Long-lived interaction between hydrothermal and magmatic fluids in the Soultz-sous-Forêts granitic system (Rhine Graben, France)

Abstract The 5 km deep drilling at Soultz-sous-Forets samples a granitic intrusion under its sedimentary cover. Core samples at different depths allow study of the evolving conditions of fluid-rock interaction, from the syn-tectonic emplacement of Hercynian granites at depth until post-cooling history and alteration close to the surface. Hydrogen, carbon and oxygen isotope compositions of CO 2 and H 2 O have been measured in fluid inclusions trapped in magmatic quartz within samples collected along the drill core. Early Fluid Inclusions Assemblage (FIA) contains aqueous carbonic fluids whereas the latest FIA are H 2 O-rich. In the early FIA, the amount of CO 2 and the δ 13 C value both decrease with depth, revealing two distinct sources of carbon, one likely derived from sedimentary carbonates (δ 13 C = − 2‰ V-PDB) and another from the continental crust (δ 13 C = − 9‰ V-PDB). The carbon isotope composition of bulk granites indicates a third carbon source of organic derivation (δ 13 C = − 20‰ V-PDB). Using a δD - δ 18 O plot, we argue that the water trapped in quartz grains is mainly of meteoric origin somewhat mixed with magmatic water. The emplacement of the Soultz-sous-Forets granite pluton occurred in a North 030–040° wrench zone. After consolidation of the granite mush at ~ 600 °C, sinistral shear (γ ~ 1) concentrated the final leucocratic melt in vertical planes oriented along (σ 1 , σ 2 ). Crystallization of this residual leucocratic melt occurred while shearing was still active. At a temperature of ~ 550 °C, crystallization ended with the formation of vertical quartz veins spaced about 5 mm, and exhibiting a width of several cm. The quartz veins form a connected network of a few kilometers in height, generated during hydrothermal contraction of the intrusion. Quartz crystallization led to the exsolution of 30% by volume of the aqueous fluid. As quartz grains were the latest solid phase still plastic, shearing localized inside the connected quartz network. Aqueous fluid was thus concentrated in these vertical channels. Eventually, when the channels intersected the top of the crack network, water boiling caused the formation of primary inclusions. At the same temperature, the saline magmatic waters, which were denser than the meteoric waters, initiated thermohaline convection with the buoyant “cold” hydrothermal water layer. This mechanism can explain the mixing of surface and deep-seated fluids in the same primary inclusions trapped during the crystallization of magmatic minerals. This study, which separately considers fluid-rock interactions at the level of successive mineral facies, brings new insights into how fluids may be different, their origin and composition, and depending on tectono-thermal conditions, bears implications for eventual ore forming processes.