SULFUR- AND OXYGEN ISOTOPES OF BARITE DEPOSITS IN SEDIMENTARY BASINS: IMPLICATIONS FOR BARITE GENESIS, MASSIVE SULFIDE GENESIS, AND SULFUR REDOX REACTIONS

Sediment-hosted barite deposits are distributed globally in rocks ranging in age from Archean to Recent. Some deposits have associated Zn-Pb-Ag sulfide mineralization, but others do not. The sulfate source for the barite is typically argued to have been seawater. However, combined S- and O isotope data for many deposits reveal a poor match with the secular curve that has been determined for marine sulfate. Our interpretation of the mismatch stems partly from studies (Johnson, Kelley & Leach, GSA 2003; Econ Geol, in press) of barite bodies in the Brooks Range of Alaska, which collectively represent one of the largest barite accumulations ever discovered. Isotopic and other observations suggest that these deposits formed on and beneath the Mississippian seafloor where ascending fluids containing Ba and hydrocarbons encountered water containing seawater sulfate. Isotopic departures from the Mississippian marine sulfate composition indicate that some sulfate was reduced to H2S as barite precipitated. Sulfate reduction was bacterially-mediated; methane or light hydrocarbons likely served as the electron donor. At the Red Dog sulfide-barite deposit, the earliest sphalerite formed from this H2S. Later sphalerite replaced barite. Released Ba ions were reprecipitated distally where they encountered sulfate that had been hydrothermally shifted to lighter O-isotopic compositions.

Previously-published and new isotope data for deposits in Nevada, central Guangxi (China), Tunisia, and the southern Pennines show isotopic departures from marine sulfate that suggest barite formed while H2S was locally generated. Absence of light oxygen in Nevada and China barite is consistent with the absence of neighboring hydrothermal sulfides. Absence of light oxygen in Tunisia and the southern Pennines, where barite is associated with sulfides, may reflect cooler or higher pH metal-carrying solutions. In contrast, barites from Magcobar in Ireland and northern Pennines localities show light oxygen suggesting that an original sulfate reduction isotopic fingerprint was modified during metal sulfide precipitation. In general, barite accumulations in sedimentary sequences may reflect hydrocarbon migration events that served as ground preparation for massive sulfide formation by producing H2S from marine sulfate.