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Mt Gibson |
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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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The base metal rich Mount Gibson gold mine is located in the Retaliation Greenstone Belt of the Murchison Province in the Yilgarn Craton, Western Australia. It is some 150 km SSE of Yalgoo (and the Scuddles/Golden Grove VHMS base metal deposits) and 240 km SSW of Cue and the Big Bell and other lode gold mines.
The host sequence youngs from west to east. It has been metamorphosed to middle amphibolite facies, strikes north-south and comprises from the base, tholeiitic basalt, magnesian basalt, the Mine Sequence, magnetic quartz-andesite with minor interflow sediments and a monotonous thick sequence of quartz porphyritic felsic volcanics.
The Mine Sequence, which hosts the gold, is composed predominantly of variably differentiated tholeiitic basalts and dolerites, with lesser magnesian basalt and intrusive quartz-feldspar porphyry.
Gold mineralisation is found within a broad, steeply east dipping, north-south trending, anastomosing shear network, the ~1 km wide Mount Gibson Shear where it is developed within the Mine Sequence. Primary mineralisation is present as lenses of sulphide bearing schist, predominantly within altered mafic rocks.
Two main styles of mineralisation and hydrothermal alteration are recognised, that are temporally separated by some 300 m.y. These two types appear to represent:
• an early, string of intermittent, steeply dipping lenses of stratabound volcanic hosted massive sulphide base metal mineralisation, composed of sphaletite-galena-pyrite with lesser pyrrhotite-chalcopyrite and tetrahedrite, overlain by spessartine/almandine garnet/gahnite, and cordierite-muscovite bearing schists;
• a later, overprinting syn-orogenic shear hosted lode gold phase within a sheared quartz-biotite and sulphide altered envelope associated with pyrrhotite-pyrite and chalcopyrite.
The host rocks and early mineralisation have been dated at ~2930 Ma, while the gold appears to have been emplaced at ~2620 Ma.
Mining was from 8 pits within the NNE trending 7 km long Mt Gibson Trend or Shear Zone, comprising the Enterprise, Donkey D, Orion, Orion North, Wombat, Deep South, Hornet and Orion 2 pits. The ore mined occurred as ferruginous duricrust and gravel over an area of 6 x 0.5 km, derived from the underlying primary sulphide mineralisation (Anand and Smith, 2001).
The Mt Gibson gold mine commenced operations in 1986, based on a shallow lateritic ore reserve of 5.3 Mt at 1.6 g/t Au. Cut-off grades of 1 g/t Au or less were used at various periods. Continued exploration expanded these resources and delineated thirteen deposits, including some primary mineralisation (Gee, 1990).
To 1996 the operation had produced 17 tonnes of Au and 10 t Ag (Yeats and Groves, 1998).
Total production between 1986 and 1999 was quoted at 27 t Au from the oxide ore of the ferruginous duricrust, extracted from pits mined to a nominal depth of 80 m distributed over a 7 km strike length of the Mt Gibson Trend (Legend Mining ASX release, 2005).
Remaining Mineral Resources in 2004, predominantly primary sulphide ore were estimated to be another 27 t of Au (Oroya Gold, reported in Legend Mining ASX release, 2005).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1996.
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.
Mount Gibson Gold
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Selected References:
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Anand, R.R. and Smith, R.E., 2001 - Mount Gibson District: in Butt, C.R.M., Robertson, I.D.M., Anand, R.R., King., D.J., Mundy, T.J., Phang, C. and Smith, R.E., 2001 Geochemical exploration in areas of transported overburden, Yilgarn Craton and environs - Murchison Filed Trip, CRC LEME, Open File Report 110, pp. 1-15.
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Yeats C J and Groves D I 1998 - The Archaean Mount Gibson gold deposits, Yilgarn Craton, Western Australia: Products of combined synvolcanic and syntectonic alteration and mineralisation: in Ore Geology Reviews v13 pp 103-129
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Yeats C J, McNaughton N J, Groves D I 1996 - SHRIMP U-Pb geochronological constraints on Archean volcanic-hosted massive Sulfide and lode Gold mineralization at Mount Gibson, Yilgarn Craton, Western Australia: in Econ. Geol. v91 pp 1354-1371
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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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