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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

1998; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr98224c

2332-4899

Jane M. Hammarstrom, Bradley S. Van Gosen,

Clay minerals and soil interactions

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 data for sites that had been mined or studied in the past; (3) develop a geochemical database for rocks to aid interpretation of stream sediment or other geochemical data; and (4) provide independent, objective, baseline geochemical data for areas that are most likely to be explored or developed in the reasonably foreseeable future.Samples are grouped by mining district (table 1, fig.3), and field sample numbers are assigned sequential map key numbers for plotting.These map key numbers are used for reference throughout this report.Table 1 includes sample locations (7.5-minute quadrangle name, latitude, longitude, descriptive information), sample descriptions, and analytical job number.Sample numbers followed by an asterisk represent replicates submitted for quality control.Different symbols are used for different samples types in figure 3. Sample types are grouped into the following general categories: Archean gneiss, iron formation, quartz vein, skarn, stream sediment, ultramafic rocks, and other altered rocks.See table 1 for a detailed explanation of sample type and for detailed locations of closely-spaced samples.7.5 minute quadrangles are shown as a faint grid on figure 3. Samples from the Chinatown (fig.4) and Virginia City (fig.5) districts are plotted on more detailed maps, along with locations of gold ' : :':-Mi §l