AN ENIGMATIC CU-AU DEPOSIT IN THE LARAMIE RANGE

The CK Gold deposit (Au-Cu) in southeast Wyoming fails to fit neatly into any known deposit type and exhibits characteristics of shear-zone, porphyry, and paleoplacer-related mineralization. The deposit lies in the Silver Crown Mining District ~ 20 miles west of Cheyenne, Wyoming in the eastern foothills of the Laramie Range (Central Rocky Mountain region). We investigated this deposit to determine the age relationships between rock types and ascertain the petrogenesis of gold and copper mineralization. The study employed a combination of field mapping, core logging, textural analysis in thin section, and U-Pb geochronology. The area is underlain by four intrusive igneous units (oldest to youngest): equigranular, hornblende biotite granodiorite and hornblende tonalite, porphyritic biotite granodiorite dikes, and pegmatite dikes. These intrusions cut hornblende biotite gneiss and deformed metasedimentary deposits. The intermediate-age porphyritic biotite dikes yielded a U/Pb zircon date of 1803±7.6 Ma, implying the deposit hosted in the equigranular hornblende biotite granodiorite is this age or older. This suggests that the Cu-Au deposit is likely unrelated to the series of younger igneous bodies exposed in the area with U/Pb ages from 1780-1760 Ma. We propose that gold and copper were introduced during the accretion of arc terranes to the Wyoming Craton during the Medicine Bow Orogeny (1780-1740 Ma). Heating from the adjacent Sherman Batholith (1430–1438 Ma) and Laramie anorthosite complex (1431 to 1436 Ma), associated hydrothermal alteration, and both ductile and younger brittle deformation have remobilized and overprinted the original deposit. Preservation of this arc terrain may elucidate the immediacy of mineralization found along the eastern flank of the Laramie Range and inform future avenues of exploration.