THE GENESIS OF FIBROUS CALCITE AND SHALE-HOSTED EMERALD IN A NON-MAGMATIC HYDROTHERMAL SYSTEM, UINTA MOUNTAINS, UTAH

Large veins of fibrous, translucent calcite, up to 0.5 km long and 20 m wide, occur in Mississippian carbonate units near the South Flank Fault zone of the Uinta Mountains. Directly underlying these veins are altered zones of Proterozoic (950 Ma) Red Pine Shale Within the fault zone and shale, three emeralds have been recovered. The largest emerald is a hexagonal, prismatic, 2 cm, 30 carat crystal exhibits zonal variations in color that correspond to measured variations in Cr and V content.

The host rock for the emeralds is a 1 km thick, black (organic-bearing) shale sequence with arkosic arenite units. Abundant pyrite, barite, vein quartz, and fibrous calcite veins, bleached shale and green mica are present along the fault zone. There are no igneous intrusions within 50 km of the area and no geophysical evidence to indicate that the alteration is associated with concealed igneous activity. The isotopic composition of pyrite from the South Flank fault zone (delta 34S, +4.6 to +4.2 per mil) is not characteristic of an igneous source, but is similar to the isotopic signature of sulfur from some Tertiary oil field brines (about +5 per mil, Wasatch Formation and lower Green River Formation) present at the base of the Uinta basin. The deepest portion of the asymmetric Uinta basin is bounded by normal and reverse faults related to uplift of the Uinta Mountains. Consequently, the source of hydrothermal fluid responsible for alteration of the Red Pine Shale maybe similar to that of Columbian emeralds where basinal brines (Tertiary or older) migrated to the fault margins of the basin. These data suggest that a basinal brine may have mixed with other water along the fault zone or reacted with feldspar and/or dolomite to form sparse emerald and other alteration products.

Analyses of C and O isotopes help constrain the temperature and composition of the mineralizing fluids and indicate a temperature in the range of 200 to 300 degrees C from a high ionic strength fluid similar to an oil field brine. As sulfate in the brine was reduced by reaction with organic carbon in the shale, a carbon dioxide-rich fluid was produced. The acidic fluid dissolved portions of the overlying limestone and intermittent loss of carbon dioxide caused precipitation of fibrous calcite.