Mineralogy, textures, and relative age relationships of massive sulfide ore in the West Shasta District, California

Abstract

The Devonian massive sulfide orebodies of the West Shasta district in northern California are composed primarily of pyrite, with lesser amounts of other sulfide and gangue minerals. Examination of polished thin sections of more than 100 samples from the Mammoth, Shasta King, Early Bird, Balaklala, Keystone, and Iron Mountain mines suggests that mineralization may be divided into six paragenetic stages, the last five each separated by an episode of deformation: (1) precipitation of fine-grained, locally colloform and framboidal pyrite and sphalerite; (2) deposition of fine-grained arsenopyrite and coarse-grained pyrite, the latter enclosing tiny inclusions of pyrrhotite; (3) penetration and local replacement of sulfide minerals of stages 1 and 2 along growth zones and fractures by chalcopyrite, sphalerite, galena, ten-nantite, pyrrhotite, bornite, and idaite; (4) recrystallization and remobilization of existing minerals, locally increasing their size and euhedralism and promoting their aggregation; (5) deposition of quartz, white mica, chlorite, and calcite; and (6) formation of bornite, digenite, chalcocite, and covellite during supergene enrichment of several orebodies at the Iron Mountain mine. Despite regional greenschist facies metamorphism and local heating by intrusive bodies, enough of the original depositional features of the ore remain to suggest that the deposits in the district formed by processes similar to those that formed Kuroko- and Besshi-type massive sulfide deposits. Mineralogic and textural evidence do not support a second major episode of massive sulfide mineralization during the Permian.