MINERALOGY, ALTERATION, AND DEPOSIT CHARACTERIZATION OF THE CARIBOU-CROSS MINING AREA, BOULDER COUNTY, COLORADO

The Caribou-Cross mines located in the Grand Island mining district in Boulder County show structurally controlled precious metal bearing veins, hosted in Laramide age Caribou monzonite and Precambrian Idaho Springs gneiss. The Caribou-Cross was intermittently mined for Au and Ag since the 1870’s and is currently undergoing brownfield exploration and mine redevelopment.

One hundred samples from the district were investigated using optical petrography, micro–x-ray fluorescence, cathodoluminescence, fluid inclusion analysis, FE-SEM-BSE and -EDS, and SEM based automated mineralogy to determine igneous host-rock types, ore mineralogy, related alteration assemblages, ore and alteration zonation, and establish a conceptual deposit model. Five host rock lithologies were identified upon investigation of the heterogeneous Caribou monzonite: magnetite dunite, monzonite and quartz monzonite, amphibolite, magnetite amphibolite, and lamprophyre. Alteration consists of large-scale, early-stage hydrothermal K-feldspar alteration occurring along fractures prior to vein formation, subsequent sericitization of plagioclase with increasing intensity towards the vein, and distal chlorite alteration. Pyrite and early-stage quartz (Q1) with deformed fluid inclusions containing double-bubbles form the first vein stage, followed by ankerite, and second-stage quartz (Q2) growing into vugs. Chalcopyrite, galena, iron-poor sphalerite, stromeyerite, and electrum are formed in fractures and vugs, followed by late barren carbonate veins. A final supergene stage forms malachite, native Ag, polybasite-pyrargyrite, and other silver-sulfosalts associated with oxides. Based on primary Q2 fluid inclusions, the minimum temperature of entrapment is 240°C-250°C and the salinity of the fluid ranged from 7.86 to 13.94 weight percent NaCl equivalent.

The veins show complex textural relationships with strong evidence for vein reopening, indicating a very strong structural control on location of mineralization and overprinting of precious metal and gangue minerals with multiple changes between lithostatic and hydrostatic conditions. The Caribou-Cross is interpreted to be a deep, high to intermediate sulfidation, magmatic-hydrothermal system.