Geochemical-exploration studies in the Coeur d'Alene district, Idaho and Montana
The principal ore deposits in the Coeur d'Alene district are lead-zinc-silver replacement veins in Precambrian rocks of the Belt Supergroup. The main ore minerals are galena, tetrahedrite, and sphalerite. The host rocks are mainly quartzite, siltite, and argillite. Cretaceous quartz monzonite locally intrudes the Beltrocks. The geochemical investigations reported here were carried out to determine if geochemical methods would be useful in the search for concealed ore deposits. About 8,700 soil samples and 4,000 rock samples were collected from a 300-square-mile (780 square kilometer) area for this study. The samples were analyzed for 35 elements. Antimony, silver, lead, manganese, and copper form dispersion patterns and halos that are related to many of the ore deposits within the district, and these same elements were found to be most useful in delineating the known mineral belts that contain most of the orebodies. The dispersion patterns are probably primary, having only minor modifications due to secondary redistribution of the ore-forming elements. The mineral belts and geochemical-dispersion patterns have been laterally offset, perhaps as much as 16 miles (26 km), by postore faulting. Prefault dispersion patterns can be restored by adjusting the geochemical maps along the postore faults to match the dispersion patterns across the fault trace. The reconstructed dispersion patterns of antimony, arsenic, lead, sulfur, and the ratio of cadmium to zinc form concentric halos around the restored position of the Gem stocks. Most of the Coeur d'Alene ore has been mined from this halo. It appears that the geochemical exploration methods reported here successfully delineate the major mineral belts and indicate unexplored areas in which to search for new deposits.