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Challenger |
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South Australia, SA, Australia |
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Au
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Super Porphyry Cu and Au
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The Challenger gold deposit is hosted by Archaean pelitic migmatites in the northwest Gawler Craton, South Australia, some 740 km NW of Adelaide (#Location: 29° 52' 39"S, 133° 35' 15"E).
The open pit commenced production in June 2002 and ceased in April 2004, before underground operations started in March 2005 and have continued to the present (late 2007).
The 900x600 km Gawler Craton occuppies the core of the state of South Australia and comprises a diverse association of geologic units dating from the Archaean to the mid-Proterozoic. The Archaean para- and orthogneisses with mafic to ultramafic intrusive and extrusive rocks of the Sleaford and Mulgathing Complexes form an older basement cropping out sporadically in the western and northwestern parts of the craton.
The Challenger gold deposit is hosted by the Archaean Christie Gneiss which comprises banded iron-formation, cordierite-garnet gneiss, layered calc-silicates, marble (locally dolomite), basic gneiss and quartzite. It is the basal unit of the 3150 to 2950 Ma Mulgathing Complex.
Studies of the Christie Gneiss indicate peak granulite facies metamorphism between 2450 and 2420 Ma, although this may not be regionally consistent in the different domains of the Craton. Variably developed retrograde metamorphism is also associated with later deformation during the Kimban (~1845 to 1700 Ma) and Kararan (~1650 to 1540 Ma) orogenies.
Two major periods of deformation are recognised in the region: 1). a period of ductile deformation occurring concurrently with the granulite facies metamorphism during the Sleafordian orogeny (2450 to 2420 Ma), interpreted to have produced the dominant regional structures of tight isoclinal macrofolds refolding earlier macro- and micro-isoclinal folds, and 2). a phase of Mesoproterozoic amphibolite-greenschist facies reworking that developed extensive regional shear zones.
Gold mineralisation at Challenger, accompanied by pyrrhotite, löllingite and arsenopyrite, occurs in a series of shoots, which average 8 g/t Au and plunge at 30° with an azimuth of 030°, are contained within a broad ENE trending envelope of elevated gold (100 to 1000 ppb) and high arsenic (>200 ppm As). Below the weathered zone the shoots have a flattened cigar shape with widths of around 20 m, down plunge lengths of more than 500 m, and are developed within a monotonous pelitic garnet-biotite gneiss with varying orthopyroxene and cordierite. This lithology, the Challenger Gneiss, does not vary in bulk composition in and away from ore. However, in the vicinity of ore it typically displays abundant signs of partial melting and the development of stromatic migmatite compared to unmineralised gneiss. No obvious alteration is associated with gold mineralisation. In addition to the Challenger Gneiss there is <5% mafic pyroxene granulites after mafic dykes and sills, as well as several younger unmetamorphosed mafic dykes and sills which cut the mineralisation and hosts.
Textural relationships indicate that gold was present prior to peak metamorphism. Visible gold is now restricted to migmatitic leucosomes and, to a lesser extent, melanosomes. These polymineralic leucosomes with veinlike morphology are generally composed of quartz + perthitic K feldspar ± plagioclase ± coarse-grained biotite and typically contain isolated porphyroblasts of garnet, cordierite and more rarely, orthopyroxene. No straight godl bearing quartz veins have been encountered. In the gold- and arsenic-rich zones, pyrrhotite (typically with chalcopyrite and rarely pentlandite) and arsenopyrite-löllingite grains are disseminated within mesosomes, melanosomes and leucosomes but are concentrated in the melanosomes. Sulfides are less common within the leucosomes, but are coarser. Visible gold (500 micons) is only seen in the leucosomes.
The gold bearing leucosomes are believed to have been formed during the metamorphism of the Challenger deposit which resulted in partial melting of the gold bearing pelitic host rock and formation of silicate and polymetallic gold-rich melts which were synchronously redistributed to accumulate as leucosomes during development of a stromatic migmatite. Gold-rich shoots at Challenger parallel the plunge of ptygmatically folded leucosomes that are shown to be parasitic to a larger scale fold geometry which appears to be structurally related to the ore shoots. It has been interpreted that concurrent migration of polymetallic and silicate melt allowed concentration of gold into a series of dilational structures which developed within the larger scale fold geometry.
This summary has been generated from Tomkins & Mavrogenes, (2002).
The indicated+inferred resource at Challenger to a depth of 400 m in 2001 (Tomkins & Mavrogenes, 2002) was:
1.79 Mt @ 8.57 g/t Au for a total of 14.82 tonnes of gold.
The total indicated+inferred resource at April 2007 was 2.14 Mt @ 9.40 g/t Au (=20.12 t Au), including
a total proven+probable reserve of 1.07 Mt @ 8.50 g/t Au (Hartleys, 2007).
From the start of operation to June 30 2006, a total of 7.787 t of Au have been produced from 1.587 Mt of ore (Hartleys, 2007).
The total measured + indicated + inferred resource at 30 June 2012 was 5.01 Mt @ 6.74 g/t Au (=33.9 t Au), including
a total proven+probable reserve of 3.25 Mt @ 6.16 g/t Au (Kingsgate Consolidated, 2012).
The deposit contains at least 16 high grade shoots, not all of which have been drilled extensively.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2007.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Challenger
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Selected References:
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Tomkins A G, Dunlap W J, Mavrogenes J A 2004 - Geochronological constraints on the polymetamorphic evolution of the granulite-hosted Challenger gold deposit: implications for assembly of the northwest Gawler Craton: in Australian J. of Earth Sciences v51 pp 1-14
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Tomkins A G, Mavrogenes J A 2002 - Mobilization of Gold as a polymetallic melt during Pelite Anatexis at the Challenger deposit, South Australia: a metamorphosed Archean Gold deposit: in Econ. Geol. v97 pp 1249-1271
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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