Abstract

Using electrode spacings ranging from 2 to 10 m, we made 157 measurements of phase and resistivity at 99 sites in four open-pit porphyry copper mines, thereby sampling a wide variety of rock and alteration types. Rock samples were collected from these sites and examined in the laboratory to characterize the alteration and mineralization and to quantify the sulfide concentration. Our experience has been that in-situ electrical measurements give a better basis for quantitative correlations between electrical parameters and mineralization than do laboratory measurements on small samples.In general, resistivity declines and phase lag increases as the sulfide content increases, but the scatter in the trends precludes any useful empirical rules. However, the scatter is much reduced when the ratio of phase to resistivity is considered: the ratio increases with the square of the sulfide content. Thus it appears that in many cases the phase-to-resistivity ratio can be used to estimate sulfide content. With appropriate conversion factors, this ratio is equivalent to the quadrature component of electrical conductivity and to the metal conduction factor.This dependence of quadrature conductivity upon the square of sulfide content is substantiated by two other data sets taken from the literature. These three independent sets of measurements also seem consistent in showing that the dependence of phase upon sulfide content depends upon mineralization habit. The observed trends must be explained before a satisfactory model of electrical conduction in mineralized rock can be established. These results indicate that careful in-situ studies have much to offer for further electrical property investigations.

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