PROGRAM and DESCRIPTIONS OF ORE DEPOSITS
Image: The Olympic Dam Complex, northern South Australia.
Porter GeoConsultancy continued its International Study Tour series of professional development courses by visiting a representative selection of the most significant iron oxide copper-gold (-uranium) deposits within Australia, supported by expert workshops and field workshops outlining their occurrence and setting.
with PIRSA Minerals (SA Govt. Dept.) and GeoScience Australia (Australian Govt. Dept.),
Olympic Dam, Cu-Au-U deposit northern SA,
Prominent Hill, Cu-Au and Au deposits, northern SA,
Cloncurry Workshop, with Dr Pat Williams of James Cook Univ., Qld,
Ernest Henry, Cu-Au deposit, Cloncurry, northern Queensland.
The tour started in Adelaide, South Australia on the evening of Sunday 21 November and ended in Brisbane, Queensland on Saturday 26 November 2005, and comprised:
Workshop & Core - Adelaide .......... Monday 21 November, 2005.
A one day workshop was held in Adelaide, South Australia to provide background information and a context to the deposits to be visited over the following 2 days.
The workshop covered the regional to local scale tectonic, geologic and metallogenic setting and alteration patterns associated with IOCG mineralisation in the Gawler Craton and Curnamona Province, including the characteristics of deposits not to be visited. Expert presenters included Sue Daly and Alan Maugr from PIRSA Minerals (the South Australian State Geological Survey), and Dr Roger Skirrow of GeoScience Australia (the Australian Commonwealth Geological Survey) and representatives of Teck Cominco.
These presentations were followed by a drill core inspection session at the PIRSA Drill Core Storage Facility with core from six significant IOCG occurrences/deposits in the Mount Woods Inlier, the Stuart Shelf and the Moonta-Wallaroo region, all in Gawler Craton and from the adjacent Curnamona Province. All were from occurrences that were not visited. In addition, the recent discovery hole which contains a major IOCG intersection at the Carrapateena Prospect (south of Olympic Dam) was studied.
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Olympic Dam ...................... Tuesday 22 November, 2005.
The Olympic Dam copper-gold-uranium-REE ore deposit is located some 550 km NNW of Adelaide and 275 km NNW of Port Augusta, in northern South Australia (#Location: 30° 26' 24"S, 136° 53' 22"E).
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Olympic Dam and all of the other significant known IOCG mineralised systems of the Mesoarchaean to Mesoproterozoic Gawler Craton are hosted within Palaeo- to Mesoproterozoic rocks, and are distributed along the eastern rim of the currently preserved craton to define the Olympic IOCG Province (Skirrow et al., 2007). Olympic Dam lies below the Neoproterozoic Stuart Shelf, where >300 m of flat lying, barren, Neoproterozoic to lower Palaeozoic sedimentary rocks unconformably overlie both the craton and the ore deposit. Some 75 km to the east, this cover sequence expands over the major NNW trending Torrens Hinge Zone at the edge of the craton, into the thick succession of the north-south aligned Neoproterozoic Adelaide Geosyncline rift basin, that masks the mid- to late-Palaeoproterozoic suture between the Gawler craton and Palaeo- to Mesoproterozoic Curnamona Province to the east.
The oldest basement rocks in the Gawler craton are Meso- to Neoarchaean gneisses (to the west) and metasedimentary and meta-volcanosedimentary rocks, and deformed granites correlated with the Palaeoproterozoic 1.96 to 1.85 Ga Hutchison Group, the 1.79 to 1.74 Ga Wallaroo Group, and the 1.85 to 1.69 Ga Lincoln Complex (Donington Suite) granitoids, respectively. These rocks are intruded by the widespread Mesoproterozoic A- and I-type granitoids of the ~1.59 Ga Hiltaba Suite (with the former dominating in the Olympic IOCG Province) and are overlain by comagmatic bimodal volcanic rocks of the areally extensive Gawler Range Volcanics (GRV).
Mineralisation at Olympic Dam is hosted by the 50 km2 Olympic Dam Breccia Complex (ODBC) that is developed within the Mesoproterozoic (1600 to 1585 Ma) Roxby Downs Granite. The Roxby Downs Granite is a pink to red coloured, undeformed, unmetamorphosed, coarse to medium grained, quartz-poor syenogranite with A-type affinities that is petrologically and petrochemically similar to granitoids of the Hiltaba Suite. Other lithologies within the ODBC comprise a variety of granite- to hematite-rich breccias, sedimentary facies, felsic/mafic/ultramafic dykes, volcaniclastic units, basalts and their altered/mineralised equivalents. The ODBC and the surrounding Roxby Downs Granite form a local basement high on a broader regional basement uplift.
Within the overall alteration envelope, the distribution of mineralisation and alteration exhibits a downward and outward zonation, while the ODBC correspondingly comprises a downward narrowing, funnel-shaped body of fractured, brecciated and hydrothermally altered granite which has resulted in a great variety of granitic, hematitic and siliceous breccias. The complex has a conical, downward tapering, central "core" of barren, but intensely altered hematite-quartz-breccia, passing outwards through concentrically zoned breccia types, including heterolithic hematite breccias (with clasts dominantly of granite and recycled hematite breccias, and domains where abundant sedimentary and volcaniclastics rocks predominate locally), to monoclastic granite breccias with a magnetite/hematite matrix, to weak incipient microfracturing of the RDG on the outer margins. A halo of weakly altered and brecciated granite extends out approximately 5 to 7 km from the core in all directions to an indistinct and gradational margin with the host granite. This progression represents an outward decrease in the degree of brecciation and intensity of iron metasomatism away from the core of the complex. The quantity of recycled hematite breccia, GRV and sedimentary rock clasts within the heterolithic hematite breccias decreases from shallow to deep levels (Ehrig, 2010; McPhie et al., 2010). The areal extent of more intensely hematite altered breccias within the complex is >5 km in a NW-SE direction, up to 3 km across, and is known to extend to a depth of at least 1400 m.
The development of the ODBC, which shows textural evidence of polycyclic alteration and brecciation events, can be considered as having formed by the progressive hydrothermal brecciation and iron metasomatism of the host granite. In detail, alteration assemblages are highly variable with complex mineral distribution patterns resulting from the polycyclic nature of the hydrothermal activity. Never-the-less, there are systematic patterns of alteration that are recognised across the deposit as a whole, and at the scale of individual breccia zones, with the degree of alteration intensity being directly related to the amount of brecciation.
The bulk of the mineralisation within the Olympic Dam deposit is associated with an assemblage of hematite-sericite-fluorite-barite-chalcopyrite-bornite-chalcocite (djurleite), the outer margin of which largely corresponds to the limits of the ODBC, where a deeper magnetite-carbonate-chlorite-pyrite±chalcopyrite zone marks the transition to the more regional magnetite-K feldspar±actinolite±carbonate assemblage (Ehrig, 2010). No associated sodic metasomatism has been observed.
The better mineralisation and strongest alteration outside of the barren core corresponds to the best-developed hematite-granite breccias. The concentric, moderate to steeply inward dipping breccia zones of the ODBC are cut by a convoluted, but overall roughly horizontal, ~50 m thick layer characterised by chalcocite and bornite, ~100 to 200 m below the unconformity with the overlying Neoproterozoic cover sequence. Both the upper and lower margins of this zone are mappable. Above the upper margin, sulphides are rare and little copper mineralisation is found in the same hematitic breccias. The lower margin marks a rapid transition to chalcopyrite, which decreases in copper grade downwards, corresponding to an increase in the pyrite:chalcopyrite ratio. While this zone is largely horizontal, as it approaches the central barren core it steepens markedly, but is still evident at depths of >1 km below the Neoproterozoic unconformity (Reeve et al., 1990; Reynolds, 2000; Ehrig, 2010). The geometry of this mineral zonation, strongly suggest interaction between upwelling and downward percolating fluids. For all fluids related to hematite alteration, fluid inclusion homogenisation temperatures are mostly between 150 and 300°C and salinities range from ~1 to ~23% NaCl equiv. (Knutson et al., 1992;
Oreskes and Einaudi, 1992; Bastrakov et al., 2007).
The higher grade underground resource occurs as up to 150 separate bodies distributed within an annular zone up to 4 km in diameter surrounding the central barren hematite-quartz breccia. These bodies correspond to the overlap of the flat-lying chalcocite-bornite layer and the steeper, inwardly dipping ring of hematite-granite breccias.
The principal copper-bearing minerals are chalcopyrite, bornite, chalcocite (djurleite-digenite), which on the basis of Nd isotopic data, textural and geochemical features appear to have precipitated cogenetically. Minor native copper and other copper-bearing minerals are locally observed. The main uranium mineral is uraninite (pitchblende), with lesser coffinite and brannerite. Minor gold and silver is intimately associated with the copper sulphides. The main REE-bearing mineral is bastnaesite. Copper ore minerals occur as disseminated grains, veinlets and fragments within the breccia zones. Massive ore is rare.
At the end of 1989, after commencing mining operations in mid-1988, reported resources and reserves (Reeve et al., 1990) amounted to:
Measured + indicated resource = 450 Mt @ 2.5% Cu, 0.6 g/t Au, 6.0 g/t Ag, 0.8 kg/tonne U3O8,
Inferred resource = 2000 Mt @ 1.6% Cu, 0.6 g/t Au, 3.5 g/t Ag, 0.6 kg/tonne U3O8,
Proved reserve = 13 Mt @ 3.0% Cu, 0.3 g/t Au, 10.2 g/t Ag, 1.1 kg/tonne U3O8,
Proved gold reserve = 2.3 Mt @ 1.6% Cu, 3.6 g/t Au, 2.9 g/t Ag, 0.3 kg/tonne U3O8.
At December 2004, published (BHP Billiton, 2005) reserves and resources were:
Proved+probable reserves totalled 761 Mt @ 1.5% Cu, 0.5 g/t Au, 0.5 kg/tonne U3O8,
within a total resource of 3810 Mt @ 1.1% Cu, 0.5 g/t Au, 0.4 kg/tonne U3O8.
At 30 June 2012, the published resources (BHP Billiton, September, 2012) amounted to:
Measured resource = 1474 Mt @ 1.03% Cu, 0.35 g/t Au, 1.95 g/t Ag, 0.30 kg/tonne U3O8,
Indicated resource = 4843 Mt @ 0.84% Cu, 0.34 g/t Au, 1.50 g/t Ag, 0.27 kg/tonne U3O8,
Inferred resource = 3259 Mt @ 0.70% Cu, 0.25 g/t Au, 0.98 g/t Ag, 0.23 kg/tonne U3O8,
Total resource = 9576 Mt @ 0.82% Cu, 0.31 g/t Au, 1.39 g/t Ag, 0.26 kg/tonne U3O8.
This resource includes a total proved + probable reserve of:
629 Mt @ 1.76% Cu, 0.73 g/t Au, 3.36 g/t Ag, 0.57 kg/tonne U3O8.
At the same date, the separate non-sulphide gold resource was 364 Mt @ 0.75 g/t Au, comprising:
Measured resource = 73 Mt @ 0.85 g/t Au; Indicated resource = 255 Mt @ 0.73 g/t Au; Inferred resource = 36 Mt @ 0.70 g/t Au.
At 30 June 2015, the published resources (BHP Billiton Annual Report, 2015) amounted to:
Measured resource = 1.330 Gt @ 0.96% Cu, 0.40 g/t Au, 2.0 g/t Ag, 0.29 kg/tonne U3O8,
Indicated resource = 4.610 Gt @ 0.79% Cu, 0.32 g/t Au, 1.0 g/t Ag, 0.24 kg/tonne U3O8,
Inferred resource = 4.120 Gt @ 0.71% Cu, 0.24 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U3O8,
Total resource = 10.060 Gt @ 0.78% Cu, 0.30 g/t Au, 1.0 g/t Ag, 0.25 kg/tonne U3O8.
This resource includes a total proved + probable reserve of:
484 Mt @ 1.95% Cu, 0.74 g/t Au, 4.0 g/t Ag, 0.59 kg/tonne U3O8.
Stockpile - 44 Mt @ 0.99% Cu, 0.51 g/t Au, 2.0 g/t Ag, 0.37 kg/tonne U3O8.
At 30 June 2015, a separate non-sulphide gold resource was 283 Mt @ 0.84 g/t Au, which was not reported in 2015.
Production in 2011-12 totalled 192 600 tonnes of Cu, 3.66 t Au, 28.21 t Ag, 3885 tonnes U3O8.
Production in 2014-15 totalled 124 500 tonnes of Cu, 3.26 t Au, 22.52 t Ag, 3144 tonnes U3O8.
The mine is owned and operated by a subsidiary of BHP Billiton Ltd.
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Prominent Hill ...................... Wednesday 23 November, 2005.
The Prominent Hill iron oxide copper-gold (IOCG) deposit is located approximately 150 km north-west of Olympic Dam and 650 km NNW of Adelaide in northern South Australia (#Location: 29° 42' 45"S, 135° 34' 48"E).
Prominent Hill, Carrapateena, Olympic Dam, Moonta-Wallaroo and Hillside, and all of the other significant known IOCG mineralised systems of the Gawler craton, are hosted within Palaeo- to Mesoproterozoic rocks and are distributed along the eastern edge of the currently preserved craton, to define the Olympic IOCG Province.
Cratonic to Domain-scale Setting
See the Carrapateena record for a summary of the regional, cratonic setting of the Olympic IOCG Province.
Prominent Hill lies on the southern margin of the Mount Woods Domain (MWD), which is located within the Olympic IOCG Province in the northeast of the preserved Archaean to Mesoproterozoic Gawler Craton. The MWD comprises Palaeo- and Mesoproterozoic metamorphic and igneous rocks that have a well defined aeromagnetic signature. It is characterised by high magnetic and gravity signatures, caused by multiple iron-oxide and mafic rock sources, and its boundaries are sharp and structurally controlled. The MWD encloses a major regional ~75 x 50 km magnetic complex with an overall 'U'-shape, elongated in a NW-SE direction and open to the SE. Outcrop is sparse, with flat lying Phanerozoic cover reaching thicknesses of up to 400 m in places, although it is generally <200 m.
The MWD comprises at least two separate sedimentary successions that have been subjected to one or more amphibolite to granulite facies metamorphic events, three periods of deformation, and two (probably three) episodes of magmatism, as well as a pulse of Neoproterozoic mafic dyke intrusion. Nine basement sub-domains have been defined, within the southern half of the domain within a 40 km radius of Prominent Hill.
The core of the southern half of the MWD is occupied by an 'eye-shaped', east-west elongated, 25 x 10 km kernel of magnetically anomalous rocks, reflecting the lopolithic White Hill Igneous Complex, which is characterised by extremely high amplitude magnetic lineaments on its margins, more subtle concentric magnetic zoning in its centre, and a very pronounced and complex gravity signature. The complex (where drilled) comprises pyroxenite, norite and gabbro, with pronounced layering defined by plagioclase and pyroxene-rich layers, with interleaved disseminated to massive magnetite-ilmenite bands. The gabbros and pyroxenites are hydrous, alkaline and enriched in volatile components, and carry up to 8% modal apatite. The complex is divided by a north-south fault into the White Hill and Joe's Dam sub-domains to the east and west respectively, based on differences in the magnetic pattern. The White Hill Complex is not as well developed in the Joe's Dam Sub-domain, which mainly comprises intercalated quartzo-feldspathic and magnetite-rich gneiss, and lesser mafic intrusive rocks.
The lenses-shaped 20 x 5 km Kennedy's Dam Sub-domain forms much of the northern margin of the White Hill and Joe's Dam sub-domains, and is characterised by relatively
low magnetic susceptibility enclosing a few moderate to high amplitude, linear magnetic anomalies. Where drilled, it is composed of a sequence of quartz-feldspar-biotite-magnetite gneisses and “granulites” with very minor amphibolite and calc-silicate units, intruded locally by coarse grained granite and pegmatite.
The Skylark Sub-domain bounds the Kennedy's Dam Sub-domain to the NW, and is characterised by large intrusions with low to moderate magnetic responses, surrounded by narrow aureoles of higher magnetic intensity related to either hornfelsing or magnetite metasomatism. Granite with actinolite±magnetite veins and pyroxenites, norites and diorites dated at 1587±4 Ma (U-Pb; Jagodzinski, 2005) have been encountered in drill holes, intruding a variety of quartz-feldspar-biotite gneisses, all with accessory magnetite. Metasediments away from the intrusions include quartz-rich meta-sandstone with plagioclase, opaque oxide (magnetite and hematite), schistose biotite, fluorite and tourmaline. Samples of cordierite-garnet-bearing pelite and magnetite psammite have maximum depositional ages of ~1750 Ma respectively ( U-Pb zircon; Jagodzinski et al. 2007), and were intruded by the syn-metamorphic Engenina Adamellite (~1691±25 Ma; Finlay 1993; Daly et al. 1998). Geophysical data suggest the domain may be underlain by a continuation of the White Hill Igneous Complex.
The 50 x 5 km Taurus Sub-domain forms the NE margin the Kennedy's Dam Sub-domain, but persists further to the east, delineating the northern margins of the White Hill and then the Blue Duck sub-domains, and the southern margin of the Ware's Peak Sub-domain to the NE. This terrane, characterised by a series of short strike length curvilinear magnetic features by major discontinuities, is interpreted to represent rocks caught up between a major strike-slip shear couple.
The extensive Ware's Peak Sub-domain occupies a large area in the NE of the MWD and is characterised by a few high amplitude, strike continuous, but folded magnetic linears, and by large areas of diffuse magnetic signature suggesting moderately magnetic and eastward deepening younger cover. Drill holes testing prospects encountered a highly variable suite of magnetite, plagioclase and garnet bearing paragneisses (including graphite rich units), as well as feldspathic and quartz-poor to quartz-rich meta-igneous lithologies (including quartz diorite, monzodiorite, syenite, granite and pegmatite). Other lithologies include metasandstone, iron formation, calc-silicate, skarn, dolomite and marble units. Undeformed granite and gabbroic intrusions have also been identified in drilling. Metamorphic grades range from mid-amphibolite to granulite facies.
The curvilinear, lensoid 2 to 6 km wide and >60 km long Blue Duck Sub-domain extends along the southern margin of the Joe's Dam, White Hill, Taurus and Ware's Peak sub-domains. In the west, it forms the southernmost sub-domain of the MWD. It is thickest in the centre, where it is immediately to the north of the Neptune Sub-domain that contains the Prominent Hill deposit near the contact between the two terranes, and thins on either extremity. In the east, its southern margin is marked by the regional ENE-trending Bulgunnia Fault. It is characterised by narrow, medium amplitude magnetic linears which are continuous for several kilometres or more. Drilling at a number of locations (mostly aimed at magnetic targets), has encountered metamorphosed calcsilicate units with associated magnetite, schistose, recrystallised K feldspar-biotite-chlorite-quartz-scapolite clastic metasediments, iron-rich, pelitic carbonate rock, marble, calc-silicate, magnetite-pyroxene-quartz rock and possible meta-evaporites. These rocks have been tentatively correlated with the 1760 to 1740 Ma Wallaroo Group found elsewhere in the Olympic IOCG Province (Freeman and Tomkinson, 2010).
The elongate Neptune Sub-domain forms the southern boundary of the MWD over an interval of ~30 km, where it fringes part of the southern Blue Duck Sub-domain. It has an overall 'V' shape, turning on its eastern extremity from an east-west to WSW trend, along the northern side of the regional Bulgunnia Fault. It includes the Neptune Volcanics (which have been tentatively correlated on lithological grounds with the ~1590 Ma Gawler Range Volcanics) and the Prominent Hill Mine Sequence. Compared to terrains further to the north, it is characterised by relatively low amplitude linear aeromagnetic anomalies and contains a sequence of lower greenschist facies, relatively undeformed, mafic to felsic volcanic rocks (basalt-andesite-dacite-rhyolite), hematite-cemented quartz conglomerate, sandstone, argillite and dolostone. This sequence coarsens southwards from argillaceous and calcareous rocks into coarse grained siliciclastic rocks. The volcanic component in the structural footwall to the Prominent Hill deposit are basaltic to andesitic in composition, commonly porphyritic and amygdaloidal. Several kilometres to the west, felsic volcanic rocks (dacite to rhyolite) become much more voluminous with ‘redrock’ hematite dusting of alkali feldspar, compared to the predominantly sericite-chlorite-earthy hematite-leucoxene-carbonate alteration within the basalts and andesites of the mine sequence. Fragmental lithologies are intercalated with the coherent footwall volcanic rocks at Prominent Hill, and include agglomerate, felsic tuff or ignimbrite (Belperio et al., 2006) and volcanic clast conglomerate. They are also intercalated with mafic to intermediate volcanic rocks and hematitic, quartz-feldspar conglomerate and interbedded coarse grained sandstone. The east-west-trending Hangingwall Fault at Prominent Hill has been inferred to separate the hematite-stable sedimentary rocks of the copper-gold mineralised host sequence of the Neptune Sub-domain in the south, from a magnetite-stable hanging wall sequence of chloritic pelite and pelitic carbonate rock of the Blue Duck Sub-domain to the north. Proterozoic basement is overlain by 90 to 150 m of flat-lying Permo-Carboniferous sandstone and diamictite and Cretaceous sandstone and black claystone.
The Danae Hill Sub-domain forms the southern margin of the MWD to the southeast of the regional Bulgunnia Fault. It is characterised by a similar magnetic signature to the Neptune Volcanics, with low to moderate amplitude aeromagnetic linears. Drilling has identified a suite of altered, low-grade metamorphosed, sheared and brecciated basalts, with lesser metasediments and acid volcanic rocks, which collectively suggest a bimodal volcanic suite. These volcanics have a markedly different trace element signature similar to that of the Neptune Volcanics and are inferred to be of late Neoarchaean to Palaeoproterozoic in age.
The Christie Domain, is found to the immediate south of the of the MWD Neptune/Blue Duck and Danae sub-domains, and is dominated by latest Archaean to earliest Palaeoproterozoic metasedimentary protoliths, metamorphosed in the earliest Palaeoproterozoic at ~2450 Ma during the Sleaford Orogeny to become the Christie Gneiss of the Mulgathing Complex. The MWD and has been overthrust onto the Christies Domain from the NE, with a major north dipping thrust marking the boundary (Betts et al., 2003).
In summary, the MWD is predominantly composed of ~1760 to ~1740 Ma Palaeoproterozoic meta-sedimentary and meta-volcanic rocks, tentatively correlated with the Wallaro Group seen elsewhere in the Olympic IOCG Province, including banded iron formations (possibly equivalent to the Middleback Range BIFs further to the south). These successions, which may include inliers of older late Neoarchaean to lower Palaeoproterozoic metamorphic rocks, underwent peak metamorphism at ~1736±14 Ma to amphibolite and granulite facies. They are bounded on the southern margin of the domain by a narrow band of younger, low- to mid-greenschist facies, ~1590 Gawler Range Volcanics equivalents and intercalated volcanic and sedimentay breccias of the Neptune Sub-domain, hosting the Prominent Hill ore deposit. All of these rocks have been intruded by the ~1691±25 Ma Engenina Adamellite, the extensive Hiltaba Suite 1584±18 Ma Balta Granite and the 1587±4 Ma mafic to ultramafic White Hill Igneous Complex.
The presence of Hiltaba-aged zircon interpreted as metamorphic (Holm, OZCHRON) in quartzites and felsic gneisses suggests that metamorphic grade in the MWD at ~1590 to ~1580 Ma was significantly higher than is typical elsewhere in the Gawler Craton during that period. This, in turn, suggests that the MWD was at deeper crustal levels than the adjacent Olympic, Christie and Wilgena Domains.
Prominent Hill Deposit
The Prominent Hill deposit was discovered under approximately 100 m of cover, and is reflected by a discrete gravity anomaly, the target of the discovery drillhole (Carter et al. 2003), corresponding to the hydrothermal iron altered (magnetite-deficient) hematite matrix breccias that host the copper-gold-silver-uranium-cerium-lanthanum ore deposit within the Neptune Sub-domain and to corresponding palaeotopographic highs. The peak of the gravity anomaly coincides with a mass of massive, barren "steely" hematite-silica flooded volcanics (on the eastern end of the main ore zone), flanked to the west by a ~2 km long mineralised hematite matrix-supported breccia. The overall gravity anomaly has an east pointing "V" form, with a 1 km long northern arm trending WNW-ESE, and the southern 2 km long limb trending WSW-ENE. The southern limb reflects the main mineralised hematite breccia, with the peak of the anomaly near the hinge, and has no magnetic expression (Belperio et al., 2006).
The associated 750 m long magnetic anomaly coincides with the northern limb of the gravity feature. It is centred ~500 m to the north of the orebody, across the Hangingwall Fault in the Blue Duck Sub-domain (Hart and Freeman 2003), and reflects a package of magnetite-chlorite-tremolite-phlogopite altered metasomatic 'skarn-like' altered calc-silicate/carbonate-rich metamorphics intercalated with highly altered, intermediate, porphyritic intrusives and chlorite matrix tectonic breccias (Belperio et al., 2006).
The deposit comprises several phases of hematite alteration with associated sericite, clay minerals and chlorite, hosted by a northerly dipping series of interbedded lithic sandstone and greywacke, siltstones, coarse grained sedimentary and volcanic breccias, and dolostone. These rocks are found in the immediate footwall to the transition from the Neptune to Blue Duck sub-domains (marked by a relatively brittle, steep northward dipping fault, known as the Hangingwall Fault Zone in the mine). Magnetite is absent in the breccias which host the mineralisation but to the north of the deposit, and separated from it by components of the Hangingwall Fault, is a body of massive magnetite with associated, pyrite, actinolite, phlogopite, chlorite, serpentinite, carbonate and talc (magnetite "skarn"). This body is very poorly mineralised in copper and gold and its relationship to the Prominent Hill orebody immediately to the south is not understood. Copper, gold, uranium and REE mineralisation is relatively late stage and overprints at least one phase of massive hematite replacement of the breccias.
The intense hematite alteration within the coarse grained breccias is locally texturally destructive and this has inevitably led to varying interpretations as to their origin. A recent re-evaluation (Freeman and Tomkinson, 2010) of the breccias has shown that they originated as a sequence of very coarse to fine-grained, laminated, clastic sedimentary rocks, that have been subject to later hydrothermal replacement with only minor additional brecciation during metasomatism. Distinctly bedded breccias, with bedding defined by the alignment of clasts and by grading, occurs over intervals to ~5 to 10 m within more massive, non-layered, poorly sorted hematite-matrix breccia.
In the open pit, individual bodies of breccia occur as a series of strata-bound, steeply dipping, tabular east-west trending sheets and westerly plunging shoots, collectively bound to the north by dolostone/Hangingwall Fault Zone and to the south by volcanic rocks. At the Western Copper deposit the stratigraphy, which includes mineralised hematite breccia and carbonaceous rocks, appears to be complexly folded.
The host sequence rocks are intensely altered by hematite-sericite-chlorite-carbonate (±quartz±barite±fluorite±REE phosphates). Copper mineralisation occurs as fine grained disseminations of chalcocite, bornite and chalcopyrite in the breccia matrices and (to a lesser extent) within clasts of hematite-rich breccias. The copper sulphides display a variety of intergowth, replacement and infill textures including chalcocite-bornite and replacement of early formed pyrite.
The current weight of evidence indicates that the Prominent Hill mineralisation originated through relatively passive infiltration of hydrothermal fluids and metasomatism localised by porosity within a sequence of coarse grained sedimentary breccias. The amount of further brecciation that can be attributed to hydrothermal processes is unclear. The copper and gold mineralisation can be shown to be paragenetically very late in the sequence and a direct genetic relationship between the copper and gold and the hematite is not proven. Geochemical trends suggest a strong relationship between gold mineralisation (in the copper-rich zones of the deposit) and REE phosphates.
Global reserves and resources at the deposit as of May 2010 stood at: 278.8 Mt @ 0.98% Cu, 0.75 g/t Au and
2.5 g/t Ag.
The declared reserves and resources at Prominent Hill in mid 2008, prior to the commencement of production in 2009, were:
Copper resource - Measured + indicated + inferred resources at 0.5% Cu cut-off: 174.20 Mt @ 1.39% Cu, 0.56 g/t Au, 3.4 g/t Ag.
Gold resource - Measured + indicated + inferred resources at 0.5 g/t Au cut-off & <0.5% Cu: 109.2 Mt @ 0.09% Cu, 1.21 g/t Au, 1.0 g/t Ag.
TOTAL Resource - Measured + indicated + inferred resource: 283.4 Mt @ 0.89% Cu, 0.81 g/t Au, 2.48 g/t Ag.
Western Copper resource (additional) - Inferred resources at a 0.5% Cu cut-off: 14.5 Mt @ 1.69% Cu, 0.28 g/t Au, 3.7 g/t Ag.
The declared reserves and resources at Prominent Hill in June 2011 were (OZ Minerals, 2012):
Proved + probable reserves: 72.3 Mt @ 1.13% Cu, 0.64 g/t Au, 3.03 g/t Ag. (included within resources)
Copper resource - Measured + indicated + inferred resources: 214.9 Mt @ 1.23% Cu, 0.5 g/t Au, 2.8 g/t Ag.
Gold resource - Measured + indicated + inferred resources: 57.8 Mt @ 0.07% Cu, 1.5 g/t Au, 1.1 g/t Ag.
Remaining reserves and resources at Prominent Hill in June 2015 were (OZ Minerals, 2015):
Proved + probable reserves: 73 Mt @ 1.0% Cu, 0.6 g/t Au, 2.9 g/t Ag.
Copper-gold resource - Measured + indicated + inferred resources: 152 Mt @ 1.2% Cu, 0.6 g/t Au, 2.8 g/t Ag.
Gold resource - Measured + indicated + inferred resources: 27 Mt @ 0.1% Cu, 1.3 g/t Au, 1.5 g/t Ag.
This description is largely based on Freeman and Tomkinson, 2010.
The title to the deposit is held by Oxiana Limited who are undertaking a feasibility study.
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Cloncurry Workshop ............. Thursday 24 November, 2005.
A field, drill core and classroom workshop was held in the Cloncurry Terrane of north-west Queensland, for the full day on Thursday 24 and on Friday 25 before and after the Ernest Henry visit. It was led by Dr Patrick Williams of James Cook University (Townsville, Australia), an internationally known expert on IOCG deposits, particularly those in the Cloncurry Terrane. He addressed the setting and characteristics of these deposits in an intitial classroom session, before going into the field to study IOCG mineralisation, alteration (local and regional) and lithologies both in outcrop and in drill core from the E1, Monakoff and Great Australia IOCG deposits of Exco Resources. On Friday 25th, a field visit was made to the regionally altered exposures and outcropping magnetite mineralisation at Mount Fort Constantine near Ernest Henry, while core was examined from the Universal Resources Little Eva deposit and presentation received on its geology and mineralisation.
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Ernest Henry .................... Friday 25 November, 2005.
The Ernest Henry IOCG style Cu-Au deposit is located 35 km NE of Cloncurry, 150 km east of Mt Isa and 750 km west of Townsville in north-west Queensland (#Location: 20° 26' 40"S, 140° 42' 21"E).
The deposit lies to the east of the Cloncurry Overthrust, within the Cloncurry-Selwyn zone of the Cloncurry Terrane, which comprises the eastern exposed margin of the Mount Isa Inlier of North-west Queensland. It contains IOCG deposits that are hosted by Palaeoproterozoic (1760-1660 Ma) silici-clastic metasedimentary and metavolcanic rocks that were deposited during periods of ensialic rifting.
The Ernest Henry deposit is hosted within the Eastern Succession of the Mount Isa Inlier, that consists of a poly-deformed Palaeo- and Mesoproterozoic volcano-sedimentary succession which is largely composed of evaporite-rich Cover Sequence 2 and silici-clastic-rich Cover Sequence 3 rocks (CS2 and 3). CS2 and 3 were deposited between 1790 and 1690 Ma and from 1680 to 1610 Ma respectively. To the west, these sequences overlie an older crystalline basement and a core of predominantly Cover Sequence 1 felsic volcanic and related intrusive rocks that correspond to the 1870 to 1850 Ma Barramundi Orogeny of northern Australia. Basement is not exposed in the Cloncurry district. Both CS2 and 3 were deposited in intracontinental rift settings, although the relationship between some parts of the sequence is obscured by the deformation history. Both sequences were also accompanied by the emplacement of various intrusive and volcanic rocks.
The first significant deformation to affect CS2 (but not CS3) was the 1750 to 1735 Ma Wonga extensional event. CS2 was extensively intruded by the 1750 to 1730 Ma Wonga Granite to the west, while the coeval Mount Fort Constantine volcanics are found to the NE. Minor tonalites, granitoids and diorite emplaced between CS2 and 3 have been dated at 1686 to 1660‚ÄāMa (including the Ernest Henry Diorite).
Thin skinned deformation of the ~1600 to 1520 Ma Isan Orogeny terminated deposition of Cover Sequence 3, and resulted in gross eastward tectonic transport, interleaving of major lithostratigraphic units, and a dominant north-south tectonic grain. This deformation has been divided into: a D1 event, which involved overall north-south compression, and is characterised by large-scale thrusts and isoclinal folds, thrust reactivation of large, km-scale, basin bounding extensional faults with CS3 rocks thrust over CS2, resulting in overturned limbs and a penetrative rock mass foliation; a D2 event, involving horizontal east-west compression producing major north-south upright to isoclinal folding of CS2 and 3 rocks, and a penetrative cleavage, which peaked at 1595 to 1580 Ma with a regional greenschist to upper amphibolite facies metamorphism and the development of anatectic pegmatites; and a D3 event which includes NW- and NE-trending brittle-ductile corridors of faulting, kinking and folding with steep plunges to the NW and NE, and dominantly north-south trending shear and fault zones and associated breccia formation.
Both CS2 and 3 were intruded by the voluminous Williams and Naraku granite batholiths at 1540 to 1500 Ma (including the 1530 Ma Mt Margaret Granite immediately to the east of the E1 deposits; Marshall and Oliver, 2007; Page and Sun, 1998). These represent the youngest felsic intrusions in the inlier, and have an outcrop exposure of >1500 km2. They were emplaced in an intracratonic environment, and have a pre-, syn- and post-D3 timing, and are largely composed of alkaline to sub-alkaline, K-rich, A-type, magnetite-bearing granitoids. They range from diorite to syenogranite in composition and are typically more oxidised than similar older (~1670 Ma) granitoids in the Western Fold Belt
of the Mount Isa Inlier. Sodic intrusions of similar age are rare.
A regionally extensive Na-Ca hydrothermal system in the Cloncurry district (>1000 km2) affected all rock types, especially the resultant calc-silicate-rich lithologies of cover sequence 2. This alteration appears to have been formed by multiple periods of hydrothermal activity that locally overlapped and is most intense in breccia zones along large structural conduits and within calc-silicate-rich units. The bulk of the sodic-calcic alteration, dominantly regional albite and scapolite, was associated with fluids that were initially mostly sedimentary formation waters with lesser magmatic components, prior to and during peak metamorphism (Kendrick et al., 2008; Oliver et al., 2008; Baker et al., 2008). Subsequent more structurally controlled albite-actinolite-magnetite-titanite±clinopyroxene assemblages, were synchronous with major granite (e.g., Williams-Naraku batholiths) emplacement (Baker et al., 2008), involving a larger magmatic fluid component, and coincided with formation of the majority, but not all of the significant oxide Cu-Au deposits. These deposits may have some stratigraphic control, but are usually associated with brittle and brittle-ductile shear and fault structures which acted as conduits for the transport of high temperature (300 to 500°C) saline fluids into the host rocks (Williams, 1998).
The Ernest Henry deposit is concealed by 35 to 60 m of extensive Phanerozoic cover and does not outcrop. While the exact stratigraphic position of the host rocks is not known, they have been tentatively correlated with the 1730 ±10 Ma Mount Fort Constantine Meta-volcanics towards the top of Cover Sequence 2. The Mount Fort Constantine metavolcanics comprise dacite and andesite with subordinate metabasalts and calc-silicate metasedimentary rocks. The only other outcrop in the district is the 1480 Ma Mount Margaret granite some 12 km to the east. Within Cover Sequence 2, volcanism is common between 1790 and 1780 Ma, and 1760 to 1720 Ma, with later 1540 to 1450 Ma granitoids.
Within the immediate orebody area the principle lithologies encountered are: i). altered plagioclase phyric andesitic volcanic/hypabyssal rocks (ca 1740 Ma) which host the orebody where they are brecciated; ii). various siliciclastic, calc-silicate-rich and graphitic metasedimentary rocks that occur as <10 m thick intercalations within the metavolcanic rocks; and, iii). medium-grained metadiorite (ca 1660 Ma).
Structural analysis suggests that ore deposition accompanied reverse-fault movement between two northeast trending bounding shear zones and formed a pipe-like zone of dilation in the K-feldspathised metavolcanic rocks. The breccia pipe, plunges at ~45° to the SSESSE, nested between the ductile shear zones (Rusk et al., 2010). The orientation of this dilational zone is consistent with the shape and dip of the Ernest Henry ore breccia.
Four stages of alteration are recognised at Ernest Henry:
i). Regional pre-ore Na-Ca alteration, occurring mainly as albitic plagioclase-, magnetite-, clinopyroxene- and amphibole-rich veining and fault-related breccia-fill.
ii). Pre-mineralisation potassic-(manganese-barium) alteration which only contains minor sulphides, and is typified by multiple stages of K feldspar-, biotite-, amphibole-, magnetite-, garnet- and carbonate-bearing veins, and by fault-related breccia and alteration.
iii). Mineralisation associated alteration, characterised by K feldspar veining and alteration. K feldspar alteration is most intense in the vicinity of copper-gold mineralisation, but forms a halo extending from several hundred meters up to 2 km beyond the ore body (Mark et al., 2006), although this outer halo may represent part of pre-ore regional alteration zone. Mineralisation is divided into two main stages, characterised by similar mineral assemblages. The first stage of economic Cu-Au mineralisation was the main ore-forming event, associated with a matrix-supported hydrothermal breccia that is enveloped by crackle veined K feldspar altered meta-volcanic rocks. The second stage of mineralisation occurs as a network of veins cutting earlier infill-supported ore-breccias, and contains a largely identical mineralogy to earlier stage. The ore-bearing assemblage dominantly comprises magnetite, pyrite, chalcopyrite, carbonate and quartz, with lesser apatite, barite, titanite, actinolite, biotite and fluorite. In the upper levels of the deposit, the bulk of the ore is present as hypogene chalcopyrite infilling between K feldspar-altered breccia clasts, while at greater depths, it both infills between, and replaces clasts. Electrum and native gold are closely associated with pyrite and chalcopyrite (Foster et al., 2007).
iv). Post-ore, volumetrically minor, multiple stage calcite-dolomite- and/or quartz-rich veining and alteration which lacks magnetite, and only carries a little gold. Deeper in the deposit, breccias include rounded clasts of previously mineralised breccias containing magnetite, pyrite and chalcopyrite, indicating multiple superimposed brecciation events (Rusk et al., 2010).
Rusk et al. (2010) interpret the data from Ernest Henry to be consistent with the following genetic trend:
i). Rapid devolatilisation (of possibly both chloride-rich brines and CO2-rich fluids) within the source magma chamber;
ii). Fluid over-pressuring in the roof of the magma chamber as a result of volatile exsolution and vapour expansion, assisted by a seal created by magma solidification, sodic-calcic alteration and/or contact metamorphism in the carapace of the igneous complex;
iii). Possible leakage of over-pressured magmatic fluid along structures controlling the location of the later breccia pipe, producing a pre-ore potassic alteration halo;
iv). The eventual failure of the seal and sudden release of fluid pressure, resulting in a high-energy fluid flow event driving brecciation and upward transported and milled clasts. The resultant breccia mass permitted the mixing and/or subsequent ingress of basinal brines circulating within fractured rocks several kilometres above the magma chamber. Fluid mixing, rapid depressurisation and resultant cooling led to ore precipitation within the matrix porosity between breccia clasts at the top of the orebody, where, as the fluid flow, temperature and pressure declined the breccia was sealed;
v). At depth, closer to the heat source, the temperature and pressure gradient degraded more slowly, allowing for fluid-rock reaction to be more protracted, such that prolonged chemical interaction between K feldspar-rich host rocks and ore fluids led to replacement style mineralisation within clasts, with the same mineral assemblage as observed in the shallower parts of the deposit.
vi). At the deepest levels, repetition of the cycle may have resulted in the release of a new pulse of fluids which brecciated and tapped earlier formed magnetite-chalcopyrite rich rocks, telescoping mineralised clasts upwards into the orebody along narrow channels, thereby upgrading ore.
The brecciated volcanic mass that hosts the ore forms a plunging elongate body, some 250 m thick, 300 m average length and extending at least 1000 m down plunge to the SSE. The breccia ranges from the unbrecciated volcanics, to crackle fracture veining to clast supported and matrix supported breccia to total clast digestion (massive matrix). The breccias typically contain 5-20 mm subrounded to rounded meta-volcanic and rare biotite altered meta-sedimentary clasts. The matrix is largely composed of magnetite, calcite, pyrite, biotite, chalcopyrite, K feldspar titanite and quartz. Accessory minerals include garnet, barite, molybdenite, fluorite, amphibole, apatite, monazite, arsenopyrite, a LREE fluorcarbonate, galena, cobaltite, sphalerite, scheelite, uraninite and tourmaline. The bulk of the economic mineralisation is restricted to breccia zones with more than 10% matrix.
The total reserve + resource prior to the commencement of mining in 1998 was 166 Mt @ 1.1% Cu, 0.54 g/t Au.
As of June 2003 the remaining resource totalled 117.9 Mt @ 1.13% Cu, 0.52 g/t Au.
At 30 June 2006, the reserves and resources were (Xstrata, 2007):
Open cut proved reserves - 41 Mt @ 0.9% Cu, 0.5 g/t Au + probable reserves of 20 Mt @ 0.8% Cu, 0.4 g/t Au,
Open cut measured + indicated resources were the same as, and included the proved and probable reserves,
Open cut inferred resources - 1 Mt @ 0.4% Cu, 0.2 g/t Au,
Underground indicated resources - 21 Mt @ 1.5% Cu, 0.7 g/t Au + inferred resources of 23 Mt @ 1.4% Cu, 0.7 g/t Au,
Open pit as at December, 2011 (Xstrata, 2012):
Total resource and reserve - depleted during 2011 from 17 Mt @ 1.0% Cu, 0.5 g/t Au, 23% magnetite at December 31, 2010
Underground as at December, 2011 (Xstrata, 2012):
Measured resource - 4 Mt @ 1.3% Cu, 0.7 g/t Au, 32% magnetite
Indicated resource - 71 Mt @ 1.3% Cu, 0.7 g/t Au, 28% magnetite
Inferred resource - 13 Mt @ 1.2% Cu, 0.6 g/t Au, 26% magnetite
Total resource - 88 Mt @ 1.3% Cu, 0.7 g/t Au, 28% magnetite
Total ore reserve (all probable) - 74 Mt @ 0.95% Cu, 0.5 g/t Au, 23% magnetite.
The operation is controlled by Ernest Henry Mining Pty Ltd, a subsidiary of Glencore Xstrata Ltd.
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Travelling from Cloncurry to Brisbane .................... Saturday 26 November, 2005, arriving by 1:30 pm.
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" available from the IOCG 05 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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