PYRITE MINERALIZATION IN DEVONIAN-AGE BLACK SHALE IN SOUTHWEST VIRGINIA: IMPLICATIONS FOR ACID DRAINAGE AND PALEOENVIRONMENTAL RECONSTRUCTION

Research of pyritized black shales can be beneficial in enhancing our understanding of the ubiquitous environmental problem of Acid Rock Drainage (ARD) in SW-Virginia. The mineralogy of pyrite is such that it readily oxidizes on contact with moisture thereby causing acid runoff which can thereafter leach heavy metals from the host shales. Moreover, patterns of pyrite concentrations and morphologies can also be used as a proxy for deciphering paleoenvironmental conditions that accompanied accumulation of such clay- and organic-carbon rich sediment.

In this study we present results of an ongoing investigation of ARD, pyritization and paleoenvironment reconstruction of the Middle Devonian Millboro Shale exposed near Radford in southwestern Virginia. We extracted a total of 368 cm of the Millboro Shale strata using a portable, gasoline-operated, concrete-cutting saw. Prior to the extraction of the stratigraphic column, a total of 30 pyritic nodules, pyritized worm burrows, and various other occurrences of pyrite were sampled and analyzed for their paleoenvironmental information.

In lab, strata were analyzed for parameters including - patterns of lamination, ichnofabric indices (i.e. levels of bioturbation), point-counts of pyrite flecks and type of macro- and micro-sedimentary structures. Samples were also examined with a Scanning Electron Microscope (SEM) and a Z-stacking Dissecting Microscope (ZDM) for ascertaining primary and secondary crystal morphologies. Results highlight overall relationships between levels of bioturbation, erosional surfaces, pyrite distribution and events of benthic oxygenation possibly related to storm surge events in this setting of the Appalachian Basin.