Sulfide Variation in the Coeur Rochester Silver Deposit: Use of Geologic Block Modeling in the Prediction and Management of Mine Waste

Abstract

Abstract A critical issue in mine waste management is the management and mitigation of acid rock drainage and metal leaching particularly from waste rock. This requires good understanding and characterization of materials most likely to generate acid pH waters and generate leachable metals and metalloids, as well as the presence of minerals most likely to neutralize acidic pH such as carbonates and certain silicates. For low-sulfidation epithermal deposits in silica-rich volcanic rocks, neutralization minerals are rare. Consequently, determination of acid generation is almost entirely a function of sulfide content of the rocks. By understanding the distribution of sulfide minerals within such a deposit, an initial mine waste management plan can be developed. The Rochester mine currently exploits a low-sulfidation epithermal gold-silver deposit and is located on the southern flank of the Humboldt Range in west-central Nevada. The Rochester mine is a conventional open-pit mining operation with cyanide heap leaching and Merrill Crowe processing. Silver and lesser gold mineralization occur associated with sulfide minerals hosted in Permian-Triassic Koipato Group rhyolitic units with extensive quartz-sericite-pyrite alteration. Gangue mineralogy of the deposit is dominated by slow or unreactive silicate minerals with little or no carbonate. Silver-bearing mineralization was controlled by primary hypogene sulfides and secondary supergene oxidation processes, and most of the mineralized material mined to date has been fully or partially oxidized. Oxidation mobilizes and depletes all forms of sulfur from primary sulfide minerals. Due to the low sulfate content and low content of neutralizing gangue phases, the waste rock classification can be based on total sulfur content. Based on the analysis of pulp samples collected from within the mine area, a sulfur block model was developed and compared to the results of standard acid-base accounting and net acid generation tests. This comparison demonstrates the benefit of the block model in identifying zones of potentially acid-generating material in the mined waste. Application of this information to mine planning has allowed more efficient management of acid-generating mine waste by allowing the operator to develop areas of the waste rock facility to receive the waste in a timely fashion. Such an approach is based on good mine geologic and mineralogical characterization and the use of a representative and comprehensive geologic block model.