Hydrothermal alteration mapping of mineralogical imprints associated with subtle geothermal system using mixture tuned matched filtering approach on ASTER VNIR And SWIR data

The purpose of this study is to evaluate the applicability of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Visible near infrared (VNIR) and Shortwave infrared (SWIR) bands in discriminating hydrothermal alteration mineralogy related to thermal springs as proxy for identifying subtle Geothermal (GT) systems at Yankari Park in north eastern Nigeria. The area is characterized by a number of thermal springs including, Gwana, Dimmil, Mawulgo and Wikki which is used directly for recreation and tourism. A Decorrelation Stretch (DCS) transform was initially used on ASTER to highlight alteration zones and generate regions of interest (ROIs) which guided field validation and identification of associated exposed alteration zones. GPS field survey and sampling of hydrothermally altered rocks and laboratory analysis using Analytical Spectral Device (ASD) and X-Ray Diffraction (XRD) is conducted for verification. Observed and validated alteration sites (ROIs) are subsequently used to extract mean image spectra from the ASTER data. We then explored the utility of mean image spectra for mapping subtle mineralogical imprints associated to geothermal systems as proxy for identifying targets in unexplored regions by using the Mixture Tuned Match Filtering (MTMF) algorithm on ASTER VNIR to SWIR spectral subsets. The results indicate that ASTER data could reliably be used for prefeasibility stage narrowing of targets and mapping of subtle alterations using image derived spectra. These could have significant implications especially for mapping unconventional GT systems in unexplored regions.