We have determined the chemical composition of ∼350-m.y.-old solutions extracted from fluid inclusions, and strontium isotopic compositions of hydrothermal minerals from the Irish zinc-lead orefield. These data show that ore-forming fluids were derived from evaporated seawater and acquired metals by deep circulation within fractures in continental crust. Mineralization occurred in the near-seafloor environment when these solutions returned to the surface via thermohaline convection and mixed with brines rich in H2S produced by bacterial reduction of seawater sulfate. The results indicate that deep penetration of seawater or evaporated seawater into the continental crust can occur in rift zones or extending passive margins and that this process can generate large volumes of base metal ore-forming solutions. Our results are inconsistent with topographic flow models for mineralization in the district, and support deep convection models for ore formation. The widespread development of evaporitic brines on the Laurussian continental margin under late Paleozoic greenhouse conditions is likely to have been critical for generating numerous accumulations of base metals in sedimentary basins at this time.