26 10. Map showing average orientation of joint sets in the southern Black Hills___________________________________________________________ 30 11. Stratigraphic sections of the Minnelusa Formation, showing correlation of brecciated rocks in outcrop with anhydrite-bearing strata of the subsurface in Custer County, S. Dak.
Thia report COilC8l'D8 work doDe oa behalf of tbe U. S. Atomic Enerp Commiaion and is publiahed with the~ of the Commiuion
Analyses of surface samples from rocks throughout the Cripple Creek district show that several extensive gold-silver-tellurium nnomalies exist. These anomalies are coextensive or partly coextensive with anomalies of iron, lanthanum, lead, mercury, antimony, arsenic, and vanadium. Manganese seems to be most concentrated in a zone peripheral to the highest gold concentrations. Hydrothermal alteration has resulted in the replacement of sodium in the sodium-rich minerals by potassium. The highest concentrations of gold, silver, tellurium, and iron conform well with the areas of greatest potassic alteration. In general, the abundance of the metals is in inverse proportion to their abundance in the crust of the earth. The largest gold anomalies are suggested as exploration targets for large low-grade gold deposits. 100 MILES I FwunE I.-Index map of Colorado, showing area of this report. 1962, however, the last major mill and the last of the active mines were closed, thereby threatening an end to the productive life of the district. As a result, the U.S. Geological Survey started geochemical studies in the district in 1964 to test for possible ex1tensions of known deposits or low-grade deposits that could be mined from the surface. The investigation consisted principally of a study of the distribution of gold, silver, and tellurium, but it also included some study of several other metals (table 1). The geochemical distribution of the mobile elements mercury, antimony, and arsenic was studied to determine whether they were useful indicators of gold and silver. Also, analyses were made of several other elements to determine whether other metals were imported by the mineralizing solutions. We express our thanks for the warm cooperation of the late Max Bowen, President, and Charles Carlton, Manager of Mines, of the Golden Cycle Corp. We. also thank Prof. Gunther Friedrich, University of Aachen, West Germany, for his assistance during the field investigations. The technical assistance of the following analysts is gratefully acknowledged:
The principal ore deposits in the Coeur d'Alene district are leadzinc-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 Belt rocks.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. about the geology and ore deposits of the Coeur d'Alene district. We particularly wish to thank the American Smelting and Refining Co.,
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