MAPPING MINERALOGY AND HYDROTHERMAL ALTERATION PRODUCTS AT THE CRIPPLE CREEK & VICTOR GOLD MINE, CRIPPLE CREEK, COLORADO, USING HYPERSPECTRAL CAMERAS

Imaging spectroscopy (hyperspectral imaging) has been used in mineral exploration for decades. Here we present spectroscopy-based mineral maps of the Cresson Pit at the Cripple Creek & Victor (CC&V) gold mine in Cripple Creek, Colorado. Imaging spectroscopy can be used to map a wide variety of minerals having absorption features in the visible to shortwave infrared region of the electromagnetic spectrum (400 to 2500 nm) on a pixel-by-pixel basis, including subtle changes in chemistry. While indistinguishable to the naked eye, common hydrothermal alteration products muscovite, sericite, and illite, grouped here as ‘white micas’, have a diagnostic absorption feature at approximately 2200 nm. The position of this feature is influenced in large part by Al, Fe, and Mg substitutions, with a shift of ~1 nm shortward or longward indicating a change in Al content of ± 0.019 Al per O equivalents, a variation of approximately ±1.05%. These changes in white mica cation content are controlled by alteration intensity, hydrothermal fluid chemistry, and host rock lithology. Maps of the Cresson Pit wall (at ~60 cm pixel size) indicate the presence of white micas, kaolinite, montmorillonite, goethite, and jarosite among other minerals, which agrees with previous spectral and geological studies of the area. Goethite and jarosite most likely formed as the result of pyrite weathering. Most white micas in the Cresson Pit have an absorption feature that range from 2201 nm (higher Al content) to 2217 nm (lower Al content). Because the causative intrusion at CC&V is a lamprophyre, which are typically high in Mg, we would expect white micas proximal to the intrusion to have a longer wavelength feature. A general trend from shorter wavelength white micas near the surface to longer wavelength white micas at depth was mapped. This trend may represent increasing proximity to the mineralization-causative lamprophyre intrusion at depth, since Mg-rich white micas have longer wavelength features.