Abstract

The West Shasta copper-zinc district, Shasta County, California, contains many volcanogenic sulfide deposits within Middle Devonian rhyolites that have not been highly metamorphosed. The district was selected by the U.S. Geological Survey for intensive geological, geochemical, and geophysical study under the Development of Assessment Techniques (DAT) project because accessible exposures have been created by erosion and mining. This report describes the geophysical methods applied to characterize the electrical properties of selected West Shasta massive sulfide deposits and their host rocks, at both small (less than 25 ft) and large (greater than 25 ft) scales. The electrical techniques used galvanic (spectral induced polarization--SIP) and induction (very low frequency--VLF, slingram, and time domain electro-magnetics--TDEM) methods.In situ spectral induced polarization measurements were carried out to determine whether or not conductive anomalies in the district could be differentiated by their polarization signatures. The sulfide, in situ, induced polarization-phase spectral signatures (the induced polarization effect as a function of frequency) have much less character and lack the distinctive shape reported for other massive sulfide deposits; however, they do have some identifiable massive sulfide traits, such as low resistivity and variable polarizability. The nondescript sulfide spectral signature is attributed to the poor development of polarization processes due to a high percentage of resistive, nonpolarizable gangue minerals, lack of pore space, and limited electrolytic fluids. Large-scale spectral induced polarization measurements over the Hornet orebody have a greater polarization than the in situ measurements. This observation, in addition to the fact that much of the Hornet sulfide body has been removed by previous mining activity, suggests that the dominant polarization processes occur at the ground-water-sulfide interface.Combined use of induction techniques, which have different depths of penetration, were used to locate conductive anomalies and determine their shape and depth. All the induction surveys over the Hornet orebody detected the conductive tabular-shaped massive pyritic sulfide deposit hosted in resistive rhyolite. Shallow penetrating induction methods near the Keystone mine detected a conductive fault zone where a block of shale has been downfaulted into volcanic rock. Integrated interpretation of deeper penetrating induction data over this conductive fault zone indicates that parts of the shale are also conductive, demonstrating that the integrated use of several induction methods provides better conductor definition than a single method.

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