Abstract
The origin of V, U, REE and other metals in the Permian Phosphoria Formation have been speculated and studied by numerous scientists. The exceptionally high concentrations of metals have been interpreted to reflect fundamental transitions from anoxic to oxic marine conditions. Much of the oil in the Bighorn Basin, is sourced by the Phosphoria Formation. Two of the top 10 producing oil fields in Wyoming are located approximately 50 km west of two abandoned U-V mining districts in the northern portion of the basin. These fields produce from basin margin anticlinal structures from Mississippian age reservoir rock. Samples collected from abandoned U-V mines and prospects hosted in Mississippian aged paleokarst in Montana and Wyoming have hydrocarbon residue present and contain anomalous high concentrations of many metals that are found in similar concentrations in the Phosphoria Formation. As, Hg, Mo, Pb, Tl, U, V and Zn, often metals of environmental concern occur in high concentrations in Phosphoria Formation samples and had values ranging from 30–1295 ppm As, 0.179–12.8 ppm Hg, 2–791 ppm Mo, <2–146 ppm Pb, 10–490 ppm Tl, 907–86,800 ppm U, 1240–18,900 ppm V, and 7–2230 ppm Zn, in mineralized samples from this study. The REE plus Y composition of Madison Limestone- and limestone breccia hosted-bitumen reflect similar patterns to both mineralized samples from this study and to U.S. Geological Survey rock samples from studies of the Phosphoria Formation. Geochemical, mineralogical and field data were used to investigate past theories for mineralization of these deposits to determine if U present in home wells and Hg content of fish from rivers on the proximal Crow Indian Reservation may have been derived from these deposits or related to their mode of mineralization.
Highlights
The original impetus for this study was to try and determine the source of Hg and Pb in the Bighorn River
Geochemical, mineralogical and field data were used to investigate past theories for mineralization of these deposits to determine if U present in home wells and Hg content of fish from rivers on the proximal Crow Indian Reservation may have been derived from these deposits or related to their mode of mineralization
We suggest that breaching and subsequent U mineralization in the Pryor Mountain U-V Mining District (PMD) was earlier than in the Little Mountain U-V Mining District (LMD) based on elevation and structural maturity of the folds
Summary
The original impetus for this study was to try and determine the source of Hg and Pb in the Bighorn River. The Bighorn River is listed as a 303d impaired water body downstream of the Crow. Indian Reservation due to elevated Pb and Hg and the source of contamination is unknown [1]. In a 2015 United States Geological Survey (USGS) study, fish tissue from the Bighorn Reservoir had the highest Hg levels observed in the entire Yellowstone River Basin (0.676 μg/g wet weight) [2]. The Bighorn River and its tributaries drain both the Pryor Mountain U-V Mining District (PMD), Montana (MT) and the Little Mountain U-V Mining District (LMD), Wyoming (WY) (Figure 1).
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