Date of Award
Master of Science (MS)
Adrian Van Rythoven
Carbonatite-related REE-Nb mineralization in southern Ravalli County, Montana was first discovered in the late 1950s and early 1960s. Over 30 discontinuous carbonatite bodies occur in a ~ 10 km2 area centered on Sheep Creek, a tributary to the headwaters of the West Fork of the Bitterroot River (WFBR). The deposits are contained in a belt that extends to the southeast into the Mineral Hill district of Lemhi County, Idaho. Country rocks include a deformed and metamorphosed bimodal igneous suite of diabase and A-type granite, both dated to 1.37 Ga by previous workers. These rocks are locally metamorphosed to amphibolite and augen gneiss. Although first explored for columbite, the districts are now gaining attention for their locally high-grade REE mineralization. This study sampled all known carbonatite bodies in the Sheep Creek district and performed additional analyses using XRD, automated SEM-EDS, electron probe microanalysis (EPMA), stable isotopes of matrix carbonate minerals, and Re-Os geochronology. The combined studies show that the deposits of Sheep Creek contain a complex assemblage of REE-Nb bearing minerals, including monazite, REE-carbonates (ancylite, bastnaesite, parisite, synchysite, sahamalite), REE-silicates (allanite, ferriperbøeite, biraite, törnebohmite), columbite, baotite, and trace Nb-REE minerals (aeschynite, fersmite). The principle gangue minerals are ferroan dolomite, calcite, apatite, barite, quartz, Ba-rich feldspars, actinolite, and phlogopite. Most highly mineralized carbonatite bodies have a calcite matrix, and also show fenite alteration of country rock amphibolite to a phlogopite-albite-calcite schist. Geochemical analyses of bulk samples from outcrops and mine dumps (n = 40) show locally high REE concentrations (> 10 wt % ΣREE), with a strong enrichment in the light REE, relative to heavy REE, and a lack of any Ce or Eu anomalies. This type of REE pattern is typical of carbonatite deposits worldwide. The carbonate isotope data are consistent with a primarily igneous origin for early dolomite, with oxygen-18 isotope shifts in the negative and positive directions for later calcite, most likely caused by hydrothermal overprinting and supergene weathering, respectively. Sulfides, including molybdenite, were found sparsely disseminated through the carbonate matrix. Two molybdenite samples were analyzed to obtain accurate age dates using the Re-Os method. The dates obtained are 149 Ma, and 109 Ma. These dates are unexpected as no known igneous or hydrothermal activity took place in SW Montana at this time. The deposits of Sheep Creek have a complex history involving igneous emplacement overprinted by deformation, hydrothermal remobilization, and weathering. Therefore, further geochronological work is recommended. The deposits, although limited in size and continuity along strike, are highly enriched in LREE and Nb, and are worth further exploration.
Risedorf, Sarah, "INVESTIGATION OF THE SHEEP CREEK CARBONATITES, RAVALLI COUNTY, MONTANA" (2023). Graduate Theses & Non-Theses. 296.