Diagenesis, Sulphur and Giant Zn Deposits

Zinc is among the top 4 mined metals in the world and societal demands require a long-term stable supply for a growing market. The largest Zn deposits in the world are massive sulphide deposits formed in carbonaceous mudstones; many are of Paleoproterozoic and Lower Paleozoic age. In the deposits of the North American Cordillera, the sulphide bodies are spatially associated with stratabound barite. The commonly accepted genetic model for these deposits involves the exhalation of a metal-bearing hydrothermal fluid onto the seafloor where it mixes with reduced and oxidized sulphur in a stratified seawater column (in a restricted basin). In this presentation I will show that, in fact, barite and pyrite are formed by diagenetic processes in the sediment that pre-date the hydrothermal system. They form below the seafloor at the sulphate methane transition zone. In situ S isotopic data from pyrites show that anaerobic oxidation of methane plays an important role in the generation of a sulphide “trap”. The hydrothermal system is superimposed on this diagenetic environment but does not typically exhale onto the seafloor. As a result, complex biological-chemical-physical interactions in time and space in the diagenetic environment have an important control on the genesis and size of these important deposits.