Explained by Alan R. Wallace and Virginia McLemore
On the choice of deposit models
Quartz-alunite gold deposits are related to felsic volcanic and subvolcanic rocks, which are abundant in southwesern New Mexico. Based on the characteristics of known occurrences and prospects, we used the descriptive model of Berger (1986) and the grade-tonnage model of Mosier and Menzie (1986).
On the delineation of permissive tracts
Felsic volcanic rocks are common in southwestern New Mexico, and acid-sulfate alteration has been reported in these rocks. Felsic to intermediate volcanic and related subvolcanic rocks and their shallow extensions make up the permissive tract.
Important examples of this type of deposit
Alum Mountain north of Silver City and prospects in the Steeple Rock district are quartz-alunite prospects. In addition, quartz-alunite deposits were recognized by industry in the Wilcox district (J.C. Ratté, oral commun., 1994) and the Cooke's Range (V.T. McLemore, written commun., 1993).
On the numerical estimates made
Although the permissive tract is not large, acid-sulfate alteration is evident in at least four districts, several prospects are present, and exploration companies have specifically looked for this type of deposit here. Most districts in New Mexico have not been adequately explored at depth, and drilling and other exploration programs are needed to better evaluate the potential for undiscovered deposits (V.T. McLemore, written commun., 1993). For the 90th, 50th, 10th, 5th, and 1st percentiles, the team made estimates of 0, 0, 1, 2, and 4 or more deposits consistent with the grade and tonnage model for quartz-alunite deposits of Mosier and Menzie (1986).
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.
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.