Explained by Alan R. Wallace
On the choice of deposit models
Proterozoic medium-grade (upper greenschist to amphibolite grades) metamorphic rocks are cut by shear zones and faults that are demonstrably of Proterozoic age.
On the delineation of permissive tracts
The criteria for inclusion included: (1) the presence of supracrustal (volcanic or sedimentary) host rocks of Proterozoic age; (2) medium-grade (greenschist to lower-amphibolite facies) regional metamorphism; (3) the presence of gold deposits, placers, prospects, and occurrences; (4) evidence of large-scale shearing and faulting; and (5) alteration mineral assemblages within and adjacent to faults and shear zones containing quartz and carbonate veining, phyllosilicate minerals sericite, chlorite, and (or) biotite, and the sulfide minerals pyrite, arsenopyrite, and (or) pyrrhotite. Areas underlain principally by granitoids or gneisses were excluded. The most favorable areas are in Wyoming, in the Medicine Bow and Sierra Madre Mountains and in the Laramie Range.
Important examples of this type of deposit
No vein deposits in Colorado are demonstrably of Proterozoic age. Small deposits occur along faults that cut syngenetic massive sulfide and sedimentary exhalative deposits, such as in the Gunnison gold belt and at the Homestake mine west of Tennessee Pass, but the ages of these deposits are unknown and may be as young as Tertiary. In Wyoming, roughly 4,500 oz of gold were produced in the late 1860s from the Centennial Ridge district in the Medicine Bow Mountains (Hausel, 1989).
On the numerical estimates made
Although the Proterozoic basement of Colorado contains low- to medium-grade metamorphic rocks and is cut by various faults and shear zones, none of the gold deposits along those faults are of the appropriate mineralogy nor of demonstrably Proterozoic age, unlike similar deposits in Wyoming and South Dakota. We estimated that there may be two low-sulfide Au-quartz vein deposits in the area of Centennial Ridge district in the Medicine Bow Mountains. Although the total amount of gold ore recovered in the district is relatively low, there are numerous gold occurrences throughout the district indicating that widespread mineralization occurred along faults and in shear zones. The bedrock of this region is poorly exposed, and, therefore, we believe that the potential for concealed gold-quartz vein deposits remains relatively high. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 2, 3, 4, and 5 or more deposits consistent with the low-sulfide Au-quartz grade and tonnage model of Berger (1986) (Mark3 index 27); most of them are believed to be in the part of the tract in southern Wyoming.
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.
Hausel, W.D., 1989, The geology of Wyoming's precious metal lode and placer deposits: Geological Survey of Wyoming Bulletin 68, 248 p.