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Unmanned aerial vehicle and laboratory-based hyperspectral imaging to unravel modern and ancient hydrothermal systems
Al-Attar, Zaid
Al-Attar, Zaid
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2024
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Understanding hydrothermal activity is crucial for mineral exploration and geothermal energy development, as it significantly influences the formation of mineral deposits and geothermal reservoirs. Modern geothermal systems serve as present-day analogs of epithermal ore-forming systems, providing insights into hydrothermal mineralization processes and fluid circulation patterns in the upper crust. This study utilizes hyperspectral imaging to detect and map alteration minerals and their spatial distribution in two distinct geological settings: the Castle Mountain low-sulfidation epithermal gold deposit and the Coso Geothermal Field. At the Castle Mountain deposit, a UAV-based hyperspectral survey was conducted to map alteration minerals. Ground truthing was performed using laboratory-based hyperspectral analysis on ground control samples from the surveyed area. Multiple mineral identification algorithms, including USGS PRISM MICA, Minimum Wavelength Mapping (MWL), and Spectral Angle Mapper (SAM) were utilized and compared to map surface alteration at the Castle Mountain deposit. Our findings indicate that the UAV-based hyperspectral scanning effectively identifies alteration minerals, reflecting the classic zonation in epithermal systems. The USGS PRISM MICA algorithm outperformed others by providing the most accurate results with the least complexity. In the Coso Geothermal Field, five reverse circulation chip drill holes were analyzed using laboratory-based short-wave infrared (SWIR) scanning and SEM-based automated mineralogy. The Illite Spectral Maturity (ISM) index and the position of the 2200 nm absorption feature were utilized to differentiate between illite, smectite, and their mixtures, enabling the construction of thermal gradients for the studied drill holes. Our results confirm the dominance of diorites and granodiorites with occasional granite intrusions. The unsystematic distribution of alteration minerals suggests multiple hydrothermal events. The ISM results correlated well with previous XRD studies. Smectite and illite-smectite mixtures were found at greater depths than their thermal stability, suggesting heating events within the geothermal system. This study validates the effectiveness of hyperspectral imaging for mapping alteration. The proposed methodologies offer a faster, non-invasive approach for mineral identification while enhancing data processing efficiency and allowing detailed mineralogical and lithological analyses. Which in turn improved our understanding of this complex geothermal system.
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