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Walhalla, Cohens Reef |
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Victoria, Vic, 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 Walhalla district and gold deposits are located approximately 180 km east of Melbourne in Victoria, Australia. The district has produced over 68 t of gold, mainly from quartz reefs since1863. The main producer has been Cohens Reef.
The reefs at Walhalla are hosted by the lower Devonian Walhalla Group which is part of a regional, conformable sequence of marine sediments deposited from the lower Ordovician to middle Devonian. During the Devonian Tabberabberan Orogeny the sediments were folded into the Walhalla synclinorium and the Woods Point Dyke Swarm (dioritic to lamprophyric) was emplaced in the more highly deformed central part of the synclinorium. The dykes were subsequently faulted and fractured and during the late Devonian to eaarly Carboniferous the Baw Baw Granodiorite was emplaced.
The Walhalla Group is approximately 2800 m thick and comprises thick bedded siltstone and thinly bedded sandstone and shale.
Two folding styles are represented, namely: 1). large, upright open folds and 2). tight asymmetric folds which occur within the Cohens Reef Shear Zone produced during a late phase of reverse faulting. The Cohens Reef Shear Zone is defined by hangingwall and footwall faults. The shear zone, which shows reverse movement, is 20 to 50 m wide, 3 km long and persists for 1500 m down dip. It contains the Cohens Reef (a laminated vein), several sheared dykes (including the Cohens Dyke which is a highly altered and sheared hornblende diorite dyke with a spatial relation to the reef), and deformed mine sequence sediments.
Five distinct styles of quartz veining have been recognised but only the laminated veins and associated stockworks contain significant gold. Cohens Reef is a laminated vein from a few cm to 3 m thick composed of quartz, calcite and country rock inclusion, dyke fragents, sulphides and gold, with selvages of country rock parallel to the vein margin. The dykes are older than the gold mineralisation. Pyrite occurs as a halo up to 10 m out from the reef, while arsenopyrite is only found within 2 m of the vein margin. Both sulphides are equally distributed in zones of stockwork development. Gold occurs in fractured arsenopyrite and pyrite crystals and as free gold in fractured quartz associated with chalcopyrite, galena and sphalerite and was introduced at a late stage in vein development.
The Cohens Reef ore shoot is regular, pitching north at 30° to 400 m depth before becoming vertical and then pitching south at 70° for 500 m before changing to vertical again to the deepest level mined of 1150 m. It has a strike length of around 400 m at any one level.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1990.
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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Hughes, M.J. and Phillips, G.N., 2015 - Mineralogical domains within gold provinces: in Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc. v.124, pp. 191-204.
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Tomlinson K M 1990 - Cohens Reef Gold deposit, Walhalla: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v2 pp 1303-1305
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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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