Faulting and Hydrothermal Alteration in a Proterozoic Rapakivi Batholith: Coupling Geothermal Reservoir Formation and Element Mobility

Abstract Crystalline rocks are increasingly recognized as geothermal targets, where reservoir parameters including porosity and permeability strongly reflect the coupled effects of brittle deformation and hydrothermal alteration. To quantify how these effects modify reservoir properties in crystalline rocks, we integrate petrophysical, mineralogical, geochemical, geochronological, and structural data from the well exposed Vehmaa Rapakivi Batholith in southwestern Finland, an analogue for fault-hosted geothermal reservoirs in crystalline settings. The batholith records five alteration stages: (1) late-magmatic F-rich, (2) high-temperature cementation, (3) propylitic alteration, (4) quartz cementation, and (5) low-temperature oxidizing overprint. The strongest reservoir development is associated with Stage 3, where mineral dissolution, replacement, and microfracturing generated porosities of up to 18%, whereas Stage 2 produced dense cemented rocks with little preserved porosity. Subsequent quartz cementation and low-temperature oxidizing alteration further modified reservoir properties through localized mineral dissolution and fracture sealing. U-Pb ages of titanite and zircon confirm that Stages 1 and 2 began during the late crystallization to early cooling of the batholith at ~ 1.57 Ga, while a younger ~ 1.0 Ga age suggests much later fluid circulation or thermal resetting. Notably, kilometer-scale strike-slip faults and alteration networks transect much of the batholith, indicating that these processes can operate over rock volumes relevant to geothermal development. These processes also redistributed economically relevant elements, including REE, Sc, Sr, Sn, and Zn concentrated along the faults. Our results show that fracturing, dissolution, and cementation control crystalline reservoir quality, while fluid–rock reactions govern element mobility and mineralization, linking geothermal potential with critical element resources.