Battle Mountain - Phoenix, Copper Basin, Copper Canyon, Fortitude

Nevada, USA

Main commodities: Au Cu Ag
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The Battle Mountain complex of skarn and intrusive related gold deposits lie within the NNW Battle Mountain-Eureka gold trend, some 50 km south-east of Winnemucca in Lander County, Nevada, USA.   The operations at Battle Mountain are also referred to as the Phoenix operation.

For details of the geological setting see the Battle Mountain - Eureka Gold Trend - Geology record which includes links to descriptions of other deposits in that trend.

While the Battle Mountain deposits are skarn and intrusive related, the Battle Mountain-Eureka gold trend also includes both sediment-hosted micron-gold deposits such as Cortez and Pipeline and younger Miocene volcanic related epithermal ores as at Buckhorn.

The Battle Mountain complex of deposits are localised in two areas, namely at Copper Canyon and Copper Basin which are about 10 km apart.

At Copper Canyon an Eocene to Oligocene granodiorite-porphyry plug cuts a complex sequence of Palaeozoic sediments and lesser volcanics. This sequence includes the lower to middle Palaeozoic Western Siliceous Assemblage rocks of the Roberts Mountains Allochthon which was thrust eastward over the Roberts Mountains Thrust to rest on time equivalent sediments of the Eastern or Carbonate Assemblage. The latter is not exposed in the immediate Battle Mountain District. Following this thrusting, which was a result of the Devono-Carboniferous Antler Orogeny, a sequence of Permo-Carboniferous sediments were unconformably overlain onto the older Palaeozoic of both assemblages. These sediments, known as the Overlap Assemblage, were eroded from the Antler Highland which was produced by the overthrusting on the Roberts Mountains Allochthon. During the Permo-Triassic the Sonoma Orogeny thrust the upper Palaeozoic silici-clastics of the Havallah Basin eastward above the Golconda Thrust to overlie all of the sequences described above. These rocks formed the Golconda Allochthon.

The Copper Canyon deposits which are distributed over an area of some 2500 x 500 m, are elongated in a NNW direction and are spatially related to the 750 m diameter late Eocene (38 Ma) Copper Canyon Stock of porphyritic granodiorite composition. Mineralisation is zoned outwards from the stock, with Cu predominating immediately adjacent to the intrusive, while gold increases and copper decreases outwards.

Individual and connected orebodies related to the Copper Canyon Stock include the East and West Orebodies, Fortitude-Phoenix, North-east Extension (or Independence Mine Area), Tomboy & Minnie and Reona.

The East Orebody on the rim of the stock contained 12.6 Mt @ 0.79% Cu, 0.86 g/t Au, while the Lower Fortitude orebody, 1 km from the main stock, yielded 10.3 Mt @ 6.96 g/t Au, 0.12% Cu, 25 g/t Ag.

At the end of 1995, the total production plus reserves at Battle Mountain totalled around 180 t Au. Reserves in the open pit Phoenix project at the end of 1998 amounted to more than 90 t (3 Moz). This orebody is the continuation of the mined out open pit Fortitude deposit and extends up to 1 to 1.5 km from the main stock, but is adjacent to a north-south fault occupied by a dyke from the stock.

Reserve and resource figures published by Newmont for the Phoenix operation at Dec. 2005 were:
    Proven + probable reserves - 314.6 Mt @ 0.89 g/t Au = 278 t Au
    Measured + indicated resources - 22.6 Mt @ 0.70 g/t Au = 15.8 t Au
    Inferred resources - 16.8 Mt @ 0.80 g/t Au = 13.5 t Au
    Proven + probable reserves - 316 Mt @ 0.15% Cu
    Measured + indicated resources - 21.01 Mt @ 0.11% Cu
    Inferred resources - 15.5 Mt @ 0.17% Cu

Mineralisation at Fortitude-Phoenix is basically a 30 m thick, stratabound gold-silver bearing, sulphide replacement of pervasively calc-silicate/skarn altered upper Carboniferous to early Permian carbonates. The hosts are mainly silty limestones and belong to the Overlap Assemblage which post dated the Devono-Carboniferous Antler Orogeny that resulted in the Western Siliceous Assemblage being thrust over the Eastern Carbonate Assemblage on the Roberts Mountain Thrust. The ore is characterised by an average of 10% sulphides, mainly pyrrhotite with lesser pyrite/marcasite, chalcopyrite and sphalerite, and patchy arsenopyrite. Gold is present as free particles, averaging 75 microns along inter-grain boundaries. The skarn alteration is represented by two prograde zones comprising an early garnet dominant assemblage with andraditic grandite and subordinate diopsidic pyroxene and epidote, and a later hedenbergitic pyroxene zone with lesser garnet. The prograde zones are replaced by an intermediate actinolite/tremolite quart-calcite phase accompanying the main stage sulphide and gold mineralisation. Native bismuth appears to accompany a late stage retrograde stage of clays.

At Copper Basin, a number of Cu and Au ore deposits have been mined, approximately 10 km to the north of Copper Canyon. The geological setting at Copper Basin consists of Palaeozoic sediments, dominated by the quartzites, shales and minor calc-silicated or skarn rocks that belong to the Cambrian Harmony Formation. These sediments have been intruded, altered and mineralised by both Cretaceous and Tertiary intrusives. The Harmony Formation is the prime host to mineralisation in the Copper Basin area. In the eastern part of the area the Harmony Formation is unconformably overlain by the upper Carboniferous to Permian Antler Peak Sequence. This sequence, which is represented by the Lower and Middle Battle Formation and by the Antler Peak Limestone, is only of minor importance as a host to ore, in contrast to the Copper Canyon area.

Copper mineralisation at Copper Basin is believed to be associated with the emplacement of the composite intrusives that are associated with the very large (around 1 Gt) but low grade Buckingham porphyry molybdenum occurrence which is found both within and surrounding a Cretaceous intrusive centre. A series of small stocks, plugs and dykes, similar in composition and texture to the Buckingham intrusive centre, form a general east-west trending belt through the Copper Basin. The Cu deposits closely follow the distribution of these Cretaceous intrusives, and are characterised by supergene enriched blankets, or structural fillings, derived from generally low grade, disseminated and micro-veinlet chalcopyrite-pyrite protoliths. Ore mineralogy comprises chalcocite, cuprite and chrysocolla, with lesser malachite, azurite and turquoise.

Economic gold and silver are, in contrast, distributed in a generally north-south alignment in the eastern portion of the district. They are typically associated with high angle structural breaks that tend to form the contact between the Harmony Formation and Antler Sequence rocks. They are also localised by skarn and calc-silicate alteration that has formed within carbonate lithologies near the top of the Harmony Formation. Gold and silver are present as electrum, occurring with limonite, quartz and secondary Cu mineralisation. The Tertiary activity culminated in the formation of the 34 Ma Caetano Tuff which comprises a blanket of quartz-latite and crystal-lithic tuff.

The Surprise deposit is part of a Cretaceous to Tertiary mineralised system which includes the Buckingham mineralisation. An adjacent Cu rich section is interpreted to have been displaced from above the Mo zone by low angle thrusting. The main molybdenum and copper mineralisation is believed to be primarily associated with the Cretaceous intrusive phase, although Cu is also associated with the Au mineralisation in several of the deposits. In contrast to the generally east-west trending Mo and Cu mineralisation, Au forms in a separate north-south zone of NNE elongated lenses. The gold ore has lower temperature features such as chalcedonic/opaline quartz, vugs and open space filling. It has been interpreted as being of Tertiary age overprinting the Cretaceous skarn sulphides of the original outer quartz-sulphide zone of the porphyry system. Within the north-south elongated gold zone there are a series of Tertiary dykes of granodioritic composition, while a larger Tertiary granodioritic plug, apparently un-related to mineralisation, is found 1.5 km to the north.

The property was owned by the Battle Mountain Gold Company in 1998, which has subsequently been incorporated into Newmont.

For more information consult the reference(s) listed below.

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.

Copper Basin


  References & Additional Information
   Selected References:
Theodore T G, Blake D W, Kretschmer E L  1983 - Geology of the Copper Canyon porphyry copper deposits: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 543-550
Wotruba P R, Benson R G, Schmidt K W  1987 - The Fortitude gold-silver deposit, Copper Canyon, Lander County, Nevada: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada    pp 343-347
Wotruba P R, Benson R G, Schmidt K W,  1988 - Geology of the Fortitude gold-silver skarn deposit, Copper Canyon, Lander County, Nevada: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno,    pp 159-171

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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