INTERPRETING MINERALOGICAL ASSEMBLAGES OF THE SILVER CLIFF CALDERA

Volcanism that produced the Silver Cliff Caldera, Custer County, Colorado from approximately 25mya-23mya, resulted in igneous activity from tectonics associated with the initiation of the Rio Grande Rift System of Colorado and New Mexico. The volcanics are characterized by felsic petrology consisting of 20 differentiated rock types. To elucidate the processes that occurred at this site and to assess the future possibilities of economic value of the area, the mineral assemblages are interpreted. From these 20 rock types, five mineralogical groups describe the petrology of this locality. The five assemblages are silicates, oxides and hydroxides, sulfides and associated minerals (tellurides), native elements (gold and silver), and other accessory minerals (thorium and zirconium).

Silicate minerals dominate in rhyolitic flow deposits of varying emplacement mechanism and crystallinity. These encompass all but five of the mapped units and contain quartz, sanidine, mixed feldspars, various clays, and bromargyrite. Oxides and hydroxides cryptomelane, goethite, and chalcophanite are widespread alteration products associated with dendrites of argentiferous manganese oxide. Sulfides and associated minerals are present in the form of galena, sphalerite, pyrite, with a potential of tellurides. Epithermal emplacement of native elements, gold and silver, led to profitable mining ventures beginning in the late 1800’s and continuing through the 1950’s. Other accessory minerals found within the boundaries of the Silver Cliff Caldera include fluorite, silver chlorides, carbonates, and rare earth minerals.

Field study of the area, in addition to analytical work performed on collected samples, confirms current literature concerning the above mineralogical suites. This was achieved by selecting and mapping previous mining areas thought to be suitable representations of the petrology of Silver Cliff. In the laboratory, multiple thin sections of each sample were made for observation under the petrographic microscope to corroborate hand sample identification. Additional testing was also conducted utilizing Raman spectroscopy. The results of the investigation trend toward the conclusion that the area of Silver Cliff continues to possess value as a potentially profitable site for mineral exploration.