Waste Rock Dump Characterization Studies at the Bingham Canyon Mine
Authors: Schofield Ian S. , Brown Paul L. , Logsdon Mark J. , Wickham Matthew P. 
Published: October 2024

The Bingham Canyon Mine, located near Salt Lake City, Utah, is surrounded by more than 6 billion tons of waste rock developed over the open cut mining history from 1903 to present; the surface area of the waste rock is approximately 5000 acres. Waste rock dumps have a thickness of more than 1 200 feet from crest to toe. From 1930 to 2000, selected portions of the waste rock dumps were commercially leached using a ferric-sulfate-based lixiviant to extract copper, whereas other portions have only received meteoric leaching. From 2011 to present, Rio Tinto Kennecott has studied the evolution and geochemical controls on water chemistry associated with the waste rock dumps at the Bingham Canyon Mine. In this program, the waste rock dumps have been characterized in detail from the field logging of, and data collected from instrumentation installed within, 13 paired borings. At 12 of the 13 locations, the borings penetrated the full depth of the dumps, through the pre-mine soil contact, and into bedrock. Borings were installed to depths approaching 900 feet below ground surface using roto-sonic drilling methods to enable (1) core recovery and (2) measurement of near in situ properties. Field logging of the borings included Unified Soil Classification System descriptions, clast lithology, relative oxidation, paste pH, and geophysical methods (gyroscopic, temperature, neutron, and gamma). Core from the borings was analyzed for geotechnical properties (density, grain size distribution, moisture content, plasticity index and limit, and direct and block shear), quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), modified acid-base accounting (ABA), modified synthetic precipitation leaching procedure (SPLP), and hyperspectral analysis by CoreScan. If water was encountered during the drilling process at sufficient volumes for collection from the core barrel, samples were collected for chemical analysis. Instrumentation installed within the borings included lysimeters, thermistor nodes, direct temperature sensing (DTS) fiber optic cables, time domain reflectometry (TDR), shear cables, gas (oxygen, carbon dioxide) measurement tubes, and vibrating wire piezometers (VWPs). Additionally, each drill site had multiple measurements of oxygen consumption in the surface layer of the local waste rock. Data acquired from the borings were linked with historical information (covering a period of greater than 50 years) from extensive drilling, mineralogical and litho-geochemical evaluations, hydraulic and tracer testing, and 20 years of seepage f low and water chemistry data to develop a conceptual model that describes the hydraulic, geochemical, and physical behavior of the waste rock dumps. Pyrite and other sulfide minerals in the waste rock dumps are oxidized by both diffusive and convective ingress of air, producing acidic, high-total dissolved solids effluents, and jarosite that has formed within the waste rock as a secondary phase that stores additional acidity. The dominant air ingress mechanism is convection, which accounts for greater than 90% of the sulfide oxidation within the waste rock dumps. Based on temperature profiles and water balance for the dumps, moisture loss to geochemical reactions is a significant part of the water budget.