Geochemical and Petrographic Characterization of Barite deposits in the Devonian Slaven Chert of Nevada

Abstract

Barite (BaSO4) deposits are crucial to economic geology, paleoceanographic, and paleotectonic studies, yet the genesis of the megaton-scale beds is still being debated. 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 weighting agent in drilling fluid. In addition, the isotopic compositions recorded in barite could be used to demonstrate chemical and biological changes in seawater over time. My study is focused on large barite beds and limestone deposits located at the Clipper, Northumberland, Greystone, Dana, Mountain Springs, and Shasta open pit mines of the Roberts Mountain Allochthon in north-central Nevada. These deposits formed on a continental slope along an active tectonic margin before the Antler orogeny. Preliminary data support a model where barium is removed from seawater as organic flux to the sea bed, buried and remobilized in organic-rich and highly reducing sediments, transported by methane flow, and precipitated as barite at and below the seafloor. Evidence of methane seepage is based on anomalous limestone lenses with depleted delta 13C values (-27.31 to -31.65 permil; n=13) and fossils of Dzieduszyckia—a brachiopod known to have inhabited seeps. Preliminary sulfur isotope data (n=34) show elevated delta 34S values up to 20 permil above contemporaneous Devonian seawater (23 permil). The low delta 13C of seep associated limestone point to a carbon source that is influenced by subsurface anaerobic oxidation of methane, while the enriched sulfur isotope signals of barite indicate a sulfate source linked to bacterial sulfate reduction. 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 µm avg). Poikilotopic frameworks of subhedral barite (.8 mm avg) enclosing smaller euhedral crystals (22 µm avg) are also common. The limestone is composed of small to large subhedral calcite crystals with extensive thin twinning. Brachiopod shells are recognizable and some shell fabric is preserved. Ongoing research is aimed at linking the geographical and stratigraphic occurrences of barite and limestone with specific petrofabrics and isotopic trends. The goal of this project is to detail the various pathways by which the barium, carbon, and sulfur cycles influenced barite formation and drove the localized chemosynthetic ecosystems across the continental slope in the Devonian.

AAPG Datapages/Search and Discovery Article #90339 ©2019 AAPG Pacific Section Convention 2019, Long Beach, California, April 1-3, 2019