Peak Hill

New South Wales, NSW, Australia

Main commodities: Au Cu
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The Peak Hill gold deposit is located 400 km NW of Sydney between the regional centres of Parkes and Dubbo in the Central West of New South Wales, Australia.

The deposit is located near the eastern margin of the Ordovician Junee-Narromine volcanic belt of the early Palaeozoic Macquarie Arc, in the Lachlan Orogen of eastern Australia and is close to the interpreted Parkes Thrust. The Junee-Narromine volcanic belt extends north-south for several hundred km and includes the Goonumbla Volcanic Complex which hosts the nearby Northparkes porphyry Cu-Au system. The host rocks at Peak Hill are believed to be equivalents of the Goonumbla Volcanics, but are separated from the main volcanic suite by north-south arc parallel structures, such as the Parkes Thrust.

Ordovician rocks west of the Parkes thrust are weakly deformed, with broad open folds and sub-greenschist metamorphic assemblages, while the Ordo-Silurian sequences east of the fault, including the rocks hosting the Peak Hill deposit exhibit tight to isoclinal folding, strong axial planar cleavage with greenschist metamorphic assemblages.

The volcanics at Peak Hill appear to form a fault bounded anticlinal structure overlain by the Ordovician sediments of the Cotton Formation and Mugincoble Chert, which are in turn overlain by Silurian sediments.

A major 30 km wide north-west trending structural corridor, thought to be part of the Lachlan Transverse Zone (LTZ), intersects and apparently offsets the Parkes Thrust and parallel structures near Peak Hill. The LTZ has been interpreted to extend south-easterly to the Cadia and Ridgeway deposits.

The core of the Peak Hill area is occupied by andesitic volcanic and volcaniclastic rocks which have a general north-south strike and observed easterly facings east of the mine area. Significant transported cover obscures the sequence to the west. Shaly sediments are common further to the east.

The central section of the deposit area is characterised by intense and pervasive alteration which forms an elongate 3.3 x 0.6 km zone, within which the textures of the host andesitic volcanic and volcaniclastic rocks have been almost totally obliterated. This alteration exhibits a broad zonation, with an outer propylitic epidote-chlorite-clay-hematite-pyrite assemblage, grading inward through paragonite-muscovite-kaolinite to an inner 350 x 550 m core of advanced argillic to argillic alteration comprising vughy residual silica, pyrophyllite-pyrite ± clay-sericite.

The Au-Cu mineralisation is principally associated with late silica-pyrite-barite veins, with the highest gold grades occurring mainly in microcrystalline-quartz-altered rocks in the paragonite+muscovite alteration zone, generally within 50 m outward from the boundary of the pyrophyllite and vuggy-quartz core.   However, Au mineralisation is found throughout the alteration zone, reaching ore grades in the silica-pyrite bodies which occur as both near vertical tabular and pipe like lenses. Copper is more variable and may have precipitated separately from the gold.

The main ore minerals are pyrite ± copper sulpharsenides enargite-tennantite with chalcocite-barite-alunite. Gold is mainly present as sub-micron particles within the pyrite with some evidence for at least two species of pyrite with in the ore system. The alteration and mineralisation are characteristic of a high sulphidation style epithermal deposit.

Weathering and intense oxidation have produced a kaolin-pyrophyllite assemblage with gossan like goethite-hematite-silica bodies to a depth of 100 m. Almost all of the Cu was leached from that oxide profile although Au does not appear to have been significantly remobilised.

Mining to 2006 has concentrated on the oxide zone to a depth of around 100 m, while a hypogene sulphide resource has been traced to depths of in excess of 300 m below surface without being closed off.

As at 31 December 2006, mineral resources and production were (Alkane Resources web site, 2007):
    Production from 1904 to 1917 - 0.5 Mt @ 4 g/t Au for 2 tonnes of contained Au.
    Oxide heap and dump leach production 1996 to 2006 - 5.25 Mt @ 1.56 g/t Au - 4.75 tonnes recovered Au (leaching incomplete 2006).
    Sulphide (Proprietary orebody only) at a 0.5g/t Au cut off (strike length of 300 m, to 200 m below oxide pit)
        Indicated Resources - 9.44 Mt @ 1.35 g/t Au, 0.11% Cu
        Inferred Resources - 1.83 Mt @ 0.98 g/t Au, 0.11% Cu
        Total sulphide resource - 11.27 Mt @ 1.29 g/t Au, 0.11% Cu for 14.5 tonnes of contained Au.

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:
Squire R J, Herrmann W, Pape D and Chalmers D I,  2007 - Evolution of the Peak Hill high-sulfidation epithermal Au–Cu deposit, eastern Australia : in    Mineralium Deposita   v42 pp 489-503
White, N.C., Leake, M.J., McCaughey, S.N. andd Parris, B.W.,  1995 - Epithermal gold deposits of the southwest Pacific: in    J. of Geochemical Exploration   v.54, pp. 87-136.

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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