WEATHERING OF SANDSTONE COAL-MINE SPOILS AS A SOURCE OF DISSOLVED SOLIDS RELEASED TO HEADWATER STREAMS

Petrographic comparison of Pennsylvanian sandstones in core and in rock spoils used to reclaim central Appalachian coal mining sites reveal the effects of surface weathering in releasing dissolved constituent ions to downslope streams. In unweathered sandstone samples, framework grains are dominated by quartz, feldspar, chlorite schist metamorphic fragments and muscovite. Cements consist of siderite and iron oxide rims, authigenic quartz overgrowths and rare kaolinite. Carbonates (calcite and ankerite) are patchy in their distribution with most samples lacking such cements and a few samples containing as much as 2.5% carbonate cement. Weathered sandstones are characterized by rust coloration related to oxidation of siderite, pyrite and possibly ankerite to limonite and/or goethite. In thin section, weathered sandstone samples show evidence for extensive kaolinitization of K-feldspar and plagioclase as well as local dissolution of feldspar grains. Locally, muscovite shows evidence of kaolinite replacement. Alteration and dissolution of plagioclase and K-feldspar and, rarely, muscovite is consistent with hydrolysis of K-feldspar and plagioclase related to interaction with acidic ground waters in a surface of surface or near surface environment. No evidence was recognized in thin section of vermiculite identified by Howard et al. (1988) in rock, soil and spoil. Feldspar hydrolysis is the most likely and volumetrically most important source of K, Na and Ca in river waters. Dissolution of calcite and/or ankerite is another possible source of Ca but these minerals are rare in thin section and, where present, show little evidence for dissolution. Sulfate ions in receiving streams likely are derived from oxidation of trace quantities of pyrite but potential sources of HCO₃ ions are equivocal. Chlorite schist grains in thin section display evidence for partial dissolution and represent a likely source of Mg discharge.