Explained by Dennis P. Cox
On the choice of deposit models
The Tertiary volcanic rocks in Arizona are considered permissive for quartz-alunite gold deposits. Although there are no known occurrences of quartz-alunite gold mineralization in the tract, deposits of this type are known in neighboring parts of California (Mojave district), and Nevada, (Alunite district), and occurrences of alunite alteration are known in nearby New Mexico (Steeplerock district).
On the delineation of permissive tracts
The permissive tract was compiled using the 1988 Geologic Map of Arizona (Reynolds, 1988, map 26). The rocks included Triassic and Jurassic through Quaternary andesites through rhyolites as well as associated subvolcanic intrusions and volcano-sedimentary rocks that are the products of both calc-alkaline and bimodal volcanism. These volcanic fields delineate the permissive tract for epithermal mineral deposits.
The permissive tract was extended under younger units where justified by geologic interpretation of the State map. More speculative extensions were made by assuming a 10° dip and extending the unit either to the point where it would be 1 km deep (a lateral distance of about 6 km) or to a point halfway between the volcanic outcrop and another, nonvolcanic unit. In general, pre-Tertiary units are too discontinuous to extend with confidence under cover. Rocks are not delineated permissive where depth to bedrock is greater than 1 km, based on the Depth to Bedrock Map for Arizona (Oppenheimer and Sumner, 1980). The resulting tract is essentially a map of volcanic rocks, but also includes those parts of epithermal districts that do not plot precisely in the volcanic terrains.
On the numerical estimates made
Because of the lack of known deposits in the tract, we considered that the probability of occurrence of hot-spring gold deposits was less than 0.01 percent, and we made no estimate.
Oppenheimer, J.M., and Sumner, J.S., 1980, Depth-to-bedrock map, Basin and Range province, Arizona: Tucson, University of Arizona, Laboratory of Geophysics.
Reynolds, S.J., 1988, Geologic map of Arizona: Arizona Geological Survey Map 26, scale 1:1,000,000.