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Fluorescent sphalerite rich in tungsten, copper, gallium, silver, and other elements from the Cordilleran-style, polymetallic veins of Philipsburg, Montana

Year: 2024
Type: article
Abstract: • Philipsburg is a Cordilleran-style, polymetallic, porphyry-lode deposit. • Some – not all − sphalerite is strongly fluorescent under long-wave UV light. • Fluorescent sphalerite has low Fe content and is formed at a high sulfidation state. • Trace elements with + 1, +3, +4, and + 6 valence enter ZnS by coupled substitution. • Tungsten concentrations up to 1980 mg/kg exist in bright red fluorescent bands. • High-W sphalerite shows a systematic shift in Raman spectra peaks. Sphalerite from the central, high-sulfidation zone (enargite-stable) of the Philipsburg polymetallic mining district, southwest Montana, displays unusually bright fluorescence (red, orange, yellow, blue, purple, green) under longwave UV light (365 nm). LA-ICP-MS analysis reveals the fluorescent sphalerite has very low Fe (average < 100 ppm) and variable content of other trace elements that correlate to luminescence color banding. Mean/maximum content (in ppm) in fluorescent sphalerite for selected elements are 5.7/7900 Ag, 107/11800 As, 1400/4730 Cd, 917/30400 Cu, 381/5000 Ga, 32/696 Ge, 119/2130 In, 230/8190 Mn, 43/3000 Pb, 16/1700 Sb, and 89/1980 W. This study is the first to document elevated tungsten content (>10 ppm) in sphalerite. Copper is closely correlated with Ga, consistent with the coupled substitution: Cu + + Ga 3+ = 2Zn 2+ . Similar coupled substitution reactions can be written for Ag + , In 3+ , As 3+ , Sb 3+ , Bi 3+ , and Ge 4+ . However, the brightest red fluorescent bands are most closely related to the unexpected presence of W. Sphalerite with high Cu and Ga but lacking W fluoresces yellow and shows a single Raman peak at 349 cm −1 corresponding to pure sphalerite. In contrast, red-fluorescent sphalerite shows the presence of a second peak at 427 cm −1 that increases in intensity with increased W content. We propose that tungsten enters the sphalerite lattice as W 6+ via a substitution such as W 6+ + 4Cu + = 5Zn 2+ and that this substitution creates lattice strain that results in the anomalous fluorescence and Raman signals. Sphalerite bands with low concentrations of Cu and Ga fluoresce blue or green. Vivid blue fluorescence is displayed by sphalerite with high Cd (>1000 ppm) but low concentrations of all other trace elements. Sphalerite from the low-sulfidation peripheral mines of the Philipsburg district contains high Fe (>10,000 ppm) and does not fluoresce. Nonetheless, this sphalerite is also highly enriched in trace metals, including Ag (mean 2480/max 8660 ppm), Cu (1610/3440), Mn (7020/8100), and Sb (1960/6390). The results of this study underscore the importance of including tungsten in the list of analytes in future studies of trace elements in sphalerite. In addition, a hand-held UV lamp may be a rapid and cost-effective method to screen sphalerite of variable composition in outcrop or drill core. It may be a useful exploration tool to vector towards a high-sulfidation zone of a zoned porphyry or epithermal deposit, when it is present.
DOI: 10.1016/j.oregeorev.2024.106267
Language: English
Open Access status: gold
Estimated APC paid: $2,620
OpenAlex ID: W4402982013
FWCI: 0.871
Cites: 33
Cited by: 4
Related to: 10