Explained by David Frishman

On the choice of deposit models

We chose to evaluate Montana for epithermal quartz-adularia vein deposits because the geology of Montana seems to be permissive for the occurrence of this type of deposit. Several important examples are known. We did not feel that we could distinguish between the different styles of mineralization described by Mosier, Menzie, and Kleinhampl (1986), and so we evaluated the possible occurrence of epithermal vein districts using a combination of the Comstock and Sado grade and tonnage models (Mark3 index 25).

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

Possible representatives include the Flathead and West Flathead mines and related Oligocene deposits in the Hog Heaven volcanic field west of Flathead Lake, some of the andesite-hosted veins (age unknown) in the Virginia City district, an epithermal vein in the Elkhorn Mountains, and chalcedony veins in the Clancy district south of Helena. It should be noted that some deposits, for example, the epithermal Pb-Ag deposits at the Flathead mine, do not possess exactly the same attributes as the published models. The deposits at the Flathead mine apparently occur at the very uppermost levels of the hydrothermal system, yet they are dominantly Pb-Ag deposits, not gold-rich deposits as might be expected according to some models.

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.

References

Mosier, D.L., Menzie, W.D., and Kleinhampl, F.J., 1986, Geologic and grade-tonnage information on Tertiary epithermal precious- and base-metal vein districts associated with volcanic rocks: U.S. Geological Survey Bulletin 1666, 39 p.

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