Formation of Phanerozoic stratiform sediment-hosted zinc-lead deposits: Evidence for the critical role of ocean anoxia☆

Abstract Stratiform sediment-hosted zinc-lead deposits form as hydrothermal sediments on the sea floor related to venting of metalliferous fluids. These deposits are widespread within organic-rich strata of Proterozoic, early Paleozoic, and to a much lesser extent Jurassic age. The absence of stratiform deposits from late Paleozoic, Cretaceous and Tertiary strata cannot be attributed to lack of requisite rift/wrench tectonic setting or inability of hydrothermal systems to transport Zn and Pb during these periods. In the modern ocean, anoxic conditions occur in areas of upwelling or within the stratified water masses of restricted basins. Euxinic waters (i.e. H 2 S-bearing) are largely limited to stratified restricted basins. Within widespread Cambrian to Mississippian organic-rich strata, sediment texture, content of benthic fauna, Ce content of benthic material, ratio of reduced sulphur to organic carbon, secular trends of δ 34 S and δ 13 C suggest widespread anoxic conditions. While predicted paleo-upwelling zones do not explain the distribution of these organic-rich strata, the greenhouse atmosphere and elevated global temperatures inferred for the early Paleozoic favour widespread ocean stagnation. In contrast, late Paleozoic strata were deposited under cooler and more oxygenated conditions. The abundance of Cambrian to Mississippian stratiform deposits and absence of late Paleozoic deposits, combined with evidence that the source of sulphide sulphur in most stratiform deposits is reduced seawater, suggests that formation of stratiform deposits requires the euxinic conditions common to stagnant reduced oceans. The occurrence of such euxinic environments exerts a fundamental control on the distribution of stratiform deposits in geologic time.