Explained by Warren Day
On the choice of deposit models
Proterozoic medium-grade (upper greenschist to amphibolite grade) metamorphic rocks are cut by Proterozoic shear zones and faults. Scattered prospects and occurrences of Au-bearing quartz veins are present.
On the delineation of permissive tracts
To delineate the permissive tract, we used the following criteria: (1) the presence of supracrustal (volcanic or sedimentary) and granitoid host rocks of Proterozoic age; (2) medium-grade (greenschist to lower-amphibolite facies) regional metamorphism; (3) the presence of gold deposits, prospects, and occurrences; (4) evidence of large-scale shearing and faulting, and; (5) alteration mineral assemblages within and adjacent to faults and shear zones that contain quartz and carbonate veining, phyllosilicate minerals (sericite, chlorite, and/or biotite), and sulfide minerals (pyrite, arsenopyrite, and/or pyrrhotite). As in the Archean examples for Wyoming, there are no known low-sulfide Au-quartz vein deposits of significant size in Proterozoic granitoids or gneissic terranes, and areas underlain by these rock types were not considered permissive. Proterozoic rocks are found only in the southern part of the State.
Important examples of this type of deposit
A small amount of high-grade (up to 67 g/t) ore was produced from the Gold Hill district on the western flank of the Snowy Range in the central Medicine Bow Mountains (Hausel, 1989).
On the numerical estimates made
The Gold Hill district on the western flank of the Snowy Range in the central Medicine Bow Mountains is estimated to have about an 8 percent chance of hosting an undiscovered low-sulfide gold deposit. The small bedrock mines of the district were developed on quartz veins associated with mafic dikes. 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 low-sulfide Au-quartz vein deposits of Bliss (1986).
Bliss, J.D., 1986, Grade and tonnage model of low-sulfide Au-quartz veins, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 239–243.
Hausel, W.D., 1989, The geology of Wyoming's precious metal lode and placer deposits: Geological Survey of Wyoming Bulletin 68, 248 p.