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Plutonic

Western Australia, WA, Australia

Main commodities: Au
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The Plutonic gold deposit is located in the SW corner of the Archaean Plutonic Well Greenstone Belt of the Marymia Inlier to the north of the main Yilgarn Craton. It is situated ~175 km NNW of Wiluna and 180 km NE of Meekatharra in Western Australia (#Location: 25° 18' 59"S, 119° 26' 42"E)

The greenstone belt was first recognised by International Nickel Australia during a nickel exploration program from 1969 to 1977. After taking an option on an existing gold exploration title over the area in in 1986, Great Central Mines discovered the Plutonic deposit in 1988. In 1989, the project was sold to Plutonic Resources. Open pit operations commenced in 1990, followed by underground mining from 1995. By early 1996 the mine had produced 31 t (1 Moz) of gold. The open pit ceased production in 2007. Plutonic Resources were subsequently acquired by the Homestake Mining Company in 1998. The project passed to Barrick Gold at the end of 2001 when that company merged with Homestake. Barrick sold the mine to Northern Star Resources Limited in February 2014. Canadian based Superior Gold Inc purchased the operation in October 2016. Underground mining has continued throughout this period.

The 50 x 10 km, ENE-WSW trending, 2740 to 2660 Ma Plutonic Well Greenstone Belt lies within the Marymia Inlier, a discrete, fault bounded gneiss-greenstone-granitoid domain. This inlier is surrounded by Palaeoproterozoic sedimentary rocks of the southern Capricorn Orogen to the north of the main Yilgarn Craton. It comprises mafic, ultra-mafic, sedimentary and felsic rocks. Banded iron formation (BIF) units are found in the north-eastern section of the belt. All of these rocks have been metamorphosed to upper greenschist, and locally to amphibolite facies.

The gold ores are hosted by metabasalt in an Archaean greenstone sequence made up of mainly low potassium tholeiitic meta-basalt, ultra-mafics (meta-peridotite and meta-komatiitic basalt), with minor siliceous, graphitic and sulphidic meta-sediments. The sequence is locally cross-cut by felsic porphyries and Proterozoic dolerite dykes.

In more detail, the host sequence is as follows, from the base (after Vickery et al. 1998):
Basal Meta-sediments - the lowest recognised unit, comprising a fine grained pelitic metasedimentary sequence of unknown thickness, consisting of finely laminated siltstone and minor graphitic shale, which are biotite rich, pyrrhotitic and commonly garnetiferous. Minor intercalations of amphibolite and felsic porphyry are common, and the sequence is intruded by Proterozoic dolerite dykes.
Basal Mafic Unit, an ~150 m thick package which overlies the Basal Meta-sediment and is characterised by an intense, pervasive layer-parallel foliation with concordant carbonate and quartz-carbonate veining. Intense silica, epidote and biotite alteration occurs locally in foliation parallel bands. The mineral assemblage of the mafic unit includes hornblende, tremolite, actinolite, plagioclase and garnet. Sulphides that typically occur as fine disseminations and in sulphide-rich veins and breccia zones, include pyrrhotite, pyrite, chalcopyrite, galena and sphalerite.
Mine Mafic Unit, which comprises a sequence of upper greenschist to lower amphibolite facies basaltic flows of variable thickness, sandwiched between the Hanging Wall and Footwall ultramafic units. The mineral assemblage includes hornblende, actinolite, plagioclase and garnet with alteration assemblages of chlorite, biotite, carbonate, epidote and albite. Sulphides include pyrrhotite, pyrite and arsenopyrite and rare sphalerite and galena. The constituent units comprise:
  - Footwall Ultramafic Unit - a thick sequence (~200 m) of metakomatiitic basalt, metaperidotite and metapyroxenite flows. The mineral assemblages include chlorite, talc, carbonate and tremolite with accessory actinolite and magnetite. Relict olivine is evident in thin section, partially to completely converted to serpentinite. Serpentinisation is common in the metakomatiite flows but is restricted to zones of <5 m in thickness. Carbonate veining is abundant in the talc-rich meta-komatiites and spinifex textures are commonly folded and contorted. This and the Hanging wall ultramafic unit exhibit evidence of intense, predominantly ductile deformation. Folding, crenulation, boudinaging of carbonate veining and locally developed ultramafic melanges are common.
  - Lower Shale - which is <10 m thick and comprises fine grained, well laminated, very siliceous, graphitic and sulphidic shale that is strongly deformed, with intrafolial folds, and contains minor garnet porphyroblasts. Small scale faulting and offsets are common and unit contacts are typically sheared. Sulphides include pyrrhotite, pyrite and sphalerite.
  - Lower Mine Mafic Unit - which is <100 m thick and is characterised by large porphyroblastic garnets with associated carbonate and pyrrhotite alteration, and by coarse grained recrystallised amphiboles (typically hornblende). This unit includes the thin Middle Shale, which is as for the Lower Shale;
  - Double Shale - as for the Lower Shale;
  - Mafic/Ultramafic Unit - an approximately 20 to 30 m thick band of mafic and ultramafic rocks similar to those described above;
  - Banded Zone - a 0.5 to 1 m thick banded cherty horizon that may represent an altered interflow tuffaceous felsic unit;
  - Upper Mine Mafic Unit - composed of low potassium tholeiitic basalts that are massive to moderately foliated and comprise distinct (though unmappable) volcanic flows;
  - Hornblende Amphibolite - which is the uppermost 5 to 30 m of the Mine Mafic Unit, and is composed of a distinctive coarse grained hornblende-rich amphibolite which typically has a high-magnesium basalt composition;
  - Hanging wall Ultramafic Unit - which is generally <100 m thick and is very similar to the Footwall Ultramafic Unit.
Overthrust Mafic Unit a tectonically placed unit of variable thickness, comprising upper greenschist to lower amphibolite facies metabasalt interbedded with thin zones of siliceous and graphitic sediment and rare ultramafic rocks. It is medium grained and massive to moderately foliated, consisting of an assemblage of hornblende and interstitial plagioclase and quartz with alteration characterised by carbonate, biotite, chlorite, epidote and silica. It has undergone moderate to strong pervasive chlorite and carbonate alteration throughout, and contains zones of locally intense biotite and rare siliceous alteration. Carbonate veins are generally folded and contorted, usually parallel to the foliation, with dominant pyrrhotite, chalcopyrite and pyrite.
Overthrust Archaean Granite, which overlies the Overthrust mafic unit to the NW of the mine. Predominantly of coarse grained, weakly foliated to massive biotite monzogranite (Gee, 1987). The contact with the underlying greenstone package is strongly sheared and a narrow mylonitic zone is developed.

Mineralisation at Plutonic is exclusively hosted by the Mine Mafic Unit, composed, as described above, of basaltic flows sandwiched between hangingwall and footwall ultramafic units. The Mine Mafic Package was deformed as a relatively brittle layer, bounded by the two ductile ultra-mafics during a period of thrusting producing layer parallel shears on the contacts, and localised linking shears within the more brittle Mine Mafic Unit. The main Plutonic deposit appears to occur as strings of lode swarms forming semi-continuous, irregular ore zones within the Mine Mafic Unit that extends for over 2 km to the NNE and >1.5 km SW from the open pit orebody over widths of up to several hundred metres (inferred from diagrams in Gazley et al. 2017).

Gold in the main Plutonic deposit occurs as a series of discrete sub-parallel, NW trending, multiple lodes dipping at 45 to 50°NE. The lodes vary from 1 to 10 m in thickness and single lodes of strike continuity of several hundred metres are common. Typically they have strongly albitised cores with 5 to 10% sulphides, predominantly arsenopyrite and pyrrhotite, with lesser pyrite, chalcopyrite and sphalerite (Vickery et al. 1998). These lodes commonly have a gradational phlogopite selvage with subordinate calcite, chlorite and amphibole. Gold, predominantly as native metal is disseminated throughout the groundmass or within the silica gangue, or occasionally within the arsenopyrite lattice but rarely within pyrite (Vickery et al. 1998).

Gazleyet al. (2017) describe the main mineralisation style at Plutonic as typically comprising thin (1 to 3 m) thick lodes of quartz-biotite-amphibole-epidote-arsenopyrite-pyrrhotite ±titanite ±carbonate ±chalcopyrite ±gold. Where well developed, these Au-bearing zones tend to be parallel to subparallel to the stratigraphy as marked by sedimentary interbeds and the foliation. The metasedimentary interbeds tend to be well mineralised occurring as lodes with a cherty appearance. Visual alteration halos are typically narrow, although K- and S-rich halos may extend for tens of metres into the wall rock. The distribution of gold is not apparently structurally controlled, but appears to be localised by rheology variations in the primary stratigraphy with late stage brittle structures acting as conduits for auriferous fluids. More quartz-rich units have been preferentially fractured with small Au-on;y veins developed perpendicular to the fabric of the rock. Geochronology data indicates multiple stages of gold mineralisation, with lead isotope data from Au-associated sulphides suggeting gold deposition/remobilisation occurred at i). ~2630 (at the end of the ~2660 to 2630 Ma period of amphibolite facies metamorphism at ≥8±2 kb and ~600±50°C; Vieilreicher and McNaughton, 2002), ii). 2300 to 2100, iii). during the 2005 to 1950 Ma Glenburgh and iv). 1820 to 1770 Ma Capricorn orogenies and finally v). at 1730 to 1660 Ma. Visible gold is late stage, probably associated with the Capricorn Orogeny or final event (Gazley et al. 2017).

The main Plutonic deposit described above was the first of the more than eight smaller Plutonic group deposits in the greenstone belt. Others include the Area 4, Perch, Salmon, Zone 124, Zone 550, Zone 019 and Trout deposits within 4 km to the east and north of Plutonic.

Reserve and resource figures at 31 December 2007, as quoted by Barrick, 2008, were:
  Proved + probable reserves - 12.35 Mt @ 4.8 g/t Au, for 57 t of contained gold,
  Measured + indicated resources - 19.19 Mt @ 4.6 g/t Au, for 85 t of contained gold,
    Production in 2007 - 6.47 t of recovered Au.

Remaining Ore Reserve and Mineral Resources at 31 December 2018, as quoted by Superior Gold, 2018, were:
  Proved + Probable reserves - 2.26 Mt @ 4.73 g/t Au, for 10.7 t of contained gold,
  Measured + Indicated resources - 4.73 Mt @ 6.59 g/t Au, for 31.2 t of contained gold, (inclusive of reserves)
  Inferred resources - 10.45 Mt @ 4.90 g/t Au, for 51.2 t of contained gold.

Cumulative production from the main Plutonic deposit to the end of 2016 totalled ~78 t from the open pit, and 87 t from underground (Gazley et al., 2017). At the same time, the estimated Mineral Resource was 45 t of contained gold, indicating a minimum endowment of 210 t of gold. The 2018 resources above suggest this may be as high as 250 t of gold.

The most recent source geological information used to prepare this summary was dated: 2007.    
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:
Gazley, M.F., Duclaux, G., Pearce, M.A., Fisher, L.A., du Plessis, E., Murray, S. and Hough, R.M.,  2017 - Plutonic goldfield: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The AusIMM, Melbourne,   Mono 32, pp. 307-312.
Vickery N M, Buckley P M, Kellett R J  1998 - Plutonic gold deposit: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne   Mono 22 pp 71-79


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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