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Formation of a Cemented Subsurface Horizon in Sulfidic Minewaste

Dep. of Soil Science, 1525 Observatory Drive, Univ. of Wisconsin, Madison, WI 53706;

F. W. Madison

Wisconsin Geological and Natural History Survey and Dep. of Soil Science, Univ. of Wisconsin, Madison, WI 53706.

^* Corresponding author.

ABSTRACT

In the early 1900s, Pb-Zn ores in southwest Wisconsin were roasted for metal extraction. In the process, Fe-bearing minerals were converted to a magnetic state for separation; metallic oxides were recovered and wastes consisting of about 30% Fe and 15% S, as well as numerous minor constituents, were deposited on the land surface in close proximity to the processing plants. Although the waste materials remain unvegetated today, significant changes have occurred in their mineralogical composition, which are reflected in the formation of discrete horizons. Micromorphological techniques were used to determine the pore space relationships of the unconsolidated material and the elemental and mineral distribution in the various horizons. Mineralogical transformations have been documented, which are largely dictated by the hydrology, porosity, and redox chemistry of the waste. The transformations include subsurface precipitation of mainly gypsum and Fe oxides and hydroxides in inter-aggregate pores and mainly hydrated Fe-sulfates such as copiapite and kalinite within intra-aggregate pores. The net result is a dense, cemented horizon, which probably has a low permeability. The presence of this cemented layer has important implications for developing reclamation strategies, because it may form a potential barrier to transport of pollutants to the groundwater.

Key Words: Acid mine drainage • Iron sulfide • Roaster process tailings • Cemented horizon

Research supported partially by a grant from the U.S. Bureau of Mines (J0225009) and the College of Agricultural and Life Sciences, Univ. of Wisconsin-Madison.

Received for publication May 12, 1987.

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