VOLCANOGENIC MASSIVE SULFIDE DEPOSITS AND ANOXIA IN THE PHANEROZOIC OCEANS

The behavior of sulfur in the oceans is strongly affected by the devel opment of anoxic water. Sulfide minerals are preserved in contact with anoxic water, bacterial reduction of sulfate with consequent isotopic fractionation becomes feasible, and sulfate may be removed, partly to nearly completely. Anoxia in the modem oceans is limited to restricte d basins such as the Black Sea and, in the open oceans, to intermediat e levels in the tropics. In the Lower Paleozoic and the Jurassic-Creta ceous there is evidence in the form of widespread, correlative black s hale, which has been interpreted in terms of more widespread and persi stent anoxia. Sulfide deposits formed on the modern sea-floor are oxid ized unless protected from dissolved oxygen by rapid burial; preservat ion of ancient sea-floor sulfide would have been most probable in anox ic water. Aspects of the age distribution, and mineralogical and isoto pic variation of volcanogenic massive sulfide (VMS) deposits can be ex plained by hydrothermal recharge from and discharge into anoxic bottom water. The Ordovician is the pre-eminent period of formation of VMS d eposits in the Phanerozoic, probably owing to a favorable coincidence of anoxia and tectonic factors in the Iapetus ocean. Sulfate-free depo sits are largely confined to intervals of widespread anoxia; exception s show geological evidence - pyrrhotite and black shale - of local ano xia. Sulfate-bearing deposits are distributed throughout the Phanerozo ic, but require anoxia as a preservation mechanism and are commonly as sociated with black shale. Certain hydrothermal systems may have gener ated their own local anoxia in ponded hydrothermal fluid that became n egatively buoyant relative to seawater. The delta(34)S ranges of sulfi des in most VMS deposits show evidence for some incorporation of reduc ed seawater sulfate, but a few show no evidence of sulfate incorporati on, consistent with the potential variability of sulfate content in an oxic water.