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This paper was presented at The AusIMM PACRIM 2004 Conference, and published in the November/December 2004 issue of The AusIMM Bulletin. The AusIMM holds all copyright for this paper. For information on this or any other AusIMM publication, please contact our Publications Department on +61 3 9662 3166.

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Common Geological Characteristics of Prominent Hill and Olympic Dam
— Implications for Iron Oxide Copper-Gold Exploration Models
By Dr Antonio Belperio FAusIMM and Hamish Freeman
Minotaur Resources Ltd

INTRODUCTION
Since the discovery of the super-giant Olympic Dam orebody in 1975, iron oxide copper-gold (IOCG) deposits have been attractive exploration targets for mineral exploration companies worldwide. Numerous exploration models have been used, based primarily on a spectrum of associated iron ranging from haematite to magnetite end members. A number of IOCG discoveries have resulted (Haynes, 2002), but the discovery of another haematite-dominant end member of this family has had limited success and the suggestion that Olympic Dam was somehow unique has been increasingly invoked.
The exploration model that lead to the discovery of Prominent Hill by Minotaur Resources was specifically targeted at the haematite end member of the IOCG family of deposits (Belperio, 2001), where deposit characteristics indicate higher grades and greater potential for world-class reserve status viz:

· Olympic Dam (haematite system)
717 Mt @ 1.7% Cu, 0.6 g/t Au, 3.6 g/t Ag, 0.5 kg/t U308
· Candelaria (magnetite-haematite)
470 Mt @ 0.95% Cu, 0.2 g/t Au, 3.1 g/t Ag
· Salobo (magnetite system)
450 Mt @ 1.15% Cu, 0.5 g/t Au

Although a resource is yet to be announced for Prominent Hill, the extent and tenor of mineralisation recorded to date with significant down-hole intercepts such as 209 m @ 1.54 per cent Cu and 0.93 g/t Au (DP005), 57 m @ 7.7 g/t Au (DP010) and 41 m @ 6.06 per cent Cu and 0.31 g/t Au (RP017) (Minotaur Resources, 2002, 2003) underscores the significance of the discovery.

The Prominent Hill discovery was a vindication of both the exploration strategy and models utilised by Minotaur. Confirmation that Prominent Hill is directly comparable in its style and genetic history to Olympic Dam would dispel the notion that Olympic Dam is a unique or singular occurrence. More importantly the deposit can provide key comparative information to that available from Olympic Dam on the geological setting, timing, processes, mineralogy and alteration, thus allowing significant refinement of global IOCG exploration models.

REGIONAL GEOLOGICAL SETTING
The Gawler Craton is an Archaean to Mesoproterozoic terrane occupying 600 000 square kilometres of southern Australia (Figure 1). Outcrop is limited and geotectonic interpretations rely heavily on drillhole and geophysical data sets (Daly, Fanning and Fairclough, 1998). The Craton hosts the world-class Olympic Dam iron oxide Cu-Au orebody, the newly discovered Prominent Hill deposit, and a number of other sub-economic discoveries of IOCG mineralisation (Acropolis, Wirrda Well, Gunson, Manxman). Each of these deposits has been related to the same major thermal event (the Hiltaba Suite – Gawler Range Volcanics magmatic event) that affected the central and eastern part of the Gawler Craton from about 1600 - 1580 Ma (Flint, 1993; Daly, Fanning and Fairclough, 1998; Ferris, Schwarz and Heithersay, 2002).

Three major areas of IOCG alteration and mineralisation are recognised along the eastern part of the Gawler Craton; the Olympic Dam area, the Moonta-Wallaroo district, and the Mt Woods Inlier. The three areas are interpreted to represent separate footprints of crustal-scale thermal anomalies and include both magnetite-biotite and haematite-sericite alteration assemblages (Skirrow et al, 2002) formed at different temperatures and pressures (crustal levels).

KEY CHARACTERISTICS OF PROMINENT HILL
Details of the Prominent Hill deposit have been progressively released since November 2001 through Company ASX announcements, annual reports, conference presentations and publications (Minotaur Resources, 2002, 2003; Carter, Belperio and Freeman, 2003; Hart and Freeman, 2003).

Geophysically, the deposit is characterised by a discrete gravity anomaly (Figure 2) the size of which varies depending on survey station spacing and the methodology used to remove regional gradients. Its size is approximately 2.5 mgals (at 1 km station spacing), increasing to 4.5 mgals (400 × 200 m spacing) and 6 mgals (100 × 100 m spacing). The source of an adjacent intense magnetic anomaly is a package of magnetite- chlorite-tremolite- phlogopite altered metasomatic rocks with skarn affinities, and intercalated highly altered, porphyritic intermediate intrusive and chlorite-matrix tectonic breccias. The gravity response reflects the overall package of hydrothermal iron altered rocks including mineralised haematite-matrix breccias. The mineralised portion of the breccias is not directly detectable by gravity or by other geophysical techniques (Hart and Freeman, 2003), being masked by complex dense host rock geology and other alteration features (sediments, volcanics, faulted skarn blocks). Northwest- and northeast- trending crustal penetrating structures are visible in magnetic and filtered gravity imagery in close proximity to Prominent Hill, and lower order equivalent structural control is evident in grade variations at the deposit level. Hiltaba-age bimodal igneous intrusions and sub-volcanic activity are present in close proximity to the deposit.

Mineralisation comprises copper-gold-silver-uranium-cerium -lanthanum intimately associated with haematite-matrix breccias. The deposit occurs below ~100 m of cover within a broader breccia complex that intrudes a sequence of low metamorphic grade mafic to felsic volcanics (c.1590 Ma Lower Gawler Range Volcanics – basalt, andesite, rhyolite) and associated volcaniclastic sediments (Mentor Formation). The volcanics and volcaniclastics occupy an arcuate graben (ENE to ESE) that was subsequently subjected to intense and pervasive hydrothermal iron alteration, replacement and brecciation (Figure 3). The breccias display a spectrum from clast-supported simple jigsaw breccia types to matrix-supported multiple – brecciated, heterolithic varieties. Drilling to the north of the main prospect area has intersected lower amphibolite to granulite facies metamorphic rocks more typical of the main Mt Woods Block.

The centre of the linear ENE-trending gravity anomaly coincides with a large area of massive, barren haematite – silica flooded volcanics that records a strong IP chargeability anomaly. This is flanked to the north by an area totalling approximately 2 × 0.5 km of haematite-rich breccia sheets. Layering within the breccias suggests that sheets are sub-vertical, strike ENE to ESE, and were emplaced along structural and lithological contacts. A halo of lesser haematite alteration and host-rock brecciation extends up to a further 0.5 km away. Iron alteration is almost exclusively haematitic. Magnetite occurs within late dolerite dykes (Gairdner Dykes, ca. 800 Ma). Sericite, silica, fluorite, barite and carbonate are common, either as breccia matrix components with haematite, or as late stage veins with or without haematite. The haematite breccias and associated alteration and mineralisation are believed to have developed through repetitive hydrothermal venting, with fluids emanating from the same linear volcanic axis that generated the host volcanics and graben.

Lithologies
Key lithologies include mafic to intermediate volcanic units and intrusive dykes, volcaniclastic sediments, dolomitic carbonates, hydrothermal breccias, intensely iron-silica altered variants (‘steely haematite’), and lower intensity earthy haematite altered to in situ crackle-veined variants (Figure 3).

Volcanics of mafic, intermediate and acid composition (basalt, basaltic andesite, andesite, dacite) are ubiquitous and form the base of a volcano-sedimentary succession. Coarser grained varieties may represent sub-volcanic dykes or sills. Late-stage fine-grained volcanic dykes (andesitic) are present within the breccia complex. Fragmental volcanic lithologies include haematitic agglomerates, conglomerates, lapilli tuffs and pepperites. The volcanics exhibit numerous low-temperature alteration assemblages involving albite, sericite, haematite (after magnetite, martite), chlorite and carbonate. More felsic volcanic lithologies (dacite to rhyodacite) occur at the western end of the prospect.

A package of low metamorphic grade sediments occupies the ENE to ESE trending graben affected by the hydrothermal event. A south to north succession of volcanics ® greywacke ® argillite ® dolomite (footwall to hangingwall) is observed across the deposit. Together with the volcanics, the sediments define the original host graben. Sandstone and greywacke units, in contact with volcanics to the south, thin significantly in the western domain, replaced by broader sequences of argillite and dolomite.

Dolomitic units are Ca-Mg- carbonate-rich, poorly bedded to massive and white, red-grey or tan in colour. They exhibit haematite (-chlorite) crackle-veining and variable silicification, sericite alteration and quartz veining. Argillites are generally fine grained, pale red-brown sericite-quartz-rich lithologies. They are well laminated throughout, although commonly exhibit disruption of original layering, and tensional fracturing and veining by haematite and sericite (Figure 5h). Other textures are indicative of slumping, suggesting that the sediments were only partially lithified during alteration/mineralisation. This also indicates deposition during an evolving rifting/volcanic event and associated volcano-sedimentary graben development. Greywacke units are generally poorly sorted, poorly bedded, fine to coarse grained, sericitic and quartzose. They are very common as clasts within the mineralised haematite breccias.

Haematite breccias are generally confined to the sequence of greywacke and sandstone and the volcanics to the immediate south. They are highly variable in composition and texture but invariably contain haematite as a matrix component. Matrix varieties include dark grey, blue-grey, purple, red-brown, jet-black or steel grey haematite. Quartz, copper sulfides (chalcocite, bornite, chalcopyrite), carbonate and fluorite are also common matrix components. The haematite breccias can be subdivided further based on variations in grain size of the matrix components (in particular haematite).

Disseminated copper mineralisation is generally most abundant in (dark grey to black) crystalline matrix haematite breccia. They are ‘high-energy’ fluidised breccias characterised by a relatively coarse grained haematite (-sericite-quartz- fluorite-carbonate) matrix and commonly exhibit a wide variety of clast types, including greywacke, altered andesite to basalt, haematite breccia and haematite rock of unknown precursor. Textures vary from milled to compositional layered. Embayment of haematised or sericitised clasts is common. They have a high matrix percentage and are correspondingly high in Fe, REE, Co, U, Cu and Au.

Earthy haematite matrix breccias consist of a fine-grained, granular, dark brown to red haematite matrix. They are interpreted to reflect a relatively low-temperature, highly oxidised regime in comparison to those breccias with a coarse, crystalline haematite matrix. Compositional layering is less common. They lack high contents of Cu-sulfides and examples of layered breccia with alternating granular earthy and dark grey crystalline haematite matrix show a decrease in sulfide mineralisation within the earthy haematite layers. Free (visible) gold and native copper may be present in earthy matrix breccias west of the main copper mineralised zone. They have higher clast component (lower matrix percentage) and correspondingly lower levels of Fe, REE and U.

Steely haematite replacement of breccia occurs primarily within the eastern part of the deposit (coincident with the centre of the eastern gravity anomaly, Figure 2C), but also occurs sporadically within or adjacent to breccia sheets. These zones are characterised by a dense, non-porous matrix of very fine-grained steel grey haematite and silica with densities in excess of 4.0 gcm-3 and an intense hardness attributed to the ultra fine silicification. Limited Cu-sulfide intersected in these breccias to date consists of coarse blebs of bornite disseminated within the matrix. As at Olympic Dam, high-grade gold mineralisation is more common in these silicified zones. The steely nature is due to extremely fine-grained haematite intergrown with cryptocrystalline silica. Clasts consist of chert and intensely silicified rock fragments of unknown precursor. Geochemically the steely haematite zone is distinctive and high in Fe (>40 per cent), As, Ba and REE, and very low in Co, Cu, K, Mg, Na, Ni, Ti, Zn and Y.

Steely haematite replacement of volcanics occurs along an ENE axis that coincides with a linear gravity shoulder (Figure 2C) that corresponds with the southern margin of the graben (Figure 3). Original volcanic textures are sporadically preserved within this iron-silica alteration zone, indicating both primary and brecciated volcanic rocks (basalt to andesite precursor) containing possible sedimentary fragments (shale or siltstone), sediments, pseudo-breccias and true breccias. These are intensely overprinted by steely-grey massive haematite veins (to a few metres thickness), post-dated again by specular haematite and barite veinlets (generally <30 mm thickness). Only minor sulfide is present (pyrite and chalcopyrite) usually within fine (<1 mm) late-stage veinlets, but a zone of enhanced gold mineralisation a few tens of metres thick is commonly encountered around the outer margin of the alteration envelope.

Alteration and veining
Metasomatitic rock assemblages north of the main mineralised zone are varied but include abundant chlorite, magnetite, phlogopite, actinolite-tremolite, pyrite (up to 20 per cent), serpentine, talc, albite, former scapolite, sericite, and haematite alteration. The metasomatites regularly contain low-grade copper (~0.1 per cent Cu as chalcopyrite) but may attain higher grades where density of late stage carbonate- (quartz-chalcopyrite- haematite-fluorite) veins is higher. Chlorite is abundant within this zone as a matrix component of tectonic breccias and in altered intermediate intrusive. Chlorite alteration also occurs within the footwall volcanics along with haematite and sericite, but its abundance decreases towards the centre of the deposit.

The main alteration products associated with Cu-Au mineralisation at Prominent Hill are haematite, silica/quartz and sericite. The zone affected by intense haematite alteration is characterised by extreme sodium depletion and absence of magnetite. Haematite is ubiquitous within Cu and Au mineralised zones. Martite (former magnetite) grains are common on the peripheries of the deposit.

Sericite is most common within the sequence of greywacke and argillite host rocks where it is a co-dominant constituent with quartz. It occurs in lesser concentrations with albite, carbonate, chlorite and haematite in the footwall volcanics, occurring as infill in vesicles and replacement in phenocrysts after feldspar as well as the groundmass. It also constitutes a major component of some mineralised breccias, both within clasts and within a haematitic matrix. Some volcanic dykes and/or sills within the breccia complex have undergone intense sericitisation.

Late stage vein types are widely varied in morphology and mineralogical composition. Veining is generally lower frequency in Cu-mineralised haematite breccias compared to host units. Common vein constituents include: carbonate (calcite and dolomite), copper sulfide, barite, quartz, fluorite and specular haematite. Carbonates at Prominent Hill are Ca and Mg rich whereas the most common carbonate at Olympic Dam is siderite. Chalcocite, bornite and chalcopyrite all occur as discreet late vein phases, commonly with haematite or carbonate.

Mineralisation
High grades of mixed Cu-Au mineralisation occur almost exclusively within the breccias where copper and gold occur as granules within the haematite-dominated matrix. Gold, in addition to its widespread association with copper in the haematite breccias, also occurs in higher-grade zones around the haematite-silica core, and in haematite crackle-veined haloes in dolomite, argillite and greywacke host rocks adjacent to breccia (Figure 4). The mineralised breccias reach the top of bedrock in the eastern part of the deposit, but to the west increasing interbeds of host sediments and lower grades of mineralisation are encountered.

At the broadest level, copper mineralisation displays a lateral and vertical zonation from chalcocite in the upper and more central portions of the breccia sheets to bornite, chalcopyrite and pyrite with depth (Figure 5). In detail, significant variations occur between discrete breccia bodies that are juxtaposed and variability of grade is also associated with metre-scale hydrothermal flow banding. A large part of the breccia system, though highly anomalous in Cu, U, Au and REE, does not carry economic grades of mineralisation. Chalcopyrite (-uraninite, -pyrite) mineralised breccias are volumetrically the most significant in terms of mineralisation intersected to date but generally of lower grade than chalcocite or bornite dominated intervals.

Uranium occurs in discrete parts of the system, often at deeper levels associated with fluorite and chalcopyrite, as small grains of coffinite and uraninite. In these zones, U concentrations may reach several thousand ppm, although through much of the breccias, the concentration is less than 100 ppm. This is a significant difference compared with Olympic Dam where Uranium is present in greater proportion, about 30 per cent of the in-ground value compared to ten per cent at PH, and reportedly occurs as pitchblende in the chalcocite-bornite zone and at the bornite-chalcopyrite interface. Cerium and lanthanum are present as Ce-apatite and Ce- and La- monazite and oxides (Figure 6). The main mineralised components (Cu, Au, Ur) all show an intimate relationship with haematite, with which they are believed to be genetically coeval.

Gold at Prominent Hill is present in greater proportion than Olympic Dam, forming about 30 per cent of the in-ground value at PH compared with ten per cent at OD, and is found in three distinct settings (Figure 4). The main hydrothermal breccia body contains a consistent level of gold over hundreds of metres of drill intercepts at the 0.2 to 2.0 gram/tonne level. Gold is also enhanced along the margin of intensive Fe-Si alteration that forms the ironstone ‘footwall’ to the Prominent Hill mineralised breccias. High gold levels have been intersected in crackle-veined dolomitic sediments in a halo above and around the copper mineralised breccias. This style of gold-only mineralisation awaits detailed investigation and drill follow-up.

Gold grains are present in a broad range of grain sizes, from grains of 800 microns readily observable by eye, down to five microns or less detected by electron microscopy. Preliminary petrological and SEM observations of the gold grains indicate a persistent association of gold, copper and iron oxide, implying a common genetic association. In particular, gold grains are rimmed with a copper rind, and contain minute copper inclusions (Figure 7). As a consequence, gold ‘floats’ readily in standard copper floatation tests with extremely high recovery- to-concentrate rates.

KEY CHARACTERISTICS OF OLYMPIC DAM
Key elements of the Olympic Dam deposit presented by Cross et al (1993), Esdale et al (2003), Gow et al (1994), Haynes et al (1995), Oreskes and Einaudi (1990, 1992), Reeve (1990a,b), Reeve et al (1990) and Reynolds (2000, 2001) are summarised below.

Olympic Dam contains ore reserves in excess of 700 Mt averaging 1.7 per cent Cu, 0.6 g/t Au, 3.6 g/t Ag and 0.5 kg/t U3O8 intimately associated with massive hydrothermal haematite alteration and brecciation. The 2600 Mt global resource also contains approximately 26 per cent Fe and 0.5 per cent Rare Earth Elements (REE), principally cerium and lanthanum, that are not recovered. The deposit occurs below 300 m of cover, within the Olympic Dam Breccia Complex, that is in turn hosted by the Roxby Downs Granite (Figure 8). Associated with the breccia complex are largely contemporaneous, mafic and felsic dykes, volcaniclastics, ash fall tuffs and associated high crustal level, near-vent sediments. U-Pb zircon ages on volcaniclastic tuffs and dykes associated with mineralised breccias are indistinguishable from the host granite intrusive and the Gawler Range Volcanic extrusive ages (c. 1590 Ma).

Geophysically, the deposit is characterised by a large discrete bouguer gravity and residual gravity anomaly of approximately 17 mgals (500 × 100 m grid spacing). A magnetic anomaly sourced from deeper below the deposit (c. 4 km) may represent a mafic intrusive complex. The gravity response directly reflects the hydrothermal haematite brecciation and alteration of the granitic host rocks, and the overprinted hydrothermal system is in turn directly related to the volcanogenic vent setting. Significant northwest- and northeast- trending crustal penetrating structures occur in the vicinity of the deposit, and lower order equivalent structures clearly control deposit geometry and grade (Figure 8). High-level Hiltaba granite (c. 1588 Ma) and associated sub-volcanic activity are coeval (within dating limits) with hydrothermal alteration and mineralisation.

A central haematite-silica-barite core barren of mineralisation is surrounded by an area of approximately 3 by 3.5 km of well-developed haematite-matrix breccias. A halo of variably altered and brecciated granite (clast-supported breccia) extends up to a further 3 km out where it grades in to less altered granite. In detail, the deposit is composed of a large number of irregular, discrete breccia bodies. Breccias display a spectrum from clast-supported simple jigsaw breccia types to matrix-supported heterolithic varieties. Repetitive lithification and rebrecciation have created a complex intermix of variably altered host and breccia fragments intermixed with haematite matrix and a variety of copper minerals largely as clasts and mineralised matrix. An IP chargeability anomaly is recorded over the WSW to WNW trending Fe-Si flooded volcanics and steely haematite-matrix breccias.

Alteration is dominantly sericite and haematite with lesser silica, chlorite, carbonate and magnetite alteration, and intensity is correlated with degree of brecciation. Early magnetite alteration has been largely overprinted by haematite. Silica alteration dominates the haematite-quartz core where barite is also enhanced. The iron rich mineralising fluids are believed to have a significant mantle component as well as a superimposed shallow crustal meteoric component (Johnson and McCulloch, 1995; Campbell et al, 1998).

Sulfide mineralisation displays a broad lateral and vertical zonation from chalcocite in the upper and more central portions of the breccia complex to bornite, chalcopyrite and pyrite with depth and lateral extent. Ore zones form only a small proportion of the breccia complex, though anomalous Cu, U, Au and REE occur throughout. Copper minerals occur mostly as disseminated grains within the breccia matrix. Gold, in addition to its widespread association with copper in the breccia, also occurs in higher-grade zones around the haematite-silica core and in the upper parts of the chalcocite-bornite zone. Mineralisation is intimately associated, and considered contemporaneous with, introduction of iron.

Strong structural control is evident in the locus of high grade mineralised zones. Both NE-SW and NW-SE trending structures are present, with individual breccia bodies having a NW trend within a WNW envelope.

COMPARISONS BETWEEN PROMINENT HILL AND OLYMPIC DAM – IMPLICATIONS FOR EXPLORATION
Exploration for Olympic Dam-style deposits on the Gawler in the 1980s and 1990s was based on limited published data, incomplete geological and geophysical information and poorly defined models (eg Roberts and Hudson, 1983; Rutter and Esdale, 1985). With additional information now available for Olympic Dam (Reynolds, 2000, 2001; Esdale et al, 2003), high quality regional data sets over the Gawler Craton from the South Australian Geological Survey, a better understanding of mineralising fluids (Skirrow et al, 2002) and a significant body of new information released for the Prominent Hill deposit, major improvements are being made in practical aspects of exploration for these types of deposits. Most exploration companies have reviewed and updated their key exploration parameters in the light of the Prominent Hill data.

Table 1 compares key exploration criteria for each of Olympic Dam and Prominent Hill. In addition to dispelling the notion that Olympic Dam is somehow a unique or singular occurrence, the information arising from the Prominent Hill highlights the key common criteria required for establishing more rigorous exploration models. In particular, the comparative data highlight a common association with near-vent volcanism and associated hydrothermal alteration and brecciation. They also highlight very similar alteration and mineralisation assemblages, proximal and distal haloes, and common zonation patterns.

Significant differences relate to the nature of the host rocks and the relative proportions of Cu: Au:U that may be expected in the mineralising system. Significant refinement of model-based exploration strategies is being implemented by Companies exploring for IOCG deposits. The remarkable similarities in ore mineralogy, zonation and paragenesis between Olympic Dam and Prominent Hill, and the similar tectonic setting, indicate these two deposits formed by very similar genetic processes. Together they define the highly prized, haematite-dominated end member of the iron oxide copper gold family of deposits.

Key pointers for explorers are the relationship with bimodal extrusives, the association with regional crustal structures, the common presence of intercalated sediments with volcanics, indicating high structural levels, and the dominant iron alteration event with which the mineralisation is associated.

Other defining features of this deposit type that can aid exploration include the extreme Na depletion associated with the haematite alteration event, the absence of magnetite in the mineralised zone, and alteration vectors involving chlorite, silica, sericite and haematite.

ACKNOWLEDGEMENTS
Data on Prominent Hill was collected as part of extensive exploration activities undertaken by Minotaur Resources Limited from 2001 to 2003, in partnership variously with Billiton Exploration Australia, BHP Billiton, Normandy Exploration and Oxiana Ltd. Numerous people from these companies assisted with data collection and interpretation. Petrological assistance and discussion were provided by Ian Pontifex, Kathy Ehrig and BHP Newcastle Laboratories. Geophysical input was provided by John Hart and geological interpretations by consultants Richard Flint and Leigh Rankin.

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CAPTIONS
Fig 1 - Tectonic map of the Gawler Craton, South Australia (simplified after Daly, Fanning and Fairclough, 1998), showing location of Prominent Hill and Olympic Dam.
Fig 2 - Geophysical responses of the Prominent Hill breccia complex. A. Residual gravity; B. 1VD gravity; C. 1VD gravity contours on TMI image with traces of diamond drillholes in yellow.
Fig 3 - Geological interpretation of the Prominent Hill breccia complex.
Fig 4 - Schematic geological model for Prominent Hill.
Fig 5 - Thin sections of main alteration and breccia styles at Prominent Hill. A. Sericitised and silicified corroded volcanic clasts in auriferous earthy haematite breccia (DP002, 200.60 m). B. Volcanic rock pervasively altered by dense, steely haematite (DP002, 440.5 m). C. Chalcocite mineralisation within bluish-grey haematite-silica matrix supported breccia (DP003, 463.5 m). D. Chalcopyrite mineralisation and compositional layering within dark grey haematite breccia (DP003, 543.2 m). E. Andesite with abundant aligned feldspar phenocrysts and late qz-cb veining (DP004, 188.5 m). F. Agglomerate with a variety of volcanic clasts in an Fe-rich volcanic (?) matrix, and fine, white plagioclase phenocrysts (DP004, 273.9 m). G. Altered quartz diorite (DP007, 122.7 m). H. Tensional fracturing within argillite (DP006, 413.9 m).
Fig 6 - Backscattered electron micrographs of breccia components, Prominent Hill.
Fig 7 - Photomicrographs of gold grains from Prominent Hill drillhole DP010 showing inter-relationships between haematite (hm), chalcopyrite (cp), gold (Au) and carbonate (cb).
Fig 8 - Geological interpretation of the Olympic Dam breccia complex (Reynolds, 2001).
Table 1 Summary of key features – Prominent Hill and Olympic Dam IOCG deposits.