Abstract The δ 34S 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.