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Edna May, Westonia
Western Australia, WA, Australia
Main commodities: Au

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The Westonia or Edna May gold deposit is located 60 km west of Southern Cross and is approximately half way between Kalgoorlie and Perth in Western Australia.

Mining at Westonia has been carried out over four separate periods since 1911: i). from 1911 to 1922, as an underground operation that produced 9.95 t of gold; ii). from 1935 to 1947, when 1.25 t of gold was produced, again from an underground operation; iii). an open cut that operated from 1986 to 1991 and produced 8.5 t of contained gold; iv). a restart of the open cut operation from 2010 through 2019, with >15 t produced over those years.

The deposit is located towards the NW end of a discontinuous, 100 km long, NW trending greenstone belt sub-parallel to the Southern Cross Belt. However, while the Southern Cross Belt lies within the Southern Cross Domain of the Youanmi Terrane, the Westonia Greenstone Belt straddles the boundary between the South West Terrane and the Murchison Domain of the Youanmi Terrane of the Archaean Yilgarn Craton.

The host sequence of the greenstone belt at Westonia is around 5 km wide and composed of amphibolite, felsic and mafic gneiss, ultramafics and late stage pegmatites microgranite and dolerite, all cut by granites.

In the mine area the sequence comprises:  i). a lower quartz-biotite schist unit,  ii). a massive to foliated amphibolite,  iii). the footwall ultramafic composed of metapyroxenite and olivine metapyroxenite,  iv). the up to 200 m thick (average ~140 m) Edna May Gneiss, a tonalitic quartz-feldspar-biotite gneiss characterised by a complex of quartz reefs and by intensive lateritic weathering near surface - it hosts the majority of the ore;  v). amphibolite similar to the second unit, and  vi). hangingwall ultramafics, originally probably pyroxenites.

The area has been influenced by a long period of weathering and lateritisation and covered by younger channel fill sediments and soil.

Virtually all of the hypogene mineralisation is within the 45°N dipping Edna May Gneiss, which have been dated at 2730 ±41 Ma.   This gneiss is interpreted to be a strongly metamorphosed granitic intrusion and has been shown to have a strike length of ~1 km and down dip extent of ~700 m.   Gold mineralisation is related to two quartz vein sets.   One set comprises of widespread, thin, sheeted veins in a ladder of stockwork style, generally sub-parallel to stratigraphy.   The second set are less frequent, larger veins which cross-cut the EMG and have much greater continuity. There are 8 principal reefs of this latter type, each with the same form, namely a thin initiation against the footwall contact, a thicker curved nose cutting across the Gneiss zone and a thinning trailing limb higher in the Gneiss unit. These lodes have typical dimensions of 2 to 5 m thickness, strike lengths of 80 to 110 m and down dip extents of 200 m. Associated visible gold is commonly seen in drill core.

Compared to other Archaean gold deposits in the craton, Westonia has a paucity of carbonate alteration, and lacks arsenic and boron.   Gold is associated with quartz veining.   The vein assemblage, in decreasing abundance, is pyrrhotite, pyrite, marcasite, chalcopyrite, ilmenite, wolframite, galena, molybdenite, sphalerite, rutile, digenite, gold, uraninite and hessite.

Weathering has produced an upper pisolitic zone with 0.5 to 1.2 g/t Au, overlying a gold depleted pallid zone in the next 15 to 20 m, then by a limonite zone with gold flooding in the gneiss between the reefs to 1.0 to 1.4 g/t Au.   The limonite zone is transitional to bedrock.

Production and reserve figures at 1990 included (Drummond and Beilby, 1990:
  Open pit production to 1990 - 2.355 Mt @ 2.3 g/t Au,
  Leach ore mined - 2.5 Mt @ 1.3 g/t Au,
  Underground reserve - 1.0 Mt @ 10 g/t Au.

Remaining Ore Reserves and Mineral Resources as at 30th June 2018 (Ramelius Resources Annual Report, 2018) were:
  Measured + Indicated + Inferred Resources - 33.158 Mt @ 0.9 g/t Au for 29.8 t of contained gold,
  including 4.3 Mt @ 1.1 g/t Au in the Greenfinch extension to the west.
  Proved + Probable Reserves - 5.509 Mt @ 1.1 g/t Au for 6 t of contained gold,
  including 1.652 Mt @ 1.2 g/t Au in the Greenfinch extension to the west

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this decription was dated: 2018.     Record last updated: 27/8/2019
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.

  References & Additional Information
   Selected References:
Drummond A J, Beilby G R  1990 - Westonia Gold deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 289-295

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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