Nevoria - Southern Cross Belt

Western Australia, WA, Australia

Main commodities: Au
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The Nevoria gold deposits are ~39 and 11 km SE of Southern Cross and Marvel Loch respectively, within the Southern Cross Greenstone Belt, in the Southern Cross Domain of the Youanmi Terrane, in the Yilgarn Craton, Western Australia (#Location: 31° 30' 26"S, 119° 35' 12"E).

Mining in the Nevoria area is recorded from 1905, although the leases were only consolidated as the Nevoria Gold Mine in 1933. Little production was undertaken in the remainder of the 1930s and 1940s as funds were sought to develop the mine. A mine was operated between 1956 and 1966 by a subsidiary of Western Mining Corporation. It was subsequently operated by Sons of Gwalia Limited in the 1980s until that company went into liquidation, before being purchased by St Barbara Limited in 2005 who continued mining until 2009. The deposit was subsequently onsold to Minjar Gold Pty Ltd in 2013. Including small outlier pits such as Kurrajong, the Nevoria mine produced 55 033 t of underground oxide ore (from 1907 to 1936), 322 345 t of sulphide ore (from 1952 to 1963) and 3.12 Mt of mixed open-pit ore (from 1981 to 1996) at recovered grades of 7.34, 6.18 and 3.04 g/t Au respectively (Mueller et al., 2004). Production figures for the period 1996 to 2009 have not been encountered.

The Nevoria deposit is hosted by the Luke Creek Greenstone succession within the Banker Saddle, the saddle shaped structure between the granitic orthogneiss Ghooli and Parker Domes to the NNW and SSE respectively. For details of the regional setting and geology of the Southern Cross Greenstone Belt, the distribution of the greenstone sequence, Banker Saddle and granitic domes, see the Yilgarn Star record. Yilgarn Star is ~5 km to the ESE of Nevoria. For more information on the Southern Cross Belt and its deposits see also the Transvaal, Yilgarn Star, Marvel Loch, Bounty, Fraser and Copperhead records.

Magnetic and gravity data indicate that the Banker Saddle separating the two granitic domes is 13 km wide and 2.5 to 4.0 km deep (Rout et al., 1995), and contains the most complete preserved succession of Luke Creek Greenstones in the Southern Cross belt. This succession is dated at >2934±7 Ma, the age of a crosscutting quartz porphyry sill in the nearby Southern Star mine (U-Pb zircon; Mueller and McNaughton, 2000). Elsewhere, rocks belonging to this succession have a minimum age of 3023±10 Ma, defined by the age of an intrusive feldspar porphyry (zircon U-Pb; Nelson, 1999).

The Nevoria orebodies are hosted within part of a 4.5 km zone of altered iron formation which occurs toward the top of the Luke Creek Greenstone succession. The deposit is on the southern limb of the subsequently cross-folded, doubly plunging, NE to WNW trending Nevoria Anticline, located in the northern half of the Banker Saddle. This iron formation is correlated with the regional Golconda Iron Formation that is found throughout the Southern Cross Belt and Marda-Diemals Greenstone Belt ~150 km to the north. The Nevoria Anticline is defined by two to three beds of grunerite-quartz±magnetite iron formation, and is refolded around the southern curved contact of the 2775 to 2724 Ma granitoid of the Ghooli Dome. Bedding is subvertical on the northern limb, but on the southern limb dips at 75 to 85° SW or SE. The iron formations are underlain by tholeiitic amphibolites (8 wt.% MgO) and overlain by a mixed volcanic sequence, predominantly composed of metakomatiites (21 wt.% MgO). The volcanic rocks are overlain by graphitic andalusite-cordierite gneiss (interpreted to represent a graded-bedding greywacke), exposed in isoclinal folds west of the Nevoria mine (Mueller et al., 2004). Elsewhere in the greenstone belt, similar sedimentary rocks are shown to unconformably overlie the volcanic sequence.

At Nevoria, the iron formation comprises three east-west trending, steeply south dipping, grunerite-quartz banded iron formation, that vary from 0.2 to 20 m (averaging 5 m) in thickness. The three bands merge into two east of the ENE-WSW trending Main fault. Where least altered, the iron formations are dark green-grey, fine-grained (0.1 to 1 mm) with 1 to 20 mm thick bands of alternating grunerite and quartz. Quartz banded iron formation (30 to 40% quartz) is interlayered with up to 1 m thick beds of massive grunerite (>90 vol.% grunerite). Quartz mesobands have a mosaic texture with abundant microbands of grunerite. Graphite is rare, whilst minor (1 to 10%) magnetite is present as both disseminations and as 0.1 to 1 mm microbands to 10 to 20 cm thick beds. Magnetite grains are devoid of ilmenite exsolution lamellae, and the least altered iron formation only contains ~0.04% Ti2/span>. The iron formation at Nevoria is characterised by high Fe, high Si2 and low Al2O3 typical of other iron formations in this part of the Yilgarn Craton (Mueller et al., 2004).

Tholeiitic amphibolites, similar to the rocks found below the iron formations, are the main volcanic rocks interbedded within and between the iron formations. They are composed of aggregates of 50 to 60% olive-green actinolitic hornblende and hornblende; 35 to 50% plagioclase and 1 to 5% quartz. These amphibolites are low in Mg, Cr and Ni and have a tholeiitic composition. The only oxide is ilmenite. Komatiitic volcanic lithologies, represented by ultramafic amphibole-chlorite rocks and lesser intercalated komatiitic amphibolite are also found between the middle and upper iron formations and higher in the sequence. The amphibole-chlorite rocks are characterised by high Mg, Cr and Ni contents. Disseminated oxides are ilmenite (~0.5%) and locally magnetite. Many of the ultramafic rocks in the hanging wall of the iron formation have a prominent spotted texture due to 2 to 10 vol.%, 3 to 10 mm, poikilitic olivine porphyroblasts that are partially replaced by serpentine and magnetite. These have an elevated magnetic signature, but grade southward, higher in the sequence, to non-magnetic komatiites and intercalated komatiitic amphibolites. These magnesian amphibolites are composed of actinolite and actinolitic hornblende, interstitial plagioclase, with ilmenite as the only oxide (Mueller et al., 2004).

Brownish-black, fine-grained graphitic schist occurs as discreet 0.02 to 2 m thick beds that are intercalated with the banded iron formations. These schists also occur as a stratigraphic equivalent of iron formations that have pinched out, and at the contact between the lower and the middle iron formation where the two merge. The least altered of these graphitic schists are composed of up to 30% biotite, Fe-Mg chlorite, 40% quartz, 10 to 20% graphite, 1 to 5% almandine porphyroblasts and 3% pyrite and pyrrhotite, with accessory plagioclase, sphalerite and ilmenite. At Nevoria, these schists represent the metamorphosed equivalents of iron rich shales interbedded with silicate facies iron formations elsewhere in this part of the Yilgarn Craton (Mueller et al., 2004).

Numerous 0.1 to 45 m thick dykes of massive or zoned pegmatite cut the mineralised iron formations to a depth of 200 to 250 m where the greenstone sequence and all ore shoots are truncated by a floor of garnet and fluorite bearing muscovite-biotite pegmatite and peraluminous two-mica granite of the undeformed, post-orogenic, 2634±4 Ma Nevoria granite-pegmatite complex. This complex cuts across both the Nevoria anticline and the contact of the Ghooli Dome. The elongate outcrop area and the shallow dip to the south suggest that the main intrusion is a sheetlike body ~500 m thick. The granite-pegmatite complex and ore shoots are displaced by faults oriented at 75 to 80° and dipping at 80 to 90°S which also disrupt the mineralised zone.

Gold mineralisation at Nevoria occurs in four types, all closely associated with banded iron formations (after Cullen et al., 1990):
i). hypogene mineralisation within sulphidic banded iron formation (BIF), with the gold content appearing to be directly related to the pyrrhotite content in and adjacent to quartz veins localised by shears or fold hinges (Cullen et al., 1990). Hypogene mineralisation within banded iron formation occurs as tabular or pipe shaped orebodies which are confined to the contacts with amphibolite and are controlled by steeply east-plunging folds. These tabular and pipe-like orebodies are individually <100 to 200 m in strike length and persist down plunge for 200 to 250 m to the underlying post-ore intrusions of the Nevoria granite-pegmatite complex (Mueller, 1997).
  The orebodies can be divided into those composed of pyrrhotite-rich hedenbergite-actinolite and almandine-hornblende assemblages. These predominantly represent calcic, potassium poor and highly reduced assemblages. Grains of native gold are enclosed in hedenbergite, actinolite, almandine and quartz and are intergrown with pyrrhotite. They occur together with accessory scheelite, chalcopyrite, maldonite, bismuth tellurides and zoned loellingite-arsenopyrite grains (Mueller, 1997). This skarnoid alteration and mineralisation can be traced over a kilometre of strike length in open pits and underground workings, but persists as lower grade mineralisation to the west and east. Subeconomic calcic-potassic alteration assemblages occur in adjacent wallrock amphibolites and metakomatiites, the most continuous development of which persists for >600 m along the footwall contact of the lower iron formation, forming a 17 m wide zone composed of numerous bands of biotite-rich diopside-hornblende, almandine-hornblende, and cummingtonite-plagioclase skarnoid. The latter type contains 1 to 5 vol.% porphyroblasts of almandine and, locally, domains of quartz-cordierite- anthophyllite rock. Thin grossular-andradite-diopside veins are scattered between the skarnoid zones (Mueller, 1997). Striations in altered iron formation (55 to 70°E), amphibole lineations in quartz-veined altered rock (71 to 74°E) and arrays of tensional quartz veins indicate a reverse movement south-side up and laterally to the west during late-stage wall-rock replacement (Mueller, 1997).
ii). several generations of quartz and quartz-sulphide veins which crosscut the BIF. These veins vary from 1 cm to 1 m in thickness and are usually massive and contain pyrrhotite with minor arsenopyrite and pyrite, and also may carry visible gold. Quartz veins associating with steep structures are also observed, varying in dip from 30 to 90°. Wall rock alteration associated with these veins includes bleaching and sulphides, while grunerite is altered to actinolite.
iii). gold also occurs in oxidised BIF at the base of oxidation, generally at depths of 35 to 45 m below surface, and in weathered quartz-sulphide veins in the same zone;
iv). Gold is dispersed in superficial laterite, calcrete and clayey soil to a depth of ~4 m.

The conditions under which the alteration of the banded iron formations and the greenstone sequence in the Nevoria mine took place, as determined from almandine-biotite pairs in graphitic schist and hornblende-plagioclase pairs in amphibolite, indicate a peak metamorphic temperature of 610±50°C, based on the calibrations of Ferry and Spear (1978), Perchuk and Lavrenteva (1983), and Holland and Blundy (1994) as employed by Mueller (1997). The calibration of Hodges and Spear (1982) provides a temperature within error but higher than the other three. Published estimates of 400 MPa regional pressure indicate a burial depth of about 14 km during mineralisation (3.6 km/100 MPa; Spear, 1993).

The mineralisation has been dated at 2635.7±1.2 Ma (concordant U-Pb age of allanite inclusions in garnet), and 2630±13 Ma by a less precise Pb-Pb errorchron age (Mueller et al., 2004). These dates suggest the mineralisation and skarnoid alteration assemblages formed 90 m.y. after amphibolite-facies metamorphism in the Southern Cross greenstone belt and are closely coeval with the emplacement of the post-ore Nevoria granite-pegmatite complex. This implies both were related to the same tectono-thermal event.

Production and Resources

Ore was mined from 6 interconnected pits, from east to west, Norton, Parbo, Nevoria East, Silver, Woolcock and the separate Bottledump pit over a strike length of 2 km.

For details of historic production and grades, see the the historical details paragraph at the top of this description. When the mine closed in 2009, the remaining Indicated + Inferred Mineral Resource was 4.08 Mt @ 3.8 g/t Au for 15.5 t of contained gold (St Barbara Annual report, 2009).

The most recent source geological information used to prepare this summary was dated: 2004.    
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:
Bloem E J M, Dalstra H J, Groves D I and Ridley J R  1994 - Metamorphic and structural setting of Archaean amphibolite hosted gold deposits near Southern Cross, Southern Cross province, Yilgarn Block, Western Australia: in    Ore Geology Reviews   v9 pp 183-208
Cullen I, Jones M, Baxter J L  1990 - Nevoria 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 301-305
Doublier, M.P., Thebaud, N., Wingate, M.T.D., Romano, S.S., Kirkland, C.L., Gessnar, K., Mole, D.R. and Evans, N.,   2014 - Structure and timing of Neoarchean gold mineralization in the Southern Cross district (Yilgarn Craton, Western Australia) suggest leading role of late Low-Ca I-type granite intrusions: in    J. of Structural Geology   v.67, pp. 205-221.
Fan H-R, Groves D I, Mikucki E J, McNaughton N J  2000 - Contrasting fluid types at the Nevoria Gold deposit in the Southern Cross Greenstone Belt, Western Australia: implications of auriferous fluids depositing ores within an Archean banded Iron-formation: in    Econ. Geol.   v95 pp 1527-1536
Mueller A G, McNaughton N J  2000 - U-Pb ages constraining batholith emplacement, contact metamorphism, and the formation of Gold and W-Mo Skarns in the Southern Cross Area, Yilgarn Craton, Western Australia: in    Econ. Geol.   v95 pp 1231-1257
Mueller, A.G.,  1997 - The Nevoria Gold Skarn deposit in Archean Iron-formation Southern Cross Greenstone Belt, Western Australia: 1. Tectonic setting, petrography, and classification: in    Econ. Geol.   v.92, pp. 181-209.
Mueller, A.G., Nemchin, A.A. and Frei, R.F.,  2004 - The Nevoria Gold Skarn Deposit, Southern Cross Greenstone Belt, Western Australia: II. Pressure-Temperature-Time Path and Relationship to Postorogenic Granites: in    Econ. Geol.   v.99, pp. 453-478.

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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