Pairing mineralogical and multispectral satellite imagery analysis to map the hydrothermal alteration on Hasandağ Volcano (Aksaray, Türkiye)

The Greater and Lesser Hasandağ summits have both active and extinct fumarole vents, with evident hydrothermal alteration in their vicinity. Active fumaroles were documented on the western summit flanks of the Greater Hasandağ crater, at altitudes between 3000 and 3150 meters. Several hydrothermally altered zones are located inside the double craters of Greater Hasandağ and on the Lesser Hasandağ summit. In order to characterize the both completed and ongoing hydrothermal alteration process, a total of 27 rock samples were gathered from the hydrothermally altered rocks. The mineralogical compositions were examined by X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and Fourier transform infrared spectroscopy (FT-IR) from the selected parts of altered rocks. Presence of common low–grade temperature (0 – 100 °C) secondary mineralization such as alunite, natro-alunite, halloysite, and opal-AN with scarce jarosite indicate existence of a continuous alteration process in a systematic order of crystallization. Recurrent temperature measurements from the fumarole vents also confirm the mineralogical findings. FT-IR analysis is also used synchronously to identify the spectral characteristics of hydrothermally altered rocks together with other analytical methods. Spatial distributions of the alteration minerals of Hasandağ were monitored comprehensively by using ASTER and Sentinel-2 imagery, relying on the reflectance and absorbance characteristics observed in the FT-IR data obtained from the altered samples. The combined results allowed us to produce precise hydrothermal alteration maps. Significantly, the zones where the alteration is prevalent are located along the Karacaören and Hasandağ Faults, which extend in the northeast-southwest and northwest-southeast directions, respectively.