A MINERALOGICAL INVESTIGATION OF AN OUTCROP OF THE BEREA SANDSTONE WITHIN THE SERPENT MOUND STRUCTURE, SOUTHWESTERN OHIO

The Serpent Mound Structure in southwestern, Ohio is a meteorite impact structure that has been investigated largely in the contexts of geophysics, stratigraphy, structural geology and mineralogy. Mineralogical investigations in the region have focused on shock deformed quartz and general investigations of sulfide mineralization. Aside from studies of simple sulfides, little attention has been given to low temperature or hydrothermal mineralization in the structure, particularly using scanning electron microscopy (SEM). An outcrop of the Berea sandstone near the edge of the structure exhibits alteration textures and was investigated to assess if unusual mineralization was present. The outcrop is approximately 9 meters thick and consists of an upward coarsening trend of fine mud/shale to fine sandstone. The sandstone beds strike approximately N45°E and dip 9 – 11° SW.

SEM investigation indicates a complex mineral assemblage is present. Subhedral kaolinite and illite commonly coat grains and occur as cement. Fe oxides are present as well. Back-scatter imaging indicates that an unidentified subhedral to anhedral aluminosilicate occurs as does an unusual Ag-bearing aluminum phosphate. Chlorine is also detected in particles near the Ag-bearing aluminum phosphate and this is consistent with low temperature hydrothermal mobilization and deposition of Ag. Some aluminosilicate grains may be detrital however some appear to post date kaolinite and illite mineralization. The Ag-bearing phosphate has textures consistent with late phase mineralization and does post date other textures. Fe-oxides appear to occur as multiple generations.

The aluminosilicate may be impact related and if enough sample material can be extracted definitive X-ray investigation will enable phase identification, potentially placing more constraints on impact conditions. Further studies of high pressure minerals may be warranted.

The Serpent Mound Structure occurs in an economically disadvantaged agricultural region. Illite, kaolinite and Fe-oxides are known to adsorb numerous agricultural chemicals and likely play a role in surface water and groundwater quality in the region. The occurrence of silver warrants further investigation in the area as mineral resources may spur economic growth.