DETECTION AND ASSESSMENT OF SUBTLE HYDROTHERMAL ALTERATION IN THE BLACK RIVER GROUP CARBONATES

The Black River Group (BRG) of New York is comprised of middle Ordovician carbonates bounded by the older Beekmantown Limestone and younger Utica shale. In southern New York, the BRG supports large oil and gas traps created by deep seated hydrothermal alteration of carbonates. This study investigates the possible northern extension of these plays by exploring evidence of deep hydrothermal alteration and bituminous residues. Core and wall rocks from the Tug Hill quarry near Champion, New York were analyzed to determine the presence of alteration features and their correlation within this larger regional context. On-site observation and sketch mapping of quarry walls, drill core analysis, petrographic study, scanning electron microscopy and spectral radiometric data were the primary methods used for this research.

Preliminary analysis indicates that repetitive northeast to southwest trending vertical joints served as conduits for precipitated carbonate veins, some of which are up to four inches thick. Smaller veins and vugs are frequently lined by carbonaceous material and include calcite, with some of these crystals displaying apparent exsolution lamellae, and ferroan calcite rhombohedra. Vug-filling is prevalent with larger crystals of calcite, some of which contain thin veneers of vitreous carbonaceous material. Finer grained vug filling consists of calcite and quartz intergrowths. Other subtle expressions of deep hydrothermal alteration are limited to the presence of ferroan calcite matrix in core samples. As of yet, no evidence of hydrothermal dolomitization, the prominent feature of hydrothermal carbonate systems of the BRG to the south, has been positively identified. While data are preliminary, subtle evidence of hydrothermal alteration suggests that at least some of the trends further to the south extend into the study area.