Paragenesis of iron sulphides in a Black Hills deposit

A small prospect mine near Deadwood, South Dakota, shows striking selective replacement of dolomite beds by quartz, pyrite, sphalerite, arsenopyrite, galena, chalcopyrite, and pyrrhotite. The occurrence of arsenopyrite, pyrrhotite, and gold values is significant in view of the occurrence of these minerals in the Homestake Mine, three miles distant.The remarkable feature of the ores is the abundant occurrence of four types of iron sulphide aside from small amounts of pyrrhotite. These varieties are: aggregates of anhedral grains of pyrite; euhedral pyrite crystals; stippled, brownish, fine-grained pyrite, and marcasite. The stippled brownish variety of pyrite is believed to have formed by alteration of pyrrhotite at a late stage in the mineralization apparently after the partial replacement of pyrrhotite by marcasite.