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Sons of Gwalia, Gwalia, Tower Hill

Western Australia, WA, Australia

Main commodities: Au
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Deposit Description

The Sons of Gwalia or Gwalia, and Tower Hill gold deposits are located 230 km north of Kalgoorlie and 3 and 1 km south respectively of Leonora within the Mount Margaret Goldfield.   It lies within the Agnew-Wiluna segment of the greater Norseman-Wiluna greenstone belt of the Eastern Goldfields in the Archaean Yilgarn Craton of Western Australia.

The greenstone belt sequence in the district comprises two associations, namely Association 1, an older western mafic-ultramafic succession of magnesian tholeiitic basalt, intrusives, minor pyroxenites and mafic to ultramafic schists, which on its western margin are cut by intrusive adamellite.   The top of this association is occupied by a 30 m thick volcanogenic conglomerate breccia.

Association 2 is composed of dominant sedimentary wackes with subsidiary extrusive volcanics, associated intrusives and black-shale chert units.

The Gwalia deposit is on the western margin of the broad, north-south trending Keith-Kilkenny Tectonic Zone.   Gold occurs within a north-south striking, 40 to 45°E dipping, 120 to 150 m thick zone of intensely sheared mafic volcanics comprising narrow mica rich (muscovite and lesser chlorite, often with associated tourmaline) bands and wider zones of granoblastic quartz-carbonate rocks.   These are the Mine Schists and are intruded by an adamellite batholith to the west.   The eastern or hanging wall of the Mine Schist is occupied by 30 to 80 m of tholeiitic basalt.

Gold mineralisation is present as banded, partially silicified sericite-muscovite-chlorite schist with disseminated sulphides (predominantly pyrite, with traces of chalcopyrite and pyrrhotite), and sulphide bearing quartz lenses and stringers in the higher grade zones.   Individual ore shoots originally mined underground (Main Lode, West Lode, South Gwalia Series, Hangingwall Series and Southwest branch) were high grade and lenticular.   The Gwalia lode system has a strike length of 500 and persists down pitch for at least 1750 m.   Within this lode system, individual lodes average 1.8 to 2.4 m in thickness.   The open pit operation exploits a broad envelope to the original high grade lodes, which occurs as a generally horseshoe shaped body some 10 to 50 m thick.

The Tower Hill deposit, which, with the Gwalia open pit and Gwalia Deeps deposits, is part of the Saint Barbara Leonora operation. It is located 2 km north of the Gwalia pit, within the same Archaean greenstone sequence, immediately adjacent to the Leonora dome. At Tower Hill the sequence comprises, from the base: i). granitoid; ii). ultramafic schist with minor metadolerite and porphyry intrusions; iii). mafic schist after dolerite and basalt with minor sediments and quartz-chlorite-sericite-chloritoid rocks in the upper sections; iv). quarz-chlorite-sericite-chloritoid rocks, similar to the previous division with minor andalusite rich zones and shale; and v). felsic schist.

Essentially conformable quartz lodes are found towards the base of the ultramafic unit at Tower Hill, within 100 m of the granitoid contact, embraced by chlorite-talc-biotite-carbonate schist and ubiquitous carbonate veining. Overlying the ultramafic unit, banded mafic chlorite-biotite-sericite-fuchsite-carbonate schist contain thin (1 to 4 mm) conformable quartz stringers. Locally a zone of quartz stockwork veining without gold is hosted by carbonate-sericite-biotite-chlorite-talc rock. Following the stringer zone is a relatively thick talc-chlorite-carbonate schist occurs with small irregular bodies of metadolerite and granitoid. Gold occurs in several sub-parallel quartz veins which extend over a strike length of 600 m and a cross stike interval of 150 m. The longest individual lode is 500 m long. The thickest lode has a mean thickness of 6 to 8 m, and maximum of 18 m. The larger veins are bifurcated. They strike N to NNW abnnd dip at 45°E. Down dip extents are of the order of 400 m to a depth of 250 m below the surface. The gold is contained within the quartz veins, which are milky, but not within the host ultramafic. The main sulphides are pyrite, chalcopyrite, molybdenite and bismuthinite, with gold, argentian-gold, tetrahedrite, arsenopyrite and other sulphides. Visible gold is rare, occurring mostly as <20µm inclusions in sulphide composites, particularly in tetrahedrite. Gold is rarely associated with pyrite or with silicates or carbonates.

Until its closure in 1963 the original mine produced - 7.254 Mt @ 11.3 g/t Au for 77.8 t Au.
Open pit mining from 1984 to 1989 yielded - 2.34 Mt @ 3.54 g/t Au for 8.294 t Au
Proven+probable reserves in 1989 were 4.28 Mt @ 3.4 g/t Au.

Proved + probable reserves at Gwalia and Gwalia Deeps in June 2007 (Saint Barbara Ltd) were: 5.6 Mt @ 9.4 g/t Au
Measured + indicated + inferred resources for Gwalia Deeps at the same date were: 12.37 Mt @ 8.9 g/t Au.

In June 2007, the Tower Hill deposit had measured + indicated + inferred resources of: 18.43 Mt @ 2.4 g/t Au.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2006.    
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
 References to this deposit in the PGC Literature Service Collection: Want any of these papers ? Click Here
Kalnejais J  1990 - Sons of Gwalia Gold deposit, Leonora: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 353-355
Schiller J C and Hanna J P,  1990 - Tower Hill gold deposit, Leonora: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 349-352


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