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Master of Science (MS)
Pevear, David R.
Babcock, R. Scott (Randall Scott)
Three "siliceous reef" uraniferous vein deposits of the Boulder batholith were studied. Two of these, the Free Enterprise and the W. Wilson, occur within hydrothermally altered Late Cretaceous Butte Quartz Monzonite and related rocks. Both deposits contain fine-grained iron and base-metal sulfides and uraninite in a matrix of microcrystalline quartz and chalcedony. The deposition of uraninite in these veins was contemporaneous with the majority of the sulfide minerals. The third deposit studied, the uraniferous Red Rock Mine, is similar in many respects to the other two occurrences, yet occurs within altered Late Cretaceous Elkhorn Mountains Volcanics.
All three occurrences formed under physical and chemical conditions similar to those of present day hot springs near Boulder, Montana. Hydrothermal alteration products and precipitate mineralogy (including uraniferous carbonates) of these hot springs are similar to those found associated with the veins. An estimated hot spring subsurface temperature of around 150°C is considered a reasonable maximum for vein formation. At this temperature the pH-f02 conditions at the time of deposition suggest vein formation either by (1) boiling of an ascending hydro- thermal solution, or (2) mixing of a hydrothermal solution with ground water. The first method is favored for the formation of the Boulder Hot Springs uraniferous precipitates and the Red Rock Mine deposit; and is supported by the presence of adularia and calcite at both occurrences. Vein mineralogy and phase relationship studies relating to the Free Enterprise and W. Wilson deposits suggest vein formation by the mixing of two solutions: (1) an iron, base-metal, sulfide-rich, reducing, hot-spring solution, and (2) a cooler, oxidizing uranium-enriched solution.
Potential sources of leachable uranium for the vein deposits are the Boulder batholith and the Oligocene post-Lowland Creek volcanics. Pb206 /U283 age dates of 45 ± 10 m.y. on ore samples from the W. Wilson suggest that the overlying, locally uranium-enriched, volcanics were the main supplier of uranium for the Free Enterprise and W. Wilson vein deposits. Lead isotope data indicate that the lead associated with these two deposits was derived from the leaching of Butte Quartz Monzonite rocks by circulating meteoric waters with subsequent incorporation into hot spring systems. The apparent age of deposition at the Red Rock Mine (50-70 m.y.) points to the Boulder batholith rocks as the uranium source.
Uranium bearing vein deposits
Western Washington University
Subject – LCSH
Uranium ores--Montana--Boulder Region; Petrology--Montana--Boulder Region; Faults (Geology)--Montana--Boulder Region; Hot springs--Montana--Boulder Region
Boulder Region (Mont).
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Dodd, Stanton Parker, "The Mineralogy, Petrology and Geochemistry of the Uranium-Bearing Vein Deposits Near Boulder, Montana, and Their Relationship to Faulting and Hot Spring Activity" (1981). WWU Graduate School Collection. 780.