Explained by David Frishman

On the choice of deposit models

We chose to evaluate Montana for epithermal quartz-alunite gold deposits of the type described by Berger (1986) because the geology of Montana seems to be permissive for the occurrence of this type of deposit. We used the grade and tonnage model of Mosier and Menzie (1986).

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, volcanic rocks near Lincoln (McDonald Meadows property), volcanic rocks near Helena and Avon, 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 parts of Montana adjacent to the Yellowstone caldera in Wyoming.

Important examples of this type of deposit

We know of no deposits of this type in the State, nor any sizeable areas that have undergone quartz-alunite alteration, although alunite is present locally in some of the geyser basins in Yellowstone National Park (White and others, 1988) and quartz-kaolinite-alunite alteration occurs in the Hog Heaven volcanic field (Cossaboom, 1981).

On the numerical estimates made

Because there are no known prospects or areas of alteration and because the tectonic regime during volcanism was apparently never extensional, it was judged that the chance for the occurrence of quartz-alunite deposits in the permissive tract was very low. The team assigned a 1 percent chance for the occurrence of an undiscovered quartz-alunite gold deposit like those described by Mosier and Menzie (1986).

References

Berger, B.R., 1986, Descriptive model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 158.

Cossaboom, C.C., 1981, Alteration, petrology, and mineralization of the Flathead mine, Hog Heaven mining district, Flathead County, Montana: Missoula, University of Montana, Masters thesis, 103 p.

Mosier, D.L., and Menzie, W.D., 1986, Grade and tonnage model of epithermal quartz-alunite Au, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 159-161.

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

White, D.E., Hutchinson, R.A., and Keith, T.E.C., 1988, The geology and remarkable thermal activity of Norris Geyser Basin, Yellowstone National Park, Wyoming: U.S. Geological Survey Professional Paper 1456, 84 p.