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Iron Sharpens Iron: Using Hydrothermal Alteration Data from Crystalline Basement to Illuminate Modern Sub-Volcanic Geothermal Reservoirs

Alan Bischoff 12, Daniel Carbajal-Martinez2, Nicklas Nordbäck2, Ester M. Jolis2, Jukka Kuva2, and Mike Heap3

  • Affiliations:  1University of Turku, Finland, 2Geological Survey of Finland, 3University of Strasbourg, France 

  • Presentation type: Talk

  • Presentation time: Tuesday 08:30 - 08:45, Room S160

  • Programme No: 5.1.1

  • Theme 5 > Session 1

Abstract

This study investigates hydrothermal alteration processes in crystalline basement terrains, providing an analogue for sub-volcanic geothermal reservoirs affected by faulting and hydrothermal activity. In these basement rocks, hydrothermal fluids interact with primary minerals, leading to significant alteration and the creation of secondary porosity above 30% and permeability values reaching over 10-12 m². Key alteration features include the replacement of mafic minerals such as biotite, pyroxene, and amphibole by chlorite and epidote, indicative of high-temperature (200--300°C) hydrothermal processes. Altered granites, migmatites, and gneisses display a consistent sequence of mineral transformations, including garnet-biotite-chlorite, quartz dissolution, and the precipitation of zeolites, prehnite, and calcite, which further alters porosity and permeability. Fault networks within these rocks provide pathways for fluid circulation, driving alteration and contributing to the formation of crystalline reservoirs. High-resolution CT scanning reveals up to 96% pore connectivity in the most altered rocks. Additionally, drone photogrammetry reveals kilometer-scale networks of faults and fractures transecting plutons and batholiths, while cross-flow pumping tests show hydraulic connection over distances of at least 500 meters, emphasizing the large-scale hydraulic connectivity of these fault zones. The study underscores the dynamic and multi-stage process of crystalline reservoir formation, involving fracturing, alteration, and reactivation under varying tectonic and thermal conditions, a common feature of sub-volcanic environments. These findings offer valuable insights into sub-volcanic geothermal systems and their potential conventional reservoirs, with implications for constructing unconventional EGS in similar settings worldwide.