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Costerfield

Victoria, Vic, Australia

Main commodities: Au Sb
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The Costerfield gold and antimony field is centred on the small settlement of Costerfield in Central Victoria, Australia, ~10 Km NE of Heathcote, 50 km SE of Bendigo and 100 km NW of Melbourne, Victoria (#Location: 36° 53' 13"S, 144° 47' 33"E).

The veins of the Costerfield district were discovered in the 1860s by two prospectors, Coster and Field, and together with additional veins subsequently discovered, were mined for both gold and antimony, most extensively during two periods: 1860 to 1883 and 1904 to 1925. Small scale production took place intermittently between 1934 and 1950. Since 1975 the has undergone more extensive, modern exploration, maimy geochemical sampling of bedrock under shallow alluvial cover and drilling. This resulted in the discovery of the Augusta deposit in 1975. Construction of a processing plant in 1995 facilitated the re-treatment of tailings and the oxide portion of the Brunswick deposit which was mined by open pit method. Resource definition drilling at Augusta culminated in the development of the Augusta underground mine in 2006, utilizing the Brunswick processing facility. Two main deposits, the Augusta and Cuffley lodes are exploited from the Augusta Mine (Mandalay Resources, 2017). The total endowment of the field at December 31 2016 is estimated to be ~15 t of Au and 65 000 t Sb (Davis et al., 2017).

The Costerfield district is located on the northern margins of the Darraweit Guim Province, adjacent to the western boundary of the Melbourne Zone of the Lachlan Orogen. The eastern margin is defined by the NE trending Cambrian Heathcote Volcanic Belt and the crustal scale Mt William Fault that separates the Bendigo and Melbourne zones (Davis et al., 2017).

The geology of the district consists of Early Silurian, weakly bedded calcareous mudstone with lesser siltstone and sandstone of the Costerfield Formation, estimate to be between 450 and 550 m thick. These are overlain by siliciclastic rocks of the Wapentake Formation, followed by mudstones of the Dargile Formation. Veins at Costerfield, which have been mined over an ~8 km north to south interval, are hosted by the Silurian Costerfield siltstone, exposed in the core of the Costerfield Anticline. The anticline, which strikes north-south, is gently bowed toward the west and domal, plunging to both the north and south. The Costerfield Anticline is truncated to the east by the reverse Moormbool Fault, above which the Costerfield Formation is thrust to the east over Devonian sequences of the Murrindindi Supergroup (Davis et al., 2017; Mandalay Resources, 2017).

The Augusta and Cuffley lodes are located between two low angle, west-dipping regional-scale thrust faults, the King Cobra (lower) and Adder (upper) fault systems. These structures offset stratigraphy and earlier structures by ~300 m, and are reflected by intensely sheared and fractured rock up to 20 m in thickness. They are interpreted to related to the regional scale Moormbool thrust-fault zone. All of the major current and historical Costerfield vein systems are interpreted to lie within the bounds of this shear zone, with the exception of the Brunswick deposit located in the hanging wall of the Adder fault (Davis et al., 2017).

The Augusta lodes occur at the southern end of the Costerfield Anticline. The deposit consists of three major veins, the E, W and N lodes. These lodes occur as either discrete veins or in mineralized zones that range in thickness from a few mm up to a maximum of 4 m. Despite their narrow width, the veins tend to be persistent along strike and down dip. Many associated second-order veins are found as splay structures branching from the primary vein systems. The veining strikes NNW and dips towards the west and runs subparallel to bedding in the upper reaches of the E and W lodes, but becomes subvertical as the main corridors of mineralisation cut through the shallowing bedding at ~150 m depth. The N Lode has a subvertical dip with small local variations over a strike extent of ~600 m. The NW dipping Brown fault has a large effect on vein width and metal concentration. This structure marks the northern limit of the W Lode, but also truncates the N and E lodes, with localised warping of the vein structures and increased concentration of mineralisation. Around the Brown FauIt and other smaller controlling structures, the vein widths reach ~2.4 m, although the average vein width is ~0.3 m (Davis et al., 2017).

The Cuffley lodes are represented, by a subvertical panel of mineralisation with an ~600 m strike length and an average mineralised width~0.4 m. It does not outcrop, and is below the historically mined Alison Reefs. The upper reaches of the deposit, which comprises of several distinct lodes, is separated from the Alison Reefs by a 30 to 40° west-dipping fault (the Flat Fault). The Cuffley system persists over a vertical interval of 350 m until terminated by the King Cobra Fault. Similarly its lateral continuation is interrupted by the NW East Fault. Immediately north of the East Fault there is a zone of subecononic mineralisation within veinlets hosted in a 5 to 200 mm thick sheared zone. To the north, this passes into the N zone where it continues for ~150 m before pinching out. The southern section of the Cuffley system continues for ~150 m southwards of the East Fault before also pinching out. Local splays occur where the Main Cuffley shear diverges from the under-developed cleavage planes of the country rock. Proximal to the intersection of splays and the main shear, increased vein dilation is often observed with accompanying increase of stibnite. Elevated gold grades have aIso been observed proximal to splay intersections where abundant gold is found along the margins of, and within, associated quartz. Enrichment along these steep, north plunging and structural intersections has been mapped over strike lengths of ~20 m and downdip for ~150 m (Davis et al., 2017).

Individual ore shoots within the district, both historic and in the currently mined lodes have been traced over strike lengths of up to 800 m and worked down to 300 m depth (Mandalay Resources, 2017).

Stibnite commonly occurs as microcrystalline (<1 mm) to coarsely crystalline (>1 cm) masses with individual crystals of more than 50 mm in length. Crystalline stibnite contains aurostibite and minor native antinmony. Acicular stibnite forms as rosettes on ioint planes and within remnant open-space cavities related to brittle fractures. Stibnite appears to fill remnant void space in quartz and quartz-carbonate veins, suggesting very late deposition and therefore, one of the last stages of mineralisation (Davis et al., 2017).

Gold is closely associated with quartz veins, stibnite and disseminated arsenopyrite and arsenian-pyrite. Gold recovered from these veins is free gold, typically ranging in size from 50 to 500 µm. Gold also occurs in the rare Au-Sb compound aurostibite (AuSb2) which breaks down to a rusty, reddish-brown colour (Stillwell 1952). This rare gold phase flanks and rims native gold grains. Gold within arsenopyrite and arsenian-pyrite has also been recognised within the Cuffley deposit as both refractory and fine-grained, <50 µm free gold. Deep drilling and mining of the Cuffley deposit suggests that the mineralised system becomes depleted in antimony at depth (Davis et al., 2017).

Mineral resources at Costerfield at 31 December, 2016 were estimated as follows (Davis et al., 2017):
    Measured resource - 0.286 Mt @ 9.5 g/t Au, 4.0% Sb;
    Indicated resource - 0.812 Mt @ 5.9 g/t Au, 2.5% Sb;
    Inferred resource - 0.611 Mt @ 5.5 g/t Au, 1.5% Sb;
    TOTAL resource - 1.710 Mt @ 6.4 g/t Au, 2.4% Sb;
    Ore reserve - 0.619 Mt @ 6.5 g/t Au, 2.8% Sb (included within Mineral Resources).

The most recent source geological information used to prepare this summary was dated: 2017.    
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.


Costerfield

  References & Additional Information
   Selected References:
Davis, C.T.M., Westcott, A.L., Fromhold, T.A., Eagle, R.M., Gneil, C.J., Cuffley, B.W. and Gregory, C.J.,  2017 - Costerfield antimony goldfield: in Phillips, G.N., 2017 Australian Ore Deposits, The Australasian Institute of Mining and Metallurgy,    pp. 797-800.


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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