Abstract

The Rock Canyon Creek carbonate-hosted REE-F-Ba deposit has tectonic, stratigraphic, and structural similarities with Mississippi Valley-type and sparry magnesite deposits in the southeast Rocky Mountains. The main REE-fluorite zone is a steeply dipping body, extending 1100 m along strike, 50 m wide, and 100 m deep. It spatially coincides with pre-existing crackle breccias in carbonate rocks, and consists of dolomite, fluorite, barite, pyrite, quartz, K-feldspar, calcite, porous apatite, REE-fluorocarbonates, and REE-phosphates. The main fluorocarbonates are bastnaesite, parisite, and synchysite. Monazite, crandallite group minerals, and apatite are the main phosphates. Fluorite content varies from less than 1 to 13.5% (by weight) and ∑REE+Y concentrations vary from trace to 1.95% (by weight). The mineralized zone is heterogeneous on the deposit scale, as indicated by three-dimensional geochemical modeling combined with a geochemical assessment based on 89 mineralized samples and detailed downhole mineral and geochemical profiles of a key borehole. Chemical heterogeneity and key elemental co-variations are explained by strong mineralogical control and have implications for the design of exploration and development programs for this type of deposit. The chondrite-normalized REE pattern of samples from the mineralized zone shows enrichment in LREE, similar to typical carbonatite-related mineralization; however, no carbonatite is exposed nearby.

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