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Fosterville

Victoria, Vic, Australia

Main commodities: Au
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The Fosterville gold deposit is located 20 km to the ENE of Bendigo in north-central Victoria.   It comprises fault controlled deposits developed along and adjacent to a series of major, steeply dipping mineralised structures developed as shoots over intervals of up to 8 km.

Gold was first discovered in the Fosterville area in 1894 with mining activity continuing until 1903 for a total production of 0.87 t of gold. Apart from a minor tailings retreatment in the 1930's, activity resumed in 1988 with a further tailings retreatment program conducted by Bendigo Gold Associates which ceased in 1989. Mining recommenced in 1991 when Brunswick Mining NL and then Perseverance Corporation Ltd (from 1992) commenced heap leaching operations from shallow oxide open pits. Between 1988 and the cessation of oxide mining in 2001, a total of 7.46 t of gold were poured (Roberts et al., 2003). Perseverance completed a feasibility study into a sulphide mining operation in completed by in 2003 with construction and open pit mining commencing in early 2004. Commercial production commenced in April 2005. In February 2008, Perseverance was acquired by Northgate Minerals Corporation. In August 2011, Northgate entered into a merger agreement with AuRico Gold Inc., who assumed control of Northgate in October 2011. In March 2012 AuRico and Crocodile Gold Corp. jointly announced that Crocodile Gold would acquire the Fosterville and Stawell Mines. Crocodile Gold's ownership of Fosterville was completed on May 4, 2012. In early July 2015, Newmarket Gold Inc. merged with Crocodile Gold to form Newmarket Gold Inc. (Newmarket Gold, 2016).

The Fosterville Goldfield is located in the Bendigo Zone of the Lachlan Fold Belt, within a host sequence dominated by folded and faulted Ordovician turbidites which were subsequently deformed in the Late Ordovician (450 to 430 Ma) Benambran Orogeny. Deformation of the sediment pile was under east-west compression which produced north-south folds dislocated by a series of west-dipping reverse faults which progressively developed and flattened down dip. These faults are interpreted to have been the likely conduits for ascending mineralised fluids.

Two broad periods of gold mineralisation have been delineated across the western Lachlan Fold Belt by radiogenic dating. The first of these is associated with quartz vein hosted mineralisation from 425 to 400 Ma in the Silurian to early Devonian, extending across Victoria from Stawell in the west through Bendigo to Wattle Gully in the east. Two granitic suites intruded into the Bendigo Zone during the early Devonian and again in the late Devonian. These intrusions appear to be linked to the second phase of mineralisation between 380 and 365 Ma, which extended from Ballarat in the west to the Woods Point-Walhalla belt in the east. The mineralisation at Fosterville appears to have been formed during this phase. This mineralisation can occur in a range of styles from quartz-carbonate vein hosted free gold through to sulphide hosted refractory gold with associated arsenopyrite, pyrite and stibnite. Erosion and deep weathering in the late Tertiary produced a regional laterite profile with weathering locally to 50 m in depth.

The deposits of the Fosterville Goldfield are hosted by a turbiditic sequence of interbedded sandstones, siltstones and shales interpreted to have formed as a regime of meandering submarine channels. This sequence is dominated by shale topped sands ranging from 0.2 to 1.5 m in thickness, with lesser massive sandstone, shale and black shale, and has been metamorphosed to sub-greenschist facies and folded to produce a set of upright, open to closed folds. Axial planar and radial cleavage is best developed in fold hinges. In addition, bedding parallel laminated quartz veins, preferentially formed in shales at or close to the contact with sandstone units. These veins, which are visually similar to those that carry free gold at Bendigo (20 km away), are effectively unmineralised at Fosterville.

The Fosterville mineralisation is controlled by a series of late brittle, often bedding parallel faults which follow pre-existing laminated quartz veins, although they also crosscut bedding to link bedding parallel faults across folds. They generally follow steeply west dipping reverse faults with a series of moderately dipping reverse splay faults formed in the footwall to the main fault which are confined to a zone within 100 m east of that main fault.

Fluid inclusion studies suggest mineralisation formed at between 140 and 385°C and depths of 2.6 to 5.7 km. Felsic dykes up to 10 m thick have intruded parts of the deposit area, while a series of thin, 170 ±5 Ma deuterically altered basaltic dykes post-date all significant faulting and mineralisation. Neogene erosion, followed by valley infill and weathering produced local clay-conglomerate alluvial channels and complete oxidation to approximately 30 to 40 m below the surface. A 10 to 15 m thick zone of partial oxidation of sulphide minerals is found immediately below the base of complete oxidation, while an around 20 m thick zone of feldspar destruction and partial carbonate dissolution occurs above the base of oxidation.

The primary gold mineralisation at Fosterville takes the form of disseminated arsenopyrite and pyrite occurring as a selvage to veins in a quartz-carbonate veinlet stockwork which is in turn controlled by the late brittle faults. Arsenopyrite is present as fine grained (0.05 to 1 mm), acicular needles without a preferred orientation. The disseminated pyrite, which is intimately associated with the gold mineralisation, occurs as 0.1 to 2 mm crystalline pyritohedrons. The arsenopyrite contains 100 to 1000 ppm Au and the auriferous pyrite 10 ppm 100 ppm Au, with around 80% of the gold within the arsenopyrite and the remaining 20% hosted by pyrite. Silver grades are low, usually around one tenth of the gold grade. Diagenetic, framboidal aggregates and laminations of pyrite up to 20 mm thick, which are common, especially in black shale units, are not auriferous.

Antimony occurs as very coarse grained overgrowths of stibnite up to 20 cm across replacing late quartz - carbonate veins, and appears to contain 1 to 10 ppm Au, although high grade (15 to 40 g/t Au) arsenopyrite and pyrite mineralisation occurs as a selvage to the quartz - stibnite veins. Antimony mineralisation appears to be restricted to splay faults.

The quartz-carbonate stockwork comprises a network of tension gash veinlets which formed perpendicular to the walls of the brittle faults, and on minor slip planes parallel to the brittle faults. Additional movement on the minor slip planes offset the tension gash veinlets and gave rise to a range of geometries from planar through to highly erratic. The quartz-carbonate veinlets themselves are barren, although they have selvages of disseminated, fine grained arsenopyrite-pyrite, up to 10 times the width of the veinlet on which they are centred. Where the stockwork is densely developed, mineralised selvages coalesce to form a solid body of mineralisation.

The primary mineralisation is structurally controlled with high grade zones being localised by the geometric relationship between bedding and faulting. Typically the mineralised shoots are 4 to 15 m thick, and have widths parallel to dip of 50 to 150 m and down plunge extents of 300 to >1500 metres. Gold grades are relatively smoothly distributed with both very high values and very low values being rare.

The current sulphide operation commenced in April 2005, with the plant at Fosterville Gold Mine having produced 31.125 tonnes of gold up to the end of December 2015. This production was initially solely from Harrier open cut with underground mining beginning from late 2006. The Harrier open cut was completed in December 2007. Since then, apart from a series of pit expansions on the previously mined Harrier, John's and O'Dwyer's South Pits during 2012 the underground mine has been the primary source of ore. Since early 2013 underground operations have been the sole provider of mill feed at Fosterville. Mining activities in 2016 focused on the Central, Phoenix and Harrier underground areas at a gold production rate of 3.4 to 3.7 tonnes per annum (Newmarket Gold, 2016).

At December 1995 the resources totalled 4.79 Mt @ 1.1 g/t Au as oxide ore, plus 5.90 Mt @ 2.7 g/t Au primary sulphide ore.

At December 2008 reserves and resources at the Fosterville operation were (Northgate Minerals website, 2009):
    proved + probable reserves - 3.617 Mt @ 4.85 g/t Au,
    measured + indicated resources - 9.772 Mt @ 2.05 g/t Au,
    inferred resources - 11.114 Mt @ 3.7 g/t Au.

At 31 December 2015 reserves and resources at the Fosterville operation were (Newmarket Gold mineral resources report, 2016):
    proved + probable reserves - 1.091 Mt @ 6.95 g/t Au, (included in resources)
    measured + indicated resources - 15.073 Mt @ 4.39 g/t Au,
    inferred resources - 5.073 Mt @ 4.08 g/t Au.
    measured resources and proven + probable reserve CIL residues - 0.571 Mt @ 7.83 g/t Au.

The most recent source geological information used to prepare this summary was dated: 2015.     Record last updated: 29/11/2016
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
 References to this deposit in the PGC Literature Collection:
Anonymous  1998 - Fosterville: in    Register of Australian Mining 1997/98    p 229-230
Arne D C, Bierlein F P, McNaughton N, Wilson C J L and Morand V J,  1998 - Timing of gold mineralisation in western and central Victoria, Australia: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks: in    Ore Geology Reviews   v13 pp 251-273
Boucher, R.K., Rossiter, A.G., Fraser R.M. and Turnbull, D.G.,  2015 - Review of the structural architecture of turbidite-hosted gold deposits, Victoria, Australia: in    Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc.   v.124, pp. 136146
Hughes, M.J. and Phillips, G.N.,  2015 - Mineralogical domains within gold provinces: in    Trans. IMM (incorp. AusIMM Proc.), Section B, Appl. Earth Sc.   v.124, pp. 191-204.
McConachy G W and Swensson C G,  1990 - Fosterville Gold Field: in Hughes FE (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1297-1298
Roberts C, Jackson T, Allwood K, Shawcross M, Story J, Barbetti L, Tielen R, Boucher R and Norris N,  2003 - Fosterville - Rise of the Phoenix, the emerging goldfield at Fosterville: in   NewGenGold 2003, Conference Proceedings, Perth WA,  Louthean Media, Perth    pp 200-213
Wang G M and White S H,  1993 - Gold mineralization in shear zones within a turbidite terrane, examples from Central Victoria, S.E. Australia : in    Ore Geology Reviews   v8 pp 163-188
Zurkic N  1998 - Fosterville gold deposits: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne   Mono 22 pp 507-510


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