Iron Oxide Copper-Gold Deposits

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.

Return to top

New & Recent International
Study Tours:
Click on image for details.

Olympic Dam ...................... Tuesday 22 November, 2005.

The Olympic Dam ore deposit lies within the outer margins of the Palaoeproterozoic Gawler Craton in northern South Australia.

It falls within the Stuart Shelf where 300 m of flat lying, barren, Neoproterozoic to lower Palaeozoic sediments 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 Adelaide Fold Belt.

The oldest basement rocks in the province are metasedimentary rocks and deformed granites correlated with the Palaeoproterozoic Hutchison Group and the Lincoln Complex granitoids, respectively. These rocks are intruded by Mesoproterozoic Hiltaba Suite granitoids and locally overlain by similar aged bimodal volcanic units correlated with the areally extensive Gawler Range Volcanics. Mineralisation is hosted by the Olympic Dam Breccia Complex (ODBC) which is developed within the Mesoproterozoic (1600-1585 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 which is petrologically and petrochemically similar to granitoids of the Hiltaba Suite, which are widespread in the Gawler Craton. The ODBC and the surrounding areas of Roxby Downs Granite form a local basement high on a broader regional basement uplift.

The ODBC 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 funnel-shaped, barren, hematite-quartz breccia "core" surrounded by an irregular array of variably mineralised and broadly zoned hematite-granite breccia bodies. The strike length of more hematite altered breccias within the complex is greater than 5 km in a NW-SE direction, and it is up to 3 km across and is known to extend to a depth of at least 1400 m. Minor volcaniclastics are found in diatreme structures at the top of the complex.

The outer margins of the complex are diffuse, grading outwards from the "core" to heterolithic breccia, through granite breccia, to crackle breccia to fractured granite. 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.

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. The dominant hydrothermal alteration is sericite-hematite, with lesser chlorite, silica, carbonate (siderite) and magnetite. There is no associated sodic metasomatism. 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 overall deposit and at the scale of individual breccia zones, with the degree of alteration intensity being directly associated with the amount of brecciation. The strongest alteration is therefore localised within, and on the margins of, the hematite-granite breccia bodies which host the ore deposit.

The resource occurs as up to 150 separated bodies distributed within an annular zone up to 4 km in diameter surrounding the central barren hematite-quartz breccia. The highest grade mineralisation occurs where an up to 40 m thick, shallowly inward dipping, irregularly developed, chalcocite-bornite 'zone', cuts hematite rich breccia zones. This undulose layer grades progressively downwards and outwards into chalcopyrite and then pyrite rich zones. The chalcocite-bornite 'zone' is overlain by a barren sulphide deficient interval extending to the overlying unconformity.

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.

As at December 2004 proven+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.

In August 2006, the total resources amounted to: 4430 Mt @ 1.1% Cu, 0.5 g/t Au, 0.4 kg/tonne U3O8, comprising:
Measured resource = 680 Mt; Indicated resource = 1360 Mt; Inferred resource = 2390 Mt.

At 30 June 2009, the total resources amounted to: 9080 Mt @ 0.87% Cu, 0.32 g/t Au, 1.50 g/t Ag, 0.27 kg/tonne U3O8, comprising:
Measured resource = 1250 Mt @ 1.11% Cu, 0.35 g/t Au, 2.12 g/t Ag, 0.33 kg/tonne U3O8;
Indicated resource = 4623 Mt @ 0.88% Cu, 0.34 g/t Au, 1.60 g/t Ag, 0.28 kg/tonne U3O8;
Inferred resource = 3207 Mt @ 0.74% Cu, 0.27 g/t Au, 1.11 g/t Ag, 0.23 kg/tonne U3O8.

This resource includes a total proven + probable reserve of:
589 Mt @ 1.81% Cu, 0.66 g/t Au, 3.36 g/t Ag, 0.59 kg/tonne U3O8.

At 30 June 2009 the separate non-sulphide gold resource was 151 Mt @ 099 g/t Au, comprising:
Measured resource = 34 Mt @ 1.10 g/t Au; Indicated resource = 102 Mt @ 0.96 g/t Au; Inferred resource = 15 Mt @ 0.91 g/t Au.

Production in 2004 totalled 8.886 Mt of ore with a head grade of 2.26% Cu, 0.64 kg/tonne U3O8 from which 224 731 tonnes of copper and 4404 tonnes of U3O8 were recovered.

Production in 2009 totalled 194 100 tonnes of Cu, 3.36 t Au, 29.14 t Ag, 4007 tonnes U3O8.

The mine is owned and operated by a subsidiary of BHP Billiton Ltd.

Return to top

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, on the southern margin of the Mt Woods Inlier, part of the Archaean to Mesoproterozoic Gawler Craton in northern South Australia.

The deposit was discovered under approximately 100 metres of cover, and corresponds to a discrete gravity anomaly corresponding to the hydrothermal iron altered hematite matrix breccias which host the copper-gold-silver-uranium-cerium-lanthanum ore deposit. The centre of the gravity anomaly coincides with a mass of massive, barren "steely" hematite-silica flooded volcanics (in the central-footwall to the main ore zone), flanked to the north by a 2x0.5 km hematite-rich breccia development. This breccia is layered, reflecting a series of sub-vertical, curved, ENE to ESE trending breccia sheets following structural and lithological contacts. These breccias are flanked by lesser hematite altered breccias extending outwards for a further 0.5 km. The iron alteration is almost exclusively hematite, with little magnetite, but is accompanied by sericite, silica, fluorite, barite and carbonate. Martite (after magnetite) however, is common on the margins of the deposit and of the hematite zone.

The nearby magnetic anomaly reflects a package of magnetite-chlorite-tremolite-phlogopite altered metasomatic skarn like altered metamorphics intercalated with highly altered, intermediate, porphyritic intrusives and chlorite matrix tectonic breccias.

The deposit is part of a more extensive breccia complex superimposed upon a sequence of approximately 1590 Ma, low metamorphic grade, mafic to intermediate to felsic volcanics and intrusive dykes of the lower Gawler Range Volcanics, composed of mainly basalt, andesite and rhyolite, passing up into fragmental volcanics (hematitic agglomerates, conglomerates, lapilli tuffs and pepperites), overlain by the associated Mentor Formation volcaniclastic sediments which pass up into dolomitic carbonates. Overall the sequence varies upwards, from footwall to hangingwall and from south to north from volcanics to greywacke to argillite to dolomite.

The upper parts of the volcanics and volcaniclastics and the overlying sediments occur as an inlier of steeply north dipping rocks within an arcuate ENE to ESE trending graben bounded by footwall volcanics to the south and by the magnetite-chlorite-tremolite-phlogopite altered intrusives and older sediments to the north. The lithologies within the fault bounded graben subsequently underwent pervasive but intense hydrothermal alteration, replacement and brecciation. The breccias range from crackle breccias on the peripheries, through jigsaw to matrix supported heterolithic breccias near the core. These breccias, which were developed from both the volcanics, volcaniclastics and sediments include intensely "steely" iron-silica altered variants to lower intensity earthy hematite altered in situ crackle-veined variants. The main alteration associated with mineralisation comprises hematite, silica/quartz and sericite. Hematite breccias are generally confined to the greywackes and sandstone and the underlying volcanics immediately to the south.

High grade mixed Cu-Au mineralisation is almost exclusively within the breccias with Cu and Au occurring as granules within the hematite dominated matrix. The copper minerals are zoned from chalcocite in the upper (both vertically and hangingwall) and more central parts of the breccia sheet, to bornite, chalcopyrite and then pyrite at depth and in the footwall. There is considerable variation in grade and mineralisation from different juxtaposed breccia sheets and within metre scale hydrothermal banding within breccia sheets. Uranium averages around 100 ppm throughout the orebody. Ce and La are present in apatite and monazite and as oxides, and are also intimately related to the hematite. Gold is found at levels of 0.2 to 2 g/t throughout the main ore zone with Cu, but also on its own on the margins of the "steely" hematite-silica footwall zone and as a halo of crackle veined dolomitic sediments in the hangingwall.

The declared reserves and resources at Prominent Hill in mid 2006 were:

Proved + probable reserves: 68.2 Mt @ 1.31% Cu, 0.59 g/t Au, 3.25 g/t Ag.
Measured + indicated + inferred resources at a 0.3% Cu cut-off: 118.68 Mt @ 1.30% Cu, 0.49 g/t Au.
Measured + indicated + inferred resources at a 0.5 g/t Au cut-off and <0.5% Cu: 22.54 Mt @ 0.1% Cu, 1.24 g/t Au.

The Cu resource includes the Cu reserve. The Au resource is NOT included in the Cu resource.

The declared reserves and resources at Prominent Hill in mid 2008 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 - 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 title to the deposit is held by Oxiana Limited who are undertaking a feasibility study.

Return to top

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.

Return to top

Ernest Henry .................... Friday 25 November, 2005.

The Ernest Henry IOCG style Cu-Au deposit is located 35 km north-east of Cloncurry, 150 km east of Mt Isa and 750 km west of Townsville in north-west Queensland.

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 succession is largely composed of evaporite-rich cover sequence 2 rocks and silici-clastic-rich cover sequence 3 rocks (ca 1740 Ma and ca 1670 Ma, respectively) which overlie crystalline basement formed during the Barramundi Orogeny (ca 1880-1850 Ma). Basement is not exposed in the Cloncurry district. The deposition of cover sequence 2 and 3 rocks was also accompanied by the emplacement of various intrusions and volcanic rocks. The cover sequences were deformed and metamorphosed during the Isan orogeny, which peaked with a regional greenschist to upper amphibolite facies metamorphism associated with a major horizontal east-west compression (D2).

The Williams and Naraku Batholiths (ca 1550-1500 Ma) that largely post-date D2 represent the youngest felsic intrusions in the inlier, and have an outcrop exposure of >1500 sq. km. 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 oxidized than similar older (ca. 1670 Ma) granitoids in the Western Fold Belt of the Mount Isa Inlier. Sodic intrusions of similar age are rare. The K-rich igneous rocks are similar to the intrusions that host Olympic Dam, and form part of a larger series of geochemically-similar Proterozoic intrusions that have a global distribution.

A regionally extensive Na-Ca hydrothermal system in the Cloncurry district (>100 sq. km) affected all rock types, especially the calc-silicate-rich rocks within cover sequence 2, and formed over the same period as the Williams and Naraku Batholiths. The regionally extensive Na and Na-Ca (e.g. actinolite, albite and diopside) 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. It affects the host rocks to all major Cu-Au deposits prior to sulphide mineralisation.

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 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 bounding shear zones and formed a pipe-like zone of dilation in the K-feldspathised metavolcanic rocks. 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 Na-Ca alteration, occurring mainly as albitic plagioclase-, magnetite-, clinopyroxene- and amphibole-rich veining and fault-related breccia-fill. ii). Pre-mineralisation 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. 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. iv). Post-ore multiple stage calcite-dolomite- and/or quartz-rich veining and alteration.

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,

The operation is controlled by Ernest Henry Mining Pty Ltd, a subsidiary of Xstrata Ltd.

Return to top

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.

		Another PGC International Study Tour Developed & Managed by Porter GeoConsultancy IOCG 05 Iron Oxide Copper-Gold Deposits 21 to 26 November 2005
		Porter GeoConsultancy Home \| More on This Tour \| Other Tours \| New Tours \| Contact us