EFFECT OF PYRITE COMPOSITION, TEXTURE, AND FORM ON ACID MINE DRAINAGE POTENTIAL IN COAL-BEARING STRATA OF THE CENTRAL APPALACHIAN BASIN

To determine controls on acid mine drainage (AMD) potential, we are examining stratigraphic variation in pyrite content, morphology, composition, and textural relations in Pennsylvanian coal-bearing strata of the central Appalachian Basin. The section investigated includes the Middle Pennsylvanian Kanawha and Allegheny Formations and the Upper Pennsylvanian Conemaugh Group and Monongahela Formation. This section contains economically important coal beds such as the Coalburg; Lower, Middle, and Upper Kittanning; Lower and Upper Freeport; and Pittsburgh, listed in ascending stratigraphic order. Compared to coal beds in the lower part of the section, coal beds above the Kanawha Formation show a fundamental shift to higher sulfur values, defining a zone of high acid-forming potential that is concentric about the north-central portion of the Basin.

Initial work has concentrated on sulfides in coal roof rocks and underclays in the Allegheny Formation, which shows the most pronounced post-mining acid generation. Subhedral pyrite shows enrichment in trace metals relative to framboidal and fracture-filling pyrite forms. Provisional ranges for both arsenic and nickel in subhedral pyrite extend from below the detection limit (0.01 wt. percent) to more than 1.1 weight percent. Arsenic is thought to destabilize the pyrite structure and may contribute to increased rates of weathering and enhanced AMD potential. Pyrite is also a source of mercury, as shown by reconnaissance laser-ablation ICP-MS analysis. In some intervals, pyrite is encased in a less reactive host such as siderite, potentially making these horizons less prone to pyrite decomposition. As this study proceeds, we will further define the stratigraphic variation in pyrite contents and compositions, assess the relative stability of different pyrite forms, and investigate the effect of trace-metal substitution on pyrite stability.