Explained by Alan R. Wallace and Virginia McLemore

On the choice of deposit models

Known quartz-adularia precious-metal veins occur in felsic to intermediate volcanic rocks in this area. 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. Base metals are not prominent in the known examples, and, as a result, the team assessed the New Mexico quartz-adularia veins using a composite model of Comstock and Sado vein deposits (Mark3 index 25).

On the delineation of permissive tracts

Extensive felsic to intermediate Tertiary volcanic fields in the area contain known quartz-adularia deposits, prospects, and occurrences. These volcanic fields are variably cut by younger intrusive rocks, and faults and caldera ring fractures are common. Therefore, permissive tracts were delineated to consist of areas of volcanic rocks of intermediate to felsic composition. The absence of faults or other fluid-focusing structures on the maps we used was not considered a negative criterion for assessment. This tract is exclusive of southwestern New Mexico, which is part of the Southern Basin and Range Province.

Important examples of this type of deposit

Cochiti (about 1.3 metric tons Au, 6.5 metric tons Ag) and Chloride are significant districts. In addition, 22 other districts and prospects were identified that are small or did not have adequate grade and tonnage information.

On the numerical estimates made

The team assessed each of the 22 prospects and arrived at a total expected value of about 0.7 deposits consistent with the combined Comstock-Sado grade and tonnage model. Additional estimates for the Black Range and for the covered area in the Jemez caldera area added 0.7 deposits. After reviewing the size of the known deposits and prospects, the extent of volcanic rocks, and the extent of covered areas, the team decided that the first estimates were uniformly between 50 and 100 percent too low. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 2, 5, 6, and 7 or more districts consistent with the combined Comstock-Sado grade and tonnage model.