Reflectance Spectroscopy of Hydrothermal Alteration Zones Developed Around Auriferous Reefs Hosted in Hutti-Maski Greenstone Belt, India: A Tool for Exploring Precious Metals

This work attempts to study the spectral characteristics of the hydrothermal alteration zones developed around the auriferous reefs in the spectral range of 350 nm to 2500 nm. The samples representing proximal alteration, distal alteration and unaltered host rock were collected from four reefs (Oakley’s reef, Strike reef, Zone I reef and Middle reefs) exposed in the Hutti Underground Mine, India. All the auriferous reefs are hosted in amphibolite, except the Middle reef, which occurs in acid volcanic rock. Spectral characterization of the continuum-removed average whole-rock reflectance spectra of the samples was carried out with ENVI software, followed by spectral deconvolution in Origin Pro software. The resulting absorption features were correlated with the mineralogy obtained from XRD and petrographic studies. It is noted that the abundance of chlorite, amphibole and calcite are key to differentiate the alteration zones developed around the reefs hosted in amphibolite. The diagnostic spectral features can identify chlorite and amphibolite at 2250 nm, 2340 nm, and 2310 and 2390 nm, respectively. A comparison of absorption band depth and XRD-derived semi-quantitative mineral abundances showed that band depths of 2250 nm and 2390 nm can be used to measure the abundance of chlorite and amphibole in the sample. The 2340 nm feature is influenced by chlorite ± biotite + calcite. The abundance of chlorite shows a decrease from the proximal zone towards the unaltered host rock, whereas the abundance of amphibole is low in the proximal alteration zone and increases towards the unaltered host rock. In the case of acid volcanic rocks, the sericite controls the spectral characteristics, with a diagnostic absorption feature at 2206 nm. The above spectral characteristics can be used as a guide for exploring gold mineralization.