Comprehensive characterization of drill core, and geochemical alteration pathways of drill core from the Humu’ula groundwater research project, Hawaii, USA: II. Keamuku maneuver area

In 2013–2015, two deep wells, PTA-2 and KMA-1, were drilled in the Humu‘ula Saddle region of Hawai‘i for groundwater and geothermal exploration as part of a broader effort to understand the geological history and resource potential of subsurface environments on the island of Hawai‘i. This study focuses on the alteration mineralogy of the KMA-1 drill hole, complementing similar investigations conducted on PTA-2. Mineralogical analysis of KMA-1 involved a multi-scale characterization of aqueous alteration using bulk and high-resolution visible and short-wave infrared (VIS-SWIR) and long-wave infrared (LWIR) spectral imaging, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). Several types of secondary phases, including alunite, jarosite, secondary hematite, and smectites, were identified. Spectroscopic data and the occurrence of alteration minerals suggest a correlation with confined groundwater units. Field surveys showed widespread alteration and abundant sulfates, while high-resolution techniques like XRD and XRF detected smaller quantities in bulk rock. This discrepancy suggests that spectroscopic methods are highly sensitive to strongly absorbing, albeit minor, mineral phases. Collectively, our analysis points to possible fumarole systems buried over time or acid-steam alteration during post-shield volcanic phases. This study contributes to ongoing efforts to characterize the mineralization associated with geothermal and groundwater systems in the Hawaiian subsurface, providing further insight into hydrothermal processes and alteration pathways.