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Hillgrove

New South Wales, NSW, Australia

Main commodities: Au Sb
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The Hillgrove antimony-gold deposit is located 24 km east of Armidale in the New England region of northern New South Wales, Australia.

Mining in this district was most intense between 1877 and 1921 when the last old mines were exhausted, before a revival in 1970 when the Hillgrove antimony mine was re-opened.

The Hillgrove deposits are hosted by both:  1). late Carboniferous, monotonous, volcanic derived turbidites which include mudstones, siliceous siltstone, greywacke and minor chert, and  2). The Permian Hillgrove Plutonic Suite granitoids which includes the 280 to 290 ±6 Ma Hillgrove Adamellite (a medium to coarse foliated biotite adamellite with blue quartz and large feldspar phenocrysts) and the 286 ±6 Ma Bakers Creek Diorite (a complex intrusive body ranging from adamellite to diorite).   The field is also cut by numerous dykes ranging from microgranite to lamprophyre, most predating the mineralisation.

The district has a strong set of north and NW trending shears which are usually mineralised, as well as post mineralisation NE and east shearing.   These shears are generally subvertical although they may locally flatten to as shallow as 30°.   Shears that are up to 5 m wide and 1.1 km long are known although they seldom exceed 1.5 m in width and 700 m in length.   In the adamellite narrow shears, usually <0.1 m are the norm.

Three distinct veining episodes are recorded, namely:
1). an early barren quartz episode,
2). the mineralised episode which in turn comprises 4 phases the last two of which may be transitional -
     i). quartz-scheelite,  ii). quartz-arsenopyrite-pyrite-gold,  iii). quartz-stibnite-gold-silver,  iv). quartz-stibnite-calcite, and
3). a late barren quartz-calcite-chlorite episode.

The scheelite phase usually occurs alone having sealed the fissures, while the quartz-arsenopyrite-pyrite-gold stage is ubiquitous, except where shears have been sealed by the scheelite phase, while the two stibnite phases are usually also present, except in the scheelite veins.   The quartz-arsenopyrite-pyrite-gold phase normally forms a 10 cm selvage to veins as sericitisation and silicification with disseminated sulphides.   The other phases are generally injected along the margins of shears and form either massive single veins or zones of quartz stringers through quartz-wall rock breccias, with scheelite or stibnite as either a semi-continuous core or as irregularly distributed masses throughout the vein.

The distribution of grades and metals in the veins is very variable.   However, economically viable ore usually occurs as vertical to steeply south plunging shoots that vary from 15 to 200 m in length.   There is no systematic metal zoning in the district, although 97% of the gold has come from shears in sediments and 90% of the scheelite from within the Hillgrove Adamellite.   Little mineralisation of note is from within the Bakers Creek Diorite.

Production and reserve figures quoted are as follows:
    Historic production to 1987 - 18 tonnes of recovered Au.
    Total proven+probable reserves 1987 - 0.129 Mt @ 2.28% Sb, 0.35% As, 8.1 g/t Au.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1990.    
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:
Ashley P M, Craw D  2004 - Structural controls on hydrothermal alteration and gold-antimony mineralisation in the Hillgrove area, NSW, Australia: in    Mineralium Deposita   v39 pp 223-239
Ashley P M, Creagh C J, Ryan C G  2000 - Invisible gold in ore and mineral concentrates from the Hillgrove gold-antimony deposits, NSW, Australia: in    Mineralium Deposita   v35 pp 285-301
Boyle G O  1990 - Hillgrove Antimony-Gold deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1425-1427
Switzer C K, Ashley P M, Hooper B and Roach B  2004 - The Hillgrove gold-antimony-tungsten district, NSW, Australia: in   Hi Tech and World Competitive Mineral Success Stories Around the Pacific Rim, Proc. Pacrim 2004 Conference, Adelaide, 19-22 September, 2004, AusIMM, Melbourne,     pp 381-383


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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