Explained by Alan R. Wallace
On the choice of deposit models
Quartz-alunite gold deposits are related to felsic volcanic and subvolcanic rocks, which are found in several volcanic fields in the State. The existence of the deposit at Summitville, an archetypal, if infamous, example led us to use the descriptive model of Berger (1986) and the grade and tonnage model of Mosier and Menzie (1986).
On the delineation of permissive tracts
Quartz-alunite gold deposits are associated with felsic volcanic and subvolcanic rocks, which in Colorado are of Tertiary age. The permissive tract encompasses Tertiary felsic volcanic rocks as shown on the State map. The principal area is the San Juan volcanic field in southwestern Colorado.
Important examples of this type of deposit
Summitville is a large known quartz-alunite gold deposit (Steven and Ratté, 1960; Perkins and Nieman, 1982). Within the San Juan Mountains, known prospects include Red Mountain near Lake City, Crater Lake-Quartz Creek near Summitville, Piedra Peak-Red Mountain south of Creede, and Calico Peak near Rico; the San Juan National Forest assessment (VFoley and others, in press) also identified seven additional areas that had potential for undiscovered quartz-alunite gold deposits. Minor alunite alteration has been reported at other areas in the State, such as the San Luis Hills southwest of Alamosa (Bartlett, 1984).
On the numerical estimates made
The state of knowledge about volcanic-hosted precious-metal deposits in Colorado is fairly good, and many areas have been prospected for this and other types of volcanic-hosted mineral deposits. Some deposits could be concealed by younger volcanic rocks in areas such as the San Juan Mountains. Very few, if any, occurrences of quartz-alunite alteration are known outside of the San Juan volcanic field, so, although felsic volcanic rocks are present outside of that area, they appear not to be as favorable as those in the San Juans. In making our estimates, we drew, in part on those made for the San Juan National Forest assessment (Foley and others, in press). For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 3, and 5 or more deposits consistent with the grade and tonnage model for quartz-alunite gold deposits of Mosier and Menzie (1986).
Bartlett, R.D., 1984, Geology of an Oligocene-age acid hot spring, San Luis Hills, Conejos and Costilla Counties, Colorado: Fort Collins, Colorado State University, unpublished Master's thesis, 111 p.
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.
Foley, N. K., Van Loenen, R.E., Nash, J.T., and Gibbons, A.B., in press, Mineral resource potential—locatable minerals,in Van Loenen, R.E. and Gibbons, A.B., eds., Mineral resource potential and geology of the San Juan National Forest, Colorado: U.S. Geological Survey Bulletin, 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.
Perkins, M., and Nieman, G.W., 1982, Epithermal gold mineralization in the South Mountain Dome, Summitville, Colorado: Denver Region Exploration Geologists Society Symposium on the Genesis of Rocky Mountain Ore Deposits—Changes with time and tectonics, Denver, Colorado, Proceedings, p. 165–172.
Steven, T.A., and Ratté, J.C., 1960, Geology and ore deposits of the Summitville district, San Juan Mountains, Colorado: U.S. Geological Survey Professional Paper 343, 70 p.