Paper No. 36-5
Presentation Time: 9:00 AM-3:30 PM
A HYDROCARBON SEEP MODEL OF LARGE BEDDED BARITE DEPOSITS IN THE DEVONIAN SLAVEN CHERT OF CENTRAL-NORTHERN NEVADA
Large, sedimentary barite deposits comprise the major economic source for this mineral at a global scale, yet the genesis of the beds is still being debated. Barite (BaSO4) is important to economic geology, paleooceanographic, and paleotectonic studies. The mineral is an economic commodity because its inert properties, low solubility, and high specific gravity of 4.5 g/cm3 allow it to act as a weighing agent in drilling fluid. The isotopic compositions recorded in barite could be used to demonstrate chemical and biological changes in seawater over time. My study is focused on the large barite deposits of the Devonian Slaven Chert, north-central Nevada. These deposits formed on a continental slope along an active tectonic margin. Preliminary data support a model where barite is remobilized in organic-rich and highly reducing sediments, barium is transported by methane seeps, and barite is precipitated at or below the seafloor. Evidence of methane seepage is based on anomalous limestone lenses with depleted d13C values (-27.31 to -31.65‰; n=13) and fossils of Dzieduszyckia—a brachiopod known to have inhabited seeps. Preliminary sulfur isotope data (n=34) show elevated d34S values up to 20‰ above contemporaneous Devonian seawater (23‰). Together, these data indicate that the sulfate pool was influenced by microbial sulfate reduction associated with anaerobic oxidation of methane in an anoxic environment. Initial petrographic analysis shows a variety of barite fabrics ranging from 1.1-3.4 mm rosettes of euhedral acicular crystals to inequigranular xenotopic mosaics (76 mm avg.). Poikilotopic frameworks of subhedral barite (.8 mm avg.) enclosing smaller euhedral crystals (22 mm avg.) are also common. The limestone is composed of small to large subhedral calcite crystals with extensive thin twinning and enfacial junctions. Curiously, brachiopod shells are recognizable and some shell fabric is preserved even though the calcite appears metamorphic. Ongoing research is aimed at linking the geographical and stratigraphic occurrences of barite and limestone with specific petrofabrics and isotopic trends. The goal is to detail the various pathways by which barium and reduced fluids influenced barite formation and drove the localized chemosynthetic ecosystems across the continental slope in the Devonian.