Hall, Hall - Tonopah, Nevada Moly

Nevada, USA

Main commodities: Mo Cu
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The Hall, Hall-Tonopah Molybdenum and Copper or Nevada Moly Mine deposit is located in Nye County, south western Nevada, USA.

The deposit was initially mined by open pit between 1982 and 1985 by the Anaconda Minerals Company and subsequently by Cyprus Mines from 1988 to 1991 for Mo, then by Equatorial Tonopah, Inc. for Cu from 1999 to 2000. It was subsequently purchased by Idaho General Mines Inc in 2006.

The geology of Central and south-western Nevada is characterised by two Palaeozoic sequences, namely the: i). Western Assemblage of deep water siliceous and volcanic sequences, particularly carbonaceous shale, mudstone, chert and volcanic rocks; and ii). Eastern Assemblage rock consisting of thick shelf sequences composed of carbonates and lesser clastic rocks.

During the Devono-Carboniferous Antler Orogeny the Western Assemblage was thrust over the Eastern Assemblage on the Roberts Mountain Thrust Zone. The subsequent Permo-Triassic Sonoma Orogeny developed a similar thrust, the Golconda structure to the west.   Following the Antler Orogeny, a Permian Overlap Sequence was deposited over both assemblages.   The Hall deposit lies between the Roberts Mountain and Golconda thrusts.

The oldest rocks in the Hall-Tonapah district are sparse outcrops of the lower Devonian dolomitic limestones of the Nevada Formation which belong to the Eastern or Carbonate Assemblage, followed by the more widespread metamorphosed turbiditic clastic and volcaniclastic sediments (with andesite porphyry clasts), and interbedded andesitic to latitic tuffs, possibly of the Permian Mina Formation.

The Mina Formation, which belongs to the Overlap Sequence, is intruded by the Hall stocks, which are overlain by Tertiary volcanic rocks, including three Oligocene units of mafic rich and poor ignimbrites and andesite flows and pyroclastics, which are in turn intruded by Miocene rhyodacitic plugs and overlain by lapilli tuffs and latite flows (Shaver, 1990).

Two molybdenum bearing, quartz monzonite stocks were emplaced between 70 and 66 Ma at the Hall Mo deposit. These stocks are texturally concentrically zoned, and each contains three stacked Mo orebodies and overlapping zones of alteration and mineralisation. The upper and outer phases are more silicic, finer grained and groundmass rich, grading to coarser and more equigranular varieties at depth in the core. This is taken to indicate continuous inward crystallisation which was interrupted three times by the successive deep release of hydrothermal fluids.

The western and eastern stocks have surface outcrop areas of 750x900 and 550x850 m respectively. The main orebody is associated with the western stock, which is sub-divided into a northern and southern stock, the two mineralised stocks referred to above, are separated by a narrow 50 m wide, partially faulted septum of sediments. The orebodies are generally flat lying to gently dipping and are entirely within the stocks, extending to depths of no more than 350 m below the surface, and occupy an area comparable to that of the stocks. A well developed leached cap (for Cu) of the order of 40 to 120 m thick, overlies a supergene blanket up to 150 m thick, with grades of up to 0.2% Cu as secondary chalcocite coatings on pyrite. The supergene blanket is exposed at surface in places by erosion. Primary Cu grades accompanying the Mo ore are of the order of 350 to 1000 ppm (Shaver, 1990).

Studies indicate that in each mineralisation/alteration pulse, early quartz-molybdenite + chalcopyrite ±pyrite veinlets with K-feldspar envelopes, were followed by muscovite ±quartz ±fluorite veinlets with envelopes of coarse grained muscovite. Later fluids in some pulses are indicated as having formed quartz-magnetite veinlets or chlorite enveloped quartz-pyrite-chalcopyrite+magnetite veins. All pulses were ultimately closed by base metal veins with quartz-galena-sphalerite-chalcopyrite+magnetite, with no alteration envelopes. Molybdenum orebodies are related to both abundance of, and molybdenite concentration in, quartz-molybdenite veinlets, with successively deeper orebodies, related to successively deeper phases of each stock and are consistently lower in grade (Shaver, 1990).

Published reserves are estimated at 200 Mt @ 0.098 % Mo (after Anaconda drilling in the 1980's)

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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