Formation of a cemented subsurface horizon in sulfidic minewaste

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. 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. Additional aspects of the study are discussed.