PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF A DRILL HOLE IN REUTER CANYON, BEAR LODGE MOUNTAINS, CROOK COUNTY, WYOMING: IMPLICATIONS FOR TERTIARY-AGED CARLIN-LIKE GOLD MINERALIZATION

The Bear Lodge Complex is located in northeastern Wyoming along a WNW- trending zone of Tertiary alkalic intrusions that extends into the northern Black Hills where intrusion-hosted and Carlin-type gold-silver deposits have been mined since the late 1800s. The sedimentary hosts are Paleozoic, and consist of shelf carbonates and clastics. A recent drill-hole SUN 024c near Reuter Canyon extends 1600 feet at an inclination of 60° and bearing of 280° from Mississippian-aged Pahasapa Limestone through Cambrian-aged Deadwood Formation. The sequence is inflated by numerous sills and laccoliths of phonolite, trachyte and latite. A stratigraphic column correlated with geochemistry assays reveals that high gold grades occurred in the upper 114 foot interval of the Deadwood Formation inflated by 137 feet of igneous intrusions. There is a strong correlation between gold and arsenic, and a weak correlation of gold with silver and tellurium. Silver tellurides are also locally present, evident from the correlation of silver and tellurium. Analysis of thin sections discloses that at least two different stages of pyrite emplacement occurred. The first generation is anhedral and disseminated throughout the rock with local alteration to limonite. Higher gold grades are associated with the second generation of pyrite which is euhedral and occurs along veins of quartz ± K-feldspar ± calcite, indicative of potassic alteration. The geochemical and petrographic data support the essential influence of the Tertiary magmas on fluid source and mobilization and source of metals.