Explained by Alan R. Wallace
On the choice of deposit models
Modern hot springs are common in Colorado, and geothermal activity was undoubtedly associated with the many Tertiary volcanic centers in Colorado. Erosion has probably stripped away many potential deposits, which form at or near the surface, but others many be concealed in volcanic piles such as the San Juan volcanic field or still preserved due to their relative youth. 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
Two areas were deemed permissive for hot-spring gold-silver deposits: (1) thick volcanic piles, such as the Flat Tops, Thirtynine Mile, and San Juan volcanic fields, where hot-spring deposits related to volcanic activity could have been concealed by subsequent eruptions, and (2) areas of modern geothermal activity (Pearl, 1980), some of which may be generating or have already generated a gold deposit.
Important examples of this type of deposit
No true hot-spring gold-silver deposits are known in the State. Possible examples of partially eroded deposits may include the Golden Wonder mine, in the Lake City district (Kalliokoski and Rehn, 1987), the San Luis gold deposit near San Luis, prospects along the west side of the Sangre de Cristo Mountains between Crestone and the Great Sand Dunes, and Poncha Hot Springs, where one sample of fluorspar ore contained 100 ppb Au (A. Wallace, unpub. data, 1993).
On the numerical estimates made
Colorado is known for its past and present high heat flow, and the permissive tracts easily could conceal a geothermal center, although very few such centers produce gold-silver deposits of the size consistent with the grade and tonnage model. Nevertheless, the amount of concealed volcanic rocks in the various volcanic fields and the presence of modern geothermal activity permits some chance for the existence of a hot-spring gold-silver deposit in the permissive areas. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 2 or more deposits consistent with the grade and tonnage model for hot-spring gold-silver deposits (Berger and Singer, 1992).
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 for hot-spring Au-Ag, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.
Kalliokoski, J., and Rehn, Patty, 1987, Geology of the veins and vein sediments of the Golden Wonder Mine, Lake City Colorado; an epithermal hot springs gold-alunite deposit: U.S. Geological Survey Open-File Report 87-344, 47 p.
Pearl, R.H., 1980, Geothermal resources of Colorado: Colorado Geological Survey Map Series MS-14, scale 1:500,000.