Explained by Douglas B. Stoeser
On the choice of deposit models
Quartz-alunite gold deposits are associated with felsic to intermediate volcanic centers with associated shallow intrusions. Although no deposits are known in the State, hydrothermal alunite is present in the Marysvale district, and volcanic rocks are abundant in western Utah. We used 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 belong to the epithermal class of mineral deposits, and typically form within a few hundred meters of the Earth's surface. They occur mainly within volcanic vent areas. Permissive rocks for this type of deposit consist of volcanic fields which have undergone minimal erosion. These are present in three east-trending belts in western Utah. In addition to the Miocene and older calc-alkaline belts that also have porphyry copper potential, the younger (20 Ma to present) extension-related, evolved, silicic systems are also prospective for epithermal gold deposits. Most of the silicic extensional magmatism also occurs within the three same areas that contain intrusion-related deposits (porphyry copper, polymetallic replacement, and skarn). Utah was considered permissive for this deposit type even though there was little direct evidence for its occurrence in western Utah.
Important examples of this type of deposit
No known quartz-alunite gold deposits occur in Utah, but areas of hydrothermal alteration that include alunite deposits occur in the Tushar Mountains area of the Marysvale volcanic field.
On the numerical estimates made
Despite the paucity of deposits or prospects in the State, the team felt that the extent of volcanic rocks and the amount of alluvial cover permitted the possible occurrence of this type of deposit. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 1, and 2 or more deposits consistent with the grade and tonnage model for quartz-alunite gold 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.