Explained by Alan R. Wallace and Virginia McLemore
On the choice of deposit models
Numerous known quartz-adularia precious-metal veins occur in felsic to intermediate volcanic rocks in southwest New Mexico. The general lack of information on grades and tonnages precluded classifying known deposits and potential undiscovered deposits into separate Creede, Comstock, and Sado deposit models. Therefore, a general volcanic-hosted quartz-adularia model was adopted, which is a combination of the Comstock and Sado data (Mark3 index 25).
On the delineation of permissive tracts
Extensive felsic to intermediate Tertiary volcanic fields in the area contain known quartz-adularia deposits. These volcanic fields are variably cut by younger intrusive rocks, and faults and caldera ring fractures are common. Therefore, permissive tracts were delineated to include areas of volcanic rocks of intermediate to felsic composition. The absence of faults or other fluid-focusing structures on maps was not considered a negative criteria for assessment.
Important examples of this type of deposit
Mogollon is a known district, and Steeple Rock is a significant prospect that exhibits characteristics of both quartz-adularia and quartz-alunite deposits. In addition, six additional prospects—Wilcox, Kimball/Steins, San Francisco, Gila Fluorspar, Carrizalillo, and Gillespie, —were identified in southwestern New Mexico.
On the numerical estimates made
The team assessed each of the seven prospects and first arrived at an expected value of about 1.3, including 1 deposit represented by unannounced but known reserves at Steeple Rock. Additional estimates for the greater Steeple Rock district and for the covered area in Hidalgo County added 0.4 deposits for a total expected value of about 1.7. The team reviewed the size of the known deposits and prospects and the extent of volcanic rocks, decided that the estimates were uniformly betwen 50 and 100 percent too low, and further revised the estimates. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 2, 3, 6, 7, and 8 or more districts consistent with the grade and tonnage model for the generalized quartz-adularia deposit type.