A PETROLOGIC AND GEOCHEMICAL INVESTIGATION OF MINERALIZATION AT THE YELLOW JACKET MINE, OURAY COUNTY, COLORADO

The Yellow Jacket mine, located in the historic Uncompahgre mining district, is part of the hydrothermal-vein system that developed after collapse of the ~27.5 Ma Silverton caldera. The mine was promoted around 1915 but was never developed even though samples can contain ~60% sulfide minerals. The vein is exposed on strike for several hundred meters and is about 2 meters thick, but the vertical extent is not constrained. One of the most striking features of the mineralized vein is the presence of distinct masses of red-orange sphalerite. This is not a typical occurrence in the western San Juan Mountains where green to black sphalerite is most common.

In this investigation samples from the Yellow Jacket vein were studied to assess the mineral associations, chemistry, and paragenesis. This was done to gain an understanding of the history of mineralization at the mine, and to explore the chemistry of the red-orange sphalerite. The ore is dominated by pyrite, sphalerite, galena, and chalcopyrite that are encapsulated by calcite with minor fluorite. Reflected light microscopy reveals a paragenesis defined by two generations of sphalerite with pyrite and galena sequentially crystallizing after the first generation. Fluorite is only distinguishable in thin section along the grain boundaries of calcite.

Electron microprobe analyses indicate that galena is argentiferous with most spots yielding 300 to 400 ppm Ag. Pyrite typically contains 100 to 800 ppm Cu but has no detectable Au. Sphalerite contains 65 to 68 weight percent Zn along with variable Fe (100 to 1800 ppm), As (0 to 600 ppm), Cu (0 to 3900), Cd (600 to 5800 ppm), and Sb (0 to 2500 ppm). Pronounced chemical zonation is evident in the sphalerite. The red-orange zones dominate in the core and contain elevated concentrations of As, Cu, Cd, and Sb. These concentrations amplify with increasing intensity of the red-orange color. Brown to green zones generally have higher concentrations of Fe and significantly lower concentrations of all other elements.

A bulk-sample assay for Yellow Jacket ore yielded 0.026 opt Au and 1.86 opt Ag. Results of this investigation indicate that the Yellow Jacket mine has a low potential for Au and Ag, but base metal concentrations are elevated and significant. Future exploration may be able to define a small but profitable deposit at this location.