Sons of Gwalia, Gwalia, Tower Hill, King of the Hills
Western Australia, WA, Australia
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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 (#Location: 28° 55' 10"S, 121° 20' 4"E).
The Gwalia deposit is on the western margin of the broad, north-south trending Keith-Kilkenny Tectonic Zone, which separates structurally complex rocks to the southwest from
less deformed sequences to the east. The Mount George Lineament and Kilkenny Fault form the western and eastern boundaries of the Keith-Kilkenny High-strain Zone respectively. The package to the SW comprises a strongly deformed, partly migmatitic granitic batholith. Greenstones adjoining the batholith to the east and north are amphibolite-facies metamorphosed mafic igneous rocks. East of the Mount George Lineament, within the Keith-Kilkenny Tectonic Zone, the geology changes to a mixture of felsic and mafic igneous rocks with abundant intercalated sandstone, shale and conglomerate, intruded by a largely undeformed discordant granite batholith in the northeast. The western margin of the Keith-Kilkenny Zone may represent a reactivated tectonic basin margin separating sequences to the west and east (Williams, 1999).
Four major regional and two minor episodes of deformation are recognised in the area. The earliest episode ('De') produced gently dipping mylonite, orthogneiss and schist, with usually north-directed movement directions. Sedimentary basins and felsic sequences are bounded by regional 'De' faults. The second event (D1) was also largely bedding-parallel, and produced structural inversion of the previously deformed rocks, and is correlated with regional north-south thrusting known throughout the Eastern Goldfields Province. The third episode (D2) resulted in a widespread north to NW-striking crenulation of the early schistosity and long wavelength open upright regional folds, affecting the early orthogneiss. Regional NW-striking strike-slip faults, eg., the Kilkenny Fault on the east margin of the Keith-Kilkenny Zone, were reactivated as sinistral faults during the third (D2) and fourth (D3) deformation events. ENE striking (D4) faults cross-cut all the earlier structures, whilst in several localities a SE striking widely-spaced, commonly gently dipping crenulation, is evidence of a sixth episode (D5). The well-exposed gently-dipping extensional faults are evidence of an extensional structural event which exerted regionally important structural controls on the development of the stratigraphy and architecture of the greenstone belts (Williams, 1999).
East of the Mount George Lineament, the region is dominated by north-northwesterly-striking faults and folds in a fold-thrust belt.
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 volcanic rocks, associated intrusive rocks and black-shale chert units. The Sons of Gwalia and Tower Hill deposits are within the lower part of Association 1 (Kalnejais, 1990).
The Sons of Gwalia deposits lie within the Sons of Gwalia Shear Zone (De or D1), a distinctive arcuate high-grade shear which broadly follows the boundary of the Auckland Pluton of the Raeside Batholith, and is immediately west of the The Mount George Lineament and main Keith-Kilkenny Tectonic Zone. The Auckland Pluton comprises medium-grained, equigranular to slightly feldspar-phyric, fairly dark, biotite granodiorite, with some pegmatitic patches (Williams, 1999).
Williams et al. (1989) showed deformation in the shear zone was earlier than the upright folding, which is in turn earlier than NNW striking strike-slip faults. Rocks within the shear zone are mainly talc-chlorite and chlorite-sericite-quartz (±pyrite) schist, with several lenses of undeformed basalt, gabbro and amphibolite in the Sons of Gwalia area, and amphibolite and granite-gneiss in the western part of the domain. Carbonate alteration is common, and numerous concordant lenses and dykes of granitoid and porphyry are present (Williams, 1999).
At the Gwalia mine, gold occurs within a north-south striking ductile mylonite zone, within a sequence of greenschist facies Archaean pillow metabasalts and minor metasediments which are variably sheared. This zone occurs as a 40 to 45°E dipping, 120 to 150 m thick zone of intensely sheared mafic volcanic rocks, comprising narrow mica rich bands (muscovite and lesser chlorite, often with associated tourmaline) and wider zones of granoblastic quartz-carbonate as veins parallel to foliation, boudins, and locally as massive quartz-carbonate. These are the Mine Schists and are intruded by an adamellite batholith to the west. On the western, footwall side of the Mine schists, there is a sequence of dolerite and ultramafic schists that are intruded to the west by the Raeside Batholith. The eastern or hanging wall of the Mine Schist is occupied by 30 to 80 m of tholeiitic basalt. This sequence is composed of pillowed basalt, sepataed by flows, or intrusions of gabbro and thin interflow volcaniclastic sedimentary rocks. This pile is overlain by bimodal volcanic rocks, between basalt and periodotite flows (Kalnejais, 1990).
Gold mineralisation is present as banded, partially silicified sericite-muscovite-chlorite schist with disseminated sulphides (pyrite is the dominant sulphide, with traces of chalcopyrite and pyrrhotite), and sulphide bearing quartz lenses and stringers in the higher grade zones. Individual ore shoots/lodes mined underground are high grade and lenticular. The Gwalia lode system, comprises a number of en echelon, moderately east dipping foliation parallel lodes which occur within strongly potassic altered mafic rocks and extend over a cumulative strike length of ~500 m and to a vertical depth of at least 2000 m. The average dip of the lode system and foliation is 45° towards 106°, whilst the average pitch of the orebody within the foliation is 70° to the south (Finucane 1965; Kalnejais 1990). Five lodes have been identified, the Main Lode, West Lode, South Gwalia Series, Hangingwall Series and Southwest branch. Individual lodes average 1.8 to 2.4 m in thickness. Within this lode system, individual lodes averaged 1.8 to 2.4 m in thickness. A feature of the Sons of Gwalia mineralisation is the change in strike of the lithological boundaries in the vicinity of mineralisation. 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
Proved + 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.
Ore reserve and mineral resource estimates in June 2013 (St Barbara Ltd ASX release 22 August, 2013) were:
Measured + indicated + inferred resource
Gwalia Deeps - 14.41 Mt @ 8.2 g/t Au (2.5 g/t Au cut-off)
King of the Hills - 11.843 Mt @ 6.4 g/t Au (3.0 g/t Au cut-off)
Tower Hill - 5.093 Mt @ 3.8 g/t Au (2.5 g/t Au cut-off)
Kailis - 1.075 Mt @ 3.3 g/t Au (0.8 g/t Au cut-off)
Proved + probable reserves (included in resources)
Gwalia Deeps - 6.57 Mt @ 8.3 g/t Au (2.5 g/t Au cut-off)
King of the Hills - 2.572 Mt @ 3.7 g/t Au (3.0 g/t Au cut-off)
Tower Hill - 0.496 Mt @ 4.3 g/t Au (2.5 g/t Au cut-off).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 3/12/2014
This description is a summary from published sources, the chief of which are listed below.
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Gwalia and Tower Hill (2 km NNW)
References to this deposit in the PGC Literature Collection:
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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