GEOLOGY, HYDROTHERMAL ALTERATION, AND GOLD MINERALIZATION IN THE VICINITY OF TAYLOR RIDGE AND CARBON HILL, BEAR LODGE MOUNTAINS, CROOK COUNTY, WYOMING

Gold mineralization and associated hydrothermal alteration in the Bear Lodge Mountains, Crook County, Wyoming, have not been examined previously in detail. The Bear Lodge Mountains are a domal uplift, which is the northernmost extension of the Black Hills of South Dakota and Wyoming. The dome is cored by Eocene alkaline igneous rocks and carbonatite dikes, containing Precambrian granite xenoliths and surrounded by Phanerozoic sedimentary rocks. The Bear Lodge Dome is host to fluorite, gold-tellurium, rare-earth, thorium, and copper mineralization. Field mapping and detailed petrographic study utilizing the cathodoluminescence technique, scanning electron microscopy, electron microprobe analysis, laser ablation mass spectrometry, X-ray diffraction, radiometric and magnetic surveys, and geochemical analysis of the area around Taylor Ridge and Carbon Hill reveal gold-tellurium mineralization, which is related to structurally controlled, incipient and massive potassic fenitization. Potassic fenitization is manifest as potassium-feldspar + pyrite ± biotite alteration. Hydrothermal potassium-feldspar with Fe³⁺-activated red cathodoluminescence precipitated in veins and replaced Ti⁴⁺-activated blue-luminescent potassium-feldspar and Mn²⁺-activated green-luminescent plagioclase feldspar; and F and Sm to Lu-enriched apatite replaced and overgrew LREE-enriched igneous apatite. Lithologic contacts, breccia pipes, and faults are preferentially Au-Te-mineralized due to increased permeability that channels hydrothermal fluid flow. The study area is enriched in rare-earth elements and base metals, representing a transition from low-temperature mineralization typical of the Bear Lodge Dome and alkaline epithermal gold deposits in general, to relatively high-temperature epithermal mineralization, indicating that there may be porphyry-style mineralization at depth. Some gold deposited during episodes of potassic fenitization was remobilized during subsequent clay alteration, oxidation, and silicification.