Explained by David Frishman

On the choice of deposit models

We chose to evaluate Montana for hot-spring Au-Ag deposits because the geology is permissive for the occurrence of this type of deposit and because deposits of this type occur in the State. We used the descriptive model of Berger (1986) and the grade and tonnage model of Berger and Singer (1992).

On the delineation of permissive tracts

For the purpose of delineating the permissive tract, the three subtypes of epithermal precious-metal vein deposits (quartz-adularia, quartz alunite, and hot-spring) were treated the same. It was judged that the criteria available to discriminate between the subtypes were too imprecise to use effectively. Probabilities of occurrence for the individual types were assigned separately, however.

Permissive areas that constitute the tract are generally small, and are scattered throughout western Montana. They were defined by the location of known active or fossil hot springs, the location of mines, prospects, and occurrences believed to represent epithermal deposits, and the location of Tertiary and Quaternary volcanic and hypabyssal rocks as shown on the Montana geologic map (Ross and others, 1955).

Areas identified include the Hog Heaven volcanic field, the Lincoln volcanics (McDonald Meadows property), the Helena-Avon volcanics, several areas of the Lowland Creek volcanics southwest of Helena and north and west of Butte, the Virginia City-Alder Gulch area, an area of Challis volcanics exposed in the Horse Prairie area southwest of Dillon, the Marysville district, and small portions of Montana adjacent to the Yellowstone caldera in Wyoming.

Important examples of this type of deposit

Possible representatives of this class of deposit are all apparently Tertiary in age (<50 Ma) and include the volcanic-hosted McDonald Meadows deposit, south of Lincoln in Lewis and Clark County; the Pauper's Dream property (or Basin Creek mine), southwest of Helena; and the Tuxedo prospect, west-northwest of Butte.

On the numerical estimates made

For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 3, 5, 6, and 9 or more deposits consistent with the grade and tonnage model of Berger and Singer (1992).

References

Berger, B.R., 1986, Descriptive model of hot-spring Au-Ag deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 143.

Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model of hot-spring Au-Ag, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.

Ross, C.P., Andrews, D.A., and Witkind, I.J., 1955, Geologic map of Montana: U.S. Geological Survey, scale 1:500,000.