Electrical Properties of Leach Ore, Cripple Creek Mining District, Colorado

Abstract

The Cripple Creek and Victor Gold Mining Company in Victor, Colorado is a leading gold producer in North America. The mine is an open pit/heap leach recovery facility where the ore is placed in an unconsolidated pile and treated with a dilute cyanide solution. The pregnant solution is then pumped to an adsorption facility for gold extraction. Annual recoveries of gold reach 365,000 oz/yr at a mine cutoff of 0.018 opt. The project investigated 1) saturation characteristics within the heap itself through the use of a series of Schlumberger soundings at locations with different solution saturation (e.g., fresh ore with no solution as opposed to fully leached rock), and 2) laboratory induced polarization spectra of ore samples to determine electrical properties that may characterize saturation conditions. Spectral induced polarization electrical frequencies used were from 0.01 to 500 Hz. If there were a characteristic difference in the electrical properties of saturated vs. unsaturated ores then these differences could be detected by surface measurements, revealing subsurface regions of under-saturation and insufficient leaching. Findings indicate the competent, low-sulfide rock is highly resistive and has very low IP response. Conclusions that could be drawn from the Schlumberger arrays on the heap showed that subtle variations in bulk resistivity could be differentiated using a vertical electrical sounding (VES) program in addition to conventional log/log plots. Results of resistivity and induced polarization measurements on individual samples exhibit wide variation in resistivity, but when normalized to 1Hz all samples regardless of mineralogical composition showed little variation. With respect to IP effect little variation existed but more study needs to be performed to determine the dominant mechanism (ionic or electrode polarization). Column tests showed distinct differences in resistivities based on whether the ore was non-leached (fresh) or leached which is consistent with heap characteristics described by the Schlumberger soundings. IP effect was consistent with field result also. An important conclusion that can be drawn from the column tests is that it is possible to replicate field conditions in the laboratory.