Sulphide in sedimentary ore deposits may be derived directly from the mantle or by reduction of sulphate. Sulphide in modem hydrothermal deposits is thought to have formed abiologically by reactions between sulphate and ferrous iron or organic matter during deep circulation of sea water and meteoric waters through the crust at temperatures above about 200°C. Below about 70°C sulphate reduction is catalysed by bacteria. This process is widespread in anoxic basins and reducing sediments, and is dependent upon, and often controlled by, the supply of organic matter. The importance of sulphate reduction in carbon turnover has been demonstrated by comparisons of organic carbon production and sulphate reduction in marine sediments and by the discovery of several genera of sulphate-reducing bacteria capable of using a wide range of organic molecules. Recent kinetic studies of sulphate reduction in anoxic marine sediments have confirmed that sulphide accumulations equivalent to those in several major stratiform ores could be generated biologically. Fixed sulphide contents in normal unconsolidated reduced marine sediments, however, are generally an order of magnitude less than those of deposits such as at McArthur River and Mount Isa in the Northern Territory of Australia. The only modern examples of sediments with large fixed sulphide concentrations are those in which sulphide is of hydrothermal origin. One constraint on the concentration of fixed sulphides in sediments is the availability of reactable metals. It is possible that high concentrations of biogenic sulphides can be produced when metalliferous hydrothermal fluids are exhaled into biologically active anoxic basins or depressions.
