Relationship between primary iron sulphide species,sulphur source,depth of formation and age of submarine exhalative sulphide deposits

Older and deeper-water submarine exhalative ore deposits tend to be pyrrhotite-bearing whereas pyrite is favoured in recent and shallow-water deposits. It is proposed that increases in sea water sulphur concentrations and Eh through geologic time and at shallower sea water depths, and also the tendency for solutions exhaled at greater water depths to be hotter may account for this observed relationship. Available sulphur isotope data for submarine exhalative deposits are compatible with this hypothesis. Many exceptions are noted to the commonly held view that pyrite breaks down to pyrrhotite at higher grades of metamorphism. Pyrrhotite bearing deposits tend to be older and formed in deep water tectonically active zones and are therefore more often metamorphosed. A loose correlation between metamorphism and pyrrhotite occurrence has thus resulted.