteration and mineralisation pattern vectors. The main Ridgeway discovery hole in December 1996 intersected two intervals of 145 m @ 4.3 g/t Au and 1.2% Cu, from 598 m downhole, and 84 m @ 7.4 g/t Au and 1.3% Cu from 821 m. By 1998, a resource of 44 Mt @ 2.8 g/t Au, 0.82% Cu had been estimated at Ridgeway. Drilling at Cadia Far East was at a reduced rate during the main Ridgeway program, and was based on investigating various vectoring factors, eventually understanding that better grades corresponded to an increase in Au:Cu ratio. By 2001, an initial inferred resource at Cadia Far East of 200 Mt @ 1.1 g/t Au, 0.41% Cu had been estimated (Wood, 2012).
Open pit mining commenced at Cadia Hill in 1998, subsequently extending into Cadia Quarry. Between 1998 and 2011, these pits produced 678.6 Mt @ 0.74 g/t Au, 0.19% Cu, with metallurgical recoveries of 76.4% Au, 84.5% Cu. Sub-level caving underground operations were initiated at Ridgeway in 2002, followed in 2009 by the Ridgeway Deeps block cave at a lower RL. Production between 2002 and 2011 totalled 51.2 Mt @ 2.07 g/t Au, 0.73% Cu, with metallurgical recoveries of 84.5% Au, 90.2% Cu (Thomas et al., 2011). The Cadia East underground panel cave mine began production in January 2013, based on a probable ore reserve of 960 Mt @ 0.61 g/t Au, 0.33% Cu in the combined Cadia East and Cadia Far East. Current remaining reserves and resources are detailed in Reserves and Resources section below.
The Cadia deposits are hosted within both Ordovician volcanic and volcaniclastic country rocks and the related intrusives of the Molong Volcanic Belt, one of four segments of the dismembered intra-oceanic Macquarie Volcanic Arc which falls within the Eastern Subprovince of the Lachlan Orogen. The Macquarie Arc was developed in response to west-dipping intra-oceanic subduction along part of the boundary between eastern Gondwana and the proto-Pacific Plate and was situated on the Gondwana Plate, some 1000 km east of Precambrian continental crust. The intervening area was occupied by a back arc basin that developed on oceanic crust as the proto-Pacific Plate retreated eastwards after the Middle Cambrian Delamerian Orogeny. Subsequent extension, strike-slip translation and thin-skinned tectonics have structurally dissected the single arc into four north to NNE trending structural volcanic belts of Ordovician calc-alkaline rocks that are separated largely by younger rift basins and in part by coeval craton-derived turbidites. Two of these volcanic belts host relatively undeformed, shoshonitic, Ordovician volcano-intrusive complexes that host porphyry and high sulphidation epithermal gold mineralisation.
Geophysical (seismic-reflection: Glen et al., 2002) and geochemical (εNd: Wyborn and Sun, 1993; Pb isotopes: Carr et al., 1995) data suggest that the Macquarie Volcanic Arc developed over a basement of oceanic crust, with little evidence for input from continental crust. Evidence from the two larger volcanic segments (the Junee-Narromine and Molong volcanic belts) Glen et al. (2003) proposed the Macquarie arc developed episodically over a period of about 50 m.y. The three pulses of volcanic activity observed in these belts (Early Ordovician, ~490 to 475 Ma; Middle Ordovician, 464 to 455 Ma; and Late Ordovician to Early Silurian, 450 to 439 Ma) were separated by two major volcanic hiatuses. Glen et al. (2003) also recognised four episodes of porphyritic intrusives in the Macquarie arc: i). ~484 Ma high K calc-alkalic to shoshonitic monzonites; ii). 465 to 455 Ma high K calc-alkalic monzogabbros to monzonites; iii). 455 to 450 Ma medium K calc-alkalic dacites; and iv). ~439 Ma shoshonitic monzodiorites to quartz monzonite porphyries.
These intrusive episodes have associated ore deposits of differing styles, including alkalic to shoshonitic porphyry Cu-Au deposits at Cadia (456 to 454 and 438 Ma), Kaiser and Northparkes (episodic activity from 484 to 439 Ma); alkalic epithermal Au-Zn at Cowal (~439 Ma K diorite to quartz monzonites); calc-alkaline porphyry Cu-Au mineralisation at Copper Hill (~450 Ma), Cargo (~450 Ma), E39, The Dam and Marsden (~439 Ma); and Au skarn at Junction reefs; and high sulphidation Au at Peak Hill and Gidginbung.
The currently exploited porphyry gold-copper deposits are localised in two tight clusters in the Cadia and Goonumbla districts, which are approximately 100 km apart, and fall within a major, long-lived, NW- to WNW-trending, semi-continental scale, structural corridor known as the Lachlan Transverse Zone. The CVO ore deposits formed immediately prior to and during the Late Ordovician to Early Silurian Benambran Orogeny that accreted the Macquarie Arc to Gondwana.
The up to 2.5 km thick Ordovician sequence preserved in the Cadia district commences with the Weemalla Formation which is at least 1000 m thick and comprises a fine grained unit of thinly laminated, carbonaceous to volcanic siltstones, with minor arenaceous volcanic beds (Holliday et al., 2002). The conformably and gradationally overlying Forest Reefs Volcanics are composed of five lithofacies: (1) intermediate volcanic lithic conglomerates, breccias and sandstones, comprising the bulk of the formation; (2) bedded calcareous volcanic sandstone; (3) laminated siliceous volcanic siltstone; (4) massive basaltic to basaltic-andesite flows; and (5) porphyritic basaltic to andesitic hypabyssal to sub-volcanic intrusions, with either pyroxene or plagioclase phenocrysts (Wilson et al., 2003).
Porphyry-style mineralisation is centred on a compositionally zoned, multiphase pluton of dioritic to monzondioritic, monozonitic and quartz monzonitic intrusions, with syenitic phases, that constitutes the Cadia Igneous Complex (CIC). Narrow pipe-like stocks and dykes cutting both the volcanosedimentary rocks and the central intrusive complex are associated with gold and copper mineralisation. The CIC youngs eastwards across the Cadia Valley. U-Pb dating of igneous minerals (Wilson et al., 2007b) indicate the monzonitic intrusives at Ridgeway and a quartz monzonite porphyry stock that lies immediately SW of Cadia Quarry are early Late Ordovician (456 to 454 Ma), whilst the quartz monzonite porphyry stock that hosts the Cadia Quarry and Cadia Hill orebodies, and a similar composition inter-mineral porphyry dyke at Cadia East, are all of Early Silurian (~438 Ma) age.
The unconformably overlying Silurian Waugoola Group is at least 200 m thick, and is predominantly composed of dark grey to green, fine-grained siltstones, with subunits of fine grained, light grey quartz sandstone and a pink crinoidal limestone band. Basalts of the middle Miocene Canobolas Volcanic Complex that are 50 to 80 m thick cover the Paleozoic rocks in the north and east of the district, whilst Tertiary gravels predominate to the south (Holliday et al., 2002).
The CVO deposit cluster is centred on a small NW-trending volcano-sedimentary sub-basin, the controlling faults of which predate porphyry mineralisation and are oriented parallel to a major arc-transverse lineament. Porphyry Au-Cu mineralisation was influenced by reactivation of the sub-basin during an extensional pulse, with dilation of the controlling faults facilitating emplacement of alkalic porphyry dykes and associated sheeted quartz-sulphide veins. During the Middle Silurian, a successor north-trending fault-bound marine basin buried Cadia East, preserving the higher levels of the ore system. These north-trending faults were reactivated as the Cadiangullong thrust system during east-vergent Devonian compression at the end of the Benambran Orogeny, progressively imbricating and juxtaposing blocks containing Cadia Hill, Cadia Quarry and Ridgeway over Cadia East, thereby superposing different levels of the porphyry Au-Cu system (Fox et al., 2015).
The Cadia-Ridgeway cluster of deposits are principally associated with a 3 x 1.5 km late Ordovician composite quartz-monzonite to dioritic porphyry stock and its probable co-magmatic volcanic wall rocks and intercalated volcaniclastics that together form part of an Ordovician volcano-intrusive Cadia Intrusive Complex (CIC). The intrusive complex is represented as the stock at Cadia Hill and Cadia Quarry, a narrow restricted pipe-like intrusion at Ridgeway and as a series of dykes at Cadia East. Overall the stock has an alkaline composition, with mineralisation and alteration being associated with porphyritic quartz-monzonite phases that are altered over an area of 5.5 x 3 km and to a depth of up to 1.6 km, defining a NW trending corridor that encloses the known deposits.
There are five components to the Cadia porphyry system within the mineralised corridor, namely:
(i) Intrusion- and volcanic wall rock hosted sheeted veins at Cadia Hill. Alteration is principally propylitic with little recognised potassic developments, while a late stage phyllic phase was restricted to zones of faulting and is followed by late carbonates. Mineralisation is mainly chalcopyrite and pyrite with lesser bornite within and disseminated around sheeted 1 to 20 mm thick quartz veins in a 100 to 350 m wide, 65° dipping zone that is 1 km long and has not been closed at depth;
(ii) Volcanic wall rock hosted disseminated and sheeted vein mineralisation at Cadia East within moderately to strongly altered lavas and volcaniclastic breccias. Alteration and mineralisation is centred on a steeply dipping, 300 m wide, east plunging core of steeply dipping sheeted quartz-calcite ±chalcopyrite ±bornite ±molybdenite ±covellite ±pyrite ±magnetite veins within a disseminated envelope of chalcopyrite, bornite and pyrite. This core persists down plunge for at least 1.6 km. Alteration types include weak propylitic, weak sericite-silica-albite, moderate to strong silica-albite flooding with hematite and K feldspar, and strong sericite-albite with silica-albite flooding ±tourmaline;
(iii) Intrusion hosted sheeted veins at Cadia Quarry, developed as a 1 km long by 200 m wide package controlled by faulting and fracturing;
(iv) The up to 70 m thick distal, stratabound hematite-magnetite skarns at Big and Little Cadia. Chalcopyrite is the dominant sulphide, with pyrite and calcite interstitial to the magnetite and hematite blades;
(v) Probable late stage distal veins.
Cadia Hill was the first of the deposits to be mined on a large scale as part of the present Newcrest Mining Ltd Cadia Valley Operations. The ore grade mineralisation is predominantly hosted by a quartz monzonite porphyry phase of the CIC, although a small portion cuts a roof pendant of Forest Reefs Volcanics at the eastern end of the deposit (Holliday et al., 2002).
The deposit was exploited via a large tonnage low grade open pit mine. The Cadia Hill deposit is bounded on three sides by postmineral faulting. To the west, a west-dipping reverse imbricate system, the Cadiangullong Fault, which encloses slivers of the Silurian Waugoola Group, truncates the ore and juxtaposes a block of quartz monzonite porphyry hosting the Cadia Quarry deposit over the Cadia Hill mineralisation. On its eastern margin, the quartz monzonite porphyry hosting the Cadia Hill deposit is thrust over Forest Reefs Volcanics carrying the Cadia East mineralisation, by the west dipping reverse Gibb Fault which has a displacement of at least 300 m. The northern side of the deposit is bounded by a NE-striking, steeply NW-dipping fault. Fault dislocation is also evident within the deposit where disparate ore zones with varying metal ratios, grades and vein densities are juxtaposed across fault planes (Holliday et al., 2002).
Mineralisation at Cadia Hill occurs as chalcopyrite, native gold, lesser pyrite and bornite, which are disseminated within and immediately adjacent to the quartz-carbonate veins of a low density sheeted vein array hosted almost entirely within quartz monzonite porphyry of the CIC, with just a small roof pendant of Forest Reefs Volcanics on the eastern end of the deposit. Post-Silurian faulting has bounded and internally dismembered the mineralisation that now occurs as an imbricate thrust slice truncated by faulting in all directions, forming a 300 m wide tabular envelope dipping at 60° to the SW. This envelope persists over a length of ~900 m and to a depth of at least 800 m beneath the surface, although grades diminish below 600 m (Holliday et al., 2002). Within the envelope, veins range from 1 to 100 mm in width, with densities from 2 to 10 per metre, but locally in the core of the deposit may exceed 15 per metre. Gold grades can be broadly correlated with the intensity of chalcopyrite bearing veins, irrespective of the host lithology. In general, the higher copper grades are found in the core of the deposit where chalcopyrite dominates over pyrite. This zone is flanked by decreasing chalcopyrite:pyrite ratios, both outwards from the core and down dip/plunge. The chalcopyrite:pyrite ratio, however increases up dip and to the NW where zones carrying bornite become increasingly abundant. A higher grade copper zone is localised at the northwestern end of the deposit, with grades of up to 0.5% Cu being encountered in an interval where bornite and chalcopyrite occur as minor infill in a crackle brecciated quartz monzonite porphyry (Holliday et al., 2002).
A pervasive, rarely texture destructive, propylitic alteration comprising a chlorite, albite, epidote and calcite assemblage is the most widespread overprint. The quartz monzonite porphyry has a pervasive pink colouration due to disseminated, sub-microscopic, hematite in both feldspar phenocrysts and in the groundmass, a feature common to the CIC in the Cadia Valley deposits. Potassic (orthoclase) alteration is manifested as narrow selvages to chalcopyrite and bornite bearing quartz veins and as ragged patches partially replacing some plagioclase phenocrysts and overprinting the earlier albite and chlorite phase and its associated magnetite veining. In addition, late- to postmineral, milled, jigsaw-fit breccias have chlorite altered rock flour cement. Sericite-pyrite alteration, with localised sphalerite and galena is also found, in association with NWstriking late mineral faults, while weakly developed postmineral crackle breccias have a laumontite-epidote-calciteorthoclase±fluorite cement and are found throughout the deposit (Holliday et al., 2002).
Cadia Quarry (now known as Cadia Extended) lies in the hangingwall block of the west-dipping Cadiangullong reverse fault, and is located immediately to the NW of the Cadia Hill pit. It is almost entirely hosted by quartz monzonite porphyry (Holliday et al., 2002). The deposit was exploited via a high tonnage, low grade open pit, which is an extension of the Cadia Hill mine. Mineralisation and alteration is largely similar to that described above for Cadia Hill. However, in addition to the sheeted quartz-carbonate vein mineralisation, there are locally high copper-molybdenum zones containing coarse grained chalcopyrite and molybdenite, which are intergrown with quartz-orthoclase-biotite-calcite-pyrite as cement in open space pegmatitic breccias within the host quartz monzonite porphyry. The breccias follow the NW to NNW-structural grain of the Cadia district and take the form of elongate pipes/dykes up to 150 m long and 10 m wide, which persist to depths of as much as 500 m. The clasts within the breccias are strongly sericite altered quartz monzonite porphyry, while the pegmatitic textures and the mineralogy are suggestive of high temperature formation (Holliday et al., 2002). The Cadia Quarry mineralisation has a grade boundary to the west, where its tenor decreases to that of a geochemical anomaly which persists under cover for some 2 km to the west, to beyond the Ridgeway deposit. To the north, the deposit is terminated at the steep intrusive contact between the host quartz monzonite porphyry and the Forest Reefs Volcanics. This contact contains some localised, weakly gold-copper mineralised epidote-garnet-magnetite skarn. To the south, copper and gold grades gradually decrease as the quartz monzonite porphyry grades into a more mafic phase of the CIC (Holliday et al., 2002).
Cadia East and Cadia Far East, are exploited by the Cadia East mine. Together they extend SE to ESE over an interval of ~2500 m in strike length, 200 to 300 m in width and >1900 m in vertical extent, plunging to the SE. It is located to the east of, and structurally below the Cadia Hill deposit. The composite deposit is hosted by a more than 2000 m thick, shallow to flat dipping sequence of the Forest Reefs Volcanics, comprising predominantly volcaniclastic breccias and conglomerates (known as lithofacies 1) and lesser pyroxene- and feldspar-phyric lavas (known as lithofacies 4). Minor monzodiorite to quartz monzonite stocks and dykes belonging to the CIC intrude these Forest Reefs Volcanics units, and in part host mineralisation at depth in Cadia Far East. The Ordovician rocks and the mineralisation are unconformably overlain by up to 200 m of the Silurian Waugoola Group (Holliday et al., 2002).
Mineralisation occurs a two broad, overlapping zones, namely:
• An upper zone of disseminated, copper dominant mineralisation within a 200 to 300 m thick, shallow dipping, unit of volcaniclastic breccia (lithofacies 1) where it is sandwiched between two coherent porphyritic volcanic bands (of lithofacies 4) - an upper feldspar porphyry and a lower pyroxene-phyric unit. This zone comprises the shallow western sections of the Cadia East open pit deposit. Within this zone, disseminated chalcopyrite-bornite forms a core zone, capped by chalcopyrite-pyrite mineralisation (Holliday et al., 2002).
• A deeper, central gold rich zone with sheeted veins, which is localised around a core of steeply dipping sheeted quartz-calcite-bornite-chalcopyritemolybdenite±covellite±magnetite veins. The highest grade gold is associated with the widest bornite-bearing veins, where native gold is commonly intergrown with bornite (Holliday et al., 2002).
Elevated molybdenite levels are mostly associated with the upper disseminated copper zone, although molybdenum continues below this zone at depth, where it also occurs along both the hangingwall and footwall of the gold rich sheeted vein interval (Holliday et al., 2002).
Three alteration styles and zones were recognised by Holliday et al., (2002), as follows:
i). Intense silica-albite±orthoclase±tourmaline, with a late sericite-carbonate overprint. Pyrite and minor fluorite are observed, although no magnetite remains. This zone forms a layer at shallower depths, that is semi-conformable with the Forest Reefs Volcanics stratigraphy, replacing more permeable volcaniclastic breccias. It is mainly the product of late sericite-carbonate and tourmaline overprinting of zone 2 type alteration and the destruction of magnetite. The upper disseminated copper rich mineralisation falls within this alteration zone.
ii). Moderate to intense, grey, silica-albite-orthoclase flooding with minor hematite staining. Hydrothermal magnetite is common and chlorite occurs as a late overprint. This style of alteration grades into an outer propylitic zone of chlorite-epidote±actinolite±calcite.
iii). Pervasive potassic alteration comprising albite-orthoclase-quartz-biotite-actinolite-epidote-magnetite with sulphides. Late chlorite is an overprint on biotite. Albite replaces magmatic plagioclase, while orthoclase occurs as an alteration selvage to mineralised veins. This zone occurs at greater depths, and overprints and passes out and upward into zone ii. The mineralised sheeted veins, particularly the gold rich zone, are accompanied by the most intense developments of this potassic zone, although the sheeted veins also persist into zone ii alteration.
Cadia East and Cadia Far East have been dislocated by at least three significant fault zones. Reverse movement on the major NE-trending, west dipping, Gibb Fault truncates the mineralised system and juxtaposes the Cadia Hill deposit over the Cadia East mineralisation on its western margin. A second, un-named, east trending reverse fault with a steep north dip occurs around 1 km to the east of the Gibb Fault and has displaced mineralisation by at least 100 m. A third significant fault is the east trending Pyrite Fault Zone which lies parallel to the main mineralisation direction at Cadia Far East, and has both syn- and post-mineralisation movement as indicated by milled clasts of pyrite, quartz and carbonate within a locally sericitic fault gouge (Holliday et al., 2002).
Cadia Skarns - Two gold-copper-hematite-magnetite skarns, Big Cadia (also previously known as Iron Duke) and Little Cadia, have long been known in the Cadia Valley. Prior to the discovery of Cadia Hill, Iron Duke (Big Cadia) had been by far the largest producer in the district, having yielded more than 100 000 t of secondary copper ore @ 5 to 7% Cu from underground operations from 1882 to 1898, and 1905 and 1917, and 1.5 Mt of iron ore @ approximately 50% Fe from 1918 to 1929 and 1941-1943 (Welsh, 1975). Based on drilling during the 1960's, there is an estimated potential of 30 Mt @ 0.4 g/t Au, 0.5% Cu for 12 tonnes of contained gold at Big Cadia and 8 Mt @ 0.3 g/t Au, 0.4% Cu for 2.4 tonnes of contained gold at Little Cadia (Holliday et al., 2002).
Big Cadia lies about 100 m north of the drill intersected contact of CIC monzonite and is some 200 m north of Cadia Quarry, while Little Cadia is some 800 m north of the Cadia Far East deposit (Holliday et al., 2002) and 2 km SE of Big Cadia (Holliday et al., 2002). Both skarn zones are around 1000 m long, 250 m wide and average 40 m thick, although in the centre of Big Cadia it reaches 70 m and is 50 to 85 m thick at Little Cadia. Weathering has resulted in the oxidation and slight secondary enrichment of each of the skarns (Welsh, 1975; Holliday et al., 2002). Primary gold-copper mineralisation at both occurs in association with the hematite-magnetite skarn that formed in the impure bedded calcareous volcanic sandstones of lithofacies 2, at the top of the Forest Reefs Volcanics. Elevated copper and gold grades are found in both the skarn and in a surrounding alteration envelope of epidote-quartz-actinolite-chlorite-sericite-calcite-rutile imposed on volcanic conglomerates of the underlying lithofacies 1 of the Forest Reefs Volcanics. Where best developed, the skarn comprises intergrowths of fine to coarse bladed hematite (partially replaced by magnetite) with interstitial calcite±chlorite±pyrite/chalcopyrite. Green (1999) presented mineralogic and isotopic evidence that suggested fluids infiltrated northwards from the CIC, along the volcaniclastic unit, to form Big Cadia. At Little Cadia many drill holes have intersected monzonite possibly belonging to the CIC below the skarn (Holliday et al., 2002).
Ridgeway is a high grade gold-copper porphyry deposit. It is the deepest formed and highest grade of the four main deposits within the Cadia-Ridgeway mineralised corridor. The deposit is an upright, bulbous body of stockwork quartz veining and alteration zoned around a 50 to 100 m diameter, vertically attenuated, alkalic intrusive plug of porphyritic Cadia Hill Monzonite, which is of monzodioritic to quartz monzonitic composition and is part of the CIC, but some 500 m NW of exposures of the main CIC body, and concealed at a depth of 450 m below the present surface (Wilson et al., 2003). Mineralisation and alteration are hosted both by the intrusive and by the surrounding volcanic rocks of the Forest Reefs Volcanics, at and just above, the contact with the underlying Weemalla Formation. The dominant volcanic host occurs as massive bands that are >50 m thick of intercalated volcanic lithic conglomerates to breccias, and bedded volcanic sandstone. Intercalated with these bands are up to 100 m thick packages of plagioclase, crystal-rich volcanic sandstones that
may locally, but not commonly, show graded bedding on scales of metres to tens of metres. Other minor lithofacies include clinopyroxene-phyric basaltic to basaltic andesite flows and a series of steeply north to NE dipping clinopyroxene-phyric basaltic to plagioclasephyric andesitic dykes (Wilson et al., 2003).
The Ridgeway complex of intrusions are physically separated from, but are petrographically and compositionally identical to, and is believed to be connected at depth to, the main Cadia Igneous Complex (CIC). The earliest phase of the Ridgeway intrusions is an equigranular monzodiorite occurring as a NW elongated, steep north dipping, 200 x 50 x 500 m body with an elliptical cross section, located on the southwestern margin of the Ridgeway orebody. In detail it occurs as two lobes, cut by the mineralisation, and is interpreted to be pre-mineral (Wilson et al., 2003).
The main mineralisation at Ridgeway is spatially related to three groups of monzonite intrusions (early-, inter- and late-mineral), all of which are post-monzodiorite. They form an irregularly shaped composite plug with dimensions of 70 x 100 x 600 m, immediately to the NE of the
monzodiorite. The individual bodies of the composite mass having dimensions from metres to tens of metres horizontally and up to 200 m vertically. Multiple intrusion and mineralising phases are indicated by truncation of contacts and veins (Wilson et al., 2003).
The highest grade gold accompanies the most intense alteration and stockwork development immediately adjacent to the monzonite porphyry, with the best being localised directly above the plug compared to grades on its lateral margins. Grades decrease laterally outwards and inwards from the intrusive contact.
The top of the Ridgeway deposit (defined by the 0.2 g/t Au cut-off) is some 500 m below the current surface, and takes the form of a subvertical, pipe like, quartz-sulphide vein stockwork body, with a WNW elongated axis and an elliptical 150 x 250 m horizontal shape which persists over a vertical interval of more than 600 m. Distinct styles of veining and alteration are related to each of the three monzonitic intrusive phases of the igneous complex. The metal grades and intensity of alteration decrease from the early- to the late-mineral phases of the intrusive (Wilson et al., 2003).
Early-mineral intrusion is accompanied by intense actinolite-magnetite-biotite (calc-potassic) alteration and up to four stages of high grade quartz-magnetite-sulphide veins, all of which contain abundant magnetite, actinolite and bornite with variable amounts of chlorite, biotite, chalcopyrite, pyrite, quartz and orthoclase. Bornite, which is the most abundant sulphide, correlates closely with gold. Magnetite dominates in the earliest vein stage, while in the last, chalcopyrite becomes more important. Some of these veins persist for up to 350 m outwards from the Ridgeway Igneous Complex (Wilson et al., 2003).
Moderate- to weak-intensity potassic alteration as orthoclase-biotite±magnetite accompanies both the inter- and late-mineral intrusions and is associated with chalcopyrite- and pyrite-rich quartz-orthoclase veining. The veining and alteration accompanying the inter-mineral phase intrusives is referred to as transitional-stage veining and transitional-stage alteration respectively. Transitionalstage alteration assemblages are characterised by orthoclase, biotite (mostly retrograde altered to chlorite) and magnetite with minor quartz, titanite and apatite. The transitional-stage veining occurs as up to 4 styles which contain variable amounts of magnetite, chalcopyrite and pyrite with quartz and orthoclase, while bornite is rare to absent. The late-mineral monzonite intrusives is accompanied by weak late-stage alteration, occurring as weak pervasive potassic (orthoclase) development around late-stage veins, and chlorite alteration of mafic components of the monzonite. The late-stage veins are characterised by pyrite±chalcopyrite with fluorite, but no bornite or actinolite, and gangue progressing from quartz to sericite to chlorite-calcite from early to late phases (Wilson et al., 2003).
Three discrete and partially zoned hydrothermal alteration suites are found on the periphery of the Ridgeway deposit, namely: i). an inner propylitic; ii). an outer propylitic; and iii). a sodic assemblage. These are peripheral to, and locally overprint, the potassic phase. Peripheral veins are characterised by epidote, prehnite, quartz and calcite in varying proportions with varying sulphides, depending on the position within the deposit. Some of the outer veins, up to 200 m beyond the inner propylitic zone, carry chlorite/ calcite-sphalerite-chalcopyrite ±galena. Phyllic alteration is only found on the margins of late stage faults (Wilson et al., 2003).
Reserves and Resources
The total pre-mining resources were:
Cadia Hill in 1997 - 352 Mt @ 0.63 g/t Au, 0.16% Cu for 221.3 t of contained Au;
Cadia Quarry in 2003 - 50 Mt @ 0.40 g/t Au, 0.21% Cu for 21.7 t of contained Au;
Ridgeway in 2002 - 54 Mt @ 2.5 g/t Au, 0.77% Cu for 132.6 t of contained Au.
Cadia East was un-mined in 2010.
The remaining proved+probable reserves in August 2010 (Newcrest website) were:
Cadia Hill - 116 Mt @ 0.60 g/t Au, 0.14% Cu;
Ridgeway underground - 101 Mt @ 0.81 g/t Au, 0.38% Cu;
Cadia East underground - 1073 Mt @ 0.60 g/t Au, 0.32% Cu.
The total measured+indicated+inferred resources at the same date were:
Cadia Hill - 408 Mt @ 0.42 g/t Au, 0.12% Cu;
Cadia Extended - 83 Mt @ 0.35 g/t Au, 0.20% Cu;
Ridgeway underground - 155 Mt @ 0.73 g/t Au, 0.38% Cu;
Big Cadia - 42 Mt @ 0.38 g/t Au, 0.40% Cu;
Cadia East underground - 2347 Mt @ 0.44 g/t Au, 0.28% Cu.
The total declared measured+indicated+inferred resource in the Cadia district was estimated in 2010 to contain 1360 tonnes (43.7 Moz) of gold and 7.99 Mt of copper.
Total production from Cadia Hill and Cadia Quarry prior to the latter being put on care and maintenance in June 2012 (Newcrest Mining reports), was:
687.37 Mt @ 0.74 g/t Au, 0.19% Cu;
Total production from Ridgeway prior to the latter being put on care and maintenance in March 2012 (Newcrest Mining reports), was:
76.69 Mt @ 1.83 g/t Au, 0.63% Cu;
The remaining mineral resources and ore reserves at 31 December 2016 (Newcrest Mining Reserves and Resources report, 2017) were:
Cadia East - 0.18 Mt @ 1.1 g/t Au, 0.33% Cu;
Ridgeway - nil;
Other - 140 Mt @ 0.47 g/t Au, 0.13% Cu;
Cadia East - 3000 Mt @ 0.38 g/t Au, 0.26% Cu;
Ridgeway - 110 Mt @ 0.56 g/t Au, 0.30% Cu;
Other - 120 Mt @ 0.38 g/t Au, 0.17% Cu;
Cadia East - nil;
Ridgeway - 41 Mt @ 0.38 g/t Au, 0.40% Cu;
Other - 39 Mt @ 0.4 g/t Au, 0.25% Cu;
TOTAL Resources - 3450 Mt @ 0.39 g/t Au, 0.25% Cu
Cadia East - nil;
Ridgeway - nil;
Other - 23 Mt @ 0.30 g/t Au, 0.14% Cu;
Cadia East - 1500 Mt @ 0.48 g/t Au, 0.28% Cu;
Ridgeway - 80 Mt @ 0.54 g/t Au, 0.28% Cu;
Other - 67 Mt @ 0.59 g/t Au, 0.15% Cu;
TOTAL Reserves - 1670 Mt @ 0.48 g/t Au, 0.27% Cu
The Cadia-Ridgeway mines are operated by Newcrest Mining Ltd.
Boddington - Western Australia, .................................... Wed. 16 November, 2011
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The Boddington gold-copper deposit is one of the largest gold producers in Australia. It is located 13 km WNW of the township of Boddington, and 95 km SE of Perth in Western Australia (#Location: 32° 44' 50"S, 116° 54' 19"E).
The deposit lies within the northern half of the small Saddleback greenstone belt in the South West Terrane of the Archaean Yilgarn craton. It also falls within the southern section of the Darling Plateau bauxite province that includes a cluster of large bauxite deposits, several of which overlie the Saddleback greenstone belt. Gold mineralisation has been exploited at Boddington in two major phases. The first was between 1986 and 2001, mainly from the bauxitic laterite and saprolitic profile, complemented by underground high grade, hard rock, lode ore from the Jarrah quartz veins. The second phase of mining commenced in 2009 and continues to the present, exploiting the large, hard-rock Wandoo deposit within the underlying greenstone basement.
The Saddleback greenstone belt was first recognised in 1976 (Wilde, 1976) during regional mapping by the Geological Survey of Western Australia (GSWA). A program of stream sediment and follow-up soil and rock sampling by the GSWA in 1978 over sections of the greenstone belt returned anomalous values of Au (up to 1.6 ppm), As, Cu, Pb, Mo and Zn (Davy, 1979). These results encouraged Reynolds Australia Mines Pty Ltd to undertake a program of surface laterite sampling for gold in 1980, and selected re-assaying of vacuum drill holes for gold from a bauxite program conducted over the area by Alwest Pty. Ltd. in 1977 and 1978. This resulted in the estimation of a resource of 15 Mt @ 2.77 g/t Au by 1981, followed from 1981 to 1984 by a 50 x 50 m program of reverse circulation core drilling to bedrock, outlining a pre-JORC recoverable reserve of ore and marginal ore of 45.1 Mt @ 1.80 g/t Au in 1984 (Symons et al., 1990).
Mining commenced in 1986 at a rate of 3 Mtpa, with the first gold pour in July (Collings and El Ansary, 1987). The bulk of the ore mined was in clay, which did not require blasting. Worsley Alumina Pty Ltd, who also operated the nearby Boddington Bauxite Mine, managed the Boddington Gold Mine on behalf of a joint venture between Reynolds Australia Alumina Ltd (40%), the Shell Company of Australia Ltd (30%), BHP Minerals Ltd (20%) and Kobe Alumina Associates (Australia) Pty Ltd (10%). By 1994, after a series of corporate divestments, amalgamations and asset sales, the partners were Normandy Poseidon (44.44%), Acacia Resources (33.33%) and Newcrest Mining (22.23%).
The Hedges Gold Mine exploited the northwestern section of the oxide deposit on the Alcoa of Australia bauxite mining leases from 1988 at a rate of 2 Mtpa, operating separately until purchased by the Boddington Gold Mine partners in 1998.
In 1990, the high grade Jarrah Quartz Veins were discovered and put into production through mining at the bottom of the laterite pit and as an underground decline operation, to recover 9.3 tonnes of gold between October 1992 and March 1997.
Mining was suspended on 30 November 2001 after the known oxide ore resource had been processed and ~189 tonnes of gold and 6500 tonnes of copper had been recovered from ~134 Mt of oxide and ~5 Mt of ‘hard rock’ ore from Hedges and Boddington (Newmont staff, pers. com. 2011). The lateritic orebody was mined by selective open cut methods over an area of 4.5 x 1 km. Gold was extracted by a carbon in leach processing, with an average metallurgical recovery of 95% (Symons et al., 1990).
Although the laterite resource had been depleted, the presence of low grade basement mineralisation had been known since the late 1980s, and a program to define and develop the hard rock Wandoo resource had been initiated in 1994 and was ongoing in 2001.
Newmont acquired Normandy Mining Limited in 2002, then the Newcrest share of the joint venture in 2006. AngloGold had merged with Acacia Resources in late 1999, and sold its Boddington interest to Newmont in 2009, who then became the sole owner of the operation. Testing over this period had progressively enlarged the resource, which by 31 December 2009, totalled a proved + probable reserve of 875 Mt @ 0.75 g/t Au, 0.11% Cu, for 650 tonnes of contained gold with an estimated metallurgical recovery of 82% (Newmont 2009 Reserves and Non-Reserve Mineralization report). Following a positive feasibility study, Newmont commenced the second phase of mining in 2009 as two major open pits over a length of 3.75 km within the earlier laterite pit, with the first gold poured in November of that year. The current mill capacity of the operation is 39 Mtpa. Remaining reserves and resources are outlined below.
The Boddington Au-Cu mine and Saddleback greenstone belt are located within the Archaean South West terrane, which covers an area of ~100 000 km2, and was amalgamated with the adjacent Youanmi terrane at ~2.65 Ga (Cassidy et al., 2006). It is characterised by a paucity of preserved greenstone belts compared to adjacent terranes of the Yilgarn craton, and those that are known, are comparatively small.
The South West terrane is predominantly composed of granitic rocks, the majority of which are divided into five main overlapping suites based on geochemical characteristics, emplaced between 2.75 and 2.62 Ga with a volumetrically smaller group dated at 2.85 to 2.70 Ga (Qiu and McNaughton, 1999). The bulk of these granitic rocks are younger than 2.69 Ga, and mainly comprise monzogranite, granodiorite and alkali feldspar granites. The younger granitoids were emplaced between 2648 and 2626 Ma, with continued cooling and Pb loss from zircons, from 2628 to 2616 Ma. Post-tectonic, ~2580 Ma granitoids also occur towards the eastern margin of the terrane (Wilde et al., 1996; Nemchin and Pidgeon, 1997).
Extensive granulite facies metamorphism with associated migmatite and charnockitic granites were formed between ~2.64 and 2.62 Ga producing a series of high-grade metamorphic domains. The most significant are the Balingup and Jimperding metamorphic belts. An extensive, but irregular, corridor of granitic gneiss and migmatites, with associated small layered mafic intrusions, continues SE from the latter to the southern edge of the Yilgarn Craton. These metamorphic domains appear to be related to broad zones of shallow, west-vergent, thrusting (Wilde et al., 1996).
Supracrustal sequences are mainly represented by 3.2 to 2.8 Ga quartz-feldspar-biotite gneisses that are predominantly after siliciclastic and chemically-precipitated metasedimentary rocks protoliths and orthoquartzites (Cassidy et al., 2006). These sequences generally predate the granitoids and are mostly preserved within extensive zones of granulites, migmatites and granite gneisses, particularly within the cores of the Balingup and Jimperding metamorphic belts.
Preserved greenstone belts are limited in extent and number, and have a range of ages. In the west, the Wongan Hills greenstone belt to the north has been dated at ~3.01 to 2.82 Ga, similar to the metasedimentary gneisses, and comprises mafic and felsic volcanic rocks, chert, BIF and small ultramafic intrusions. The Morangup and Saddleback greenstone belts are younger, composed of basaltic, intermediate and lesser felsic volcanic rocks, ultramafic dykes and granitoids emplaced between 2714 and 2612 Ma. Both are partially fringed by granulites and older metasedimentary gneisses. The latter are interpreted to represent older sialic basement onto which the greenstone belt rocks have been emplaced. Both are otherwise truncated by younger granitoids (Wilde et al., 1996).
Saddleback Greenstone Belt
The main Saddleback greenstone belt (SGB) trends NNW-SSE over a strike length of 42 km. It is straddled by two major late shear zones, with a width that is generally ~6 km, but is up to 8.75 km in the southern half. Gravity data indicates that the vertical thickness of the belt increases from around 1 km in the vicinity of Boddington Gold Mine, to over 4 km in the south of the SGB. It is enclosed by younger granites in the northern section, but to the south has contacts with both older metasedimentary gneisses and with granulite gneisses and migmatites. Wilde (1976) observed both faulted and intrusive contacts defining the outer margin of the SGB.
Wilde (1976) divided the sequence within the SGB into three units. The first is the Hotham Formation, which is restricted to a 3 km2 triangular area on the south-eastern margin of the SGB, and is largely composed of meta-sedimentary rocks. The sequence comprises a suite of quartz-mica schist, meta-siltstone, meta-tuff and fine-grained meta-greywacke, followed to the east by the dominant meta-siltstones with a few thin bands of agglomerate, which in turn, passes conformably via a transitional boundary into the volcanic rocks of the Wells Formation.
The Wells Formation, which is ~5500 m thick in the Boddington Gold Mine area, comprises interfingering andesitic, dacitic and rhyodacitic lavas, porphyritic lavas, tuffs, breccias, agglomerates and minor sedimentary rocks, all of which are variously deformed and metamorphosed. In general, lavas are more abundant in the north whilst pyroclastic rocks predominate in the southern half of the SGB. Relatively thick bands of dacite and andesite have been differentiated in mapping towards the northeast, hosting the ore deposit. This formation corresponds to the interleaved basaltic, intermediate to felsic, dacitic and andesitic volcanic lithofacies divisions shown on the accompanying image.
The Marradong Formation is 3000 to 8000 m thick and dominated by meta-basaltic rocks with only minor dark metasedimentary intercalations and rare bands of more evolved volcanic lithologies. The contact with the Wells Formation is reasonably abrupt and conformable, although thin bands and lenses of meta-dacite occur within the western basaltic rocks close to the contact. Laterite cover of this formation is almost complete and as a consequence exposure is poor.
Within the northern SGB, the Wells and Marradong formations are cut by shallow granitoid plutons which include weakly to strongly porphyritic diorite and microdiorite, monzodiorite, quartz-diorite, granodiorite and tonalite, as well as ultramafic dykes and late monzogranite.
According to Allibone et al. (1998), geologic mapping and radiometric dating in the Boddington Gold Mine area suggest the Wells and Marradong Formations, and associated intrusions, are the result of at least five discrete Archaean magmatic events, two of which are apparent within the Wells Formation, suggesting a temporal break within that sequence. These are as follows: (i) From ~2714 to 2696 Ma, indicated by intrusive monzodiorite to granodiorite dated at 2714±2 and 2696±4 Ma in the central Boddington Gold Mine, intruding host volcanic rocks of the Wells Formation, which must therefore fall within the same age range or be older; (ii) Between ~2696 and 2675 Ma, represented by a set of NNW trending peridotite-pyroxenite-dolerite to gabbroic dykes that may be up to a few tens of metres thick; (iii) At ~2675 Ma, when a second suite of intermediate to basaltic volcanic and volcaniclastic rocks were deposited to the north and east of the deposit, visually indistinguishable from the earlier suite, accompanied by coeval granodiorite-tonalite intrusions; (iv) Numerous pyroxenite dykes that cut rocks from all three previous events, but predate the fifth; (v) ~2612 Ma, represented by the un-mineralised Wourahming monzogranite to the northeast of the Boddington Gold Mine, cutting rocks associated with all previous events. This intrusion is distinct in terms of its texture, magnetic signature, radiometric signature and age.
All of these magmatic rocks are cut by at least three generations of Proterozoic dolerite dykes that are locally up to 80 m thick.
Wilde (1976) mapped the Marradong, Wells and Hotham formations as three time stratigraphic units younging to the NE. However, McCuaig et al. (2001) suggest the Marradong Formation is the oldest, temporally overlapping the Wells Formation, both formed during the ~2714 to 2696 Ma magmatic event, whilst the 2675 event also contributed to the Wells Formation, and the Hotham formation post-dates both.
Three generations of NNW to NW striking ductile shear zones are recognised in the Boddington Gold Mine area, each characterised by a distinct mineral assemblage and an incipient to strongly developed foliation, generally subparallel to the primary lithologic layering. D1 and D2 are both indicated to have northeast-side up, dominantly dip-slip displacement and are separated by the first pulse of ultramafic dykes, whilst D3 postdates the ~2675 Ma magmatic event. A fourth deformation D4, produced numerous narrow WSW oriented brittle faults that accompanied emplacement of the second generation of ultramafic dykes and cut all the rocks emplaced at or before -2675 Ma. This set of parallel, en echelon D4 faults are restricted to a kilometre-wide, generally NW-SE elongated rectangular zone in the core part of the Boddington Gold Mine. It also reactivated D1/D2 structures (Allibone et al., 1998).
Mineral assemblages within the SGB rocks indicate metamorphism to upper greenschist and lower amphibolite facies under relatively low-pressure conditions between 2650 and 2630 Ma, in contrast to the granulite facies of the enclosing older rocks framing the SGB in the south.
Regolith and Oxide Mineralisation
The regolith at the Boddington Gold Mine was developed by in situ weathering of Archaean bedrock, with only minor transport of the residuum. It is divided into upper laterite and lower saprolite zones that are 2 to 15 and 25 to 80 m thick respectively, separated by a 1 to 5 m thick transitional ferruginous clay zone. From the surface, the laterite zone comprises 1 to 10 m of topsoil and loose gravel, which includes nodules and pisoliths of hematite and maghemite. These are underlain by 1 to 2 m of ferruginous duricrust which forms an almost continuous blanket over large areas and may be either pisolitic or fragmental. The latter contains fragments from a few mm to >30 mm across in a matrix of gibbsite and goethite. The underlying B-zone forms the base of the laterite profile and comprises a yellow-brown gibbsitic bauxite with goethite, hematite and minor kaolinite. It varies from 1 to 10 but averages ~4 m in thickness, is locally mottled and contains patches of saprolite and incipient nodules.
The underlying saprolite is 25 to 80 m thick, and ranges from white to multi-coloured, mottled and ferruginous kaolinitic clays, frequently with preserved rock textures and less destructive and very varied weathering in contrast to the laterite zone. There is a relatively abrupt saprock transition over a few metres into bedrock. The saprock is characterised by green smectitic clays, with rock fragments and well preserved textures. This interval is characterised by its retention of essentially all alkali elements (Symons et al., 1990; Anand, 2005; Collings and El-Ansary, 1987).
Bauxite mineralisation is hosted by the lateritic gravels, duricrust and B-zone. A semi-continuous blanket of gold mineralisation within the duricrust, B-zone laterites and in the underlying ferruginous clay zone accounts for ~30% of the gold in the regolith. This blanket has relative homogeneity and apparent continuity over lateral distances of hundreds of metres. The overlying lateritic gravels contained only anomalous, but not economic, concentrations of gold, and were generally discarded as overburden (Symons et al., 1990). Within the laterite zone, gold occurs in intimate association with iron and aluminium hydroxides, and was precipitated at a redox front at or near the Tertiary water table, whilst gold is depleted from the upper duricrust, suggesting ongoing leaching. Gold occurs as discrete grains, usually from 1 to 10 µm in diameter, with a fineness of >990 (Symons et al., 1990).
The remaining 70% of the gold in the regolith is hosted within the saprolitic clay and saprock zones. In contrast to the laterite zone, gold in the saprolite clays is relatively inhomogenous and has poorer continuity. It occurs in primary quartz veins, in clays immediately adjacent to mineralised quartz veins, and in secondary, shallow dipping, goethitic horizons. The latter are interpreted to represent precipitation at paleo-water tables. However, clays with little or no quartz or ferruginisation may also be well mineralised (Symons et al., 1990). Symons et al. (1990) noted that, whilst concentrations of secondary gold in shallow dipping goethitic horizons are locally dominant, detailed sampling confirms that the distribution of gold within the clay zone essentially reflects a primary control. As such remobilization due to weathering is less marked than in the laterite zone. In the saprock zone, the distribution of gold mineralisation is closely related to primary basement mineralisation. Supergene gold-copper-silver mineralisation has also been recorded in the saprock zone, with a downward zoning of copper minerals from malachite through cuprite, native copper and chalcocite to primary sulphides, principally chalcopyrite (Symons et al., 1990).
The bulk of the oxide mineralisation directly overlies a complex of stockwork veining and incipient hydrothermal alteration which covers an area of >25 km2 within the SGB, centered on the late, barren, ~2612 Ma, Wourahming monzogranite. The highest concentration of veining, which encloses the Wandoo deposit, is found to the west of this monzogranite. It occurs within a kilometre-wide package of sheared andesitic and dacitic volcanic and dioritic intrusive rocks focused on the exposed western cusp of a large composite diorite stock, the upper sections of which are sporadically exposed over an area of 15 km2. The core of the composite stock comprises medium-grained aphyric diorite intruded by variably quartz-feldspar porphyritic diorites, and then by a later-stage, blue-quartz ptygmatic-veined quartz diorite.
The Wandoo deposit comprises two main, NW-SE elongated resource shells that are ~2 x 1 and 2 x 0.7 to 1 km to the SE and NW respectively within the corridor of strong veining, separated by a 500 m wide gap. These shells are further divided into at least eight domains based on lithology and grade. Mineralisation is inhomogeneous, with localised concentrations of from >1 to 19 g/t Au in veins, lenses and stock-works. It is characterised by a reduced assemblage, with chalcopyrite and pyrrhotite the dominant sulphides, and minor pyrite, sphalerite, cubanite, cobaltite, arsenopyrite and pentlandite. Molybdenite is locally a major mineral throughout the deposit. There is a strong correlation of Cu, Au, Mo, Bi and W in the core of the mineralised system, with Pb, Zn and Ag enriched on the periphery (Kalleske, 2010).
McCuaig et al. (2001) recognised two stages of mineralisation at Boddington, supported by age dating of associated molybdenite by Stein et al. (2001). The earliest is associated with the pre-2696 Ma diorite intrusion, and is accompanied by extensive silica-biotite alteration, occurring as complex quartz +albite +molybdenite ±clinozoisite ±chalcopyrite ±gold veins variably deformed by D1 ductile shearing. Re-Os ages from molybdenite in these veins indicates a formation age of ~2700 Ma. Subsequently, a ductile D1/D2 event was accompanied by silica-sericite-pyrite alteration, whilst ductile D3 shear zones produced mylonite zones with silica-albite-epidote-pyrite alteration.
McCuaig et al. (2001) observed the second and main stage of mineralisation post-dated all of the magmatic events, with the exception of the late 2612 Ma monzogranite, and accompanied the D4 brittle deformation. It has been divided into the following paragenesis: (i) complex quartz +albite +molybdenite ±muscovite ±biotite ±fluorite ±clinozoisite ±chalcopyrite veins, comprising the main late Mo stage; (ii) Clinozoisite-sulphide-quartz-biotite veining containing the bulk of the low-grade skarn altered Au-Cu mineralisation; (iii) Actinolite ±sulphide ±quartz, carbonate-chlorite-sulphide, and sulphide veins which constitutes the main high grade mineralisation. Re-Os dating of these mineral assemblages (Stein et al., 2001) return a weighted mean model age of 2615±9 Ma, broadly synchronous with the Wourahming monzogranite.
Based on the integration of available geochemical and geological datsets, McCuaig et al. (2001) concluded that the main controls on ore were, in increasing order of importance: (i) The late ductile-brittle NW and WNW striking, subvertical D3-D4 fault network; (ii) Intersection of this fault network with structurally competent lithologies; (iii) Intersection of late faults with early ductile D1/D2 quartz-sericite shear zones. (4) NE-striking faults which compartmentalise the deposit and offset favourable host rocks pre-mineralisation.
High grade vein accumulations also occur within, and external to, the Wandoo resource, such as the Jarrah veins on the northern extremity of the Boddington Gold Mine. This vein system was predominantly within andesitic and to a lesser degree, felsic tuffaceous fragmental rocks and dioritic intrusions, hosted by the Jarrah Shear, a 50 to 150 m wide zone of shearing and deformation of varying intensity and age. The shear contained folded, recrystallised, extensional quartz veins, crosscut by later, brecciated fault structures hosting gold and base metal sulphides.
The Wandoo deposit is interpreted as a structurally-controlled, intrusion-related, Au-Cu deposit, formed by two overprinting magmatic-hydrothermal systems. The first has ‘porphyry-like’ characteristics, associated with dioritic intrusion at ~2700 Ma, the other coeval with a barren post-tectonic granitoid at ~2612 Ma. Although gold is associated with both events, the main stage, particularly the high-grade mineralisation, appears synchronous with the latter (McCuaig et al., 2001).
Reserves and Resources
The basement mineralisation in 2004 comprised (Newcrest Annual Report, 2005):
Proven + Probable Reserve - 395 Mt @ 0.87 g/t Au, 0.13% Cu; (reserves included in resources),
Measured + Indicated Resource - 505 Mt @ 0.86 g/t Au, 0.12% Cu, for 435 t Au;
Inferred Resource - 232 Mt @ 0.80 g/t Au, 0.09% Cu, for 185 t Au.
Following further drilling and development, the reserve and resources at 31 December 2009 (Newmont 2010) were:
Proven + Probable Reserve - 966.4 Mt @ 0.69 g/t Au, 0.11% Cu, for 664 t Au, (reserves additional to resources)
Measured + Indicated Resource - 364.8 Mt @ 0.44 g/t Au, 0.08% Cu, for 159 t Au,
Inferred Resource - 292.9 Mt @ 0.5 g/t Au, 0.11% Cu, for 146 t Au.
Remaining ore reserve and mineral resources at 31 December 2016 (Newmont 2017) were:
Proved Reserve - 205.4 Mt @ 0.76 g/t Au, 0.11% Cu, for 362 t Au,
Probable Reserve - 218.8 Mt @ 0.75 g/t Au, 0.11% Cu, for 362 t Au,
TOTAL Proven + Probable Reserve - 424.2 Mt @ 0.76 g/t Au, 0.11% Cu, for 322.4 t Au, (reserves additional to resources)
Measured Resource - 108.7 Mt @ 0.48 g/t Au, 0.11% Cu,
Indicated Resource - 245.5 Mt @ 0.53 g/t Au, 0.10% Cu,
Measured + Indicated Resource - 354.2 Mt @ 0.51 g/t Au, 0.11% Cu, for 181 t Au,
Inferred Resource - 7.5 Mt @ 0.58 g/t Au, 0.11% Cu, for 4.35 t Au.
TOTAL Resource - 361.7 Mt @ 0.76 g/t Au, 0.11% Cu, for 275 t Au.
Kalgoorlie Superpit - Western Australia .................................... Thu. 17 November, 2011
The KCGM Super Pit at Fimiston exploits the Golden Mile deposit within the Kalgoorlie Gold Field, ~600 km ENE of Perth. The Mount Charlotte deposit, 3 km to the NW, is made up of four orebodies Reward, Charlotte, Maritana and Charlotte Deeps. Other deposits mined in the gold field include Mount Percy, ~1.5 km north of Mount Charlotte and Hannan's North, which is 750 m NW of Mount Percy (#Location: Fimiston Superpit 30° 46' 39"S, 121° 30' 10"E).
Gold was first discovered in what was to become the Kalgoorlie Gold Field in June 1983 by three Irish prospectors, Dan Shea, Tom Flanagan and Paddy Hannan. Hannan and Flanagan had originally come to the area during the Coolgardie gold rush in 1889. By 1893, the alluvial gold at Coolgardie was largely exhausted and only one significant reef was being exploited. News of a 'good discovery' at a place called Mount Yuille started a rush in June 1893, and Hannan, Flannagan and O'Shea joined in. En route they prospected what they regarded as prospective ground, and on June 10, 3 days after leaving Coolgardie they found 'colours of gold' and then encountered 'good gold' from the north end of Mount Charlotte to south of Maritana Hill. In a few days they collected 3 kg of gold at surface, and Hannan set out for Coolgardie and lodged a Reward Claim application on June 17 1893. This discovery sparked a new rush with initial prospecting and mining activity concentrated at the northern end of the Kalgoorlie field where gold occurred in quartz veins and as surficial alluvial/colluvial concentrations familiar to prospectors. Two years elapsed before the full significance of the relatively inconspicuous ferruginous, oxidised outcrops after sulphide and carbonate rich ores to the south was recognised. However, when it was, that part of the gold field, which became known as the Golden Mile, was rapidly developed (Clout et al., 1990). The gold field has been exploited continuously since Hannan's discovery in1893.
In 1896 Kalgoorlie was connected to Perth by rail. In 1903 the Goldfields Water Supply Scheme was completed with a pipeline to provide water from the Mundaring Weir 566 km to the SW, near Perth. By 1908, ~100 headframes studded the Golden Mile and beneath the ground there were >3000 km of workings. Production from the gold field reached a peak in 1903 with 38.124 t of gold extracted from ore at an average grade of 41.1 g/t Au, predominantly from gold-telluride lodes (Blainey, 1993). Subsequently, production declined due to a combination of factors, including fixing of the gold price, rising costs as lodes were followed underground and ores became more refractory. At the same time, many smaller mines were acquired and consolidated into the larger operations. By then the gold field had become well established and production continued from ten major shafts and numerous other smaller operations. Increasing inflation and lack of manpower during and following the First World War had a negative influence on mining activity, although a re-evaluation of gold in 1932-33 sparked a short-lived boom. Production declined again in the 1960s, leading to cessation of mining on the Golden Mile in 1975, although production continued from the Mt Charlotte mine due to the introduction of low-cost mechanised mining. The floating of the gold price in late 1973, and the marked rise in price from 1979 sparked a resurgence in mining, and the Golden Mile operations progressively reopened with new open-cut mines developed.
Throughout its history, the mines of the Kalgoorlie Gold Field had been progressively consolidated into fewer and larger operations until in 1989, when Bond International Gold (Bond) attempted to consolidate the final three main remaining companies, Gold Mines of Kalgoorlie Ltd (GMK), North Kalgurlie Mines Ltd (NKM) and Kalgoorlie Lake View Pty Ltd (KLV) into one operation. By 1989 Bond had gained control of GMK and KLV and a number of other joint ventures over much of the Golden Mile. In April 1989 Bond sold half of its interests to Homestake Gold of Australia Limited, who subsequently separately purchased all the shares of NKM. Bond and Homestake then each controlled 50% of three joint ventures, Mount Percy, Fimiston/Paringa and Kalgoorlie Mining Associates (KMA) that controlled a complex mosaic of mining leases, mining operations and treatment facilities that covered the Golden Mile and Mount Charlotte deposits. The Kalgoorlie Consolidated Gold Mines Joint Venture (KCGM) was formed in 1989 to amalgamate the various mining operations and treatment facilities by combining the mines of the Golden Mile into the single Fimiston Superpit and operating Mount Charlotte as a separate underground mine. In August 1989, the remainder of the Bond Kalgoorlie interests held through GMK were acquired by Normandy Australia. In December 2001, Canadian company Barrick Gold Corporation merged with Homestake and in February 2002, Normandy was acquired by Newmont Mining Corporation. Since then, Barrick and Newmont have been the equal joint venture owners of KCGM and the Golden Mile and Mount Charlotte mining leases.
The Kalgoorlie Gold Field, which occupies and area of ~10 x 2 km, lies within the southern half of the NNW-SSE aligned Neoarchaean, Kalgoorlie granite-greenstone Terrane. This terrane is the western-most tectonic element of the Eastern Goldfields Superterrane that forms the eastern half of the Yilgarn Craton. The Kalgoorlie Terrane forms a NNW-SSE trending strip that is exposed over a width of ~50 to 120 km and length of ~800 km. It is separated from the older Youanmi Terrane to the west by the major east dipping Mount Ida Fault and from the similar Kurnalpi Terrane to the east by the Ockerburry Shear Zone which also dips east. The Kalgoorlie and Kurnalpi terranes were deposited after ~2810 Ma in an extensional rift setting flanked by the Youanmi and Burtville terranes that represent the proto-Yilgarn craton. Granites occupy ~70% of the solid geology of the Eastern Goldfields Superterrane to a depth of at least 15 km, enclosing and underlying a supracrustal succession of bimodal ultramafic-mafic and felsic volcanic rocks in variable proportions, with interbedded volcaniclastic and clastic rocks. The granites and greenstone successions of the Kalgoorlie and other terranes across the Yilgarn Craton developed during three distinct thermal/magmatic events at ~3.1 to 2.9, ~2.8 and ~2.76 to 2.60 Ga.
For more detail see the Yilgarn Craton overview record.
The southern half of the Kalgoorlie Terrane, contains a Neoarchaean greenstone succession overlying and intruded by large volumes of granite, as follows, from the base (after Gauthier et al., 2005; Vielreicher et al., 2016; Phillips et al., 2017):
Kambalda Sequence which comprises a 1500 to 4000 m thick, 2715 to 2690 Ma mafic-ultramafic suite consisting of the:
• Kambalda Komatiite which consists of channelised and overbank spinifex and cumulate textured komatiitic volcanic rocks with individual flows that are up to several metres thick. It is geochemically primitive with minimal crustal contamination, and contains zircons dated at 2708±7 and 2702±7 Ma. It has been subjected to carbonate alteration in the deposit area that includes magnesite and dolomite. Only the upper part of the unit, the >700 m thick Hannan's Lake Serpentinite is is found at Kalgoorlie, where it comprises komatiite flows ranging from picrites to peridotites.
• Devon Consols Basalt, a 50 to 200 m thick sequence of siliceous, high-Mg basalts, interpreted to represent a crustally contaminated komatiite. It has a variolitic texture, with pillowed, flow breccias to massive flows. Where altered is, it has a carbonate assemblage of dolomite and ankerite.
• Kapai Slate, a 1 to 20 m thick, sulphidic, tuffaceous black shale to mudstone marker unit recognised over strike intervals of tens of kilometres. Dated at 2692±4 Ma (U-Pb zircon; Claoué-Long et al, 1988). Carbonate alteration in the deposit area includes calcite and ankerite.
• Williamstown Dolerite Sill, a 150 to 250 m thick sub-volcanic ultramafic gabbro sill, which is geochemically distinct from the tholeiitic sills in the district. It has a bimodal composition, with lower ultramafic and upper gabbroic layers, and has been dated at 2696±5 Ma (U-Pb zircon; Fletcher et al, 2001). It grades laterally into extrusive facies and in mineralised areas has been altered to dolomite and ankerite.
• Eureka Dolerite Sill, a 100 to 200 m thick, differentiated gabbroic subvolcanic sill that is co-magmatic with the high-iron tholeiitic Paringa basalt. It is characterised by a fine to medium grained ophitic texture with chlorite clots after pyroxene that are mantled by carbonates (Bateman et al., 2001). Thin granophyric layers with quartz crystals occur towards the top of the thicker part of the sill. It is intruded within, and is geochemically similar, to the high iron tholeiite unit of the Paringa Basalt (see below). The sill has a gradual normal fractionation trend, with the upper sections being more fractionated and relatively more enriched in incompatible elements such as Zr, Ti, V and P. It is also geochemically distinct from the Golden Mile Dolerite with higher contents of incompatible elements.
• Paringa Basalt, a 300 to 900 m thick pile of basaltic flows grading from a high magnesium basalt (>10 wt.% MgO) with ubiquitous variolitic and local spinifex textures at the base, to a gradually more fractionated tholeiitic basalt (3 to 10 wt.% MgO; Bateman et al., 2001). The uppermost section is characterised by pillow and flow breccia textures and a general increase in interflow sedimentary rocks, mainly finely bedded black shale. The upper section varies from 50 to 300 m in thickness from north to south within the gold field, and consists of a high-iron tholeiite, in sharp contact with the underlying tholeiitic basalt. The contact is usually occupied by a 1 to 5 m thick finely bedded black shale horizon. This high-iron tholeiite is characterised by high TiO2 of 1.5 to 1.8 wt.%, Fe2O3 of 14 to 16 wt.%; Zr of 100 to 130 ppm; P2O5 of 0.2 to 0.25 wt.%; and low MgO of 3 to 5 wt.%, and has a normal fractionation trend. The thicker section of high-iron tholeiite in the south and on the western limb of the Kalgoorlie Syncline is intruded by the ~100 m thick co-magmatic Eureka Dolerite sill. In the eastern section of the Golden Mile, the high-iron tholeiite below the Golden Mile Dolerite, hosts the bulk of the economic Fimiston style lodes (see Mineralisation section below) that occur within the Paringa Basalt (Travis et al., 1971). The basalt has been dated at 2690±5 Ma (U-Pb zircon; unpublished, quoted by Vielreicher et al., 2016). Where altered, it comprises an assemblage that includes ankerite, siderite calcite and dolomite.
The Kambalda Sequence has numerous interbedded interflow sedimentary rocks, and is overlain by the Kalgoorlie Sequence.
Kalgoorlie Sequence, deposited between 2690 and 2660 Ma (Blewett et al., 2010) as follows:
• Oroya Shale which overlies the Paringa Basalt, and where present separates it from the Golden Mile Dolerite sill. Vielreicher et al. (2016) regard this unit as the uppermost of the Kambalda Sequence, whilst Phillips et al. (2017) suggest it is the lowermost member of the Kalgoorlie Sequence, part of the Black Flag Group. It comprises a carbonaceous greywacke that is similar to rocks found above the sill in the Black Flag Group. While many authurs suggest base of the Kalgoorlie Sequence is unconformable (e.g., Blewett et al., 2010), Gauthier et al. (2004) note that deep water carbonaceous clastic sedimentary rocks occur as interflow inter-beds within the upper sections of the Paringa Basalt and associated sub-volcanic sills. These interflow beds are increasingly abundant and thicker towards the top of the Paringa Basalt reflecting the waning stages of mafic volcanism. Consequently they suggest this contact between the Black Flag Group and the underlying Paringa Basalt, at the Golden Mile deposit, is transitional rather than an unconformity (e.g., Krapez et al., 2001). East of the Golden Mile deposit, where the Golden Mile Dolerite sill is absent, the basal mudstone unit of the Black Flag Group that lies above the Paringa Basalt is conformably overlain by a >200 m thick, fine to medium grained greywacke, possibly of volcaniclastic origin (Gauthier et al., 2005).
• Golden Mile Dolerite Sill, which is 600 to 750 m thick and has been dated at 2680±9 Ma (Zircon; Rasmussen et al., 2009) and 2673±5 Ma (Zircon; Claoué-Long et al., 1988). It is a stratabound intrusive unit predominantly found along the contact between the Paringa Basalt and the unconformably overlying Black Flag Group, and is interpreted to have been emplaced at a depth of >5 km below surface. While some authors suggest it is a volcanic unit of ponded lavas (e.g., Golding, 1985), Hayman et al. (2019) present evidence that it is intrusive. It is a fractionated sill that is divided into 10 lithologically distinct units with gradational boundaries that may be traced for >10 km along strike in the immediate deposit area and for ~25 km overall. The sill has fine grained, chilled margins that are 10 to 20 m thick and have a tholeiitic basalt composition, representing units 1 and 10 respectively. Between these margins there is a pattern of crystallisation progressing from the floor through unit 2 to 7, and from Unit 9 in the roof, towards the more fractionated granophyric Unit 8 occurring towards the centre of the intrusion. It represents a pattern of iron and incompatible element enrichment, including silica, as the magma becomes progressively more fractionated, as follows:
- Units 2 and 3 are orthopyroxene-clinopyroxene-plagioclase cumulates characterised by high MgO, Ni (50 to 150 ppm) and Cr (200 to 400 ppm). Unit 2 contains olivine, whilst Unit 3 is dominantly pyroxene bearing, although the two units have gradational boundaries. Together they are 70 to 100 m thick.
- Unit 4 is ~90 m thick and is a pyroxene-phyric band with distinctive aggregates of ilmenite, and is dominated by pyroxenes and plagioclase. It is characterised by flat geochemical profiles with low TiO2, zirconium and vanadium contents. Chrome and nickel become more enriched towards the base of the unit. There is a gradational upper boundary between units 4 and 5.
- Units 5 and 6 both of which are <100 m thick and are sub-ophitic, composed of a plagioclase-orthopyroxene-clinopyroxene assemblage with the appearance of minor primary quartz. However, unit 6 marks the onset of magnetite crystallisation, characterised by 10 to 15% fine-grained magnetite, and has a sharp contact with Unit 5. While Unit 6 is fine grained and sub-ophitic, it grades upward to be weakly granophyric in texture. It is also characterised by a very high V (600 to 1200 ppm) and enriched Cu (200 to 300 ppm compared to 20 to 50 ppm in unit 5) contents. Ni and Cr typically have a small peak at the base of this unit but decrease to very low levels towards the top of the unit, each grading from 50 to 5 ppm. Conversely, Ti and Titanium are gradually enriched towards the top of Unit 6. These geochemical levels are independent of proximity to mineralisation.
- Unit 7 is 100 to 200 m thick, and like units 5 and 6, is characterised by a sub-ophitic plagioclase-orthopyroxene-clinopyroxene assemblage with minor free quartz, although it contains coarse pegmatitic bands, generally a few metres in thickness, that are compositionally similar to unit 8. It is characterised by abundant magnetite, a decreasing V trend and increasing Zr and Ti values towards the top, reflected by the presence of abundant fine grained ilmenite. Ni, Cr and Cu are very low.
- Unit 8 is 100 to 200 m thick, and composed of a granophyric textured clinopyroxene-plagioclase-quartz-ilmenite-magnetite assemblage. It contains 10 to 15 wt.% magnetite, and is characterised by a high content of incompatible elements such as Zr, P and Ti, with low, V, Cr, Ni and Cu. It has been subdivided into two further sub-units, with Unit 8a at the top, which is more areally restricted and is more siliceous, with lesser Fe and Ti than than the underlying U8b. Unit 8a is found sporadically along the western limb of the Kalgoorlie syncline and is the host of the Mount Charlotte quartz-vein stockwork mineralisation. It also hosts several other smaller quartz-stockwork style occurrences along a roughly 12 km strike length of that limb, but has not been mapped on the eastern limb (Travis et al., 1971).
- Unit 9 is 100 to 350 m thick, and comprises a plagioclase-orthopyroxene-clinopyroxene-quartz assemblage with abundant fine grained magnetite and medium to coarse grained ilmenite. It is medium to coarse grained, and locally granophyric where it grades into U8, but above that is mostly sub-ophitic in texture. There is a gradual enrichment in Zr, Ti and V downwards toward Unit 8. It has relatively elevated Cr and Ni at the top towards the upper chilled margin, and a gradual depletion towards the base of the unit 9. It has consistently higher V, Zr and Ti contents compared to units 4 and 5, with a increase in V towards the Unit 8 contact.
The Aberdare Dolerite sill mapped at the northern, eastern nargin and southern end of the Gold Field, has been shown to be equivalent to the Golden Mile Dolerite, with a gradual attenuation of the fractionation and resulting magmatic layering of the latter into the former. The two 'facies' are juxtaposed across the late Adelaide Fault to the south, but have a gradational transition to the north (Gauthier et al., 2005).
• Black Flag Group, a >3000 m thick, 2690 and 2660 Ma succession that comprises cycles of volcanic, volcaniclastic and sedimentary rocks and mostly represents rapidly reworked pyroclastic debris. In the Golden Mile deposit area, the base of the Black Flag Group comprises black
mudstone inter-bedded with siltstone and sandstone beds, similar to that of the Oroya Shale described above (Gauthier et al., 2005).
The Black Flag Group is divided into the Early Black Flag Group, which consists of massive graded to moderately stratified feldspar-rich sandstone, siltstone, felsic cobble-conglomerate, volcanic rocks and associated polymict volcaniclastic and epiclastic rocks, and rare mudstone. The volcanic rocks are mostly tonalite-trondhjemite-dacite with subordinate rhyolite and andesite. Rapidly reworked feldspar-rich pyroclastic debris predominates towards the base and intermediate to felsic volcanic and volcaniclastic rocks in the upper sections. The Late Black Flag Group unconformably overlies the Early Black Flag Group and the Golden Mile Dolerite sill. It consists of coarse mafic conglomerate facies, quartzo-feldspathic sandstone, with interbedded volcanic rocks as well as mudstone-siltstone (Oxenburgh et al., 2017; Squire et al., 2010), and includes clasts sourced from the sill in its basal sections. Two additional, similar cycles are mapped, which are variously attributed to the Black Flag Group (e.g., Vielreicher et al., 2016) or the overlying lower and late Merougil Group (e.g., Squire et al., 2010).
• Intrusions - the Kambalda and Kalgoorlie sequences are intruded by a series of dykes and sills related to the Kalgoorlie Sequence volcanism (after Gauthier et al., 2005; Vielreicher et al., 2016):
- Syn-volcanic, tholeiitic dykes - which are fine grained, chloritic and commonly contain vesicular chilled margins. They occur within the lower black shale portion of the Black Flag Group and commonly have very irregular and peperitic margins. They are commonly strongly sericite + ankerite altered
within the Golden Mile deposit and are cross cut by the feldspar porphyry dykes.
- Quartz-albite porphyry dykes and sills - A suite of syn-volcanic, structurally early, pre-gold, calc-alkaline plagioclase-phyric rhyolite and dacite sills and dykes dated at between 2673±3 and 2669±17 Ma, regarded as feeder dykes to volcanic units in the Kalgoorlie Sequence.
- Hornblende and feldspar porphyry dykes - occurring as local NNE striking and ~74° SE dipping, syn-deformational dyke swarms. These intrusions are a variably metamorphosed, ~2.65 Ga fractionated suite of sub-alkaline to alkaline hornblende-phyric andesite to feldspar-phyric porphyry dykes with Mg-monzodiorite-diorite to granodiorite-tonalite compositions respectively. These are both cut, and are cut by, Fimiston-style lodes and are cut by Charlotte-style lodes (see Mineralisation section below).
The feldspar porphyry dykes contain 10 to 40%, 2 to 10 mm plagioclase phenocrysts and 1 to 10% small rounded quartz phenocrysts within a fine grained quartzo-feldspathic groundmass. They have been dated at 2650±6 Ma (zircon; U-Pb SHRIMP; Vielreicher et al., 2010), although the same author summarises the feldspar-phyric porphyries as being emplace between 2.67 to 2.66 Ga. These dykes also contain trace to 10% hornblende phenocrysts. Feldspar porphyry dykes within the Golden Mile deposit are pervasively sericite + carbonate altered and variably hematitic. Two main large feldspar porphyry dykes occur within the Eastern part of the Golden Mile deposit where they are continuous along strike for at least 2 km, down dip for >1.5 km, and vary in thickness from 5 to 20 m (Gauthier et al., 2005).
A 100 to 400 m thick, composite, steeply west dipping feldspar porphyry dyke intrudes along the trace of the Kalgoorlie syncline to the south of the Golden Mile deposit and cuts the folded mafic volcanic rocks, Golden Mile Dolerite and overlying Black Flag Group sequence. It has wide zones of intrusive breccias, mainly along its margins, and contains several textures interpreted to indicate shallow intrusion (Morris and Witt, 1997; Ong, 1994).
The hornblende porphyry dykes are generally 1 to 5 m thick and are less abundant than feldspar porphyry dykes (Stillwell, 1929). They generally contain 10 to 40%, 2 to 10 mm hornblende phenocrysts aligned along the NW penetrative foliation planes. These dykes also contain 5 to10% plagioclase and 1 to 10% rounded quartz phenocrysts. Within the Golden Mile deposit, the are ubiquitously altered to an assemblage of carbonate and sericite. They have an alkaline affinity, as indicated by their enrichment in incompatible elements such as P2O5, Zr and Y and are interpreted to have been formed by a different magmatic event than the earlier calk-alkaline feldspar porphyry dykes (Gauthier et al., 2005). They have been dated at 2646±13 Ma (zircon; U-Pb SHRIMP; Vielreicher et al., 2010) who also quotes these dykes as being emplaced between 2.66 Ga and 2.65 Ga based on additional evidence..
- Intrusive breccia, which typically occur on the margin of large feldspar porphyry dykes, or as small individual breccias commonly found close to a larger coherent feldspar porphyry dyke. They cut all the host rocks present at the Golden Mile and typically grade from angular jig saw fit of feldspar porphyry clasts within the host rock matrix to more heterolithic and matrix supported breccias. Some exotic fragments within breccias have travelled up to 600 m away from their source. These breccias both cut and are cut by feldspar porphyry dykes, and are regarded to be intrusive, rather than tectonic.
- Late deformational mica lamprophyre and kersantite dykes dated at 2642±6 Ma (zircon; U-Pb SHRIMP; McNaughton et al., 2005) and 2637±20 Ma (monazite; U-Pb SHRIMP; McNaughton et al., 2005), and are both cut and are cut by Green Leader-style lodes and are cut by Charlotte-style lodes, but are not affected by Fimiston-stage alteration (see Mineralisation section below).
Late Clastic Basins - the Kambalda and Kalgoorlie sequences are unconformably overlain by the 2658 to 2655 Ma Kurrawang Formation sequence of polymictic conglomerate, grading upward into fine-grained sandstone and siltstone in extensional fault controlled clastic basins.
Deposition of the Kambalda and Kalgoorlie sequences took place during 2720 to 2670 Ma D1 extension. This was followed by inversion during the D2 ENE-WSW directed compressive event at ~2665 Ma producing NNW upright folding and reverse faulting, responsible for tilting of the host sequence, formation of the Kalgoorlie Syncline and Anticline, regional Boulder-Lefroy and deposit scale Golden Mile faults and a penetrative, subvertical foliation. This was followed by renewed extension during the 2665 to 2655 Ma D3 event, resulting in the development of a series of metamorphic core complexes exposing the main intrusive phase of high-Ca granites that were emplaced from 2740, continuing to 2650 Ma, and accounting for >60% of the granites in the terrane. The volumetrically lesser Low-Ca granitoids, which are reworked earlier High-Ca granites, were all formed between 2655 and 2630 Ma, subsequent to the main greenstone belt magmatism and the clastic basin deposition. Later deformation comprised D4a ENE-WSW contraction, resulting in upright folding and reverse faulting at ~2655 Ma; D4b WNW-ESE contraction, producing sinistral strike-slip shearing and thrusting from 2655 to 2650 Ma; D5 NE-SW contraction and dextral strike-slip transtension from 2650 to 2635 Ma; and D6 Low-strain vertical shortening and horizontal extension after 2630 Ma. The deformation stages D1 to D6 listed herein are as defined by Blewett and Czarnota (2007) in GeoScience Australia Record 2007/15, and resulted from the exhaustive Module 3 structural study of the pmd*CRC and AMIRA Y1-P763 Project that concluded in November 2005. A plethora of stages have been defined by other authors, including Mueller et al. (1988), Swager (1997), Nguyen (1997) and Miller (2006) and are used in current literature, such as Vielreicher et al. (2016) - see Blewett et al. (2010) for a comparison of the different stages defined.
Folding - The Kalgoorlie Gold Field follows the NNW to north-south trending, fault dislocated, upright, early D2 Kalgoorlie Anticline-Syncline pair. The Golden Mile deposit is located where the trend of the axial trace of the structural pair changes from NNW in the south, to north-south in the north. These structures have folded a core of Kambalda Sequence rocks that are intruded by the Golden Mile Dolerite sill and are surrounded by overlying and fault juxtaposed Black Flag Group rocks of the Kalgoorlie Sequence. This syncline-anticline pair dominates the structural architecture in the Golden Mile deposit area (Woodall 1965, Travis et al., 1971).
The Kalgoorlie Syncline is an asymmetric fold with an overturned western limb dipping at 80°W, whilst the eastern limb dips ~30°W in the Golden Mile deposit area. To the south of the deposit, where its strike rotates to trend NNW, the same limb changes dip to ~75°E. The axis of the Kalgoorlie Anticline, immediately to the east, is doubly plunging. In the Golden Mile deposit area, where the axial plane dips at ~80°W, the axis plunges at ~20°S, resulting in progressively older units being exposed along its axial trace towards the NNW. To the north of the deposit, the fold plunges to the north. In the immediate Golden Mile deposit area and northward, the hinge of the Kalgoorlie syncline is offset by the Golden Mile fault (Woodall, 1965), but is preserved ~2 km south of the deposit (Travis et al., 1971; Gauthier et al., 2005).
Smaller scale parasitic folds are developed on the western limb of the Kalgoorlie Anticline, spatially associated with the bend in the axial trace of the first order Kalgoorlie Syncline-Anticline fold pair and on the basis of structural fabric can be inferred to be coeval with formation of the main Kalgoorlie Syncline-Anticline.
A fault coincides with the axial plane of the Kalgoorlie Anticline and with the eastern edge of the Golden Mile Dolerite. This structure is interpreted to be a reactivated normal growth fault active during the emplacement of the intrusion. This conclusion is supporter by the absence of that intrusion on the eastern limb of the Kalgoorlie Anticline, and the distribution and thickness of its constituent units towards the axial plane.
The regional scale Boomerang Anticline is ~10 km NNW of the Golden Mile deposit, and refolds both the Kalgoorlie Syncline-Anticline pair and the Golden Mile Fault. Its axial trace is parallel to the steeply west dipping penetrative northwest foliation which overprints the earlier folds at the Golden Mile.
Early Faulting - Most of the folds described above are variously displaced by the crustal-scale, NNW trending, D2, sinistral Boulder-Lefroy Fault which has an offset of ~12 km and occurs along the western margin of the gold field. Displacement on this and relate D2 faults is also in part due to reactivation during subsequent compressional events. The Golden Mile deposit is located at a major change in the strike of this fault, corresponding to the similar bend in the axes of the Kalgoorlie Anticline-Syncline pair as described above. The regional scale sub-vertical Boulder-Lefroy Fault is oriented NNW south of the deposit and NW to its north, before returning to a NNW strike further to the north again. The parallel subsidiary D2 Golden Mile Shear/Fault passess through the deposits as do related lode bearing D2 shears and splays. These structures are dislocated by D3 extensional faults and D4 and D5 dextral shears, which combine to form a complex pattern of NNW, NNE north-south and NE faults (Vielreicher et al., 2016).
In the Golden Mile deposit area and further to the NNW, the D2 Golden Mile Fault dips steeply to the west, and has a substantial normal displacement, offsetting the hinge of the Kalgoorlie Syncline in the immediate Golden Mile deposit area (Woodall 1965). In the deposit area, it forms the eastern contact between a sliver of overturned, 85°W dipping, finely bedded, mudstones and sandstones of the Black Flag Group, and the Golden Mile Dolerite. The former is on the western limb of the Kalgoorlie Anticline, the latter dips at 30°W and is on the juxtaposed western limb of the Kalgoorlie Anticline. To the west, further Golden Mile Dolerite concordantly underlies the Black Flag Group sliver. This sliver of Back Flag Group sedimentary rocks represents the remnant core of the Kalgoorlie Syncline. It is a weak lithological layer, wedged between two competent blocks of Golden Mile Dolerite, and has been the focus of multiple pulses of displacement and shearing associated with this structure (Clout, 1989). This sedimentary sliver is also discordantly intruded by numerous steeply west dipping feldspar porphyry dykes (as described above) sub-parallel to the trace of the Golden Mile Fault (Stillwell, 1929). These dykes have a similar orientation to the feldspar porphyry dykes that cut other lithological units throughout the deposit (Gauthier et al., 2005).
Several sets of steeply east and west dipping reverse faults occur within the Black Flag sediment wedge and extend into underlying basalts and dolerites. These include an axial planar fault in the core of the Kalgoorlie Anticline. One such late fault within this set of structures (described as an example) extends over a strike length of ~2 km near the centre of the Black Flag Group wedge within the Golden Mile deposit. It is a 0.3 to 1 m thick zone of graphitic fault gouge with fabrics that demonstrates a dextral sense of movement. The fault strikes at 320° and dips 85°W, with stretching lineations that are consistently sub-horizontal. It contains completely dislocated and boudinaged quartz-carbonate veins, suggesting a complex history of repeated movement and later reactivation (Robert and Poulsen, 2001).
In the NE of the Golden Mile deposit, a set of steeply east-dipping reverse faults with offsets of several hundred metres merge into the Golden Mile Fault. Both these reverse faults and the Golden Mile Fault systematically truncate the Fimiston lodes with a dextral sense of displacement. Drilling and detailed mapping show that the trace of the Kalgoorlie Anticline axial plane is not parallel to the Golden Mile Fault, with dips of 80°W and 85°W respectively, whilst the anticline axis plunges at 20°S. As the two structures diverge to the north and south of the deposit, the closure of the Kalgoorlie Syncline reappears. In addition, detailed field observations in the Golden Mile demonstrate that the Golden Mile Fault offsets the upright Kalgoorlie Syncline and that it postdates upright folding and tilting of the host sequence (e.g., Gauthier et al., 2005). These conclusions are not consistent with previous interpretations that faulting predates tilting and that the Kalgoorlie anticline represents an overturned thrust ramp (Clout, 1990).
Shearing - A network of NW to NNW trending shear zones, predominantly steeply NE dipping and NE block up irrespective of dip, offset the stratigraphy, porphyry dykes and Fimiston lodes. Displacement is predominantly reverse, with only a minor, late, dextral strike-slip component. Most result in similar offset on the stratigraphy, lodes and dykes, although those closer to the Kalgoorlie Fault display a significantly larger component of displacement of the stratigraphy than the lodes and dykes. These shears are concentrated in the NW section of the Eastern Lode Domain, and ubiquitously cross cut Fimiston lodes. Most are narrow zones characterised by several cm-scale shear planes over a 1 to 2 m width. There are at least 5 main shear zones, which are part of complex array that includes many smaller structures with lesser displacement. Those shear zones closest to the Golden Mile Fault have the most extensive displacement, with the two main structures (the 'A' and 'C' shears) together accounting for 425 m of reverse movement and 175 m of dextral offset. They are sub-vertical to steeply west dipping at their NW end, whilst those to the NE, further from the Golden Mile Fault, have lesser displacement and are steeply NE dipping. The latter include the Kalgoorlie Shear, one of the most extensive, with a 175 m reverse offset on the stratigraphy, feldspar porphyry dykes and Fimiston lodes and <10 m of dextral strike-slip. The similar Australia East shear zone also distal to the Golden Mile Fault has a a reverse displacement of ~120 m and a 10 to 20 m sinistral offset (Gauthier et al., 2005).
These steep shears are overprinted by a younger conjugate network of shallow dipping, reverse shear zones that are ubiquitous throughout
the Golden Mile. They generally dip at 30 to 45° to either the east or west, the latter set being dominant (Ridley and Mengler, 2000). The reverse displacement on these shears is generally around a few metres at most. They commonly contain quartz veins and quartz breccias where cross cutting competent units (Ridley and Mengler, 2000). At Mt. Charlotte, the late steep north-south to NNE dextral strike slip faults cross cut the set of shallow dipping reverse shears (Ridley and Mengler, 2000), although the latter share mutually cross cutting relationship with the Mt. Charlotte quartz-carbonate vein stockwork (Clout et al., 1990, Ridley and Mengler, 2000).
Late Strike-Slip Faulting, which trend north-south to NNE and are sub-vertical, forming a regionally significant set. They represent the latest set on the Golden Mile, post-dating the Fimiston lodes and crosscut the early faulting. One of these, the Hannans Star Fault, dextrally offsets the Morrisson lode, part of the Western Lode System, by ~40 m. The Hannans Star Fault also offsets the steeply east dipping reverse Adelaide Fault, a late regional-scale, NNW trending, steeply west dipping dextral fault which is reflected by ~1 m of sericitic and carbonate-rich fault gauge. The Australia East Fault, a steeply east
dipping reverse fault which offsets the Fimiston Lodes, is itself cross cut and offset by the Adelaide Fault, establishing the latter as a late structure. Displacement on this structure varies from an ~300 m normal, to a dominantly dextral offset in the SE. The late NNW, ~60 to 70°W dipping Golden Pike Fault dextrally offsets the Boulder-Lefroy and Golden Mile faults by ~500 m, but the stratigraphy by ~2 km (Gauthier et al., 2005). These late faults are interpreted to have influenced the structural fabric that controlled the deposition of mineralisation. The dextral D5 Adelaide/Hannans Star and Golden Pike faults which are obliquely displace the main structural trend of the Golden Mile deposit were accompanied by reactivation of the earlier major structures that define that trend during gold mineralisation (Vielreicher et al., 2016; Groves et al., 2018). Similarly, as detailed below, the D5 dextral, obliquely cross cutting Charlotte and Maritana faults straddle the Mount Charlotte deposit with a similar reactivation of the Golden Mile and related D2 structures.
Gold mineralisation within the Kalgoorlie Gold Field is hosted in all rock types within the gold field. However, 70 to 80% of production comes from mineralisation hosted by the 2680±9 Ma Golden Mile Dolerite.
The Kalgoorlie Gold Field is characterised by two dominant ore types, the Fimiston- and Mount Charlotte-styles, largely corresponding to the two mining areas (Vielreicher et al., 2016).
Fimiston-style lodes within the Golden Mile deposit can be subdivided into the Eastern Lode Domain, a swarm of smaller lodes segmented by numerous steep reverse faults, and the Western Lode Domain which is less complex, with good lateral and vertical continuity, and are better defined. These domains are separated by the Golden Mile Fault and band of intensely deformed Black Flag Group rocks and basically coincide with the eastern and western limbs of the Kalgoorlie Syncline (Gauthier et al., 2005).
Fimiston-style lodes are predominantly zones of replacement characterised by brecciation and fracture fill, and comprises a complex array of ductile to brittle stringer-, vein- and breccia-lodes that evolved broadly during the syn- to late-formation of the regional NW-trending foliation. They are developed in four principal directions in both the Eastern and Western Lode Domains (Finucane, 1948) with strike and dip orientations as follows: i). Main 135 to 140°/90 to 85°SW, parallel to the main NW Golden Mile Fault; ii). Caunter at 100 to 120°/55 to 80°SSW; iii). Cross Lodes at 40 to 50°/65 to 80°SE; and iv). Easterly at 160°/70 to 90°ENE. The Main and Caunter lodes are the dominant sets in both domains. Individual lodes are generally narrow, <2 m thick, but are vertically and laterally extensive, up to 2 km long by 1.3 km in vertical extent. The vertical continuity of mineralisation has been traced to a depth of at least 1.8 km. Together, the lodes in both domains form a downward tapering array, which is sub-vertical in the Western and steeply west dipping in the Eastern lode domains (Gauthier et al., 2005).
The approximately planar, steeply dipping Fimiston-style lodes are characterised by pyrite veinlets and disseminations, fine-grained quartz-sericite-sulphide-telluride bearing quartz-carbonate veinlets, crackle- and cockade-breccias, banded chalcedonic quartz-carbonate veins, and stringer zones that grade into lenses of microbreccia and cataclasite (Vielreicher et al., 2016; after Clout, 1989; Bateman et al., 2001; Gauthier et al., 2004). Lodes are characterised by variably foliated quartz - sericite - pyrite - ankerite/dolomite/siderite/calcite - Fe/Ti-oxides (locally as boxworks after Ti-magnetite), ±Fe-chlorite, ±tourmaline alteration. This sericite, carbonate and pyrite-dominated alteration forms i). an inner proximal gold-bearing, pyrite-sericite zone with ankerite ±siderite, surrounded by ii). an ankerite and siderite zone that extends for over 100 m outwards, and is enveloped by iii). an outer 1 km wide chlorite-calcite ±ankerite assemblage. The distribution of this alteration is strongly influenced by both structure and lithology (Gauthier et al., 2005). There is a strong lithological control on alteration (Nixon et al., 2014), and as a consequence, the intensity and mineralogy of alteration is strongly controlled by the interplay of structure and lithology (Phillips, 1986; White et al., 2003).
The wide distal carbonate ±chlorite alteration zone appears to be the result of fluid flow along multiple thinner conduits in the inner pyrite-rich zones that were each accompanied by their own discrete alteration selvedges. As fluid flow continued and additional fractures opened, the selvedges expanded and coalesced to form a broad, composite, virtually continuous alteration halo (White et al., 2003).
The mineralogy of this alteration assemblage would be consistent with influx of an aqueous-CO2 dominated fluid reacting with the metamafic host rocks to cause breakdown of actinolite to chlorite and carbonate (e.g., Vielreicher et al., 2016).
Sericite in the proximal alteration zones is regarded to be the result of potassium addition from the fluid (White et al., 2003), and pyrite is the result of sulphidation of Fe in the host rocks (Neall, 1987). Mineral equilibria studies show the alteration assemblages are consistent with equilibrium with a single fluid of composition XCO2 = CO2/(CO2 + H2O) = 0.1 to 0.25 (outer zones) and 0.25 (inner zones) at temperatures of 320 to 315°C (White et al., 2003). These temperatures are within the range of from 390 to 305°C defined by chlorite and arsenopyrite geothermometry of alteration assemblages collected from the Oroya, Lake View and Great Boulder Main Lodes on the Golden Mile (Vielreicher et al., 2016). Sericite geobarometry on the same samples suggest pressures of 110 to 290 MPa (Vielreicher et al., 2016). In addition, Shackleton et al. (2003) interpreted data to suggest gold + calaverite + petzite and minor late hessite in the Golden Mile were deposited by a fluid cooling from 300 to below 170°C.
Gold within the Fimiston lodes is present in either inclusion-poor auriferous pyrite, or as micron-scale inclusions in <100 µm diameter inclusion-rich pyrite. As such, the gold is typically refractory, with gold grades directly related to the occurrence of fine-grained pyrite.
High grade zones, typically carrying >50 g/t, but locally up to 1% Au, contain native gold, tellurium in a range of telluride minerals (Shackleton et al., 2003), as well as closely associated chalcopyrite, arsenopyrite, tennantite-tetrahedrite, sphalerite and galena. These zones are characterised by the addition of CO2, K, Rb, S as well as Au (-B), Te, V and Ba (Phillips, 1986). Locally, bonanza grade shoots, known as 'Green-Leader' (Larcombe, 1912) or Oroya-style lodes with grades of up to 10% Au are spatially associated with intersections of the lodes with carbonaceous metasedimentary units. Gold in has been observed in syn-sedimentary to diagenetic zoned pyrite nodules within these graphitic units (Steadman et al., 2015). The Oroya-style lodes are characterised by abundant free gold with associated native tellurium and rarely, mercury (Weller et al., 1998), along with disseminated pyrite and numerous Au-Ag, Ni, Hg, Pb, Sb-bearing telluride minerals (Stillwell, 1929; Scantlebury, 1983; Clout et al., 1990). They occur within roscoelite-bearing (green vanadium-bearing muscovite) alteration zones with ankerite, calcite, siderite, sericite, V-Ti bearing Fe-oxides, pyrite, chalcopyrite, arsenopyrite, tennantite-tetrahedrite, sphalerite and galena (Stillwell, 1929; Scantlebury, 1983). The principal example is the 1500 m long Oroya shoot, controlled in large part by the 50°W dipping, reverse Oroya shear zone system. This style of ore occurs in the upper extremities and brecciated cores of steeply dipping Fimiston lodes, and may represent a late-stage of the D4b transpressional regime that generated the Fimiston lodes (Mueller et al., 1988) and a separate mineralisation sub-stage (Bateman et al., 2001) as described below.
Gold within the Fimiston lodes is present in either inclusion-poor auriferous pyrite, or as micron-scale inclusions in <100 µm diameter inclusion-rich pyrite. As such, the gold is typically refractory, with gold grades directly related to the occurrence of fine-grained pyrite.
Zoning is evident within the Golden Mile, with an overall increase in Sb/Au in the tennantite-tetrahedrite group minerals, as well as an increase in the Au:Ag ratio in the free gold with depth (Golding, 1978). This zonation is also associated with variations in the telluride mineralogy, with increasing montbrayite ((AuSb)2Te) laterally, and telluroantimony (Sb2Te3) at depth (Shackleton et al., 2003).
In the Western Lode Domain, Fimiston lodes form a sub-vertical network 1.6 km in strike length and up to 1.1 km in vertical extent. They mostly occur in the steeply west dipping Unit 9 of the Golden Mile Dolerite, close to, but bounded by the Black Flag Group to the east and by Unit 8 to the west. The core of the lode system in this domain is spatially associated with a bend in the Golden Mile Fault, from a strike of 135° in the north to 120° in the south and from sub-vertical near surface, to 80°W dipping at depth. This bend, or jog, corresponds to the similar bends in the axes of the Kalgoorlie Syncline-Anticline pair and in the Lefroy-Boulder Fault as described previously. The lodes within this domain are vertically zoned, e.g., the Lode #4, where the gold decreases markedly from the bonanza grades over the upper 400 m to subeconomic values below the 1100 m level (Larcombe, 1912; Clout, 1989).
The lodes of the Eastern Lode Domain form a steeply west dipping network that extends over a strike length of ~3 km associated with a corridor of alteration and gold mineralisation that persists for a further 10 km to the NNW where it encompasses several smaller gold deposits, including Mt Percy. The lode system within this domain has a pronounced narrowing downwards, which is controlled by the shallow west dip of the Golden Mile Dolerite. It thins from widths of 500 to 600 m within the Golden Mile Dolerite, to 200 to 300 m wide in the underlying Paringa Basalt. The lodes are steeply west dipping, with economic grades generally restricted to the Golden Mile Dolerite, rarely persisting for more than 50 to 100 m below in the Paringa Basalt (Woodall, 1965). Unlike in the Western Lode Domain, the Eastern Domain lodes are strongly discordant to the shallow dipping stratigraphy and stratigraphic contacts have a strong influence on the location of high grade ore shoots within lodes (Gauthier et al., 2005).
Paragenesis. The Fimiston lodes and their associated alteration reflect a complex evolving system of paragenetic stages with close spatial and temporal relationships that tend to overlap and form complex mutually cross-cutting relationships (Clout, 1989). The main paragenetic stages comprise:
• Stage 1 - Early iron-carbonate +magnetite +hematite +quartz veins and breccias which form widespread carbonate-rich breccias and veins without significant gold grades, and pre-dates the main gold bearing stages. Carbonate breccias and veins are best developed within the more competent granophyric Unit 8 and Unit 7 of the Golden Mile Dolerite and have a clear spatial association with the later stage mineralised Fimiston lodes which they clearly pre-date. The breccia, that exhibit a jigsaw pattern, have a matrix that ranges from iron-carbonate in wide zones of hydrothermal brecciation to narrower zones of ankerite +quartz +magnetite +hematite breccias and veins with locally well-developed infill textures. Magnetite varies widely, with local massive bands up to a few cm thick, and can grade locally into black hematite-rich breccias with the same textural characteristics and temporal relationships. Magnetite and quartz +magnetite veins and associated magmatic breccias have mutually cross cutting relationships with feldspar porphyry, dykes suggesting contemporaneous development.
• Stage 2 - Finely disseminated pyrite +quartz +sericite +carbonate +tourmaline. Pyrite generally occurs as broad zones of 10 to 20 wt.%, but locally up to 50 wt.% disseminations and/or narrow veinlet stockworks. Pyrite is also commonly concentrated in pillow rims and flow breccia matrix within the Paringa Basalt. Pyrite disseminations dominate within the Paringa Basalt and to a lesser extent within Unit 8 of the Golden Mile Dolerite where they form relatively broad zones in association with Fimiston lodes. The typically contain from 1 to 10 g/t Au, although local high gold grades are associated with pyrite +tourmaline veinlets. This mineralisation is characterised by a high Ag:Au ratios (commonly >1)in contrasts to the last following paragenetic stages, which have low Ag:Au ratios. Tourmaline is commonly associated with the pyrite-rich veins and dissemination zones. Pyrite dissemination zones generally grade laterally into sericite +iron carbonate alteration with diminished levels of pyrite (1 to 5 wt.%), and the appearance of magnetite. Although the margins of these pyrite zones have a similar mineralogy to the selvages of later stage veining they can be demonstrated to be due to separate events (Gauthier et al., 2005).
• Stage 2a - Green leader style disseminated pyrite +gold +tellurides within roscoelite-rich alteration zones. Tourmaline also commonly accompanies this sub-stage. It represents a sub-set of stage 2 (Clout, 1989) and forms bonanza ore shoots e.g., the Oroya lode which yielded >65 tonnes of gold at an average grade of 31 g/t Au; and the Duck Pond ore shoot that averaged 1245 g/t Au. These high grade shoots form within the upper extremities of the typical Fimiston type lodes (Clout, 1989). Both the Oroya and Duck Pond shoots occur as flat lying cigar-shaped lenses, controlled by the intersection between a steeply west dipping Fimiston lode and shallow dipping lithological contacts. Duck Pond lies within the large Lake View lode, and is associated with an infold of black shales of the Black Flag Beds in the Lake View syncline, although not formed directly in contact with the black shales (Clout, 1989; Gauthier et al., 2005). The gently-plunging pipe-like Oroya Shoot lies within the Paringa Basalt and follows a graphitic sedimentary unit, the Oroya Shale, at the contact with the Golden Mile Dolerite (Bateman et al., 2001).
• Stage 3 - Quartz ±carbonate veinlets and breccias, which comprises arrays of generally 5 to 20 mm thick quartz veinlet stockworks within Fimiston lodes. The veinlets locally grade into thicker breccias, commonly containing mineralised wall rock clasts. The veinlets are rimmed by narrow pyrite +sericite +iron carbonate alteration selvedges grading outward into sericite +iron carbonate. These veinlets and breccias host high grad gold with associated tellurides, and generally represent the high grade portion of the lodes. The quartz veinlets contain well-developed comb textures and quartz breccias display a range of infill textures. Whilst the veinlets are dominated by quartz, they also contain carbonate as discreet bands within the veins and breccias. Anhydrite is also a locally significant infill (Clout, 1989). Thee quartz veinlets clearly cross cut pyrite-rich zones of stages 2 and 2a.
• Stage 4 - Banded quartz-carbonate veins, which range from 2 to 10 cm in thickness, and are predominantly aligned at angles of 238°/65°NW and 22°/59°SE, close to the Cross Lode and Caunter trends (108°/77°NW). These veins cross cut both Stage 2 and 3 veins and are composed of finely alternating bands of quartz and carbonates with pyrite locally concentrated along individual bands. They have variably developed selvedges of pyrite +sericite +carbonates. There is a correlation between gold grades and the pyrite content of the veins. Gold grades are highly variable, with these veins commonly being barren at the periphery of the deposit.
• Post Fimiston hydrothermal events, which postdate the NW foliation and cross cut the Fimiston lodes. These include:
- Sub-vertical barren coarse-grained carbonate veins that are ubiquitous throughout the Golden Mile deposit area and are also a widely distributed regional feature. They veins are associated with the steep reverse faults and cut Fimiston Lodes.
- Mount Charlotte-style lodes which are described below. In the Golden Mile deposit these veins are mostly within the most competent Unit 8 of the Golden Mile Dolerite and also within feldspar porphyry dykes. Two concentrations of these veins form stockworks that are dominantly oriented north-south, and dip steeply west and sub-horizontal to shallow west. They occur along the late Golden Pike fault within Unit 8 in the Western Lode Domain, where they form the Golden Pike and Drysdale stockworks (Clout et al., 1990). See details of the Golden Pike fault above in the Structure section.
Age of Mineralisation - Fimiston-style gold mineralisation is constrained to ~2.64 Ga, during (late D4b? to) D5. It post-dates the 2.69 Ga Paringa Basalt, 2.68 Ga Golden Mile Dolerite and deposition of the 2.69 to 2.66 Ga Black Flag Group. A 20 m thick pre-mineral feldspar porphyry dyke that is clearly cross cut by the Australia East Lode of the Golden Mile deposit has been dated at 2676 ±3 Ma (zircon; U-Pb TIMS; Gauthier et al., 2005). Other similar dykes have yielded comparable ages and relationships to mineralisation. Alkaline hornblende porphyry dyke, dated at 2663 ±11 Ma (zircon; U-Pb TIMS; Gauthier et al., 2005) clearly cross cuts a Fimiston stage banded quartz-carbonate vein of the Golden Mile deposit. However, these dykes, which occupy similar structural sites to the gold mineralisation, are ubiquitously altered to an assemblage of carbonate and sericite within the Golden Mile deposit, implying they may be intra-mineral. An unaltered lamprophyre dyke cross cutting the Oroya lode was dated at 2638 ±6 Ma and dissects the Oroya lode (McNaughton et al. 2001). This dyke is described as unaltered except for a thin crust along its margin and contains no significant K2O (K2O = 0.15 wt.%). This is significant, as the Oroya lode is characterised by intense roscoelite alteration and substantial associated K2O enrichment (Clout, 1989).
Petrographic studies (Mikucki and Roberts, 2003) indicate gold mineralisation and wall rock alteration were just post-peak regional greenschist-facies metamorphism, which is considered to have occurred prior to ~2.65 Ga (Vielreicher et al., 2016).
Mineralisation-related, hydrothermal monazite and xenotime in ore samples from the Mt Charlotte deposit, which is interpreted to be structurally late, gave ages of 2655±13 Ma (Rasmussen et al., 2009) and 2644±11 Ma (Vielreicher et al., 2010). The most precise data from the structurally older Fimiston-style ore samples yielded an age of 2632±12 Ma (Vielreicher et al., 2010), with a minimum age of 2636 Ma for Oroya-mineralization (McNaughton et al., 2005). Published 40Ar/39Ar dates for hydrothermal, gold-related sericite from the Golden Mile (recalculated from Kent and McDougall, 1995 by Vielreicher et al., 2010) of 2641±13 Ma and 2643±13 Ma overlap all these data. From this, Vielreicher et al. (2016), concluded that as geological evidence indicates Mt Charlotte ore formed after the Fimiston ore, both ore types must have formed within 10 m.y. of each other at ~2.64 Ga.
Mount Charlotte-style lodes
Mt Charlotte type Quartz-carbonate veins are the dominant lode type in the Mount Charlotte mine, but also occur at Mount Percy and in the Golden Mile deposit (e.g. the 'Golden Pike and Drysdale stockwork lode') mostly within the most competent Unit 8 of the Golden Mile Dolerite and also within feldspar porphyry dykes (Gauthier et al., 2005).
Conversely, Fimiston-style lodes are also represented at Mount Charlotte where Mount Charlotte-style quartz vein stockworks are separated from Fimiston-Style mineralisation and alteration across barren D4b thrusts (Mueller, 2015). These lodes are more siliceous and silver-rich than those in the Golden Mile deposit and commonly contain white laminated quartz veins and grey silica-pyrite bands, and economic gold-pyrite ±telluride mineralisation is typically developed over 0.5 to 2 m widths. Similarly, Fimiston-style mineralisation is redeveloped at Hannan's North to the north of Mount Charlotte, where the lode lacks the late telluride stage of the Golden Mile. Sericite-ankerite alteration is discontinuous, and siliceous lodes are locally in direct contact with chloritic Golden Mile Dolerite (Haycraft, 1965).
Mount Charlotte-style lodes are mostly brittle, and slightly younger than the Fimiston lodes. They comprise elongate pipe-shaped networks of dilational vein-stockworks of sheeted north-dipping veins occupying extensional and shear fractures (Mueller, 2017).
The vein network that makes up the Charlotte orebody, which accounts for ~70% of the ore mined at Mount Charlotte, covers a plan area of 250 x 75 m, and vertical extent of >1000 m. The orebody strikes NW-SE, parallel to the Golden Mile Dolerite and D2 Golden Mile Fault, and is terminated and offset by the transtensional D5 dextral, strike-slip, north-south striking Charlotte, Reward and Maritana faults and related intervening structures (Mueller, 2017).
These two major structural elements control the location of Mt Charlotte-style gold mineralisation within the Mt Charlotte deposit, namely the 2625 Ma, dextral, strike-slip D5 faults, and reactivated west-dipping D2 reverse faulting. The deposit occurred where these structures were both active within competent igneous rocks that provided a lithological control (Bateman and Hagemann, 2004). These include, in particular, the most differentiated coarse-grained and competent granophyric clinopyroxene-plagioclase-quartz-ilmenite-magnetite Unit 8a of the Golden Mile Dolerite, but also within Units 7, lower 8 and 9, and porphyry dykes intruding the Hannans Lake Serpentinite. The Charlotte deposit area includes swarms of ~2.67 Ga feldspar-phyric porphyry, lesser ~2.66 to 2.65 Ga calc-alkaline hornblende-phyric and ~2.65 to 2.64 Ga lamprophyre dykes (Vielreicher et al., 2016).
The D5 faults crosscut and offset the D2 faults, although later oblique reactivation of the latter is also evident. The D5 faults have a brittle-ductile component, reflected by syn-mineralisation foliation, slickensides and stepped veins on fault surfaces, overprinted by a brittle component of fault cataclasite that always crosscuts the veins. The reactivated D2 faults are brittle-ductile, characterised by foliation, tension gashes and mineral lineations, as well as stepped veins on fault surfaces. Both brittle-ductile fault sets crosscut quartz veins, and are crosscut by them, indicating both sets were active at the time of vein formation. The interaction of the two sets of faults produced a network of extension and shear fractures, infilled by veins that are solely dilatational, with no significant consistent vein parallel sense of displacement (Mueller, 2015). They were formed by the stepped dextral displacement on the D5 faults and translation and extension on the reactivated D2 structures in a competent host rock.
The Mount Charlotte-style lodes contain coarse-grained quartz, carbonate, albite and scheelite and are spatially associated with late steeply (~70°) and gently (~45°) dipping, northerly trending brittle dextral-reverse fault sets that offset the regional NW-trending foliation (e.g. Clark, 1980; Ridley and Mengler, 2000; Weinberg et al., 2005; Mueller, 2015). The veins are composed of an inner core of quartz ±scheelite and an outer shell of quartz-dolomite-albite-pyrite (Mueller, 2017). Individual veins vary from a few millimetres to 5 m in thickness and contain multiple generations of quartz growth (Clark, 1980; Bateman and Hagemann, 2004). Quartz, which is unstrained, accounts for ~90% of the vein and is coarsely crystalline with prismatic terminations (Clout et al., 1990). The veins themselves are largely barren of gold but characteristically carry scheelite within the quartz core, occurring as grains from <1 mm to up to 10 cm across. There is a very strong oscillatory zonation in rare earth elements (REE) within these veins on a scale of 1 to 200 µm (Brugger et al., 2000; Ghaderi et al., 1999). These veins form a mutually crosscutting network, and appear to be preferentially developed within rheologically competent units, as described above.
Gold in the Mt Charlotte-style lodes is located on the outer vein margin contacts and in the pyritic selvedges, where it occurs as 5 to 15 µm grains in fractures within pyrite, or at contacts between pyrite and gangue minerals. It is predominantly free, and is associated with pyrite, pyrrhotite, telluride minerals, ankerite, sericite and quartz. Native gold fineness is above 800 (Clout 1989). The well defined selvedges surrounding veins have associated enrichment in K, Rb, Cs, Li, Ba, Ca, Sr, Mg, Ni, V, Cr, W, Te and Au (Mueller, 2015).
The stockwork mineralisation has a distinct zonation of carbonate-sericite-albite-pyrite ±pyrrhotite dominant alteration assemblages surrounding the individual veins. These selvedges are zoned with:
i). a proximal, inner halo that persists for up to ~50 cm from vein margins and is composed of an assemblage of quartz-albite-muscovite-ankerite-ilmenite-rutile-pyrite/pyrrhotite carrying from 5 to 10 g/t Au. It grades outward through
ii). a distal and diffuse gradational zone that is 50 to 100 cm from the vein margins and contains less muscovite and more chlorite and ilmenite and outwardly decreasing gold grades, before passing into
iii). the least altered protolith, carrying 50 to 200 ppb Au, peripheral to the alteration selvedge, representing the background district scale chlorite-calcite alteration of the Golden Mile Dolerite.
The alteration selvedge flanking the veins represents chlorite destruction and growth of ferroan carbonate, sericite, pyrite and native gold. This outer zone of least altered protolith may be 0.5 to >10 m in width. The mineralogy of this protolith is the result of the breakdown of actinolite in the Golden Mile Dolerite to form chlorite and carbonate, as detailed in the Fimiston-style lodes section above. Ore (>2 g/t Au) is formed where the stockwork of veins is sufficiently densely spaced that the mineralised alteration selvedges coalesce to produce a bulk mineable grade. Where this occurs, the remaining least altered chloritic rock accounts for ~25 to 30% of the resource.
The alteration mineralogy enveloping individual veins changes systematically depending on where the vein is within the deposit, as follows (Clark, 1980; Mikucki and Heinrich, 1993; Bateman and Hagemann, 2004; Vielreicher et al., 2016):
• Type 1 - Pyrite-muscovite alteration, where ankerite, muscovite, pyrite, siderite and rutile in the proximal zone and albite, chlorite, magnetite and pyrite occur in the distal zone. This alteration is characterised by the inner proximal halo becoming bleached and muscovite-rich and the dominant sulphide being pyrite. This alteration occurs in the upper and outer levels of the mine, i.e., from the surface to 450 m depth and contains high gold-grades, but at depth, mostly forms an outer shell at the periphery, outside of the bulk mineable orebody. As such it forms a bell shaped cap to the deposit
• Type 2 - Pyrite-pyrrhotite alteration results in an assemblage of quartz, ankerite, muscovite, pyrite, siderite, rutile (pyrrhotite) in the proximal zone, and albite, chlorite, magnetite and pyrrhotite in the distal zone at depths of 600 to 800 m. As such it is similar to the Type 1 alteration, but with the addition of pyrrhotite in the distal diffuse contact zone (i.e. up to 100 cm from the vein margins), and an increase of albite at the expense of muscovite in the inner bleached zones. This is the predominant alteration type associated with economic mineralisation in the uppermost sections of the deposit. It is also common towards the periphery of the orebodies at intermediate levels, and is associated with isolated, distal veins in the deeper part of the deposit, i.e., Charlotte Deeps.
• Type 3 - Pyrrhotite-albite alteration, which consists of assemblages of ankerite, albite, sericite, pyrrhotite, siderite, rutile (pyrite) in the proximal zone, and albite, chlorite, magnetite and pyrrhotite in the distal zone and occurs at depths of >800 m, where pyrrhotite is found in both the inner and outer parts of the alteration envelopes, with a marked increase in the content of albite in the inner zone. This downward expanding zone is the dominant alteration type in the mid to upper core and deeper sections of the Mount Charlotte deposit.
These three types, or alteration assemblages, mark a series of vertical changes, particularly the decrease in pyrite with depth, the reduction of muscovite and corresponding increase in albite. In the inner proximal zone (up to 50 cm from the vein margin), albite increases with depth antithetically to sericite and becomes dominant below the D4b Flanagan thrust, a shallow 330 to 335° and 50 to 55°SW dipping structure offsetting the NW-SE striking deposit near its centre. In the distal zone (50 to 100 cm from the vein margin), sericite and ankerite decline and the protolithic chlorite begins to dominate. Pyrite is the only disseminated sulphide above 450 m depth, but decreases sharply beyond the replacement front of the proximal selvedge. Between 450 and 700 m below surface, pyrite is still the only sulphide in the inner zone, but gives way to pyrrhotite in outer chloritic domains. Below the Flanagan thrust, pyrrhotite dominates over pyrite in the inner selvedge, also.
These changes in the zoned vein selvedges where gold is hosted are reflected in the grade of bulk mineable gold in the Charlotte deposit, which declines bendfrom 5.0 g/t at ~125 m below surface to ~3.0 g/t Au at a depth of ~900 m (Mueller, 2015). Fluid inclusion data and chlorite thermometry indicate that the down-plunge zonation is due to the cooling of the hydrothermal fluid from 440 to 410°C at the lower levels to 360 to 350°C at the top of the Charlotte orebody, while the greenstone terrane remained at a 250°C ambient temperature and at 300 MPa lithostatic pressure (Mueller, 2015). In the bulk ore, this decrease in temperature is also reflected in a decreasing Au/Ag ratio and increasing gold, arsenic and mercury contents in the upper sections of the deposit (Mueller, 2015; Bateman and Hagemann, 2004).
The extension and shear fracture network opened by interactions between the dextral D5 and reactivated D2 structures, as described above, was infiltrated by high-pressure H2S rich fluid at 2655 ±13 Ma (xenotime U-Pb; Rasmussen et al., 2009) and 2644 ±11 Ma (xenotime; Vielreicher et al., 2010). Gold was deposited during wall rock sulphidation in overlapping vein selvages at a temperatures gradient as detailed in the previous paragraph. The open fractures filled with barren quartz and scheelite during retrograde cooling of the hydrothermal event, through ~300°C. During the sealing of these fractures, fluid flux was periodically restricted at the lower D4b thrusts. Cycles of high and low up-flow, represented by juvenile H2O-CO2 and evolved H2O-CO2-CH4 fluid, respectively, are recorded by the REE and Sr isotope compositions of scheelite oscillatory zones (Mueller, 2015).
Mueller (2015) suggest the temperature gradient measured in the vein stockwork points to a hot (>600°C) fluid source 2 to 4 km below the mine workings, and several kilometres above the supracrustal rocks. Geochemical characteristics of the ore/alteration related elements are consistent with the local high-Mg monzodiorite/hornblende-phyric porphyry suite but not with the Golden Mile Dolerite host rock. Similarly, 87Sr/86Sr ratios of the vein scheelite are higher than the mantle ratio of the Golden Mile Dolerite and overlap those of high-Mg monzodiorite/hornblende-phyric porphyry intrusions emplaced along the Golden Mile Fault at 2662 ±6 to 2658 ±3 Ma (Mueller, 2015).
A final chlorite-stable event dated at 2603±18 Ma (xenotime U-Pb; Vielreicher et al., 2010) overprints both the Golden Mile and Mount Charlotte deposits. This age is coeval with published 40Ar/39Ar white mica data from the Golden Mile, and overlaps an ~2.61 to 2.60 Ga event recorded at most
deposits in the Kalgoorlie Terrane (Vielreicher et al., 2015). It most probably coincides with the final terrane uplift, cooling and stabilization (Witt et al., 1996).
Formation of the Kalgoorlie Gold Field
According to Vielreicher et al. (2016) both the Golden Mile and Mount Charlotte deposits comprise i). shear-zone related gold-bearing pyrite-telluride disseminations, veinlets and breccias (predominantly at Fimiston) that are structurally overprinted by ii). quartz vein stockworks with gold-bearing sericite-albite-pyrite/pyrrhotite selvedges (dominantly at Mt Charlotte) that carry a significantly lower proportion of telluride minerals. Studies of the mineralogical characteristics, fluid inclusion and isotopic studies all show this mineralisation is consistent with formation from a dominantly low-salinity, 18O enriched, potassic H2O-CO2(+CH4) fluid (or fluids). These studies all indicate that these fluids formed mineralisation at similar temperature (>400 to 300°) and pressure (110 to 300 MPa) ranges. The fluids were transmitted through openings in fractured rheologically favourable competent rocks to deposit gold by reacting with chemically favourable wall rocks, in conjunction with decreasing solubility of gold complexes during declining temperature and rapid pressure release (Vielreicher et al., 2016).
Vielreicher et al. (2016) note that the presence of methane in the ore fluid is attributed to either mixing with an externally derived fluid, or to post-entrapment fluid modification (e.g. Hagemann and Brown, 1996; Mernagh et al., 2004). Alternatively, it could be the result of fluid-rock interaction and reduction of juvenile carbon dioxide (Mikucki and Heinrich, 1993; Polito et al., 2001), generated during desulphidation of the ore fluid, which also caused destabilisation of gold thiosulphide complexes and precipitation of pyrite and gold.
Precise geochronology indicates the formation of the Fimiston and Mount Charlotte lode-styles was temporally close, within ~10 m.y. of each other, at ~2.64 Ga, during D5, after peak metamorphism at ~2.65 Ga, and ~20 to 30 m.y. after the peak of intrusion of the feldspar-phyric porphyry dykes at ~2.67 Ga, and followed deposition of the Black Flag volcaniclastic rocks at ~2.69 to 2.655 Ga; (Krapez et al., 2000). While the gold mineralisation appears to have been a single protracted event, a paragenetic evolution can be recognised, as detailed above (e.g., Groves et al., 2018).
Both the Golden Mile and Mount Charlotte deposits are each largely confined between a pair late stage dextral faults oblique to the main structural trend that have accompanied reactivation of earlier structures. These fault pairs are the Adelaide and Golden Pike faults on the Golden Mile and the Charlotte and Maritana faults at Mount Charlotte (Vielreicher et al., 2016; Groves et al., 2018).
Kalgoorlie sits near the centre of an ~180 km diameter block of preserved greenstones/supracrustals, the largest in the Eastern Gold Fields Superterrane.
The mineralised system at Kalgoorlie was formed on a kilometre scale (at least), adjacent to the active, crustal-scale, Boulder-Lefroy fault system that allowed rapid uplift and erosion of several kilometres of both the Kambalda and Kalgoorlie sequences and deposition of volcanic and clastic sedimentary basinal rocks of the Late Clastic Basin sequences at 2.658 to 2.655 Ga (Vielreicher et al., 2016). The faults of this system are also considered to have provided efficient plumbing system for magmas, including deep lithosphere derived lamprophyres found within the Gold Field. Hydrothermal fluids may have also traversed the same structures from the deep crust/lithosphere to reach the interconnected mesh of upper crustal structures in an uplifted block (inlier) of ultramafic to mafic greenstone rocks within the Kalgoorlie Gold Field. The Boulder-Lefroy fault system is interpreted to be spatially associated with a series of significant gold deposits, extending from Noresman in the south, past Kambalda-Saint Ives, New Celebration, Kalgoorlie, Kanowna and Paddington, which collectively account for ~50% of the gold produced from the Yilgarn Craton (Vielreicher et al., 2016).
Kalgoorlie sits within the core of a complex antiformal structure formed by the interaction of D2 to D5 folding and thrusting (Vielreicher et al., 2016). This antiform isolated a relatively brittle mafic sequence, including the Golden Mile Dolerite, within a relatively incompetent and impermeable envelope of Black Flag volcanosedimentary and sedimentary rocks forming a trap and depositional site for mineralised fluids (Vielreicher et al., 2016). The Kalgoorlie Gold Field is located adjacent to a NW-trending dilational jog in the regional NNW-trending Boulder-Lefroy Fault. This jog results in the trend of the latter being deflected from NNW south of the deposit, to NW to its north. This bend is also reflected in a change in the strike of the Golden Mile Fault and the axial planes of the Kalgoorlie Syncline-Anticline pair, which are also buckled to produce changes in dip (Blewett et al., 2010; Vielreicher et al., 2016).
Data from deep seismic surveys (Drummond et al., 2000), and magnetotelluric (MT) surveys (Henson and Blewett, 2006) show broad anomalies below Kalgoorlie which may reflect alteration and the passage of large volumes of fluids (e.g., Blewett et al., 2010). In the MT data, both the upper crust and upper mantle are anomalously conductive. A domed upper-mantle conductivity (MT) anomaly beneath Kalgoorlie is coincident with the edge of a deep seismic tomographic velocity anomaly. The latter may reflect a favourable deep architecture which acted as a fluid pathway (Blewett et al., 2010). The lower crust beneath Kalgoorlie is also anomalously devoid of reflections in the deep seismic data, which may also represent widespread alteration (Blewett et al., 2010; Drummond et al., 1993; Goleby et al., 1993). These data and interpretations all suggest large scale lithospheric to crustal scale fluid circulation and transport below the Kalgoorlie Gold Field, within thinned, rifted crust of the Kalgoorlie Terrane, just to the east of the older, thicker Youanmi Terrane basement margin.
Production, Reserves and Resources
The bulk of the gold from the Kalgoorlie Gold Field is mined from two operations, i). the KCGM Superpit at Fimiston that is ~3.5 x 1.5 km in area, scheduled to reach 700 m deep, and exploits the remains of >1000 named ore lodes and intervening mineralisation, and ii). the Mount Charlotte deposit, which is 3 km to the NW, made up of four orebodies Reward, Charlotte, Maritana and Charlotte Deeps, consisting of predominantly brittle-style vein stockworks that extend for ~1000 m along strike x 1100 m vertically, and up to 90 m in width.
Total production from the Kalgoorlie Gold Field, dominantly from the Golden Mile and Mount Charlotte mines between from 1893 and 2005 was ~1475 t Au (47.5 Moz), with a further ~110 t from 2005 to 2010. By 2017 the gold field had yielded ~1866 t of gold with a remaining endowment of ~363 t of gold in 2018.
Production from the northern part of the field, including the Mount Percy mine totalled ~10 t of gold from three open pits between 1985 and 1992. The Hannan's North mine, also to the north, had been worked since 1893, but was taken over and expanded by Broken Hill Proprietary from 1931. It operated until 1968, producing 0.838 Mt of ore averaging 11.1 g/t Au for 9.286 t of gold.
The total open pit and underground reserves plus resources at December 31, 2010 were (Barrick, Newmont, 2011):
Proved + probable reserves - 153 Mt @ 1.71 g/t Au, for 260 t Au; (reserves are additional to resources)
Measured + indicated resources - 95.6 Mt @ 0.76 g/t Au, for 72 t Au;
Inferred resource - 2.24 Mt @ 4.47 g/t Au, for 10 t Au.
The total open pit and underground reserves plus resources at December 31, 2018 were (Barrick, Newmont, 2018):
Proved + probable reserves - 192.776 Mt @ 1.18 g/t Au, for 227.5 t Au; (reserves are additional to resources)
Measured resources - 10.686 Mt @ 1.42 g/t Au, for 15.17 t Au;
Indicated resources - 50.910 Mt @ 1.51 g/t Au, for 76.87 t Au;
Inferred resource - 18.804 Mt @ 2.33 g/t Au, for 43.81 t Au.
TOTAL Mineral Resource - 80.40 Mt @ 1.69 g/t Au, for 187.33 t Au.
TOTAL Ore Reserve + Mineral Resource - 273.17 Mt @ 1.33 g/t Au, for 363 t Au.
The Kalgoorlie Super Pit open pit and Mt Charlotte underground operations are owned by KCGM, a 50:50 JV between Newmont Mining and Barrick Australia. Production in 2010 totalled approximately 24.5 t of recovered Au (Barrick and Newmont, 2011) and in 2018 ~22 t per annum (KCGM website).
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Sunrise Dam - Western Australia .................................... Fri. 18 November, 2011
The Sunrise Dam operation is approximately 55 km to the south of Laverton, 220 km NNE of Kalgoorlie, and 770 km NE of Perth, in the Eastern Goldfields province of the Archaean Yilgarn craton, in Western Australia.
The deposit originally straddled the title boundary between the AngloGold Ashanti Ltd Cleo mine and the adjacent Sunrise leases of the Placer Pacific Granny Smith operation, (see the Granny Smith record), with the pits of the two operations overlapping. AngloGold Ashanti subsequently took control of the complete deposit.
The ore at the ~2670 Ma Sunrise Dam deposit is developed within both Archaean basement and the overlying transported cover. The basement ore lies within the Laverton Tectonic Zone of the Yilgarn craton, characterised by major north-south shears and associated faults. The dominant host is strongly deformed, greenschist facies andesitic to basaltic/mafic-ultramafic volcaniclastic rocks and magnetite-rich shales and turbidites (banded iron formation), which have been intruded by both quartz-feldspar porphyry sills and dykes (e.g., Dolly porphyry) and localised ultramafic and lamprophyre dykes. The timing of gold mineralisation relative to the metamorphic peak has not been ascertained, although similar deposits elsewhere in the Yilgarn craton are inferred to have formed syn- to post- peak metamorphism. Gold mineralisation is found intermittently within a NE trending corridor over a length of 4.5 km, coincident with a strongly magnetic BIF rich sequence.
Gold mineralisation is structurally controlled and vein hosted, occurring in two main styles, namely: (1) gently dipping shear-related and high strain veins; and (2) stockwork zones in steeply dipping planar faults with brittle characteristics, commonly concentrated at lithofacies contacts within the volcanic stratigraphy or porphyry margins, and within hinge domains in the magnetite shales (BIF). Gold is found in all lithologies, but is best developed in the Fe rich bands, in association with pyrite replacement of BIF. These mineralisation styles occur as:
Group I orebodies, which occur in shallowly dipping foliation parallel veins within a strong penetrative fabric that consists of a sericite±chlorite cleavage and/or schistosity. Veins typically contain quartz-carbonate±pyrite±arsenopyrite and quartz-sericite-carbonate-pyrite-chlorite alteration.
Group II lodes are steeply dipping and characterised by steep veins and breccias up to a few metres in width. Breccias comprise angular clasts of sericite-altered host-rock volcanics up to several cms across, locally with jigsaw fits, set in a quartz and quartz-carbonate matrix. Veins may be up to 5 m wide, and consist of carbonate-pyrite-arsenopyrite-quartz. Gently NW-dipping, laminated quartz-carbonate veins containing gold, arsenical sulphides and tellurides are also observed within Group II orebodies, and are interpreted to have formed during D4 dextral normal faulting.
Group III orebodies are hosted within steeply dipping stockwork breccia zones up to 20 m wide, and less commonly as vein zones. Stockwork veins commonly contain carbonate-chlorite-quartz±sericite±pyrite±arsenopyrite, with adjacent alteration typically consisting of sericite-quartz-pyrite-ankerite±arsenopyrite. The breccia is characterised by sericite-altered host-rock volcanics clasts and quartz and carbonate matrix.
Group IV lodes are hosted within the quartz-feldspar Dolly porphyry, with mineralisation being typically arsenic rich, occurring and within steep narrow (0.2 to 0.5 cm wide) gold-bearing quartz-pyrite-arsenopyrite veins.
These variably oriented gold-hosting structures and mineralisation styles are the result of a complex structural and mineralisation history involving at least six phases of deformation, namely:
D1 - formed several major shallow- to moderately dipping northwest-trending shear zones (Cleo, Margies, Mako, Sunrise, Midway-GQ, and Carey). These include low-angle ductile shear zones, characterised by a penetrative S1 fabric, mostly parallel to the structures, are up to 40 m wide, and are vertically stacked above one another. Steeply dipping shear zones are also interpreted as initial D1 structures that subsequently underwent D3 reactivation.
D2 - produced north- and south-plunging upright folds with steep axial surfaces, in response to east-west to WNW-ESE shortening, and S2 cleavage which crenulates S1. No mineralisation is associated with D2 structures, although the bulk of the ore accompanied D3 and D4.
D3 - characterised by thrusting along gently dipping D1 shear zones and sinistral shearing along steeply dipping structures, accompanied by Group I and II orebodies (see below) in the respective structures. Quartz-feldspar porphyries (e.g., the Dolly dyke) are interpreted to have intruded during late D2 to D3 and locally host narrow gold-bearing quartz-pyrite veins (Group IV orebodies). These quartz porphyries are deformed by D3 shear zones and cut by S3 fabrics.
D4 - resulted in dextral faults as a response to NE-SW shortening accompanied by steeply dipping stockwork vein (D4a) and breccia systems (D4b) that comprise the Group III orebodies.
D5 - produced strike-slip faults as a result of SE compression.
D6 - characterised by dextral conjugate faults caused by east-west shortening. Neither D5 nor D6 structures are mineralised.
In the transported cover, secondary (supergene) gold with extremely high gold grades was hosted by fluvial sediments within two distinct horizons, each of 2 to 12 m in thickness over a 600 x 200 m area and at a depth of from 5 to 40 m. These were developed near the base of Tertiary palaeochannels and horizontal blankets of mineralisation related to iron redox fronts and associated palaeo-water table.
The ore below the unconformity is developed in both oxidised and fresh bedrock, occurring as a shallow west dipping zone covering a plan area of 1600 x 700 m, and extending to more than 700 m below the surface.
In December 2000 the AngloGold Cleo resource totalled 40.8 Mt @ 3.39 g/t Au, while in December 1998 the Placer Dome Granny Smith section of the deposit had a resource of 11.3 Mt @ 3.2 g/t Au. Together these total more than 170 t of contained Au.
Extensions of the ore at depth comprise the Sunrise Deeps discovery.
Total production to 2010 was 149 t (4.8 Moz) of gold at an average grade of 4.2 g/t Au.
At December 2010, (AngloGold Ashanti reserve statement):
Proved + Probable Reserves were: 13.89 Mt @ 3.08 g/t Au, and
Measured + Indicated + Inferred Resources were: 36.68 Mt @ 2.85 g/t Au (which includes the reserves, and totals 104.38 t of Au),
Additional low grade resources and stockpiles were: 22.8 Mt @ 2.70 g/t Au (for an additional 61.55 t Au)
Production in the year 2006 totalled 14.463 tonnes of Au and by 2010, 12.316 tonnes Au.
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Telfer - Western Australia .................................... Sat. 19 November, 2011
The Telfer gold-copper deposit is located within the Paterson Tectonic Province, in the Great Sandy desert, 400 km ESE of Port Hedland, and 1000 km north of Kalgoorlie, Western Australia (#Location: 21° 43' 18" S, 122° 12' 52" E).
The first recorded indication of the Telfer deposit was the discovery in 1970 of extensive small, low relief quartz-limonite gossans and limonitic siltstones outcropping as pods along lithological contacts by an independent geologist prospecting for copper in the Paterson Range. These gossans were sampled, but were not assayed for gold, and no title was taken. In 1971, Day Dawn Minerals NL undertook a regional sampling program in the district, also without title. Although anomalous copper and gold values were obtained, they were not deemed sufficient to warrant further investigation in the remote location. In mid 1972, Newmont Australia Limited, who were aware of both the original gossan discovery and the Day Dawn anomalies, visited and pegged title over the area. This was followed by sampling and then an intensive drilling program that by 1975 had defined an open pit reserve of 3.8 Mt @ 9.6 g/t Au, mainly of oxide ore. In 1975, BHP Gold Limited was introduced as a 30% joint venture partner in the project to satisfy Australian Government foreign ownership legislation. Later, in 1990, Newmont and BHP Gold merged their Australian assets to form Newcrest Mining Limited that assumed full ownership of the project.
Mining commenced during 1975 at Main Dome and reached full production of 0.5 Mtpa in 1977. Ore processing was initially by milling, cyanidation and Merrill-Crowe gold recovery. During the early 1980s, exploration maintained the high grade reserve, although the potential for a large, low grade oxide resource in both Main and West domes was also recognised and by 1986 the crushing and grinding capacity was enlarged and the Merrill-Crowe process was replaced by a carbon in leach (CIL) circuit. A dump leach operation was commenced in 1988. By the late 1990s, Telfer was treating 2.5 Mtpa through the mill and CIL circuit, and 15 Mtpa of low-grade oxides by dump leach.
In 1989, supergene-sulphide ore was being mined from the base of the oxide zone, initially from the open pit and in 1990, from underground. This required a sulphide flotation circuit to produce a copper and gold concentrate. By the late 1990s, 0.3 Mtpa of this ore was being treated.
In 1991 surface exploration diamond drilling intersected the I30 quartz reef within the Main Dome at 1000 m below surface. A mining feasibility study was completed in 1995 and a decline was commenced that reached ~500 m below surface by July 1997. An underground drilling program ensued, followed by a study of how to best extract the series of deep high grade reefs that were discovered. This work outlined a high grade core reserve in the I30 Reef of 1 Mt @ 15.6 g/t Au, 2.6% Cu, surrounded by a larger tonnage, lower grade gold and copper stockwork. This was, in turn, was part of an even larger underground resource on the eastern limb of the Main Dome associated with an additional eight hanging wall and three footwall reefs, collectively known as Telfer Deeps.
By 2000, the oxide resources were largely exhausted after extracting ~185 tonnes of gold, and mainly hypogene sulphide ore was being mined creating recovery and cost problems. As a consequence, operations were suspended pending feasibility studies into mining hypogene ore in the open pits and Telfer Deeps and modification of the flotation circuit of the existing sulphide treatment plant to accommodate the increased underground reserve and higher sulphide and copper content. In the meantime, the decline to the Telfer Deeps was deepened. The feasibility study was completed in 2002 and production recommenced in the open pits in 2004 and underground in 2006, based on reserves and resources of 320 Mt @ 1.4 g/t Au, 0.14% Cu and 444 Mt @ 1.4% Cu, 0.13% Cu respectively in the open pits and 39 Mt @ 2.7 g/t Au, 0.5% Cu and 59 Mt @ 2.8 g/t Au, 0.52% Cu respectively underground. Resources are inclusive of reserves. The main underground mining method was sub-level caving. Production has continued to the present with only minor operational pauses, for a cumulative total of >150 tonnes of recovered gold. Details in this section are largely drawn from Moorhead et al. (2013) and Newcrest Mining annual reports and releases to the ASX.
Telfer is located within the NNW to NW trending, >1000 km long by 150 to 200 km wide Paterson Orogen (Fig. 1). This orogen fringes the northeastern margin of the Archaean to Palaeoproterozoic West Australian Craton, and merges with the Musgrave Orogen to the SE. It is composed of two main elements, the Rudall Complex and Yeneena Basin. The Rudall Complex comprises ~2015 to 1765 Ma Palaeoproterozoic igneous and sedimentary rocks that were subjected to regional D1 and D2 deformation, metamorphism to granulite facies and granitic intrusion during the ~1800 Ma Yapungku Orogeny, followed by voluminous post-orogenic 1590 to 1310 Ma granitic intrusions only subjected to greenschist facies metamorphism. The complex also includes a domain of sheared peridotite, gabbro, pelitic schist and metaturbidites to the south and east (Bagas, 2004; Whitaker et al., 2010).
The >24 000 km2 Yeneena Basin contains unconformably overlying Neoproterozoic marine sedimentary rocks of the ~9 to 13 km thick Yeneena Supergroup. It represents the extensional, fault controlled, northwestern extremity of the ~2 million km2 Centralian Super-basin, developed where the latter encroached upon the Paterson Orogen. The Yeneena Supergroup is the thickest measured section in the entire super-basin, but only represents the first of four super-sequences contained therein. To the west and SW, it is in fault contact with both stratigraphically equivalent and younger rocks of the Officer Basin section of the Centralian Superbasin that lap onto the West Australian Craton. To the northeast, the Yeneena Basin and Rudall Complex are overlain by the extensive Phanerozoic Canning Basin (Huston et al., 2010).
The Yeneena Supergroup is composed of the Throssell Range and overlying Lamil groups. The Throssell Range Group is mainly exposed to the SW of Telfer, although similar rocks to the NE may be equivalents (Fig. 1). The lowest unit is the up to 4000 m thick Coolbro Sandstone, predominantly composed of grey, massive and thickly bedded fine- to coarse-grained sandstone. It is conformably overlain by the up to 3000 m thick Broadhurst Formation, a succession of interbedded fine to coarse sandstone to silty shale, including units of dark grey to black carbonaceous siltstone to shale with up to 10% pyrite and pyrrhotite, and stromatolitic dolostone. This unit hosts the ~810 to 790 Ma Nifty and Maroochydore stratabound, sediment-hosted, quartz-dolomite-sulphide vein network copper deposits, 70 km west and 60 km SSW of Telfer respectively.
The Throssell Range Group is structurally overlain by the Isdell Formation, an ~1000 m shelf sequence of fine grained, thinly bedded carbonate rocks, calcareous siltstone, minor shale and local sulphidic dark grey or pale grey-cream carbonate rocks with lesser sandstone. This sequence is conformably overlain by the Lamil Group, which has only been subjected to greenschist facies metamorphism. It comprises a shallow marine sequence, divided into the Malu, Puntapunta and Wilki formations. The Malu Formation, the principal host to ore at Telfer, is up to 2000 m of thickly bedded, fine to medium grained quartzite and quartz sandstone with occasional thin interbeds of siltstone and mudstone. It has a transitional upper boundary with the Puntapunta Formation, 1500 m of laminated to thinly bedded dark grey dolomitic sandstone, dolomitic siltstone, chert, shale and limestone. The Wilki Formation, the uppermost unit of the Lamil Group, consists of up to 1400 m of silicified sandstone and minor shale.
A suite of highly fractionated, I-type granite bodies, the Mount Crofton, Minyari, Wilki and O'Callaghans granites, intrude the Lamil Group (Fig. 1). The first three are oxidised, temporally progressing from strongly to more weakly magnetite-bearing granites, emplaced between ~645 and 630 Ma, whereas the 605 Ma O'Callaghans Granite is reduced and ilmenite-bearing (Czarnota et al., 2009; Maidment, et al., 2010). Although no granites are exposed within 5 km of the Telfer deposit, modelling of gravity data suggests large concealed intrusions, including the O'Callaghans Granite, extend beneath Telfer (Schindler et al., 2016). The latter is closely associated with the O'Callaghans scheelite-wolframite skarn altered deposit (Inferred resource of 69 Mt @ 0.34% WO3, 0.55% Zn, 0.27% Pb, 0.29% Cu).
Deposition of the Yeneena Supergroup took place during an ~850 to 824 Ma, NE-SW directed extensional event. Related mafic to intermediate intrusions have been dated at 837 to 815 Ma. Yeneena Supergroup deposition was terminated by basin inversion during the Miles Orogeny between ~820 and 810 Ma (Huston et al., 2010). This event comprises two pulses of regional deformation overprinting the Palaeoproterozoic D1 and D2 fabrics of the Rudall Complex. The first, D3, produced dextral strike-slip and reverse fault reactivation of the prominent NW to NNW trending extensional phase growth faults (e.g., Vines and Camel-Tabletop faults; Fig. 1), and recumbent folding in the Throssell Range Group. D4 was responsible for broad folds, conjugate faulting and greenschist facies metamorphism. A progressive 15° anti-clockwise rotation from NNW D3 to NW D4 axes resulted in a series of domal structures, including the Telfer Dome (Bagas, 2004). Widespread mafic dykes and sills were intruded between 750 and 700 Ma, followed by extensive 645 to 605 Ma granitic intrusion. The subsequent Paterson Orogeny involved early D5 open folds and a late D6 episode of NW trending dextral and ENE striking sinistral faults (e.g., Parallel Range and McKay faults; Fig. 1B) at ~550-540 Ma (Maidment, et al., 2010).
Deposit Geology and Mineralisation
Gold and copper mineralisation at Telfer is concentrated in the core of the NW-SE elongated, 10 x 3 km, Telfer Dome (Fig. 2), and is hosted by the Malu Formation, the stratigraphy of which may be summarised as follows:
Malu Quartzite - up to 1000 m of mainly massive and uniform metamorphosed quartz sandstone with increasing quantities of pelitic interbeds at both the top and bottom, indicating gradational contacts with the Isdell Formation and Telfer Member respectively. Much of the quartzite is pyritic and stratabound quartz veins are locally gossanous. The Malu Formation of the Telfer Dome is subdivided as follows, from the base:
• Telfer Member - which is 600 to 700 m thick and is largely a transition zone between the Malu Quartzite and the overlying, mainly carbonate rich Puntapunta Formation. With the Isdell Formation and Malu Quartzite, it is the main Au bearing formation in the region and comprises an alternating sequence of quartzite, siltstone and shale, subdivided into four quartzite and four shale/siltstone units in the Telfer area, as follows, from the base:
- Lower Vale Siltstone, 2 to 5 m thick - thinly bedded and silicified siltstone with disseminated pyrite and siderite.
- Footwall Sandstone, 20 to 50 m thick - commonly poorly sorted quartz sandstone.
- Middle Vale Siltstone, 5 to 9 m thick - fine grained and thin bedded argillaceous siltstone, claystone, mudstone, minor carbonaceous limestone and calcareous sandstone. It is pyritic and the main ore host in the original Telfer open pits hosting the Middle Vale Reef. Abundant shortite pseudomorphs are evident in this unit.
- Median Sandstone, 25 to 40 m thick - poorly stratified and thick bedded, fine grained and well sorted quartz sandstone with silty or muddy interbeds.
- Upper Vale Siltstone, 1 to 4 m thick - thinly bedded sideritic siltstone with minor fine grained sandstones, containing disseminated pyrite in places.
- Rim Sandstone, 30 to 40 m thick - a stratified sequence of interbedded quartz sandstone to subarkose and argillaceous siltstone.
- Outer Siltstone, up to 500 m thick - well stratified, thin bedded, argillaceous, calcareous and minor carbonaceous siltstones with interbedded sandstone. The lower sections host the 'E Reefs' at Telfer. In the Karakutikati Ranges this unit has laminated dolomite and dolomitic shale prominent near the top.
• Upper Malu Member, 620 m of thick-bedded, fine- to medium-grained siliciclastic sandstone; sporadic interbedded carbonate sandstones and carbonaceous
• Middle Malu Member, 290 to 320 m of very fine- to coarse-grained sandstone; interbedded with laminated quartz sandstone and siltstone; varying dolostone and
• Lower Limey Unit, 9 m of bedded fine-to-coarse carbonate sandstone.
• Lower Malu Member, 420 m of turbiditic quartz sandstone; minor siltstone interbeds; carbonaceous towards the top.
Within the Telfer Dome, this succession has been folded into two asymmetric, en echelon, doubly-plunging anticlines, the Main and West domes, separated by a tight faulted syncline (Fig. 2). Both domes have shallow to moderately dipping western and moderate to steep eastern limbs indicating NE vergence in a NE-SW directed compressive stress field during D3 folding. This is the first regional deformation evident at Telfer, and resulted in well-developed cleavage, parallel, or at a low angle, to bedding. This fabric is crenulated by a steeply SW dipping axial plane cleavage. The former is the result of bedding plane slip/shearing during folding, with total displacements of <10 m (Vearncombe and Hill, 1993). This bedding plane slip was strongly partitioned within more ductile siltstone beds and along competence-contrasting sandstone-siltstone contacts. In addition to the folding, shallow NE vergent thrusts formed. Five monocline-anticline structures have been superimposed on the two domes. Each strikes north-south, and is doubly plunging, with axial planes dipping at 35 to 50°W. They are typically up to 1 km in length and 50 to 200 m wide, often with steep east limbs that are locally overturned. A third, weakly developed, ENE trending cleavage, which dips either to the SSE or NNW, is likely related to D4.
Three different vein styles host gold and copper mineralisation within the Telfer Dome. These are:
• Bedding-concordant veins, locally termed 'reefs', that are laterally extensive over lengths of up to 1 km or more, both laterally and down dip, and are 0.1 to 10 m, averaging between 0.3 and 0.7 m thick. These reefs follow D3 bedding plane slip shear structures. In the hypogene zone, reefs are composed of quartz±dolomite with sulphide and gold, and traces of scheelite, carrying grades of between 5 and >50 g/t Au, and 0.2 to 1.5% Cu. Average reef grades are frequently >10 g/t Au. The proportion of quartz, dolomite and sulphides is variable, ranging from as much as 80% quartz, to dominantly sulphides within sericite-pyrite shear planes. There is a tendency for a higher proportion of dolomite in the lower reefs, which may be pink, grey or white. A total of 21 reef structures have been identified within the Main Dome, distributed over a stratigraphic thickness of ~1000 m, occurring as either single reefs of sheeted clusters of closely spaced veins. The main upper reefs, the E and Middle Vale reefs, hosted by the basal Outer and Middle Vale siltstones respectively, are planar sheets with surface areas of up to 20 km2, the latter of which averages 1 m in thickness, but ranges up to 3 m. These two reefs extend from the southern Main to the Western Dome. However, Reefs below the Middle Vale are less extensive, and are best developed in the anticlinal crest of the Main Dome, tapering on the flanks of the structure. The next significant vein, the M10, which is 100 m below the Middle Vale Reef in the Upper Malu Member, has a north-south strike length of 2 km and 500 m dip extent, mostly on the eastern flank of the dome. It averages 0.45 m in thickness, ranging from 0.1 on the flanks to 2 m in the axial zone. This is a member of the M-Reefs that are distributed over intervals of <10 to >150 m within the Upper Malu Member. Other M-Reefs illustrate the variation in reef characteristics. The M40 reef, 250 m below M10, is 0.2 to 0.6 m thick carrying 5 to 80, averaging 16 g/t Au, hosted within the Upper Limey Unit which has undergone strong carbonate alteration. The M45 reef, which is 25 m lower, is generally ~2 m thick, and occurs as either a sericite-pyrite shear plane or a quartz-dolomite-sulphide reef, with grade that are typically <10 g/t Au and <1% Cu. The M50 reef, which is 25 m lower, and 300 m below M10, is composed of three separate bands. The upper of these averages 0.2 m in thickness and comprises disseminated sulphides, fine white dolomite and pyrite-chalcopyrite veins within a black, fine- to medium-grained carbonaceous siltstone, carrying 6.4 and 3 g/t Au respectively above and below the base of oxidation. The middle section is a 0.1 to 2.0 m thick quartz-dolomite-sulphide reef as described in other positions, with grades varying between 5 and 94, but averaging 34.6 g/t Au. The lowest band is a 0.2 to 0.5 m thick calcareous sandstone. The A-Reef set is developed within the lower two thirds of the Middle Malu Member, being most densely packed at the base where they may be only a few metres apart, becoming more widely spaced upwards. The most significant in the lower part of the sequence is the I30 Quartz Reef at the contact of the Middle Malu Member and the Lower Limey Unit. It has an area of 875 x 160 m, elongated north south following the crest of the Main Dome, where it is 10 m thick, tapering to ~0.5 m on the flanks of the fold. Mineralisation is dominantly pyrite and chalcopyrite with a gangue of quartz, grey, white and pink dolomite, calcite and rare siderite. The geometry of this reef is controlled by the intersection of the I30 monoclinal fold, a north-south striking reverse fault and a near vertical NE trending fault corridor. The lowest concordant vein set, the B-Reefs, occur as a cluster in the top 75 m of the Lower Malu Member.
• Veins that are discordant to lithological boundaries, within the axial core of the domes, trending either NE and dipping ~45°NW, or ENE with steep NNW or SSE dips. The former comprise quartz-carbonate-sulphide-gold, and the latter carbonate-quartz-sulphide-gold, with gold and copper grades similar to the reefs. Both contain traces of scheelite. The carbonate is a pink to grey dolomite with minor calcite.
• Breccia and stockwork veining, which forms the bulk of the sulphide resource. Stockwork veins are narrow and discontinuous, cutting across stratigraphy, and are composed of dolomite-quartz-sulphide-gold, forming zones that are characterised by consistent grades of >0.3 g/t Au. The breccias consist of wall rock clasts in a matrix of dolomite-quartz-sulphide-gold. Stockwork mineralisation envelopes and links concordant reefs to form halos with a thickness from a few to as much as 200 m perpendicular to the reefs, with lateral continuities of 100 to 1500 m. These halos are best developed in the axes of the main domes, and pass outwards into patchy zones and pods of stockwork mineralisation. The other principal occurrence is as large, gently SE plunging elongate zones that follow the axial traces of monoclinal structures, and are not closely linked to the main concordant reefs, although intersecting reefs are locally enriched.
Underground mapping shows mutually crosscutting relationship between the different vein types, suggesting several pulses of hydrothermal activity forming alternating reefs, discordant and stockwork veins. The metasedimentary rocks hoating these veins have all been affected by sericitisation, silicification, and lesser carbonate (calcite-dolomite) and tourmaline replacement, which is most intensely developed immediately around the veins and breccias. Sericite-carbonate alteration is most pronounced in argillaceous and carbonate hosts, while silicification is strongest in quartz-rich arenites (Schindler et al., 2016).
Hypogene sulphides are predominantly pyrite and chalcopyrite, with lesser chalcocite and bornite, minor cobaltite and nickel-sulphide. They occur as disseminations and locally as massive zones in quartz and quartz-carbonate veins, and as disseminated blebs and euhedral pyrite crystals replacing sedimentary hosts. Two phases of pyrite growth are recognised. The first is rare subrounded and rounded aggregates of fine euhedral pyrite, possibly of diagenetic origin. It occurs in a thin laminated interval at the top of the Middle Vale Reef, and is overgrown by the dominant coarse-grained subhedral to euhedral epigenetic pyrite found throughout the deposit. Primary gold occurs as free grains on boundaries and crack infill of pyrite and chalcopyrite, in the lattice of pyrite, as Au telluride (calaverite) or Au-Ag telluride (sylvanite), or as electrum inclusions in pyrite, and to a minor degree on gangue silica grains. There is a correlation between vein frequency and gold grade.
The deposit has been oxidised to depths of from 80 up to 200 m, with an irregular oxide to hypogene boundary, sections of which are occupied by a well-developed supergene enrichment zone. The modern static water table is ~80 m below the surface, and the base of the supergene sulphide zone is at a depth of ~240 to 290 m. Oxidation of primary sulphides to boxwork textured goethite, hematite and other iron oxides is complete throughout the weathered profile, accompanied by variable amounts of vein quartz and clay minerals. Other minerals include monazite, xenotime, magnetite, cuprite, covellite, rutile and zircon. Only minor enrichment of gold appears to have occurred in the oxide zone, although there has been almost total depletion of copper. Minor redistribution of gold occurred near surface with the growth of small nuggets and wire gold in shallow gossans. In addition, erosion of the reefs produced a pediment of alluvial gold in pisolitic laterite and gravels distributed irregularly along the eastern flank of the Main Dome (Dimo, 1990).
The supergene enriched zone contains a complex assemblage of secondary oxide and sulphide minerals, including chalcocite, bornite, chrysocolla, cuprite, tenorite and malachite plus remnant native gold. This ore has similar textures and appearance to the hypogene mineralisation, except that chalcocite has extensively replaced pyrite along grain boundaries and fractures in the supergene zone. On the northeastern flank of the Main Dome, supergene and mixed supergene-oxide ores averaged 27 to 29 g/t Au and 2 to 10 wt.% Cu, whilst the underlying hypogene reef contained 1.6 to 12.8 g/t Au and up to 0.1 wt.% Cu (Dimo, 1990).
Hypogene mineralisation at Telfer post-dates D3 cleavage, as indicated by its control on veining and by euhedral pyrite replacement of minerals forming the cleavage fabric (Rowins et al., 1997). Limits on the age of this deformation event are indicated at the Nifty copper deposit, where mineralisation dated at between 810 and 790 Ma also post-dates D3 cleavage. Monazite and xenotime from Au-bearing concordant and discordant veins at Telfer have been dated at 652±7 and 645±7 Ma respectively, closely coinciding with ages of the oxidised, magnetite-bearing granitic rocks widespread within the district and indicated in gravity data to underlie the deposit at depth (Maidment et al., 2010; Schindler et al., 2016). Fluid inclusions from the various vein types contain five fluid assemblages, with trapping pressures calculated to vary from 3 to <1.5 kbar, homogenisation temperatures of from >480 to <150°C and a mix of salinities from fluids with ~50 and a second suite with 21 to 54 wt.% NaCl equiv.. Other data indicate reduced fluids dominated the system. Fluid inclusions also suggest the three vein types formed from a similar fluid with any vein composition variation reflecting reactions with differing host lithologies. The higher trapping pressure ranges measured exceed calculated lithostatic pressure at the estimated ~5 km Palaeodepth of the deposit at the time of formation, suggesting the ore fluids were over-pressured (Schindler et al., 2016). This is supported by the vein-fault geometry within the deposit which also demonstrates overpressuring.
Goellnicht et al. (1989) and Schindler et al. (2016) suggest the high content of major elements (Na, Mg, K, Ca, Fe, Mn, Cu, Zn, Pb, Co, Ni, Ag, Ce, La, Y and Mo),and high homogenisation temperatures in all fluid inclusion assemblages are compatible with involvement of a magmatic hydrothermal fluid. However, Rowins et al. (1997) present C, O, B, Pb and S isotope data from both ore sulphides and alteration minerals (carbonates and tourmaline) they interpret to indicate ore fluid solutes were derived from sedimentary rocks of the Yeneena Supergroup.
On the basis of these observations, it is postulated metal-bearing magmatic fluids were released into the permeable sedimentary rocks of the Lamil Group, sourced from the large composite granitic body indicated to underlie the district and deposit (Goellnicht et al., 1989; Schindler et al., 2016). These mingled with basinal brines, to be circulated in thermal convection cells, driven by the magmatic heat source, scavenging additional solutes from the host sequence (Rowins et al., 1997). The high temperature, over-pressured hydrothermal fluids collected in the crests of the pre-existing D3-4 Main and West domes, where they permeated bedding plane shears below fine grained aquitards, to hydraulically delaminate and inflate the sequence. During this process, Vearncombe and Hill (1993) estimate that in the Middle Vale Reef, ~40% of the matrix of the siltstone in the shear zone was dissolved and removed. Periodic over-pressure induced fault failure led to sharp pressure release, adiabatic cooling and precipitation of gold, copper, silica and dolomite to form the concordant reefs. The preceding asymmetric NE vergent D3-4 deformation had both enhanced dilation in the bedding plane shears and preferentially fractured competent sandstones on the steeper northeastern limbs of the domal structures. This acted as ground preparation, facilitating access of fluids and providing sites for deposition of the bulk of the concordant, crosscutting vein and stockwork ore. Mineralisation is, in general, best developed in areas of the greatest structural complexity (Schindler et al., 2016).
Reserves and Resources
In 1988 reserves + production accounted for 146 t Au at an average grade of 2.35 g/t Au.
In 1996 open pit resources totalled 92 Mt @ 1.1 g/t Au, while
underground resources were 3.9 Mt @ 11 g/t (indicated) + 7.5 Mt @ 6 g/t Au (inferred).
In 1997, the total measured + indicated + inferred resource was - 173 Mt @ 1.4 g/t Au.
The original mine operated from 1975 to 2000, over which period it produced ~185 t of recovered gold mainly from oxide resources. The redeveloped operation commenced with two open pit mines (Main and West domes) in November 2004 and February 2005 respectively and underground (4 Mtpa) in February 2007 mainly exploiting sulphide ore. Total production from this second period had reached 188 t Au by December 31, 2017.
In June 2005, published reserves and resources totalled (Newcrest reserve statement, 2006):
Total reserves: 360 Mt @ 1.5 g/t Au, 0.18% Cu for 535 t Au,
Total resources: 520 Mt @ 1.57 g/t Au, 0.18% Cu for 815 t Au,
including Open pit - 444 Mt @1.39 g/t Au, 0.13% Cu; Underground 59 Mt @ 2.8 g/t Au, 0.52% Cu; plus satellites and stockpiles.
At the end of June 2011, published reserves and resources totalled (Newcrest website, 2011):
Main Dome open-pit reserves: 240 Mt @ 0.80 g/t Au, 0.10% Cu for 190 t Au,
West Dome open-pit reserves: 190 Mt @ 0.64 g/t Au, 0.06% Cu for 120 t Au,
Underground reserves: 46 Mt @ 1.3 g/t Au, 0.33% Cu for 59 t Au.
TOTAL gold in reserves in June 2011 was 369 t.
Main Dome open-pit resources: 390 Mt @ 0.66 g/t Au, 0.08% Cu for 260 t Au,
West Dome open-pit resources: 370 Mt @ 0.50 g/t Au, 0.05% Cu for 185 t Au,
Underground resources: 100 Mt @ 1.2 g/t Au, 0.31% Cu for 120 t Au.
Other resources resources: 16 Mt @ 0.42 g/t Au, 0.33% Cu for 6.2 t Au,
TOTAL gold in resources in June 2011 was 575 t.
Ore Reserves are included within Mineral Resources.
Remaining Ore Reserves and Mineral Resources at 31 December, 2016 were (Newcrest Mining, 2017):
Main Dome open-pit reserves: 30 Mt @ 0.61 g/t Au, 0.097% Cu for 18.3 t Au,
West Dome open-pit reserves: 78 Mt @ 0.67 g/t Au, 0.060% Cu for 52.3 t Au,
Underground reserves: 19 Mt @ 1.4 g/t Au, 0.24% Cu for 26.6 t Au.
TOTAL reserves: 127 Mt @ 0.77 g/t Au, 0.10% Cu for 98 t Au.
Main Dome open-pit: 16 Mt @ 0.4 g/t Au, 0.1% Cu,
Main Dome open-pit: 49 Mt @ 0.83 g/t Au, 0.070% Cu,
West Dome open-pit: 180 Mt @ 0.61 g/t Au, 0.065% Cu,
Underground: 84 Mt @ 1.20 g/t Au, 0.28% Cu,
Main Dome open-pit: 0.27 Mt @ 0.65 g/t Au, 0.056% Cu,
West Dome open-pit: 7.7 Mt @ 0.60 g/t Au, 0.075% Cu,
Underground: 18 Mt @ 1.50 g/t Au, 0.44% Cu,
TOTAL resources: 355 Mt @ 0.82 g/t Au, 0.14% Cu for 291 t Au.
NOTE: Resources are inclusive of reserves.
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The summaries above were prepared by T M (Mike) Porter from a wide range of sources, both published and un-published. Most of these sources are listed on the "Tour Literature Collection", soon to be available from the OzGold 2011 Tour options page.
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T M (Mike) Porter, of Porter GeoConsultancy
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T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.
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