OCCURRENCE OF GOLD IN THE CROSS MINE, BOULDER COUNTY, COLORADO, USA

The Cross Mine, located in the Grand Island mining district in Boulder County, Colorado, USA, was mined for Au and Ag in the late 1890’s. Precious metal-bearing veins in the deposit are predominantly hosted in the Precambrian quartz-biotite-sillimanite Idaho Springs Gneiss, but are subordinately also hosted in Laramide-age monzonitic intrusions. The occurrence and sequestration of precious metal minerals, however, is not well understood, limiting our understanding of this enigmatic magmatic-hydrothermal deposit. It was the aim of this project to improve our understanding of the occurrence and distribution of ore minerals within the veins and the nature of the associated hydrothermal alteration in the Cross Mine. Select drill core intercepts with high precious metal grades were targeted for sampling. A subset of six samples from varying depths from the Idaho Springs Gneiss with concentrations of up to 9.47 ppm Au and 164 ppm Ag were investigated in the Mineral and Materials Characterization Facility in the Department of Geology and Geological Engineering at the Colorado School of Mines, Golden, CO, USA. Mapping µXRF and pXRF analyses were performed on drill core slabs prior to thick section preparation to evaluate the location and association of ore minerals while preserving their textural context. Reflected and transmitted light petrography was performed on polished thick sections, followed by FE-SEM BSE imaging and EDS analysis, and SEM-based automated mineralogy mapping. High grades correlate with the occurrence of Ag-bearing native gold forming grains that are ≤20-30 µm in size and are commonly associated with pyrite and galena as well as sphalerite and barite. Alteration haloes of mm to cm scale predominantly consist of K-feldspar and muscovite. This reconnaissance survey forms part of a larger study in the Grand Island mining district that will shed light on the processes involved in the formation of this unusual magmatic-hydrothermal deposit type and aid in further brownfield exploration efforts.