Explained by Warren Day
On the choice of deposit models
In southernmost Wyoming and northern Colorado, the Proterozoic rocks exposed in the Medicine Bow, Park, and Sierra Madre Mountains contain several small massive sulfide deposits and occurrences that have characteristics similar to Besshi-type massive sulfide deposits. The descriptive model of Cox (1986) and the grade and tonnage model of Singer (1986), based on examples from Japan, were selected and used for assessment.
On the delineation of permissive tracts
Besshi-type volcanic-hosted massive sulfide deposits produce copper and zinc, with gold and silver as common byproducts. These deposits form thin, sheetlike stratiform bodies of massive to well-laminated pyrite, pyrrhotite, and chalcopyrite with interlayers of clastic sedimentary rocks and mafic tuffs and flows (Cox, 1986). The genesis of this deposit type is uncertain due to the amount of regional deformation and metamorphism of the deposits in the type locality in Japan. However, the general exploration criteria for these deposits include: (1) clastic terrigenous marine sedimentary rocks and mafic volcanic tuffs and breccia host rocks; (2), locally are associated with black shale, oxide-facies iron formation and red chert; (3) and, form from submarine hot springs related to basaltic magmatism in rifted basins in island-arc or back-arc tectonic settings.
Klipfel (1992) suggested that the massive sulfide deposits of the Encampment district of the Sierra Madre Mountains, which extends into northern Colorado, are of Besshi type. In the western part of this district, Cu ± Zn Besshi-type deposits form in dominantly mafic volcanic host rock terrane, whereas in the eastern part, Pb - Zn ± Ag deposits occur in metamorphosed metasedimentary rocks and may be more like sedimentary exhalative deposits (Klipfel, 1992). Several of the ore deposits and occurrences are locally associated with iron-formation, ferruginous chert, and volcanic rocks, indicating synvolcanic hot-spring activity conducive for formation of Besshi-type massive sulfide deposits.
Important examples of this type of deposit
Several small massive sulfide deposits are exposed and have been exploited in the Encampment district of the Sierra Madre Mountains and in the Centennial Ridge area of the Medicine Bow Mountains.
On the numerical estimates made
The assessment team estimated that the package of rocks in the Encampment district has the highest potential for hosting an undiscovered Besshi-type massive sulfide deposit. We estimated that there is a 1 percent chance of the presence of an undiscovered Besshi-type massive sulfide deposit in the Encampment district in the Sierra Madre Mountains. In the Medicine Bow Mountains, which are adjacent to this area to the east, there are two areas that may host similar types of deposits. In the Centennial Ridge area, smaller massive sulfide deposits and occurrences in rocks of similar age and grossly similar tectonic setting. Therefore, we judged that there was a lesser chance for the presence of a deposit in this area.
For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 0, 0, and 1 or more deposits consistent with the grade and tonnage model of Singer (1986).
Cox, D.P., 1986, Descriptive model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136.
Klipfel, P.D., 1992, Geology and metallogeny of the southern portion of the Encampment district, Colorado and Wyoming: Golden, Colorado School of Mines, Unpublished Ph.D. dissertation, 244 p.
Singer, D.A., 1986, Grade and tonnage model of Besshi massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 136-138.