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
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The Cowal (previously Lake Cowal and Endeavour 42) epithermal gold deposit is located on the western edge of Lake Cowal, approximately 35 km NNE of West Wyalong in central New South Wales, Australia. The deposit falls within the Macquarie volcanic arc of the Palaeozoic Lachlan Fold Belt of southeastern Australia.
Gold mineralisation at Cowal is hosted by the Ordovician Lake Cowal Volcanic Complex, exposed as a north-south elongated 40 x 15 km window of intermediate calc-alkaline intrusives, volcanics and volcaniclastics surrounded by unconformably overlying Siluro-Devonian sediments and volcanics. Immediately to the west of this window a highly deformed north-south zone within the Siluro-Devonian defines the broad Booveri Fault Zone.
The 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.
The hosts to mineralisation comprise volcaniclastics and coeval extrusives and intrusive magmatic rocks which dip NW at 40 to 45°. This sequence has been subdivided into three conformable units, namely: i). The Great Flood unit of 250 m of monotonous beds, to 72 m of vitric volcaniclastic debris with mass flow structures. The coarser beds are intercalated with less than 3 m thick intervals of laminated shale, siltstone and mudstone, with a 20 m bed of polymictic volcanogenic conglomerate; ii). The Golden Lava unit, comprising 60 to 110 m of trachyandesite interbedded with monomictic sand to breccia
with clasts of plagioclase porphyritic fragments - believed to represent a submarine lava with associated hyaloclasite and autobreccia; iii). The Cowal Conglomerate, which is the oldest unit and is around 100 m thick with massive to graded beds of well rounded to very angular, clast supported polymict volcanic debris (andesitic) with interbedded laminated siltstone and mudstone, and evidence of mass flow.
This succession is cut by the 456 ±5 Ma holocrystalline to euqigranular to porphyritic Muddy Lake Diorite (in places a gabbro) and by porphyritic to aphanitic mafic dykes. The latter post date the diorite, are 0.2 to 20 m thick (averaging 1 to 3 m) and were emplaced in active faults.
The gold mineralisation is primarily in narrow dilatant veins of quartz-carbonate-sulphide (with common adularia) and carbonate±quartz-sulphide and narrow, healed faults with a similar mineralogy. Gold occurs to a lesser extent in pyrite stringers and as disseminations, shear chlorite-carbonate veins and chlorite-carbonate-sulphide veins. Approximately 20% of the orebody is in the oxide zone where gold is more erratic, reflecting leaching and dispersion. The veins generally strike at 305° and dip 35°SW and are at their highest density in the Golden Lava, although those in the Muddy Lake Diorite are usually thicker. The veins are typically parallel sided and from <1 to 100 mm thick. Vein alteration haloes are rare and the main sulphides are quartz, sphalerite, chalcopyrite, galena and pyrrhotite, with minor associated visible gold. The best gold accompanies sphalerite and to a lesser extent adularia.
Four alteration styles are recognised, namely i). Propylitic, in the surrounding hosts; ii). Quartz-sericite-carbonate, associated with fault zones and gold mineralisation, and is best developed in the Golden Lava and Great Flood units and grades outward from the faulz zones and associated veining to the propylitic zone - this style usually embraces veins of ankerite-quartz-pyrite-sphalerite-chalcopyrite-galena; iii). Quartz-potassium feldspar - restricted to the Golden Lava as irregular patches and zones usually associated with later chloritisation and occassionally surrounds sulphide bearing dilational veining; iv). Chlorite-carbonate-pyrite, usually associated withe the previous type of alteration and surrounding zones of quartz, K-feldspar, pyrite, sphalerite and chalcopyrite veins.
At December 31, 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 December 31, 2010, the reserve - resource estimates were (Barrick Annual Report):
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 2010 totalled 9.27 t of recovered Au.
Cadia Valley Operations - New South Wales, .................................... Tue. 15 November, 2011
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The Cadia and Ridgeway porphyry gold-copper deposits are located 20 km south of Orange in the central tablelands of New South Wales, Australia, some 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).
Cadia Hill and related adjacent resources 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 quartz veins, sheeted and stockwork quartz and quartz-sulphide veins and disseminated mineralisation with higher grade gold and associated copper mineralisation.
The Cadia deposits are hosted within Ordovician volcanic, volcaniclastic and intrusive rocks of calc-alkaline affinity in the Eastern Subprovince of the Lachlan Orogen which were formed in the intra-oceanic Macquarie Volcanic Arc. 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 rolled back 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 host porphyry and high sulphidation epithermal gold mineralisation. 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 Cadia district falls within the Molong Volcanic Belt in the eastern part of the Lachlan Orogen. 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.
The ore deposits occur as a string of mineralised centres within, and elongated parallel to, a 7 km long, NW- to WNW-trending corridor of alteration and mineralisation that is up to 2 km in width and has been intersected by drilling to a depth of more than 1600 m.
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 is present 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. This array forms a broadly tabular envelope that is approximately 300 m wide, dips SW at around 60° and strikes NW. The sheeted vein envelope persists over a length of some 900 m and to a depth of at least 800 m beneath the surface, although grades decrease below 600 m (Holliday et al., 2002). Within the envelope, veins range from a millimetre to 10 centimetres in width with densities from 2 to 10 per metre, but locally in the core of the deposit may exceed 15 per metre (Newcrest Mining presentation). 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 and its continuation, Cadia Far East, extend SE to ESE over an interval of approximately 1.7 km in length, 300 m in width and at least 1600 m down dip, plunging to the SE. 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 sericitecarbonate 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 albiteorthoclase-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 1960Ős, 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 WNW elongated, steep north dipping, 200 x 50 x 500 m body with an elliptical cross section, located on the southern 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 north 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 interand 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).
The total pre-mining resources were:
Cadia Hill in 1977 - 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. The Cadia-Ridgeway mines are operated by Newcrest Mining Ltd.
Boddington - Western Australia, .................................... Wed. 16 November, 2011
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The Boddington gold deposit is located approximately 130 km SSE of Perth in Western Australia and lies within the Saddleback greenstone belt, part of the Western Gneiss terrane, on the far south-western extremity of the Archaean Yilgarn Craton. The Saddleback greenstone belt is some 35 km long and 5 to 10 km wide (#Location: Wandoo - 32° 45' 15"S, 116° 21' 19"E).
The deposit was discovered in 1980 in the Darling Ranges Bauxite district and mining commenced in 1987. The original operation commenced on a resource of 60 Mt @ 1.6 g/t Au which occurred as very fine, generally <5 µm, gold grains spread through the 4 to 25 m thick lateritic weathering profile, sections of which are mined on a large scale for bauxite elsewhere in the district.
The lateritic gold mineralisation at Boddington occurred as a semi-continuous blanket over a strike length of more than 5 km and a width of around 1 km. Some 30% of the gold was hosted in the same vertical zones as the bauxite, namely the upper lateritic gravels, the hardcap and the B-zone laterite. The remaining 70% was found in the underlying clay and saprolite zone, which was 10m, and locally up to 120 m in thickness.
The laterite resource was exhausted in 2001 and the mine placed on care and maintenance, after having produced 105 Mt @ 1.4 g/t recovered Au for 147 t Au and 6500 t Cu.
Beneath the lateritic ore, there are a variety of other ore types present in the greenstone belt bedrock, including rich lode deposits and the large low grade, hard-rock Wandoo body. Construction of the Wandoo mine was completed in 2009, and the first gold and copper concentrate was produced in August of the same year.
The Wandoo deposit occurs as disseminations and stockworks hosted by 2715 to 2690 Ma dyke and stock-like dioritic intrusives and andesitic volcanic rocks of the Saddleback Greenstone belt. The element chemistry of the host intrusions indicates an island arc tectonic setting. The host rocks underwent a ductile deformation event (D1-D2), followed by a second period of supracrustal deposition comprising intermediate volcaniclastic rocks and intrusion of coeval granodiorite-tonalite rocks dated at around 2675 Ma. The chemistry of the latter rocks is again consistent with formation in an arc setting. All of these rocks were then metamorphosed to upper-greenschist to lower-amphibolite facies at around 2640 Ma, and deformed by brittle-ductile faults (D3-D4). A late monzogranite intrudes the greenstone belt just east of the Boddington mine. This intrusion, dated at 2612 Ma, is distinct in terms of its K feldspar-phyric texture, associated aplites and rare pegmatites, distinctive magnetic low signature, and its elevated U-Th and K radiometric character.
The core of the mineralised hydrothermal system is characterised by overlapping Cu, Au, Mo, Bi and W concentrations, with marked enrichments in Pb, Zn and Ag on the periphery.
Two stages of mineralisation are recognised at Boddington, namely:
i). An early widespread silica-biotite alteration, and complex quartz+albite+molybdenite ±clinozoisite ±chalcopyrite veins variably deformed by ductile shear zones. Re-Os ages from molybdenite in these veins indicates a formation age of ~2700 Ma.
ii). A second, main stage of mineralisation which cuts all of the above, and comprises,
a). complex quartz+albite+molybdenite ±muscovite ±biotite ±fluorite ±clinozoisite ±chalcopyrite veins (controlling the Mo distribution);
b). clinozoisite-sulphide-quartz-biotite veining (controlling the bulk of the lowgrade Au-Cu mineralisation);
c). actinolite ±sulphide ±quartz, carbonate-chlorite-sulphide, and sulphide veins (controlling high grade mineralisation). Re-Os dates from these mineral assemblages yields ages of around 2625 to 2615 Ma, broadly synchronous with the monzogranite to the east of the deposit.
Structurally, mineralisation is controlled by the following, in increasing order of importance: i). a late ductile WNW to NW striking, sub-vertical fault network with elevated mineralisation and alteration; ii). intersection of this fault network and competent lithologies; iii). intersection of the late faults and early ductile quartz-sericite shear zones; and iv). NE striking corridors which appear to compartmentalise the deposit by offsetting favourable hosts prior to mineralisation.
The Wandoo basement gold mineralisation is therefore interpreted to represent a structurally-controlled, intrusion-related Au-Cu deposit, paragenetically associated with both ~2700 and 2612 Ma events, with the main stage, higher grade mineralisation being apparently synchronous with the late, K-rich monzogranite suite.
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 in 2010 (Newmont 2011) were:
Proven + Probable Reserve - 1089 Mt @ 0.58 g/t Au, 0.11% Cu, for 631 t Au, (reserves additional to resources)
Measured + Indicated Resource - 469 Mt @ 0.40 g/t Au, 0.08% Cu, for 187 t Au,
Inferred Resource - 163 Mt @ 0.43 g/t Au, 0.11% Cu, for 70 t Au.
Remaining ore reserve and mineral resources at 31 December 2016 (Newmont 2017) were:
Proven + Probable Reserve - 514.6 Mt @ 0.70 g/t Au, 0.11% Cu, for 362 t Au, (reserves additional to resources)
Measured + Indicated Resource - 354.2 Mt @ 0.51 g/t Au, 0.10% Cu, for 181 t Au,
Inferred Resource - 7.5 Mt @ 0.58 g/t Au, 0.10% Cu, for 4.35 t Au.
The current hard rock Boddington project is owned and operated by Newmont Australia (in 2017). The original laterite mine was operated by Worsley Alumina Pty Ltd for the original joint venture owners, Newmont Mining (44.45%), AngloGold Ashanti (33.33%) and Newcrest Mining (22.22%).
The hard rock description was based on McCuaig et al., 2001 available from the GeoScience Australia website.
Kalgoorlie Superpit - Western Australia .................................... Thu. 17 November, 2011
The KCGM Super Pit lies within the Kalgoorlie Gold Field, some 600 km east of Perth. It exploits the Golden Mile string of deposits (#Location: 30° 46' 39"S, 121° 30' 10"E).
The Kalgoorlie Gold Field is developed within the Archaean Norseman-Wiluna greenstone belt of the Eastern Goldfields province in the eastern Yilgarn craton. It is hosted by greenschist facies, marine volcano-sedimentary rocks, divided into narrow blocks by NNW-trending regional wrench faults. The volcanic succession comprises, from oldest to youngest: (1) the Lunnon Basalt (~2720 Ma) - pillowed to massive tholeiitic lavas; (2) Kambalda Komatiite (2709±4 Ma) - ultramafic flows; and (3) Devon Consols and Paringa Basalts - two pillowed to massive magnesian lavas, separated by the 10 m thick, 2692±4 Ma, Kapai Slate marker horizon of sulphidic and tuffaceous shale.
This volcanic succession is overlain by black shales, greywackes, and volcaniclastic rocks of the Black Flag Group. To the south of Kalgoorlie, near Kambalda, this latter unit is intruded by the 2680±8 Ma Condenser Dolerite sill and 2678±8 Ma rhyolite porphyry dykes. In the Kalgoorlie district it is intruded by the 750 m thick Golden Mile Dolerite, which is the principal host rock to lode structures, and comprises a folded and metamorphosed, differentiated sill, emplaced at the same stratigraphic position as the Condensor Dolerite. The youngest mineralised rocks in all gold mines located between Kalgoorlie and Kambalda are 2675 to 2660 Ma stocks and dykes of hornblende-plagioclase porphyry.
Deformation of the volcano-sedimentary succession in the Kalgoorlie-Kambalda area was subdivided by Swager (1989) into four events: D1, characterised by recumbent folding and nappe-style thrusting; D2, which comprises upright folding that produced regional northwest-trending folds; D3, a post ~2660 Ma episode of sinistral wrench faulting in a transpressional regime, with major NNW-trending faults; and D4 which involved dextral wrench faulting, mostly NNE-trending.
The Kalgoorlie Gold Field comprises more than 1000 discrete lodes, controlled by brittle-ductile shear zones, clustered in a geometric array on both sides of the steeply dipping, NW to NNW-trending, sinsitral Golden Mile fault, which transects folded Golden Mile Dolerite in the centre of the mining district. The most significant of the lodes make up the Golden Mile, Mt Charlotte, Mt Percy and Hannan's South ore zones. An envelope of chlorite-calcite alteration, surrounds the entire mining area, replacing metamorphic actinolite and albite in all mafic rocks. The main lode systems are contained within a volume of these altered rocks, which is up to 5 km in length, by 1 to 2 km in width, and to a depth of 1000 m. They are controlled by a complex series of steeply dipping shears, and are largely hosted by ultra-mafic and mafic rocks and sills, the most important of which is the composite mafic sill, the Golden Mile Dolerite, and to a lesser extent the Paringa Basalt.
Two groups of structures, the Fimiston and Oroya lodes have been recognised. The Fimiston Lodes occur in steeply dipping (70 to 90°) shear zones, commonly parallel to the main Golden Mile fault, although others have different strike orientations. Individual lodes are up to 2 km long by 1.3 km in vertical extent, with high-grade shoots located at the intersections of shear zones. They are characterised by breccia bodies and cavity-fill veins surrounded by (1) an inner sericite-ankerite-siderite-quartz-hematite-pyrite±telluride alteration zone, containing most of the gold, and (2) an outer ankerite-sericite-quartz-pyrite zone where chlorite and calcite are progressively replaced. The ores are mineralogically refractory and complex, containing free native gold (often intimately associated with gold-bearing arsenical pyrite) and a significant proportion of Au-Ag-Hg-Pb telluride minerals. These Fimiston Lodes are subdivided into the Eastern and Western Lode System on the flanks of the Kalgoorlie Syncline. The Eastern Lodes System comprises a swarm of lodes (areas of pyritic and hydrothermal alteration) with mineralisation confined to shoots at lode-lode and lode-fault intersections. The Western Lode System is less complex, although the lodes are more persistent and well defined. Individual lodes occupy 20 to 50% of a lode channel and may be 30 to 1800 m long, 0.1 to 10 m thick and extend 30 to 1160 metres down dip. Mueller et al. (1988) suggest that the Fimiston Lode shear zones formed during D3 sinistral wrench faulting, whereas Bateman et al. (2001a) and Bateman and Hagemann (2004) conclude that the mineralised shear zones formed as flat, late D1 thrusting structures, subsequently rotated into their present subvertical position during D2 folding.
The Oroya Lodes represent the high-grade "green leader" ores, characterised by green vanadian muscovite, ankerite, quartz, pyrite, native gold, and gold-silver tellurides, the principal example of which is the 1500 m long Oroya shoot on the Paringa mine leases of the Golden Mile. This lode is controlled in large part by the 50°W dipping, reverse
Oroya shear zone system. This style of ore also occurs in the brecciated cores of steeply dipping Fimiston lodes, and may represent a late-stage of the D3 transpressional regime that generated the Fimiston lodes (Mueller et al., 1988), or a separate mineralisation event (Bateman et al., 2001).
Mount Charlotte occurs as a quartz vein gold deposit, present as a series of steeply plunging, pipe-like vein stockwork orebodies in massive 2692±2 and 2678 to 2670 Ma metagabbro of the Golden Mile Dolerite. These, and other related quartz-vein stockworks cross-cut the Fimiston and Oroya lodes, and are apparently controlled by district-scale NNE-trending strike-slip D4 faults. The Mt Charlotte mine exploits the main Charlotte and Reward orebodies and the satellite Maritana and Northern bodies. The Charlotte orebody extends more than 800 m vertically, from the surface to -1000 m RL, 250 m north-south along strike and over a width of 50 to100 m east-west. Reward extends from the surface to -800 m RL, 250 m north-south along strike and 50 m east-west. The orebodies are restricted to the most differentiated (and competent) unit of the host sill and are usually found adjacent to major steeply dipping faults where these cut the sill. The stockworks have two sets of veins that were developed as hydraulic fractures and were filled simultaneously and are of equal significance. Gold is in pyrite or pyrrhotite bearing metagabbro around the stockwork veins and to a lesser degree as free gold in the veins and along vein margins.
The final 'Super Pit' open pit is designed (as of 2008) to have dimensions of 3.8 x 1.35 km and a depth of 500 m below the surface by 2018. Production in 2006 was 85 Mt of mined rock, 12 Mt of which was milled. The remaining tonnage was stockpiled low grade mineralisation and waste. In 2007, the underground Mt Charlotte mine produced 1 Mt of ore @ 3 g/t Au.
Total production from 1893 to 2005 was ~1475 t Au (47.5 Moz), with a further ~110 t from 2005 to 2010.
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 in addition 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 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).
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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 deposits are within the Paterson Tectonic Province, in the Great Sandy desert, 400 km ESE of Port Hedland, and 1000 km north of Kalgoorlie, in Western Australia (#Location: 21° 46' 08" S, 122° 12' 30" E).
The orebodies are hosted by quartzites, sandstones and siltstones of the Telfer Formation, a member of the late Mesoproterozoic to early Neoproterozoic Yeneena Group. The Yeneena Group (which was metamorphosed at 1130 Ma) unconformably overlies the Palaeo- to early Mesoproterozoic metamorphics of the Rudall Complex which were metamorphosed between 1530 and 1330 Ma.
The Yeneena Group comprises, from the base:
Lower Yeneena Group
Coolbro Sandstone - around 2500 m of planar and trough cross bedded grey, massive and thickly bedded sandstone.
Broadhurst Formation - up to 2000 m of dark grey to black carbonaceous quartz mica siltstone to shale to pelitic schist which conformably overlies the Coolbro Sandstone.
Upper Yeneena Group
Isdell Formation - which comprises around 1000 m of fine grained, well bedded dolomitic and calcareous rocks.
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 and Telfer Formations respectively. Much of the quartzite is pyritic and stratabound quartz veins are locally gossanous.
Telfer Formation - 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.
- Camp Sandstone, 10 to 90 m thick - similar to the Median Sandstone and is commonly pyritic.
Puntapunta Formation - 2000 m thick, comprising an extensive unit of dolomite, limestone, calcarenite and sandstone. The dominant rock type is a well-bedded medium- to coarse-grained clastic dolostone with detrital quartz and lesser muscovite. Fine to coarse grained dark grey calcarenite interbeds within the dolostone, commonly contain pyrite. Siliceous quartz sandstone beds up to 10 m thick are distributed through the unit while cleaved shale and interbedded shale and sandstone are found near the top of the formation to the north of Telfer, marking a transition to the overlying Wilki Quartzite.
Wiki Quartzite - 1000 m thick, conformably overlies the Puntapunta Formation and comprises a continuous sequence of thick bedded, medium grained quartzites.
The structure in the mine area is dominated by broad, gentle, domal structures, with shallow bedding dips on their flanks. The domes are intruded by 680 to 620 Ma granitoids, which on the basis of isotope studies may be related to the emplacement of mineralisation. Granitoids in the district are also associated with porphyry-like Cu-Au, Au and W-Pb-Zn skarn mineralisation, as well as stratibound Au replacemenr reefs (Rowins, et al., 1998).
The ore deposit occurs as a series of silicified, generally conformable, reefs around 2 m thick, and stockwork zones within units of claystone, mudstone, carbonaceous limestone and argillaceous siltstone. These reefs are generally localised along the base of individual siltstone beds each of which overlies a sandstone unit in the lower sections of the Telfer Formation. The Middle Vale Reef (MVR), the main ore zone in the original open pits, lies within the lower 2 m of the Middle Vale Siltstone (MVS). The underlying Footwall Sandstone carries quartz veinlets with 0.1 to 1.5 g/t Au to a maximum of 20 m below the MVS. The other main ore zone, the E Reefs, are localised in the lower sections of the Outer Siltstone. The Middle Vale and E reefs, which comprised the bulk of the ore in the original pits, are remarkably conformable thin sheets of auriferous quartz-pyrite-chalcopyrite mineralisation, or their oxidised equivalents, that extend over an area of at least 20 sq. km. They are conformable in general, but transgressive in detail. The reefs are everywhere within distinctive calcareous, carbonaceous and argillaceous sediments. Significantly large pods are also associated with cross faulting. Primary mineralisation is mainly pyrite and quartz with gold, chalcopyrite and pyrrhotite, and minor bornite and chalcocite, as well as lesser galena, sphalerite, scheelite and Pb-Co-Ni sulphides. Pyrite may locally reach concentration of 20% and extend up to 20 to 30 m into the footwall. Sulphides are present as disseminated blebs and euhedral crystals of pyrite replacing metasedimentary host rocks and as disseminated and locally massive zones in quartz veins, both concordant and discordant. Pyrite and quartz commonly occur as crudely banded, conformable zones of coarse euhedral and fine anhedral grains that mimic the bedding structure in the host siltstones. Common gangue minerals include quartz, sericite, calcite, dolomite, ankerite, tourmaline and albite. Gold occurs as inclusions in pyrite, often associated with small amounts of chalcopyrite. The underground orebodies comprise a series of vertically stacked stratabound, high grade quartz-sulphide (±secondary iron oxides) centred on anticlinal hinges. The reefs are linked by lower grade stockwork vein arrays and sheeted vein sets, with a similar mineralogy to the conformable reefs. At least 8 main reefs are distributed over a stratigraphic interval of >500 m below the MVR and E Reefs (Dimo, 1990; Rowins, et al., 1998; Newcrest, 2006).
The base of supergene ore is approximately 240 to 290 m below the current surface, occurring as both leached gossan and the supergene enriched sulphide mineralisation. The supergene sulphide and primary zones of the Middle Vale Reef are similar in texture and appearance, except that chalcocite has extensively replaced pyrite along grain boundaries and fractures in the former. In some places all pyrite has been replaced, resulting in grades of up to 90 g/t Au and 20% Cu.
The initial mining exploited oxidised and higher grade supergene ore in the top 100 m below the present surface, which were underlain by hypogene mineralisation that rarely exceeded 3 g/t Au in the upper levels. The Telfer sequence is generally oxidised to a depth of 200 m below surface and to as deep as 1000 m along permeable structures. Underground, high grade hypogene ore is also exploited, with individual reefs around 50 cm thick which have grades of up to 60 g/t, diluted to 10 to 12 g/t Au, 0.8% Cu over mining widths.
Mineralisation has been defined in the Main and West domes to a depth of 1300 and 1500 m below the surface respectively. Both deposits remain open at depth and are subject to ongoing exploration (Newcrest, 2011).
The Telfer deposit was discovered in 1971 and brought into production in 1977. After producing 186 t of Au, operations were suspended in late 2000 due to commodity prices and metallurgical problems. The resource was re-assessed and re-developed, and the new open pit mine commenced operations in 2004, followed by the underground extraction in early 2006. The projected mine life (in 2008) was for the open pit to operate until 2023 and the underground mine to 2015 (Newcrest, 2008).
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 1977 to 2000, over which period it produced almost 185 t of recovered gold. 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.
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.
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,
Ore Reserves are included within Mineral Resources. Total gold in resources in June 2011 was 575 t,
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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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