TRACE METAL COMPOSITION OF FLUORESCENT SPHALERITE FROM BUTTE, MONTANA

Better known for its prodigious copper and silver endowment, the polymetallic, porphyry-lode deposits of Butte, Montana produced over 2.2 billion kg of refined zinc, making it one of the largest historic producers of this critical metal in the U.S. We have begun a systematic investigation of the trace element composition of sphalerite and other ore minerals across the zoned Butte district, with comparison to the nearby Philipsburg district, a porphyry-lode system in southwest Montana with similarities to Butte. Based on preliminary LA-ICP-MS analyses, trace metals that are enriched in Butte sphalerite (maximum values in ppm) include Ag (13000), Cu (11200), Mn (11200), Cd (6900), In (6630), Ga (5610), W (1690), As (1294), and Ge (963). Sphalerites from Butte with Fe < 1000 ppm show strong photoluminescence under long wave (365 nm) UV light, including various shades of deep blue, green, yellow, orange, and bright red fluorescent colors. Sphalerites with > 1 wt% Fe are non-fluorescent, and often have abundant small inclusions of chalcopyrite. Sphalerite that fluoresces deep blue has low concentrations of most trace elements, with the exception of Cd. Bright red fluorescence is correlated with the presence of several metals, including Cu, Ga, and W. Tungsten is believed to enter the sphalerite lattice in a coupled substitution with copper as follows: W⁶⁺ + 4Cu⁺ = 5Zn²⁺ (Beaucamp et al., Goldschmidt 2022, Abstract 11433). Red-fluorescing sphalerite grains show a shift in the dominant peak of their Raman emission spectra from 349 to 427 cm^-1. This phenomenon is also tentatively linked to coupled substitution of univalent copper, hexavalent tungsten and/or trivalent gallium into the sphalerite structure. Strongly luminescent sphalerite requires a low content of Fe (a fluorescence inhibitor), which in turn can be explained by either a high fugacity of S₂ or undersaturation with common Fe-sulfide minerals such as pyrite or pyrrhotite. Fluorescent sphalerite in the high-sulfidation “central zone” directly overlying a porphyry intrusion may be a general feature applicable to other porphyry-lode, or “Cordilleran-style”, deposits. Interpretation of new LA-ICP-MS data on Butte sulfides is ongoing, with the broader goal of using trace elements to fingerprint geochemical and critical commodity zonation within the district.