TRACE SULFATE CONCENTRATIONS AS AN INDICATOR OF DEPOSITIONAL ENVIRONMENT: EXAMINATION OF POSSIBLE CALCITIZED EVAPORITES FROM THE PROTEROZOIC ATAR GROUP, MAURITANIA

The scarcity of bedded gypsum in Proterozoic successions has been attributed to both the high solubility/low preservation potential of gypsum/anhydrite, and the difficulty of precipitating these evaporite minerals under the extremely low marine sulfate conditions that potentially characterized Proterozoic marine systems. Recognition of ancient evaporite deposits is therefore a critical step both in constraining depositional environments and in interpreting the long-term geochemical evolution of the Proterozoic biosphere.

A series of unusual limestone associated with intertidal-supratidal carbonates beds occur in the Proterozoic Serize Formation, Atar Group, Mauritania. These beds are characterized by a bright white color, nodular to irregular to massive bedding, breccia horizons, microkarstic surfaces, and interlamination with terrigenous deposits. The hypothesis that these beds may represent calcitized evaporites prompted examination using a variety of petrographic and geochemical methods. Standard petrographic analysis showed no characteristic evaporite replacement textures. Cathodoluminescence analysis showed zoned recrystallization textures and revealed clusters of unzoned crystals with 90° crystal faces, which suggest a possible halite precursor. Wet chemical analysis provides the most compelling evidence for prior evaporites. The concentration of trace amounts of sulfate (carbonate-associated sulfate, or CAS) that substitutes for the carbonate ion in the carbonate crystal lattice acts as a proxy for the sulfate concentration of seawater at the time of carbonate deposition or early diagenesis. Unusual carbonates from the Serize Formation contain CAS concentrations averaging 242 ppm, which is significantly higher than that preserved in similarly aged marine rocks, even those that are interbedded with gypsum or contain diagnostic pseudomorphs after gypsum. High CAS concentrations suggest either deposition under highly evaporative conditions or recrystallization in the presence of sulfate-rich fluids, such as those resulting from evaporite dissolution. Measurement of CAS in a variety of shallow to deep-marine carbonate facies within the Serize formation is currently underway for comparison with the suspected calcitized evaporites.