Abstract

Predominant alteration shows zoning related to distance from fractures. Adjacent to fractures, illite (sericite), quartz, and pyrite replace all feldspars, chlorite, and mixed-layer illite-montmorillonite; the chlorite and illite-montmorillonite represent alteration of primary hornblende, biotite, and calcic plagioclase, farther from fractures. Less prominent changes, related to depth, include local replacement of calcic cores of plagioclase by albite or calcite at 200 feet or more, and by K-feldspar above 300 feet. The minerals are those that theoretically would be expected at the temperatures and water compositions in the drill holes, which suggests that present waters are producing the observed alteration, and resemble an ore-forming fluid.

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