An experimental study of water–rock interaction and acid rock drainage in the Butte mining district, Montana

Abstract

This paper summarizes a laboratory study of water–rock interaction and the generation of acidic mine drainage in the Butte mining district, Montana. The Butte district includes two large open pit mines exploiting different portions of the same porphyry Cu–Mo system: (1) the well-known Berkeley Pit, which is abandoned and flooding with acidic mine water; and (2) the nearby Continental Pit, which is active. The two pits are separated by a normal fault with several thousand feet of vertical displacement, and consequently the styles of alteration and mineralization are quite different. Humidity cell and recirculating leachate experiments of crushed rock exposed on the walls of the Berkeley Pit produced effluent waters very similar in composition to the flooded Berkeley Pit lake. In contrast, the same experiments on crushed ore from the Continental Pit generated circum-neutral leachates with low metal concentrations, caused by the presence of calcite in the alteration assemblage. Because carbonate is being depleted at a faster rate than pyrite during weathering, the quality of mine drainage in the Continental Pit could eventually degrade.