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Lewis Ponds

New South Wales, NSW, Australia

Main commodities: Au Zn Cu Ag Pb
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The Lewis Ponds massive sulphide Zn­Pb­Cu­Ag­Au deposit is located in the Palaeozoic eastern Lachlan Fold Belt of central western New South Wales, in eastern Australia.

It has the characteristics of both volcanic-hosted massive sulphide (VHMS) and carbonate-hosted replacement deposits. The two main stratabound massive to disseminated sulphide ore zones (known as Main and Toms), are hosted by a tightly folded sequence of Upper Silurian marine felsic volcanic and sedimentary rocks and have a combined indicated resource of 5.7 Mt @ 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au.

The Main Zone lies within a thick unit of poorly sorted polymictic breccia, limestone-clast breccia and quartz crystal-rich sandstone, while the Toms Zone falls is hosted by the overlying siltstone. Carbonate­chalcopyrite­pyrite and quartz­pyrite stringer veins occur in the footwall porphyritic dacite, south of the Toms Zone and are predeformation. Strongly sheared dolomite­chalcopyrite­pyrrhotite veins directly underlie the Toms massive sulphide lens.

The mineralised zones are predominantly composed of pyrite, sphalerite and galena. Early framboidal, dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate are found throughout the breccia and sandstone beds that host the Main Zone, but are rarely preserved in the annealed massive sulphide in Toms Zone.

A semi-conformable hydrothermal alteration envelope surrounds both the Main and Toms zones. This envelope is characterised by texture destructive chlorite-, dolomite- and quartz-rich assemblages, with dolomite, chlorite, quartz, calcite and sulphides having selectively replaced breccia and sandstone beds in the Main Zone host sequence. In contrast the underlying porphyritic dacite is only weakly sericite altered.

Partial dissolution of the dolomite-altered sedimentary rocks produced vuggy and botryoidal textures and facilitated the growth of base metal sulphides, carbonate and quartz in open cavities. The intense chlorite-rich alteration assemblage which underlies the Toms Zone, grades outward into a weak pervasive sericite­quartz assemblage distal to the massive sulphide lens.

The Main Zone is interpretted to have formed by lateral fluid flow and sub-seafloor replacement of poorly sorted breccia and sandstone beds. In contrast, the Toms Zone was emplaced into fine-grained sediment at or near the seafloor, above a zone of focused up-flowing hydrothermal fluids. Copper-rich assemblages were deposited in the Toms Zone footwall and massive sulphide lenses in Main and Toms zones as the hydrothermal system intensified. During D1 deformation, fracture-controlled fluids within the Lewis Ponds fault zone and adjacent footwall volcanic succession remobilised sulphides into syntectonic quartz veins (Agnew et al., 2005).

The most recent source geological information used to prepare this summary was dated: 2005.    
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:
Agnew M W, Large R R and Bull S W  2005 - Lewis Ponds, a hybrid carbonate and volcanic-hosted polymetallic massive sulphide deposit, New South Wales, Australia: in    Mineralium Deposita   v39 pp 822-844


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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