Idaho, USA

Main commodities: Ag Au
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The DeLamar bulk silver-gold mine is located in Owyhee County, ~5 km SW of Silver City and some 90 km SW of Boise, in Idaho, USA (#Location: 43° 0' 29"N, 116° 50' 17"W).

The district around De Lamar is known for the discovery of placer gold in 1863 and for the production of gold and silver from veins in the Silver City area until 1914. Between 1888 and 1914 the district produced 12.5 t Au and 187 t Ag from 0.725 Mt of ore (Tooker, 1985). Between 1977 and 1993 the mine was operated by Earth Resources and NERCO/Kennecott, before being purchased by Kinross Gold in 1993. It was placed on care and maintenance in 1998 and closed in 2003 and rehabilitated over the ensuing years until complete in 2006.

The geology of the district is dominated by granodiorites of the late Cretaceous Idaho batholith with isolated small roof pendants of Palaeozoic and Mesozoic sedimentary rocks. These are overlain by Miocene alkali-olivine basalts and rhyolites of the Owyhee volcanic field which is developed on the northern margin of the north-east trending Snake River Rift/Lineament. The main host sequence is composed of Miocene rhyolites which comprise rhyolitic flows, tuff, breccia, volcaniclastic sediments and chalcedonic sinters. The Miocene Silver City Rhyolite, Miocene alkali olivine basalt and the Cretaceous granites all contain epithermal Ag-Au mineralisation, but only the rhyolites have yielded bulk tonnage ore.   The principal host is a 15.7±0.3 Ma porphyritic rhyolite emplaced as coalescing and overlapping lava domes and was preceded by cogenetic eruption of tuff breccia in the form of tuff rings (Tooker, 1985).

The ore deposit is irregular, both in shape and distribution, apparently controlled in part by structure, as well as the geomorphology of the rhyolite domes. It includes three main zones of localised mineralisation, the larger two of which cover areas of about 335 x 150 m and 300 x 180 m respectively, with mineralisation persisting to depths of 100 to 120 m as upward flaring cones. Mineralisation occurs as major veins (up to several metres wide) interspersed with high density fracture fillings and veinlets, with adularia-sericite type veining. It was deposited in zones of fracturing and within the volcanics beneath clay zones that locally capped the system. The regional structure is controlled by 10 to 20° trending oblique-slip normal faulting associated with the basin and range fault episode which is more pronounced to the south of the Snake River Lineament in Nevada (Tooker, 1985).

Mineralisation is essentially confined to stockwork veinlets, within silicified rhyolite porphyries which have been intensely silicified. There is also strong argillic alteration, represented by an assemblage of sericite, kaolinite, Na-montmorillonite, illite and pyrite. Pyrophyllite is reported as rare fracture coatings (Tooker, 1985).

The ore mineralogy comprises naumannite, argentite, native gold, marcasite and pyrite (a colliform variety, melnikovite), with trace amounts of chalcopyrite and galena. The secondary minerals include native silver, jarosite, argentojarosite, cerargyrite, goethite, hematite and lepidocrocite. The gangue is predominantly composed of silica as quartz and as chalcedony (Tooker, 1985).

Argentite occurs as coarse grains as large as 2 mm in diameter, while naumannite is found as replacement rinds on argentite and as overgrowths on quartz. Gold occurs as anhedral blebs in quartz and is associated with naumannite, either in fractures, absorbed on the surface, locked in the naumannite or in solid solution (Tooker, 1985).

The early hydrothermal mineralisation commenced with quartz and sericite, followed by pyrite (melnikovite) as overgrowths on quartz. Marcasite followed pyrite, but was less abundant, occurring as a crust on the melnikovite and as small laths in quartz. The exact timing of the chalcopyrite is uncertain, although it is commonly locked in and replaced by, naumannite. Argentite followed marcasite and is in turn succeeded by naumannite. Gold partly overlaps the naumannite (Tooker, 1985).

The mineralisation is anomalously high in Ag, Au, Hg and Se. Hg best delineates the orebodies (Tooker, 1985).

Published production and reserve figures include:

    9.1 Mt @ 0.82 g/t Au, 68.6 g/t Ag = 7.5 t Au, 624 t Ag (Reserve, 1984, USBM).
    4 Mt @ 0.82 g/t Au,= 3.3 t Au (Approximate Production, 1978 to 84, USBM).
    13.8 Mt @ 1.39 g/t Au, 42 g/t Ag = 19 t Au (Reserve, 1993, AME, 1994)

Production between 1977 and 1987 amounted to 528 t of Ag and 7 t Au from ore with an average grade of 62 g/t Ag and 1 g/t Au.   Historical undergroung mining in the district from 1863 to 1914 yielded 1300 t Ag, 18 t Au.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1997.    
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:
Halsor S P, Bornhorst T J, Beebe M, Richardson K, Strowd W  1988 - Geology of the DeLamar Silver mine, Idaho - a volcanic dome complex and genetically associated hydrothermal system: in    Econ. Geol.   v83 pp 1159-1169

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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