Hydrothermal Alteration in Some Granodiorites

Abstract The patterns of distribution of the mica and clay minerals resulting from alteration of feldspars are much less regular and well defined in the “porphyry copper” deposits than in the vein deposits such as those at Butte, Montana. There are, however, interpretative problems common to both. Chemically, the destruction of the K-feldspars may be interpreted in the light of laboratory investigations of the system potash-alumina-silica-water at various K+/H+ ratios. The experiments indicate that the K-mica (sericite) stability field lies between the K-feldspar and kaolinite fields. The K-mica field is approximately 1.5 pH unit wide, and its boundaries with K-feldspar and kaolinite are parallel below 500°C. The boundaries slope toward higher K+/H+ ratios at lower temporatures. Thus, considering the. K-feldspar—sericite—kaolinite system of minerals, one might expect field patterns showing a central zone of kaolinite bordered by a zone of K-mica which in turn grades outward into unaltered K-feidspar. Such a pattern is demonstrated by Butte’s inner diekite-kaolinite and sericite and possibly by the zoning at San Manuel described by Schwartz. Alteration of plagioclase introduces additional complications into the distribution patterns of clay and mica minerals, the chief of which is the outer argillic zone so characteristic of both vein and disseminated deposits.