Fracture-controlled multi-stage secondary mineralization and alteration in Kaldakvísl basaltic lava group, Husavik–Tjörnes Peninsula, northern Iceland: implications for subsurface CO2 storage via carbon mineralization

Successful pilot projects, e.g., CarbFix (Iceland) and Wallula (USA), where CO2 has been injected into subsurface basaltic rocks, have demonstrated the potential and advantages of mafic and ultramafic rocks (unconventional reservoirs) for long-term, safe CO2 storage by mineral trapping. Despite the advantages, the CO2 storage potential in unconventional reservoirs is relatively underexplored. Consequently, the key factors (and/or their interplay) that impact secondary mineralisation and the storage capacity of basaltic lava flows in the subsurface are less well understood. In this study, we integrate field-based geological investigation with whole-rock geochemical-, mineralogical-, and SEM- analysis, to characterise Miocene age Kaldakvísl basaltic lava flows, and associated deformation structures exposed along the western and eastern coastline of the Husavík–Tjörnes Peninsula, northern Iceland. Our results reveal at least two phases of fault activities and vein development in the western coast, associated with deformation along a major normal-dextral strike-slip fault, the Husavik-Flatey Fault Zone (HFFZ), whereas the eastern coast is far less deformed. Overall, the lava flows were largely characterized by tabular, sheet-like geometry, variable thicknesses ranging from c. 1 – 7 m, and sometimes interbedded with thin volcaniclastics and paleosols. Individual lava flows exhibited large variability in intra-flow vesicle morphology, intensity, connectivity, and mineral fill that allows us to subdivide each flow sequence into three distinct units: vesicular base-, massive core-, and vesicular top- of flow. Field observations and petrological analysis of lava flow sequences from the western coast show that the flow tops have been subjected to intense and higher degrees of hydrothermal alteration and secondary mineralization (e.g., zeolites and minor carbonates) compared to the flow base and core. Conversely, lava flow sequences from the eastern coast are generally less altered, preserving the primary composition and open vesicles of the lava flows. This suggests a strong correlation between the degree of deformation and tectonic fracturing, and the degree of hydrothermal alteration and secondary mineralization, underpinning the control the former has on the latter. Furthermore, the results of XRD analysis and optical microscopy identified zeolite minerals that formed both at lower temperatures (55-110 °C), such as chabazite and heulandite, and higher temperatures (70 °C up to 300 °C), such as stilbite and analcime. We propose that these zeolite minerals form from distinct hydrothermal events, reflecting a multi-stage rather than a continuous mineralization and alteration process. Our observations suggest that the multi-stage alteration process was most likely driven by the multiple phases of fault activities and vein formation associated with the HFFZ and subsidiary faults, which provided pathways for hydrothermal fluids. This study improves our understanding of the factors that influence hydrothermal alteration and secondary mineralization in basaltic rocks and has implications for evaluating the potential role of fractures in CO2 storage in unconventional reservoirs.