GRANITIZATION AND HYDROTHERMAL ALTERATION AT BINGHAM, UTAH

Abstract

The rocks exposed in the disseminated copper deposit at Bingham, Utah, consist of folded Pennsylvanian quartzites, limestones, and dolomites within which have been formed granite, actinolite syenite, granite porphyry with associated aplites, and biotite-quartz latite porphyry. Field and petrographic evidence shows that the granite porphyry and biotite-quartz latite porphyry are magmatic. That the granite and actinolite syenite may have been formed by granitization is strongly suggested by: (1) the presence of a microgranitic rock in the granite, recognized as being granitized quartzite, and (2) the replacement of quartzite by feldspar, observable only with the aid of the microscope, that occurs along the contact of the granite. Other chemical, textural, compositional, and other evidences support this hypothesis. The hydrothermal activity attending the ore deposition took place in seven stages following the formation of the igneous-appearing rocks. Except for the first two, which can be separated on good microscopic evidence, all stages can be differentiated by structural field relations. Stage I saw the formation of kaolinite and illite. During Stage II hydrothermal biotite and sericite were widely developed. In Stage III, chlorite and hydrothermal biotite were formed, but in restricted areas. In Stage IV, quartz and sericite formed over wide areas, while during Stare V quartz was deposited in open fissures without accompanying alteration. Stage VI is the sulfide stage, and Stage VII saw the development of sericite and allophane in open fissures.