Nature and origin of authigenic rhodochrosite and siderite from the Paleozoic aquifer, northeast Mississippi, U.S.A.

Abstract

Botryoidal rhodochrosite, siderite and pyrite are the principal authigenic phases in fractured chert comprising the Paleozoic aquifer of northeast Mississippi. Locally, alternating bands of near end-member composition rhodochrosite and siderite occur in veinlets up to 2 mm wide in the fractured chert. Carbon isotope data for the carbonates (δ 13C values ranging from −9.6 to −11.3% PDB) indicate that both inorganic and organic C were incorporated into the authigenic phases. The Paleozoic aquifer is hydraulically connected with the overlying Upper Cretaceous (Coastal Plain) strata. Groundwater in the Coastal Plain units is undersaturated with respect to calcite and has relatively high Fe and Mn contents, the latter apparently caused by the dissolution of Fe Mn oxyhydroxides. The natural downward recharge of these waters into the Paleozoic aquifer causes calcite dissolution in limestone at the base of the chert, which in turn leads to the precipitation of rhodochrosite and siderite. Additionally, pyrite is precipitated as SO 4 reduction occurs in the same zone. Rhodochrosite has not been previously documented as an authigenic phase in a fresh-water aquifer system, suggesting that the geochemical environment necessary for its precipitation is not common. The alternating bands of rhodochrosite and siderite observed in this study, coupled with available thermodynamic data, indicate that the groundwater a Mn 2+ /aFe 2+ ratio must have fluctuated over time. Variations in the ratio are best explained by variable Eh conditions at the site where Fe Mn oxyhydroxide dissolution occurs.