Supergene copper mineralization at the Lakeshore Mine, Pinal County, Arizona

Abstract

There are six copper-bearing supergene mineral assemblages at the Lakeshore porphyry copper deposit. These assemblages form zones which have been mapped throughout the deposit. The zones are named for their principal copper-bearing minerals: the chalcocite zone, the cuprite-native copper zone, the brochantite zone, the chrysocolla zone, the copper wad zone, and the goethite zone. Two distinct episodes of oxidation have affected the Lakeshore deposit. The first resulted in the creation of a chalcocite enrichment blanket and a large volume of chrysocolla mineralization. The chalcocite enrichment blanket was largely destroyed during the second episode and the brochantite zone (Cu 4 (SO 4 )(OH) 6 ) was formed. The Lakeshore porphyry copper deposit is a sulfur-poor system without a phyllic, pyrite-rich, alteration halo. There is no leached capping in the classic sense at the Lakeshore deposit. The goethite zone, which contains an average of 0.3 percent copper, occupies the relative position of a leached capping.Protore mineralization and alteration patterns governed the formation of the chrysocolla, brochantite, and chalcocite zones. The chalcocite and brochantite zones formed in rocks which previously contained abundant hypogene pyrite and chalcopyrite mineralization. The antecedents of the chrysocolla zone contained much less pyrite and chalcopyrite and more acid-consuming silicates, such as biotite and plagioclase, than the antecedents of the chalcocite and brochantite zones. The geochemical changes associated with the formation of the chrysocolla zone are consistent with the destruction of biotite and plagioclase and the precipitation of gypsum and chrysocolla. The geochemical changes associated with the formation of the chalcocite and brochantite zones are consistent with the recrystallization of biotite to smectite and vermiculite.The neutralization of copper-bearing weathering solutions by plagioclase and biotite resulted in widespread dissemination of copper into altered phenocryst sites. A complex assemblage of copper-bearing clay or chrysocolla or both is present in the altered phenocrysts. There is a continuous range of compositions from chrysocolla to copper-bearing clay in altered plagioclase phenocrysts. There is no continuous solid solution between chrysocolla and copper-bearing clay in altered biotite phenocrysts or in supergene veinlets.