Sulphate is considered to have been a major source of sulphide in strata-bound and stratiform base-metal sulphide deposits. Many of these deposits, however, appear to have been formed at moderate temperatures (<200 °C), which poses the question, By what mechanism(s) was sulphate reduced to sulphide? Two modes of reduction have been established experimentally: (1) catalysis by sulphate-reducing bacteria, which at present is only known to occur below ca. 100 °C; and (2) abiological reduction by ferrous iron or organic matter, which has only been clearly shown above ca. 250 °C.Several attempts have been made to demonstrate abiological reduction below 200 °C, and some new data are presented here. Although the results do not exclude the possibility that such a reaction may be geochemically significant, there has been no unequivocal demonstration of nett sulphide formation from sulphate at these temperatures.Recent studies of the microbiology of hydrothermal regions have opened up the prospect of bacterial sulphate reduction at much higher temperatures than had earlier been thought possible.

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