Sunday, 5 October 2008: 10:30 AM
George R. Brown Convention Center, 320ABC
Manganese oxide crusts ~1 cm thick were found in Spring Branch, located 35 miles north of Chattanooga, Tn. Surface mining of coal occurred in the Spring Branch watershed from 1985 to 1997. Mine drainage was treated in a pond that discharged to Spring Branch from an adjacent 564 acre mine. Manganese oxide crusts developed in the stream channel below the pond outfall. In order to better understand how these and other banded manganese oxide crusts form and interact with aqueous environments, the mineralogy and chemistry of these stream deposits were investigated using reflected light microscopy, powder X-ray diffraction, and scanning electron microscopy. A series of light and dark bands is repeated 5-6 times within the manganese oxide crusts. Light-colored bands contain coarse-grained radial calcite crystals, and grade into a darker, rust-colored mix of calcite, clay minerals, and manganese oxide. Thin (>100 micrometers) distinct dark bands consist of a calcium-rich buserite phase. Imprints of elongate diatom frustules ~10 to15 micrometers long and 2 to 3 micrometers wide are visible only in these thin manganese oxide-rich layers. Water samples were obtained from pond effluent and at a location downstream of the manganese oxide crusts. The pH of the pond effluent was near neutral (7.7), and the pH of the stream water remained constant from the pond to the mouth of the stream. These data suggest that precipitation of manganese oxides was not due to changes in pH. However, the dissolved manganese concentration in the stream water increased downstream from 560 to 3170 micrograms/liter suggesting that the stream water is a mix of mine drainage water with a lower manganese concentration and groundwater with higher manganese. Alternating calcite-rich and manganese-oxide/diatom-rich layers within the crust may be produced by annual or seasonal cycles in water temperature, rainfall, oxidation state, or microbial activity.