Base Metal Distribution in Granitic Rocks; data from the Rocky Hill and Lights Creek Stocks, California

Abstract

Significant systematic areal variation patterns in copper and zinc contents exist in rocks of the Rocky Hill (Tulare County) and Lights Creek (Plumas County) stocks, California. Significant areal variation patterns in copper contents of biotite and sulfide phases, total sulfide contents and other properties also exist in the Rocky Hill stock. These patterns are related, and an increase in the content of copper-bearing sulfides is commonly accompanied by a decrease in the copper content of the associated ferro-magnesian silicate phases. This relationship is usually independent of the total copper content of the rock and of the fractionation trends shown by other metals. The distribution patterns of total sulfides and the intensity of deuteric alteration, however, are related, with both increasing in content or intensity towards the core of the stock. Some copper may have been extracted from the silicate phases and fixed in sulfides during alteration. In spite of a low overall content of copper in the Rocky Hill stock, considerable concentration has occurred in a small area. The distribution of zinc in this stock does not appear to be significantly related to any of the above factors.Transfer or migration of copper within the rocks of the Rocky Hill stock is indicated by comparisons of the frequency distributions of copper and zinc and by other data. Consideration of these distribution patterns in relation to models of mineralization in granitic rocks points out that structural level and other aspects of the local geologic setting can have an important influence on the nature of such patterns. Interpretations in regard to metallogenesis and exploration should take this influence into account.Calculated activities of Cu, Zn, and annite molecule in biotite for estimated conditions of late crystallization at Rocky Hill appear to be in satisfactory agreement with observed analysis data. The total content of sulfur in the rock, including the vapor phase coexisting with it during crystallization, then becomes the critical factor in controlling sulfide-silicate partitioning of copper. Under the crystallization conditions estimated for Rocky Hill, a separate zinc sulfide phase is not stable.An analysis of the variation data for Cu and Zn in these stocks provides estimates of local scale or outcrop variation and establishes the significance of large scale (regional) variations occurring in both. It also provides a limited basis for an estimation of the type of distribution involved at the local or outcrop scale. As a result of fracture-controlled sulfide distribution in the Lights Creek stock, local scale variation is much larger than that at Rocky Hill and more dependent on sampling configuration; highly significant large scale variation in Cu and Zn is nonetheless present.