ORGANICS, BRECCIA, AND CRYSTALLINE VEINS IN HYDROTHERMAL CARBONATE MOUNDS OF THE EOCENE GREEN RIVER FORMATION, BRIDGER BASIN, WYOMING

Organic matter associated with crystalline veins in decametre-scale hydrothermal spring-mound deposits of the Laney and Wilkins Peak members of the Eocene Green River Formation, Wyoming may be linked to a regional thrust fault system and underlying organic-rich strata. The study area at Wildhorse Draw is located in the southeastern Bridger Basin, where Precambrian basement faults coincide with the paleomargin of the ancient Lake Gosiute. Reactivated as high-angle reverse faults during the Sevier-Laramide Orogeny, they may have served as pathways for hydrothermal and CO₂-charged fluid flow. Petrographic microscopy, SEM-EDS, Sr, C and O isotope analysis, and FT-IR were performed on samples to determine composition and the relative timing of crystal precipitation in a series of fluid injection events. The main spring-mound deposit and adjacent smaller mounds formed in shallow-water lake-margin environments and were fed by fluids containing mainly silica, calcium, aluminum, magnesium, and barium. Breccia clasts are found within the feeder veins, along with calcite, dolomite, analcime, chert, barite, and mega-quartz, as well as a waxy, black organic material that occurs with boxwork calcite throughout the veins. Based on these results, we hypothesize that the fluids were pressurized and hydrothermal, and that organic material was mobilized from below and transported within the vein system. Precipitation of different mineral phases may be attributable to slurry separation resulting from rapid pressure decrease upon host-rock brecciation. The ⁸⁷Sr/⁸⁶Sr ratios measured from these spring deposits are significantly less radiogenic than coeval Wilkins Peak and Laney member sediments, which indicates a source other than lake water; the fluids may have interacted with underlying bedrock. Together, the evidence indicates that contemporaneous tectonic activity may have been an important mechanism involved in the formation of the spring-mound deposits.