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Transvaal Lode - Southern Cross Belt |
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Western Australia, WA, Australia |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Transvaal gold deposit lies within the Archaean Southern Cross Greenstone Belt, Southern Cross Domain, in the Youanmi Terrane of the Yilgarn Craton, some 235 km WSW of Kalgoorlie, 375 km east of Perth and 6 km south of the town of Southern Cross, Western Australia.
For information on the Southern Cross Belt and its deposits see also the Copperhead, Fraser, Nevoria, Bounty, Yilgarn Star and Marvel Loch records.
The first recorded production at Transvaal was from underground workings between 1901 and 1911, when a total of 57 123 t of ore @ 7.9 g/t Au was mined for 0.456 t of recovered gold. Between 1988 and 1995 Reynolds Australia and Sons of Gwalia from 1995 to 1997 cumulatively mined 2.3 Mt @ 4.6 g/t Au for 10.64 t of gold from both underground and open pit mines.
The Transvaal project comprises a cluster of near north-south striking parallel lodes, namely Jupiter, Mercury, Sunbeam, Polaris, Aquarius and
Transvaal distributed over an area of ~2500 x 360 m.
The Southern Cross Greenstone Belt which hosts the Transvaal deposit is a narrow, elongate NW trending package of supracrustal rocks that extends some 300 km from Jackson in the north, to Hatters Hill in the south. The supracrustal succession has been divided into two sequences:
• a package comprising a basal quartzose sandstone, overlain by dominant metakomatiite and metabasalt rocks with thin interflow layers of quartz-biotite schist, quartz-grunerite banded iron formation and metagabbro sills; and
• an upper greywacke sequence mainly composed of pelitic metasedimentary rocks.
The succession underwent extensional domal uplift accompanied by granitic intrusion. Dextral transpression and progressive deformation has produced a range of tight to open folds, layer parallel ductile shear zones and brittle faults (Williams et al., 2008). The Transvaal Shear Zone, in which the Transvaal deposit is located, is focussed between the Rankin and Ghooli domes to the NE and NW respectively. These structural domes are two of a series of such structures cored by syn-tectonic granite. Metamorphic grade in the greenstone belt ranges from upper greenschist near the centre of the thicker portions of the supracrustal rocks, to granulite and amphibolite facies near the granite-greenstone contacts.
The Transvaal Shear Zone is a major regional structure which is defined by a broad zone of deformation, and is geochemically anomalous with elevated Au, As and Sb signatures.
At Transvaal, the supracrustal sequence trends NNW and dips steeply to the east, although vertical dips and steep westerly dips have also been recorded. It can be divided into three units (Williams et al., 2008):
i). an eastern mafic volcanic package, which are primarily meta-basalts, generally metamorphosed to actinolite-plagioclase-chlorite dominant rocks;
ii). a central ultramafic volcanic package of meta-komatiites, commonly tremolite dominated, with thin intercalated layers of graphitic quartz-biotite schist and quartz-grunerite banded iron formation and minor amounts of talc, carbonate and chlorite. Localised anthophyllite-cummingtonite alteration has been recorded and is a possible indicator of a magnesian composition or alteration. The mafic and ultramafic packages are together up to 3000 m thick and are cut by lens shaped meta-gabbro sills.
iii). an up to 1500 m thick western sedimentary unit that can be subdivided further into a pelitic and a psammitic unit. A thin zone of carbonaceous sedimentary rocks and sulphidic cherts occur at the base of this suite. The pelitic sedimentary rocks are typically fine grained and dark-grey with cm scale compositional and grain size layering, and a well developed layer parallel cleavage. They are composed of biotite-quartz-andalusite-cordierite-garnet. Wilde (1989), noted an increase in biotite, pyrrhotite and quartz in wall rocks proximal to lode positions. The psammite facies are generally quartz dominant with associated biotite.
The majority of the gold deposits of the Southern Cross Belt (eg. Marvel Loch) are in the mafic-ultramafic units, although the Transvaal lodes are just above the contact between the volcanic and sedimentary sequences. Mineralisation at Transvaal occurs within two narrow, NNW trending, sub-parallel lodes that dip steeply to the east. The eastern lode, on which the Sunbeam, Polaris, Aquarius and Transvaal pits have been mined, is adjacent, but slightly discordant to the ultramafic-sediment contact, while the western lode is located adjacent to the pelite-psammite contact, and hosts mineralisation within the Jupiter and Mercury pits (Williams et al., 2008).
The ultramafic schists and graphitic pelites that host the Transvaal lodes have been subjected to lower amphibolite facies metamorphism. The main tabular orebodies, comprise massive quartz-sulphide veins controlled by the Transvaal Shear Zone which overprints the dominant penetrative regional cleavage. It has related splays that are flanked by hydrothermally altered wall rock composed of proximal diopside-actinolite-quartz grading outwards into distal amphibole (hornblende-actinolite)-biotite-plagioclase-quartz schist in the ultramafic hosts, while in sedimentary hosts the proximal zone is represented by biotite-muscovite-quartz-graphite surrounded by a distal zone of cordierite-quartz-biotite-graphite schist. The dominant sulphides are pyrrhotite, loellingite and arsenopyrite, with associated chalcopyrite and cubanite. Gold occurs as both electrum (dominantly with 24 to 56% Ag) and native gold and is found as inclusions or fine grained blebs on arsenopyrite, pyrite, or quartz crystal margins.
The Transvaal lode and mineralised halo which is the northern-most of the cluster of lodes that make up the Transvaal deposit, is a single well defined shoot some 0.5 to 3 m thick, 150 m long, trending NNW and plunging moderately steeply to the south to a depth of more than 200 m.
Remaining Indicated + Inferred Mineral Resources at 30 June 2008 were 6.511 Mt @ 3.5 g/t Au for 22.8 t of contained gold at a 1 g/t Au cut-off (Williams et al., 2008).
Remaining Indicated + Inferred Mineral Resources at 30 June 2009 were 3.43 Mt @ 4.9 g/t Au for 16.8 t of contained gold (St Barbara Ltd Annual Report 2009). This latter estimate was calculated using a graduated cut off grade of 0.5 to 0.9 g/t in oxide and 0.7 to 1.0 g/t in fresh rock within an optimised $1200 pit
shell and a 2.6 g/t cut-off grade below the optimised pit shell.
This summary is partially drawn from "Williams, R., Potter, J. and Snow, N., 2008 - Mineral Resource Estimate, Transvaal Project, Southern Cross, Western Australia; prepared for St Barbara Limited by Runge Limited, 93p.".
The most recent source geological information used to prepare this decription was dated: 2009.
Record last updated: 28/8/2019
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.
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Selected References:
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Bloem E J M, Dalstra H J, Groves D I and Ridley J R 1994 - Metamorphic and structural setting of Archaean amphibolite hosted gold deposits near Southern Cross, Southern Cross province, Yilgarn Block, Western Australia: in Ore Geology Reviews v9 pp 183-208
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Doublier, M.P., Thebaud, N., Wingate, M.T.D., Romano, S.S., Kirkland, C.L., Gessnar, K., Mole, D.R. and Evans, N., 2014 - Structure and timing of Neoarchean gold mineralization in the Southern Cross district (Yilgarn Craton, Western Australia) suggest leading role of late Low-Ca I-type granite intrusions: in J. of Structural Geology v.67, pp. 205-221.
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Hagemann S G, Brown P E, Ridley J, Stern P, Fournelle J 1998 - Ore petrology, chemistry, and timing of electrum in the Archean hypozonal Transvaal Lode Gold deposit, Western Australia: in Econ. Geol. v93 pp 271-291
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Mueller A G, McNaughton N J 2000 - U-Pb ages constraining batholith emplacement, contact metamorphism, and the formation of Gold and W-Mo Skarns in the Southern Cross Area, Yilgarn Craton, Western Australia: in Econ. Geol. v95 pp 1231-1257
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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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