Bald Mountain

Nevada, USA

Main commodities: Au
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The Bald Mountain gold mine is located in the Southern Ruby Mountains, north-west White Pine County, Nevada, USA. It is approximately 115Ękm south-east of Elko and 160Ękm to the north-west of Ely.

Mineralisation was discovered at Bald Mountain in 1869. From 1869 to 1877 minor amounts of silver chloride, antimony, lead and copper were produced, while small quantities of gold were taken from a nearby creek. From 1876, when the Copper Basin mine was established, operations followed sporadically until the 1930's. In the 1940's and 1950's tungsten was mined from skarn deposits nearby. Significant exploration was commenced in the 1960's, with an emphasis on gold. Several 'deposits' were discovered, with a pilot scale heap leach operation being started in 1983. This was upgraded to a full production facility in 1985, with the development of the Top Area, followed in 1988 by the RBM Area orebody (Placer Dome, 1995).

The Bald Mountain Mining District contains more than 40 known gold occurrences, of which around 14 have been mined. Others include the Rat, RBM and North Pits on the western flank of Big Bald Mountain; the Top and Sage Flats on the east flank of Big Bald Mountain; the Poker Flats, Horseshoe, Bida, Saga, Galaxy, and Winrock deposits, which are located on the eastern flank of Little Bald Mountain and along the margins of the Mooney Basin. The largest known gold resource in the district is the Top deposit (Barrick Gold, 2006).

Mining is by open pit at a rate of about 1 Mt of ore+waste per month (1995). In the early 1990's Bald Mountain produced 1.5 to 1.8 t Au and 0.15 to 0.18 t Ag per year from conventional heap leach methods, comprising agglomeration, leaching, carbon recovery, stripping, electrowinning and dore production (Placer Dome, 1995).

In August 1994 the nearby Alligator Ridge mine was purchased by Placer Dome US Inc and amalgamated with the Bald Mountain operation (Placer Dome, 1995).

The Bald Mountain mine lies on the western flanks of Little Bald Mountain and Big Bald Mountain. Rocks exposed in the area comprise a conformable, generally SSE dipping sequence of Cambrian to Silurian limestones, dolomites, shales, quartzites and siltstones. These sediments have been intruded by the Jurassic (159 Ma) Bald Mountain quartz-monzonite stock, and related felsic dykes, along a major north-west structural trend. Intense pre- and post-mineral faulting has produced a complex structural regime at Bald Mountain, controlled by the intersection of deep crustal NW and NNE trending fractures which have localised and controlled the location of gold deposits. Overall , the pluton, associated dykes and Au mineralisation are all controlled by the crustal scale, NW trending Bida Trend structure. Every rock exposed in that part of the district, from the lower Cambrian Hamburg Dolomite to the Silurian Laketown Dolomite has hosted some ore (Placer Dome, 1995).

Gold mineralisation is associated with steeply dipping structures / fracture networks and occurs in both sediment hosts and intrusive rocks as micron sized particles. In favourable hosts, such as the upper member of the Windfall Formation, or in sediments within the thermal aureole of the main stock, gold is confined to the margins of the controlling structure, resulting in narrow ore zones. In contrast, in zones of structural intersections, or at permeable-impermeable sedimentary contacts, ore zones may attain appreciable widths (Placer Dome, 1995).

The alteration assemblage includes bleaching, dolomitisation and sanding (decarbonatisation) of dolomites; argillisation and silicification of quartz-feldspar porphyries and softer sediments; quartz-sericite stockwork veining; sericitisation; K-feldspar metasomatism; minor skarn development which is not related to the presence of gold mineralisation; and several iron oxide regimes (Placer Dome, 1995).

Gold generally occurs as disseminated micron-sized particles, although rare visible gold is seen in the eastern portion of the Top deposit. Most exploited ore are oxidized, although mineralised carbonaceous material are obvious in the district.

The important trace elements associated with gold mineralisation include As, Sb and Hg. Typically Au:Ag ratios approach 8:1 (Placer Dome, 1995). In 1994 production from the Bald Mountain operation was 4.1 t Au from 2.42 Mt of ore (AME, 1995).

Production and reserve statistics include:
  Production to 1990 - 4.5 Mt @ 2.5 g/t Au = 11 t Au (Placer Dome, 1995).
  Reserves 1989 - 6.1 Mt @ 2.4 g/t Au = 14 t Au (1 g/t cut off, Placer Dome, 1995).
  Proven + probable reserves 1994 - 17.0 Mt @ 1.7 g/t Au (AME, 1995).

Reserves + production as known at the end of 2005 totalled ~170 t Au, while the remaining reserve was 100 t Au (Barrick, 2006).

Published ore reserves and mineral resources at Dec 31, 2015 (Kinross Gold Reserve and resource statement, 2015) were:
        Proven + probable reserves - 54.627 Mt @ 0.6 g/t Au, plus
        Measured + indicated resources - 188.971 Mt @ 0.6 g/t Au, plus
                              inferred resources - 24.396 Mt @ 0.5 g/t Au
        for a total of ~170 t of contained Au.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1994.    
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:
Ilchik R P  1990 - Geology and geochemistry of the Vantage Gold deposits, Alligator Ridge-Bald Mountain Mining District, Nevada: in    Econ. Geol.   v85 pp 50-75
Nutt C J and Hofstra A H,  2007 - Bald Mountain Gold Mining District, Nevada: A Jurassic Reduced Intrusion-Related Gold System : in    Econ. Geol.   v102 pp 1129-1155

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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