Geology of the Brick Flat massive sulfide body, Iron Mountain cluster, West Shasta District, California

Abstract

The Brick Flat massive sulfide body is one of a group of eight individual bodies that constitute the Iron Mountain cluster in the southern part of the West Shasta district. Before they were separated by postmineral faulting, five of the eight sulfide bodies formed a single large deposit about 1,375 m long with a mass of some 23 million metric tons. The pyritic Brick Flat sulfide body is one of the five faulted segments of this deposit. It was stripped of its overlying cover and partly mined for its sulfur content in the 1950s and 1960s, and was mapped and sampled in some detail as part of the recent West Shasta project investigations.The Brick Flat massive sulfide lies within medium phenocryst rhyolite that is characteristic of the ore-bearing middle unit of the Balaklala Rhyolite. It is interpreted to be downfaulted a vertical distance of 75 to 85 m from the Old Mine sulfide-gossan orebody along the north-dipping Camden South fault. It is bounded in turn on its north side by another parallel fault, the Camden North, which drops the orebody down another 75 m to the level of the Richmond orebody. A weakly pyritized rhyolitic tuff lies a few tens of meters beneath the sulfide and a discontinuous belt of tuff lies a few tens of meters above and parallel to the N 60 degrees E-trending main belt of massive sulfides in the Iron Mountain cluster. The spatial coincidence of massive sulfide bodies and tuff beds at a stratigraphic level several tens of meters higher suggests that either a graben or a narrow elongate basin constituted the depositional environment at the time the deposits were formed.