GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 104-9
Presentation Time: 9:00 AM-6:30 PM


BECK, Bryan, COLLINS, Larry B., FELL, Heather Anne, JONES, Alesha R., KIRKLAND, Brenda L., LEWIS, Michelle, MAKAMSON, Jared, OPPEDAL, Kari and SIGLER, Roger, Department of Geosciences, Mississippi State University, P.O. Box 5448, Mississippi State, MS 39762

Pyrite concretions discovered in the Demopolis Chalk from the Upper Cretaceous were investigated to determine the role of organic matter in their origin. Samples of pyrite concretions and nodules were collected from two sites in northeastern Oktibbeha County, Mississippi and prepared for thin section analysis, X-ray diffraction, and SEM imaging. Concretions vary in size from less than 1 cm to approximately 10 cm in length and were generally tubular to spherical in shape. Nodules on the surface of these concretions were present and consisted of 2-5 mm sized bumps and range in color from a brown tone, to red, or to a dark purple. In the first outcrop, the concretions were radially spread out from an average distance of 26.4 inches within a central point. In hand sample, pyrite concretions appeared to form as elongate crystals growing from a core of porous clusters that initially appeared to be composed of irregularly clustered spheroidal shapes in the center. SEM images of the center of the nodules revealed clusters and bands of crystals. Bacillus-shaped bacteria were present, and organic debris, for example dried biofilm, was ubiquitous. Fungal hyphae were found in samples of the coccolith-rich chalk. X-ray diffraction analyses of metallic pyrite concretions exhibited the presence of pyrite and calcite at a 70:30 ratio. Elemental maps (Energy Dispersive Spectroscopy/EDS) of the chalk, carried out specifically to identify micro accumulations of pyrite, showed only calcite. Some elongate pyrite concretions are interpreted as forming in burrows, perhaps in association with mucus produced by organisms. The controlling factors of the more abundant, spherical concretions is still in question, but bacteria and other organic compounds are clearly present and could play a role. The study of pyrite concretions is potentially relevant to investigations of the Earth’s early atmosphere and the biogenesis of sulfide minerals in ore deposits.