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Occurrence and distribution of Au and Ag in the Caribou-Cross mining area, Boulder County, Colorado
Langston-Stewart, Sage
Langston-Stewart, Sage
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2022
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Abstract
The Caribou-Cross deposit is part of the historical Grand-Island mining district in southwest Boulder County, Colorado, just west of the town of Nederland, approximately 21 miles west of Boulder. The Grand Island district includes at least 15 historic silver and gold mines with the Caribou and the Cross mines being the two primary mines. The Caribou-Cross and legacy mines scattered about the property of Grand Island Resources was intermittently mined for Au and Ag since the 1870’s and is currently undergoing brownfield redevelopment by Grand Island Resources.
The Grand Island mining district is in the northernmost part of the Colorado Mineral Belt (CMB), a northeast-southwest trend of mineral deposits stretching across Colorado. Structurally controlled precious metal bearing veins are hosted in Laramide age Caribou monzonite and in Precambrian Idaho Springs gneiss. It is the goal of this study to improve the understanding of ore zonation, related alteration assemblages, the occurrence, mineralogy, and spatial distribution of precious metals, and the development of a conceptual model for the Caribou-Cross deposit in the Grand Island mining district.
One hundred samples from across the district ranging from igneous host rock lithologies, primary veins, to waste rock samples were collected and investigated using optical petrography, micro-X-ray fluorescence, cathodoluminescence microscopy, fluid inclusion petrography and microthermometry, FE-SEM-BSE and -EDS, and SEM based automated mineralogy.
Six different intrusive rock types were identified and distinguished based on petrography and automated mineralogy, including magnetite dunite, monzonite, quartz monzonite, amphibolite, magnetite amphibolite, and lamprophyre. The monzonite shows an alkaline evolution path and evidence suggests the mafic bodies originated from magmatic differentiation.
Alteration consists of large-scale, early-stage hydrothermal K-feldspar alteration along fractures prior to vein formation. Subsequent sericitization of plagioclase is present proximal to the veins whereas chlorite alteration is present distal to the vein. Early pyrite and quartz (Q1) veins were followed by ankerite and second-stage quartz (Q2). Fluid inclusions in Q1 quartz are CO2 rich suggesting that vein formation took place deep below the paleosurface. In contrast, rare fluid inclusion assemblages present in Q2 quartz are low in CO2. Homogenization temperatures range from 210 to 225°C and salinities range from 7.9 to 13.9 wt.% NaCl equivalent. Base metal minerals (chalcopyrite, galena, iron-poor sphalerite) and precious metal phases (stromeyerite and Ag-rich gold) formed in fractures and vugs contemporaneously with and after the Q2 quartz. Late barren carbonate veins and a final supergene stage, which includes malachite and azurite, conclude the formation of the Caribou-Cross deposit. It is interesting to note that Ag-bearing gold was only found at Cross, but not at Caribou.
Veins show strong structural control and complex overprinting relationships that are indicative of a system that evolved through space and time with multiple changes between lithostatic and hydrostatic conditions. The Caribou-Cross deposit is interpreted to be a deep seated magmatic-hydrothermal system that formed from fluids having an intermediate sulfidation state
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