English

Abstract

Barrick Goldstrike Mines Inc. (BGMI) has developed a comprehensive closure planning, materials characterization and in-situ monitoring program for the closure of its mine waste facilities. Facility closure techniques use topsoil and in-pit material to construct evapotranspiration (ET) cover systems to reduce or eliminate infiltration of meteoric water. One major facility at BGMI, the AA Leach Pad (AA Pad) has been closed. The Bazza Waste Rock Facility (WRF), which will hold about 1.75 billion metric tonnes (mt) of waste rock within an area of 950 hectares (ha), is undergoing concurrent closure over the next 10 years. Hydrologic performance of the AA Pad cover and the inventory, suitability and timing of in-pit materials dictated the ET cover design approach for the Bazza WRF. The AA Pad ET cover system was constructed in 2000 using two comparative cover systems; 1.2 m of fine-grained Tertiary-aged valley fill deposits of the Carlin formation, and 1.5 m of salvaged topsoil materials. The flux of meteoric water through the AA Pad cover was measured using in-situ vadose zone monitoring. Both covers had very low deep percolation rates, especially after vegetation matured. The AA Pad monitoring results were used to optimize the Bazza cover. First, an unsaturated flow model was calibrated to the in-situ monitoring data from the AA Pad. Next, the hydraulic properties of cover materials to be used for the Bazza cover were determined in large-diameter cores to minimize scale effects. Large scale in-situ hydraulic testing was also conducted to assess material variability. Finally, in-situ monitoring systems were installed in concurrent reclamation areas at the Bazza WRF. ______________________ 1 Paper presented at the 7 International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Guosheng (Johnny) Zhan is the senior corporate hydrologist, Barrick Gold Corporation, Salt Lake City, UT 84111, USA. William Schafer is the Principal, Schafer Limited LLC, 3018 Colter Ave, Bozeman, MT 59715, USA. Mike Milczarek is the technical director, GeoSystems Analysis, Inc, 2015 North Forbes Blvd, Suite 105, Tucson, AZ 85745, USA. Ken Myers is the president, The Mines Group, Inc., 1325 Airmotive Way, Reno, NV 89502, USA. Joe Giraudo and Ron Espell are the senior environmental engineer and manager, respectively, Barrick Goldstrike Mines Inc., P.O. Box 29, Elko, NV 89803, USA.