The Relationship Between Waste Rock Geochemistry, Age and Reactivity

摘要

Understanding the relationship between variables controlling the propensity for acid rock drainage in the 4000 acre historic Robinson District near Ely, NV, required a detailed understanding of dump conditions. Profiles of O_2, pH, temperature and specific conductivity were collected from various waste rock dumps that represent a continuum of oxide/sulfide, leached/unleached dumps of different ages from across the site. Young sulfide dumps were characterized by background ambient temperature and rapid oxygen consumption, resulting in the development of a steep oxygen gradient from atmospheric levels at the surface of the dump to near zero levels a few feet into the dump. The temperature of young dumps has yet to increase while recognizable decreases in pH and increases in specific conductivity are just starting. Eventually, the oxidation rate slows, resulting in oxygen levels closer to atmospheric in the dump. During this stage, the pH and SC profiles develop, indicating zones of advanced oxidation and neutralization, and the dump may also develop an elevated temperature gradient. In the mature stage, a sulfide dump exhibits close to atmospheric oxygen levels throughout, similar to an oxide dump profile, but remains exothermic. In conjunction with pH, chemical and SC profiles, these data identified dumps that require remedial measures compared to those that require only minimal action prior to closure.