Nevada, USA

Main commodities: Au
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The Northumberland deposit is located within the Toquima Range of northern Nye County, south-central Nevada, USA. It is found along the eastern margin of the Northumberland Caldera, 35 km to the NNE of the Round Mountain deposit.

The regional geology of the district is characterised by lower Palaeozoic rocks, intruded by Mesozoic granodiorite, followed by the formation of the Oligocene Northumberland Caldera. The Palaeozoic rocks comprise both the autochthonous and allochthonous assemblages of the Devono-Carboniferous Antler Orogeny. The allochthonous sequence of the upper plate is represented by the chert, shale, argillite and lesser interbedded greenstones of the Ordovician Vinini Formation. The autochthonous assemblage is composed of limestone, shale and argillite mapped as the Ordovician Pogonip Group and the Silurian Masket Shale. The main host to ore is the autochthonous assemblage. Gold and silver also occurs in the Mesozoic intrusions, while bedded barite is found in the allochthonous rocks. Silver rich quartz veins occur in Cretaceous tonalite bordering the Northumberland deposit. The silver mineralisation is believed to be older than the gold ore (Bagby & Berger, 1985; Thorman & Christensen, 1991).

Hypogene alteration is represented by an early calc-silicate skarn assemblage, silicification and intense argillisation. This was followed by supergene oxidation and clay formation. The orebodies comprise i). tabular zones along the contacts between tonalite sills and the country rock sediments; ii). diffuse mineralisation in breccia zones; and iii). stratabound bodies in sediments (Bagby & Berger, 1985). The orebody has dimensions of 1100 x 200 x 19 m (USBM).

The oxidised ore contains micron sized metallic gold, accompanied by clay and iron oxides, while the un-oxidised mineralisation is associated with carbonaceous matter, with silica, pyrite, freibergite, sphalerite, chalcopyrite and minor molybdenite. A variety of veins are recorded, including quartz veinlets cutting jasperoid, calcite veins within acid leached rocks and barite veins which are spatially associated with the jasperoids (Bagby & Berger, 1985).

The current resource (2006) occurs in 8 deposits distributed over a 2600 x 500 m area occuring as stacked, sediment hosted and finely disseminated gold-silver deposits which are generally concordant with bedding and follow three low-angle tectono-stratigraphic host horizons near the crest and within the limbs of the Northumberland anticline (NewWest Gold Corp, 2006).

Published production and reserve figures include:

    15 Mt @ 1.5 g/t Au = 23 t Au (Production + Reserve, 1983, Bagby & Berger, 1995).
    8 Mt @ 2.4 g/t Au (Production + Reserve, 1984, Romberger, 1986).
    31 Mt @ 2.05 g/t Au (Measured + indicated resource, 2006, NewWest Gold Corp, 2006).

The most recent source geological information used to prepare this summary was dated: 2006.    
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

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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