Large bodies of fibrous calcite, up to 400 m long and up to 20 m wide, are hosted by Mississippian carbonate units in the Uinta Mountains, Utah. Subjacent to the carbonate rocks, emerald mineralization has been recognized recently in the Neoproterozoic Red Pine Shale. Two types of fibrous calcites are recognized on the basis of appearance and geochemical characteristics. Fibrous orange calcite is translucent, similar to “Mexican calcite” commonly sold at gem-and-mineral shows, and is being mined and used in carvings and for other decorative purposes. Recently recognized fibrous brown calcite is also present in the same general vicinity.

Calcite veins hosted with Mississippian carbonate rocks are located a few kilometers south of the South Flank fault zone (SFFZ) in the vicinity of a swarm of north-trending faults. Parts of the Red Pine Shale along the SFFZ contain abundant secondary pyrite. Altered shale also contains quartz and calcite veins as well as albitized feldspar in some arkosic units and is variably enriched in Cu and Zn. At least three emeralds have been discovered in altered Red Pine Shale.

δ18OVSMOW values of calcite (−10 to −13‰) would be in equilibrium with basin brines (assumed to be −3 to +5‰), suggesting precipitation in the range of 100°–300° C. Our work suggests that sulfate-bearing basinal brines from the Uinta Basin may have migrated upward along the SFFZ and interacted with the carbon-rich shale. As sulfate was reduced by organic carbon to form sulfides, emerald was produced and a CO2-rich fluid migrated upward and dissolved parts of the Mississippian carbonates. The fluid experienced intermittent loss of CO2 gas, which allowed fibrous calcite with cone-in-cone textures to precipitate rapidly. This model and geologic setting have broad similarities to both Mississippi Valley and Colombian emerald deposits. The genesis of fibrous brown calcite, however, is less certain. These calcites lack critical geochemical characteristics that tie orange calcite to precipitation from hot brines; rather stable isotope data are permissive of origin from low-temperature ground water.

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