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Yilgarn Star - Southern Cross Belt

Western Australia, WA, Australia

Main commodities: Au
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The Yilgarn Star gold deposit lies within the Archaean Southern Cross Greenstone Belt, Southern Cross Domain, in the Youanmi Terrane of the Yilgarn Craton, some 235 km WSW of Kalgoorlie, 375 km east of Perth and 45 km SE of the town of Southern Cross, Western Australia.

  No historic working exist over the Yilgarn Star deposit, which was discovered by follow-up of a soil geochemical anomaly. Open pit mining began in December 1991 and was completed in October 1995, to a vertical depth of 130 m, having extracted 3.5 Mt @ 3.3 g/t Au for 11.8 t of contained gold. Underground production commenced in March 1995. Production from the underground mine to January 2001 totalled 3.32 Mt at 4.83 g/t Au for 16.2 t of contained gold. Remaining Ore Reserves, at the same time were 1.02 Mt at 7.12 g/t Au for 7.15 t of gold. The total gold endowment of the Yilgarn Star deposit, as known in 2001 was 35 t. By 2009, the St Barbara 2009 Annual Report quotes a remaining resource of 2.5 t of contained gold.

Regional Setting

  The Southern Cross and other belts at the eastern margin of the Youanmi Terrane include the submarine Luke Creek basalt-komatiite-graywacke succession, correlated on a regional scale by the Golconda Iron Formation (Watkins and Hickman, 1990). The minimum age of this succession is 3023±10 Ma, defined by the zircon U-Pb age of an intrusive feldspar porphyry (Nelson, 1999). In the Marda Greenstone Belt, ~50 km to the NE of the Southern Cross Belt, folded Luke Creek greenstones are overlain above an angular unconformity and basal transgressive conglomerate by the Marda volcano-sedimentary complex. This conglomerate fines upward into arkoses and siltstones, overlain by shallow-water and subaerial andesites, dacites, and rhyolites (Hallberg et al., 1976; Chin and Smith, 1983; Walker and Blight, 1983). Rhyolitic ignimbrite from this package has been dated at 2734±3 Ma (U-Pb zircon; Nelson, 2001), correlatable with the lowermost part of the marine Kalgoorlie-Kambalda succession to the east (Mueller et al., 2004). The mafic-ultramafic units containing banded iron formations and overlying predominantly sedimentary sequence of the Southern Cross Grenstone Belt is taken to belong to the same stratigraphic package.
  The ~300 km long, NNW-SSE aligned Southern Cross Greenstone Belt has been subdivided into a number of generally NNW-SSE elongated fault bounded domains that are largely defined on structural style. The greenstones of the belt are generally conformably arranged around and between the margins of two parellel strings of NNW-SSE elongated granitoid cored domes (e.g. the Ghooli and Parker domes to the north and south of Yilgarn Star respectively; and the Rankin Dome to the west of the Ghooli Dome), although there is also local discordance between granitoid domains and greenstones. Gravity data (Dentith et al., 1992) suggest that the Rankin Dome granites are relatively thin, and are underlain by greenstones. As such, the dome is interpreted be a thin slice of granite overlying greenstones of the adjacent greenstone domains above a thrust or detachment (Witt et al., 2001). Gravity data also suggests the total thickness of preserved greenstone sequences along the western flank of the Ghooli Dome between Southern Cross and Marvel Loch is only of the order of 1 km, increasing to about 4 km in the Banker Saddle, the saddle shaped structure between the Ghooli and Parker Domes, and within which Yilgarn Star and Nevoria are hosted (House, 1991).
  The commonly foliated metagranites of the Ghooli Dome have emplacement ages ranging from 2.72 to 2.69 Ga (U-Pb SHRIMP zircon; Qiu et al., 1999), whilst most of the surrounding batholithic granitoids have been dated in the range 2.78 to 2.63 Ga (SHRIMP zircon; Qiu et al., 1999).
  Metamorphic grades vary from domain to domain, ranging from upper greenschist to upper amphibolite facies. Peak metamorphic temperatures have been estimated at 590 and 540°C at the Marvel Loch deposit and at Southern Cross, 12 and 40 km NW of Yilgarn Star respectively (Dalstra and Ridley, 1995). At the Nevoria deposit, 5 km NW of Yilgarn Star, Mueller et al. (2004) estimate post-metamorphism (~90 m.y. after peak metamorphism) mineralisation, based on the mineralogy, took place at temperatures of 610±50°C and pressures of 400±100 MPa (14 km burial depth). However, the garnet-cordierite-andalusite assemblage in pelites at Kelly Star, ~3 km NNW of Yilgarn Star, indicates temperatures of >620°C and pressures of <2 kbar (Witt et al., 2001). Similarly, the widespread occurrence of andalusite in pelitic assemblages throughout the Southern Cross greenstones is indicative of low-pressure metamorphism (Witt et al., 2001). These sedimentary sequences are higher in the stratigraphy of the belt and further removed from the granite dome-greenstone contacts.
  The stratigraphy of the Southern Cross Greenstone Belt near Yilgarn Star has been masked by subsequent deformation and an absence of rocks that can be reliably dated. Along the southwestern margin of the Ghooli Dome, a banded iron-formation bearing unit is underlain by altered metabasalt/amphibolite, which may correlate with a massive sulphide band ay Yilgarn Star that similarly overlies altered mafic rocks.
  For more information on the Southern Cross Belt and its deposits see also the Transvaal, Nevoria, Marvel Loch, Bounty, Fraser and Copperhead records.

Geology

  The Yilgarn Star gold deposit is located toward the eastern edge of the Banker Saddle that separates the Ghooli and Parker domes to the NNW and SSE respectively. The deposit is situated on the southwestern margin of a NW-striking tremolitic amphibole-rich unit, within a highly strained band of that schist unit, locally referred to as the Yilgarn Star Shear Zone. This shear zone varies in strike from NW to WNW, and dips at 70 to 80°SW reflecting variation in thickness of the tremolite schist buttressing the surrounding schists. The Yilgarn Star gold deposit is in the WNW- trending portion of the shear zone. The footwall of the shear zone is occupied by tremolitic amphibole-rich unit whilst the hanging wall comprises metasedimentary rocks and a complexly banded, altered mafic schist, known locally as the Lower Banded Schist. The main units may be summarised as follows, from the footwall to hanging wall.
Footwall Tremolitic Schist, which has a pod like shape, tapering to the NW and SE. Outcrops near the deposit comprise banded to decussate rocks composed of 80 to 98 vol.% tremolitic amphibole, with minor plagioclase, accessory rutile and opaque oxide minerals. Geochemical analysis of similar rock between Yilgarn Star and Harris Find indicate a high-Mg basalt composition. On the northeastern wall of the Yilgarn Star open pit it contains up to 50% chlorite and up to 10% amoeboid olivine (forsterite) porphyroblasts suggesting an ultramafic precursor. It is considered possible that the 300 to 400 m thick main tremolitic pod at Yilgarn Star is a relatively magnesian feeder channel of a komatiitic flow complex, whereas the thinner tremolitic schists along strike represent less magnesian overbank deposits (Witt et al., 2001, and cf. Hill et al., 1990; Lesher, 1989).
Lower Banded Schist, which comprises a wedge shaped body of banded biotite- and amphibole-rich schist that forms the hanging wall to the Yilgarn Star Shear Zone in the central and southern parts of the deposit. Petrographic and whole-rock geochemical data has been interpreted to indicate it represents an altered mafic unit with minor thin, interleaved slices of ultramafic rock (Bodycoat, 1999). These schists are medium to coarse grained, and banded on a scale of several centimetres to about a metre. Individual bands are composed of the follow assemblages: i). pink-brown bands of quartz-cordierite-biotite(-pyrrhotite); ii). pale grey bands of cummingtonite-quartz-plagioclase (An94); iii). garnet (andraditic)-amphibole; and iv). dark grey-green bands of cordierite-anthophyllite-plagioclase-quartz-ilmenite. The first two are dominant. The banding is accentuated by deformed calc-silicate veins, typically zoned from cores that are coarse-grained and diopside-rich, margins that are actinolitic amphibole-rich. The actinolitic-diopside and amphibole of these veins is coarse grainsize (diopside >1 cm). These veins may carry minor quartz, calcite, plagioclase, titanite and pyrrhotite (Witt et al., 2001).
  The Lower Banded Schist is generally capped by a 0.1 to 8 m thick massive pyrrhotite horizon. Locally this layer is underlain by a 5 m thick coarse-grained diopside-garnet unit (Witt et al., 2001).
Metasedimentary Unit, which occurs in the hanging wall of the mineralisation at Yilgarn Star is predominantly composed of biotite-rich micaceous schist. In the deposit area it is in faulted contact with both the footwall tremolitic schist and the lower banded schist. It is a characteristically a dark grey to black schist composed of quartz-andalusite-biotite with accessory graphite and tourmaline with a linear fabric defined by elongate aggregates of biotite that average ~0.4 mm in length. Foliation defined by the micas wrap around poikiloblasts of andalusite and cordierite which average ~0.6 mm in width. Internal inclusion trails of quartz and biotite are generally parallel to the external fabric. Between 5 and 15% disseminated pyrrhotite is aligned parallel to the main foliation. Thin 2 to 3 mm stringers of pyrrhotite make up between 1 and 3% of the rock.

  The Yilgarn Star Shear Zone strikes WNW within the deposit area and NW-SE to the NW and SE, whilst the greenstones contain a pervasive, peak-metamorphic foliation that parallels the dominant greenstone trends and dips subvertically to the west. In the hangingwall metasedimentary unit, mesoscopic folds, defined by bedding, are oriented NNW to north, and dip steeply west. The axial plane foliation to folds in the metasedimentary unit, and the footwall foliation, are both oblique to the Yilgarn Star Shear Zone. A late sinistral component of strike-slip displacement was imposed on the NW-oriented Yilgarn Star Shear Zone, with the WNW section of the structure representing a dilational jog formed during peak metamorphic deformation (Mason and Archibald, 1997).

Mineralisation

  The Yilgarn Star gold deposit is ~1.2 km long. It varies from <1 to 30 m in width and dips steeply SSW to a vertical depth of 800 m, where it is cut by granitic pegmatite and pegmatitic granite. The gold occurs in lodes that are concentrated along the faulted contact between the Footwall Tremolitic Schist and the hanging wall Lower Banded Schist or Metasedimentary Unit, but also occur within the latter. High-grade ore shoots plunge at ~80°NW, colinear with late fold hinges. The Yilgarn Star Shear Zone is cut by several 300 to 320° sinistral faults, with apparent local gold enrichment at some of the intersection points (Witt et al., 2001).
  Two main styles of gold mineralisation are recognised at Yilgarn Star, primarily controlled by the nature of the host lithology. Around 50% of the gold is hosted by metasedimentary rock, while the remaining half is in ultramafic rocks (Witt et al., 2001).
Metasediment-hosted mineralisation occurs within laminated quartz-biotite-pyrrhotite veins with alteration selvages characterised by an increase in the biotite content from a background of 10 to 30%, to 50% or more. In some locations, biotite-rich alteration selvages contain small (1 to 3 mm) garnet porphyroblasts and disseminated pyrrhotite. These veins are oriented oblique to the trend of the Yilgarn Star Shear Zone and are commonly deformed by the late folding. Anastomosing zones of intense ductile deformation, preferentially located in pelitic beds, overprint these veins. These pelitic beds are relatively narrow and poorly developed, enclosed by the dominant psammitic rocks (Mason and Archibald, 1997). High-strain zones are spatially associated with local changes in the strike of the Yilgarn Star Shear Zone (Mason and Archibald, 1997). They are characterised by intense foliation and retrogression of biotite to muscovite. Fuchsite occurs where high-strain zones are within, or adjacent to, thin units of interleaved ultramafic rock. Within these high strain zones, mineralised veins are strongly deformed with boudinaged and rootless folded veins being common. The bulk of the metasedimentary rock-hosted gold at Yilgarn Star is located in these retrograde, high-strain zones (T. Davis, as recorded by Witt et al., 2001).
Ultramafic-hosted mineralisation is associated with quartz-diopside-pyrrhotite veining. Diopside is typically found as coarsely crystalline grains up to several cm across. Carbonate and forsterite as fine- to medium-grained (0.1 to 3 mm) granular intergrowths are found within some of these veins, and some of thes contain very high grade, visible gold. They are encompassed by an alteration envelope of chlorite-talc-carbonate with irregular, foliation-parallel stringers and bands of carbonate-forsterite. The carbonate is generally calcite but, locally is dolomite. The veins show much the same relationships to foliations and folds as those in the hangingwall metasedimentary rocks (Witt et al., 2001).

The Yilgarn Star deposit comprises three separate lode systems, all of which are variably developed in metasedimentary and ultramafic rocks.
Northern Lode System - a tabular zone, about 150 m long and 3 to 7 m thick. Individual lodes occur up to as much as 10 m into the footwall tremolitic schist. They carry 4 to 8 g/t Au, with the highest grades associated with quartz-diopside-calcite and forsterite-calcite-magnetite veins. Both assemblages also contain pyrrhotite. Quartz-pyrrhotite(-biotite) veins in the metasedimentary unit also carry high-grade ≤50 g/t Au with biotite enrichment in the altered metasedimentary wallrocks (Witt et al., 2001).
Central (Herringbone) Lodes - carrying 3.5 to 12 g/t Au and predominantly composed of quartz-pyrrhotite-biotite veins that occur in metasedimentary rocks, up to 30 m into the hangingwall of the Yilgarn Star Shear Zone. These veins are associated with intense shearing, preferentially developed in pelitic beds, anastomosing around psammitic interbeds and lenses. They are typically parallel the axial planes of late folds. Gold is also found in small, brittle faults that are 2 to 40 m long, characterised by sinuous milky and grey quartz veins and stringers. Tremolitic ultramafic-hosted mineralisation occurs along the faulted contact with metasedimentary rocks or with the Lower Banded Schist, with gold contained within forsterite-carbonate-magnetite-pyrrhotite veins. The faulted contact between the Lower Banded Schist and the Tremolitic Ultramafic Schist afforded a dilatent zone characterized by massive bucky white quartz. Gold is found within the veins and in association with chlorite and diopside in veinlets and ultramafic wallrocks (Witt et al., 2001).
Southern Lodes - which are preferentially developed within the Footwall Tremolitic Schist, extending up to 15 m below the Yilgarn Star Shear Zone. These lodes have a strike length of ~90 m, width of up to 20 m and extend to a depth of 370 m below surface. Gold occurs as coarse (>1 mm) grains and as fine dustings within, and proximal to, forsterite-carbonate-magnetite-pyrrhotite veins which contain both calcite and dolomite. This lode has yielded >5.3 t of gold at a grade of 97 g/t Au, based on mill recovery (Witt et al., 2001).

Although only gold is recovered at Yilgarn Star, minor disseminated galena and sphalerite occur in some of the quartz lodes with assays of up to 2.45% Zn, 0.68% Pb, 265 ppm Ag, 52 ppm Bi and 265 ppm Sb. However, these elements are more typically 50 to 300 ppm Zn, 30 to 200 ppm Pb, 0.5 to 2 ppm Ag, 0.5 to 2.5 ppm Bi, and 1 to 10 ppm Sb (Witt et al., 2001).

Production and Resources

Production and resources are as detailed in the history paragraph near the top of this summary.

The most recent source geological information used to prepare this summary was dated: 2001.    
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:
Witt, W.K., Drabble, M. and Bodycoat, F.M.,  2001 - Yilgarn Star gold deposit, Southern Cross greenstone belt, Western Australia: geological setting and characteristics of an amphibolite-facies orogenic gold deposit: in Hagemann, S.G. and Neumayr, P., (Eds.) 2001 Proceedings, 4th International Archean Symposium, Perth, W.A., Geological Survey of Western Australia    Record 2001/17 pp. 45-62.


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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