IMPACT OF LARGE-SCALE ALLEGHANIAN FLUID MIGRATION ON APPALACHIAN BASIN COAL BEDS

A variety of lines of published research document that Paleozoic sediments of the Valley and Ridge Province and Appalachian basin of the eastern U.S. contained aquifers for westward migrating fluids during a late phase of the Alleghanian orogeny (~270 MY ago). The fluids involved were likely derived in part from deep-seated metamorphic processes related to this orogeny. Geologic evidence for this fluid migration hypothesis includes westward directed tongues of elevated vitrinite reflectance values in Pennsylvanian age coals, diagenetic remagnetization of Valley and Ridge rocks and precipitation of authigenic K-feldspar all of which occurred in late Paleozoic time. A further consequence of this fluid migration event was precipitation of As-rich pyrite (up to 2.7 wt. % As) which also contains elevated levels of other potentially toxic elements including Hg (up to 25 ppm), Tl (up to 10 ppm) and Mo (up to 200 ppm). USGS coal geochemical data indicate that this arsenian pyrite is particularly abundant in the Black Warrior Basin of Alabama where the structural complexity of the basin allowed effective ingress of late fluids. Detailed petrographic studies on Black Warrior Basin samples reveal that this arsenian pyrite was paragenetically late, occurring in faults and veins in structurally disrupted coal. Arsenian pyrite structurally cross-cuts earlier pyrite generations and fills voids from the partial dissolution of early diagenetic calcite and siderite. In the central and northern Appalachian basin, arsenian pyrite is present but is less abundant than in Alabama. In samples from coals in Kentucky, the mode of occurrence and composition of the arsenian pyrite is similar, but it is elevated in Mo (up to 6000 ppm) relative to Black Warrior Basin. Evaluation of the geologic controls on arsenian pyrite distribution is important given the potential consequences to the environment from mining and combustion of As-rich coal.