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SCHROTH, ANDREW W., Northern Arizona University, Dept. of Geology, Flagstaff, AZ

ABSTRACT: A Hydrogeochemical Characterization of Acid Mine Drainage Using Secondary Mineral and Solution Compositions, Alta Mine, Colorado Mining District, MT

Acid mine drainage (AMD), a pollution problem generated from the exposure of sulfide minerals to meteoric water and atmospheric oxygen, is a serious threat to water quality throughout the intermountain west. My current research focuses on identifying and describing processes and pathways controlling the geochemistry of solutions, draining the Alta Mine, Jefferson City, MT. Solution composition data reveals elevated concentrations of metals, sulfate, and hydronium, all of which are geochemically characteristic of AMD systems. In an attempt to reduce metal concentrations draining the site, a wasterock pile containing sulfide minerals such as chalcopyrite and pyrite with which AMD comes in contact was removed. This provided a unique opportunity to examine and describe amorphous iron oxide minerals that have precipitated out of AMD in the waste rock pile. Two distinct secondary iron oxide minerals were observed in the field, one being a light orange layer and the other appearing as dark red cement. Ammonium oxalate extraction data reveals that the lighter precipitate contains higher levels of heavy metals than the darker precipitate. Precipitate heavy metal attenuation is a result of adsorption, precipitation, and co-precipitation reactions. We have observed that solution composition changes as AMD flows down-gradient through the pile, and these trends are reflected in down-gradient secondary mineral compositional trends. Additional work is required to further constrain compositional differences initially observed in the AMD secondary minerals. An upcoming water sampling trip will provide further insight into the effectiveness of the remediation effort and the influence of the waste rock pile on heavy metal mobility.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid