Selected geochemical data for the Dillon BLM Resource Area, including the Virginia City mining district, Madison and Beaverhead Counties, southwest Montana : mineral-resource and mineral-environmental considerations

Abstract

Geochemical data on rocks associated with mineral occurrences provide signatures that aid in recognition of the types of mineral deposits that may be present in an area. They also indicate metals, minor elements, or trace elements that may be mobilized from rocks to soils or to waters as rocks are exposed to natural (weathering, landslides) or human-induced (surface disturbance such as road construction or mining) processes. As part of an ongoing study to provide federal land managers with a perspective on the nature of mineral resources in the Dillon BLM Resource Area and intervening areas of the Beaverhead National Forest in Madison and Beaverhead Counties, southwest Montana (fig. 1), 100 samples of rocks and stream sediments were collected during site visits to selected active and abandoned mines, prospects, mineral occurrences, and areas of suspected hydrothermal alteration during the summers of 1994-1996. This report presents geochemical data and sample descriptions for these samples. The Dillon Resource Area covers parts of the Dillon, Bozeman, Ashton, and Dubois 1° by 2° (1:250,000-scale) quadrangles and includes 52 mining districts (fig. 2). Mining district names and boundaries are based on a recent map of mining districts of Montana (Chavez, 1994), which is available in digital form from the Montana State Library's web site (http://nris.mt.gov/nsdi/nris/ab45.eoo.zip). The area of Beaverhead and Madison Counties includes active mines that produce talc, chlorite, placer garnet, and placer gold, as well as exploration projects for gold, talc, and zeolites (McCulloch, 1994). Based on a search of the U.S. Bureau of Mines MAS/MILS database (Kaas, 1996), the area contains over 1,500 historic mines, prospects, and mineral localities (fig. 2). Gold is the principal commodity reported at over onethird of these localities. Precious and base metals, iron, and phosphate account for 75 percent of the occurrences (see inset table on fig. 2). Other commodities reported in the study area include talc, tungsten, manganese, thorium, pumice, sand and gravel, stone, aluminum minerals, silica, garnet, and clays. We focussed our sampling on mining districts where additional geochemical data would help determine the nature of the occurrences and on the districts most likely to experience minerals activity on federal lands in the reasonably foreseeable future. The districts sampled for this report are shown in color on figures 2 and 3. Parts of the Dillon Resource Area that lie within the Dillon 1° by 2° quadrangle (north of 45° 00' N, west of 112° 00' W) were not extensively sampled for this study; the Dillon quadrangle was thoroughly evaluated in previous studies (Pearson and others, 1992; Berger and others, 1983). A few samples were collected from districts in the Dillon quadrangle that have been the focus of very recent exploration. Sites in the Dillon Resource Area were chosen to: (1) elucidate the nature and types of mineral deposits that could be present in the study area; (2) obtain more complete, and modem geochemical d...

Map showing the potential for mineral deposits associated with Precambrian mafic and ultramafic rocks in the Blacktail and Henrys Lake Mountains and the Greenhorn and Ruby Ranges of southwestern Montana

“…Sample 96JH67 is a grab sample of a moderately serpentinized, pyroxene-rich ultramafic body mapped by Hadley (1969). Refer to Hammarstrom and Van Gosen (1998) for sample locations, analytical techniques, and complete analyses.…”

“…Most of the prospects (table 1) and the most detailed studies have focused on the ultramafic rocks of the Ruby Range. Geochemical data for mafic and ultramafic rocks in map areas B (Wolf Creek pluton) and F (Virginia City mining district) were sampled for this study (table 2); see Hammarstrom and Van Gosen (1998) for exact sample locations and analytical techniques.…”

“…Berg (1995) reported gold (8 ppb or less), platinum (5 ppb), and palladium (all <2 ppb) values for four samples of ultramafic rocks from the Wolf Creek pluton in the area of the Elk Creek vermiculite deposit. Hammarstrom and Van Gosen (1998) reported on the major and trace element chemistry of two pyroxene-rich ultramafic rocks from the same area (table 2) as well as samples of biotite-rich schist and amphibolite that developed at the metasomatic reaction contact that localized the vermiculite deposit. Chromium concentrations are typical (about 2,000 ppm Cr) for ultramafic rocks, but nickel concentrations are relatively low (<1,000 ppm) for ultramafic rocks (Hawkes and Webb, 1962).…”