SULFUR ISOTOPES IN EARLY PROTEROZOIC VOLCANOGENIC MASSIVE SULFIDE DEPOSITS - NEW DATA FROM ARIZONA AND IMPLICATIONS FOR OCEAN CHEMISTRY

The δ34S range of sulfides from Early Proterozoic volcanogenic massive sulfides of Arizona is narrow, 129 out of 133 measurements lying within a range of -2.8 to + 3.2‰. Other Early Proterozoic and Archean deposits of comparable type and mineralogy have similar S-isotope characteristics. These deposits, unlike Phanerozoic analogues, do not appear to have incorporated reduced sulfur derived from marine sulfate. Early Proterozoic oceans are known to have contained sulfate; in this paper it is a major tenet that sulfate was confined to an upper water layer. It is proposed that most volcanogenic massive sulfide hydrothermal systems prior to 1.7 Ga were recharged with sulfate-free water from an anoxic lower layer, and in general discharged into reducing environments. Rare deposits with exceptional mineralogy or S-isotope compositions may reflect hydrothermal recharge from the oxidized upper layer, or deposition from fluids discharging into, and mixing with, the upper water layer. © 1990.