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.
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 ÒgranulitesÓ 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 ÔredrockÕ 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.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2015)