Abstract

Large bodies of fibrous orange 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 recently been recognized in the Neoproterozoic Red Pine Shale. Fibrous orange calcite is translucent, and is being mined and used as architectural stone, in carvings, and for other decorative purposes.

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

Our work suggests that brines from the Uinta Basin may have migrated upward along the SFFZ and interacted with the organic shale, producing emerald and a CO₂–rich fluid that dissolved portions of the overlying Mississippian carbonates. The fluid experienced intermittent loss of aqueous CO₂, causing fibrous calcite to rapidly precipitate. This model and setting has similarities to both Mississippi Valley and Colombian emerald deposits.