Common occurrence of pyrite before pyrrhotite in deep-seated sulfide deposits is attributed to decrease from initially high oxygen content of ore-forming fluid rather than to decrease in availability of sulfur or increase in temperature. The high content of oxygen, early in sequence and presumably associated with high content of CO 2 , is indicated by common presence of sulfate (barite) in gangue. Late pyrite, following pyrrhotite is consequence of increased sulfur activity.

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