Data are presented on sulphur isotope ratios and Co/Ni ratios of samples from both the Cae Coch massive pyrite body and the adjacent lithologies in North Wales. The majority of the pyrite has light δ34S (-20 to -24‰ CDT) indicative of a predominantly biogenic sulphur source; a minority of heavier values are interpreted as the result of sulphate depletion during the last stages of bacterial sulphide production. These values are far lighter than from volcanic-associated, exhalative massive sulphide deposits found in similar geological settings elsewhere, and Co/Ni ratios at Cae Coch (0.2-0.5) are also far lower than usual for such deposits. An alternative syn-diagenetic inhalative origin is proposed for the sulphide mineralization at Cae Coch.

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