English

Abstract

The Barrick Goldstrike Mine is located in the Carlin Trend, about 20 miles northwest of Carlin, Nevada. The majority of the ore mined at Goldstrike is from the Post-Betze open pit, the largest open pit operation in Nevada. Nevada law requires that the water quality of temporary or permanent reservoirs that may develop in open pits be periodically tested. Pit water quality prediction is conducted to monitor water quality and the possible impact that it may have on human, terrestrial, or avian life. Water quality in the open pit at Goldstrike has been analyzed in three previous studies (BGMI 1991, Radian-Baker 1997, TRC 2000). The most recent analysis of the pit lake water quality model is discussed in this presentation. The effects of placing 516 million t of backfill waste rock into the open pit were also examined. This evaluation of pit lake water quality (Schafer and Logsdon 2003) differs from previous studies in that it utilized laboratory studies where natural groundwater from the site was mixed with water that had contacted weathered mine rock as a means of simulating the pit lake geochemical reaction path. Calibration of PHREEQC using batch test results enabled refinement of solubility data obtained from the thermodynamic mineral equilibrium database employed by PHREEQC. The prediction of pit lake hydrology and water quality involved six key elements including 1) summarizing the mine plan, and geochemistry of exposed rocks and backfill, 2) evaluating mine filling using a regional groundwater flow model, 3) conducting eight large-diameter column studies to generate representative rock contact waters, 4) performing batch mixing tests to simulate final pit water quality at various stages of filling, 5) calibrating a geochemical model based on the batch tests, and 6) using the calibrated model to predict water quality at various stages of pit lake recharge. The uncalibrated PHREEQC model accurately predicted the common ion concentrations and pH of the batch tests, and correctly simulated the precipitation of large amounts of calcite when a mixture of various mine waters, groundwater and meteoric water was evaporated (although aragonite was actually the dominant solid that formed). Agreement between the model and batch tests was poorer for some metals, however. The PHREEQC model over-predicted zinc, nickel and antimony concentrations in the pit lake while under-predicting concentrations of barium, copper and arsenic. Calibration of PHREEQC enabled accurate prediction of trace element concentrations in batch tests, therefore improving its reliability for pit water quality prediction.