Fluid mixing at the origin of a Ba-Pb mineralization in Triassic sandstones of the Geneva Basin (France–Switzerland)

Unconformity-related fluorite-barite-Pb-Zn mineralization is a key indicator of paleo-fluid circulation and tectonic evolution in sedimentary basins. This study investigates fracture-controlled barite-Pb veins in Triassic sandstones of the Geneva Basin (France–Switzerland) using petrography, stable (S, O) and radiogenic (Pb, Sr) isotopes, fluid inclusion microthermometry and elemental analysis, and basin thermal modeling. Results reveal that Triassic carbonates capping the mineralization, traditionally linked to the Middle Triassic Muschelkalk, likely date to the Late Triassic (Norian), revising prior chronostratigraphic interpretations. In the mineralized sandstone, white quartz nodules formed via pseudomorphic replacement of diagenetic anhydrite nodules during Lower Cretaceous cooling (~120–150 °C, 20.5 wt% NaCl equiv.), setting a maximum age for subsequent Ba-Zn mineralization. The barite-sulfide assemblage precipitated through the mixing of a Ba- and Pb-rich formation brine sourced from Triassic sandstone with a sulfate-rich brine possibly derived from overlying anhydrite-bearing carbonates. Thermochemical sulfate reduction, triggered by hydrocarbons expelled from underlying Permo-Carboniferous sediments, produced H 2 S and promoted sulfide precipitation. Fluid mixing is identified as the primary mechanism for mineralization, with a probable Lower Cretaceous age, though a Miocene age cannot be excluded. This model highlights intra-basinal processes in the formation of unconformity-related hydrothermal deposits, consistent with European fluorite-barite-Pb-Zn systems, and underscores the role of primary and secondary permeability in geothermal exploration in the Geneva Basin.