Celestine and powellite as redox indicators during diagenesis of molybdenum-bearing carbonate aquifers

The mineralogy and chemistry of a molybdenum-bearing carbonate aquifer matrix of Eocene age from Central Florida were investigated. Out of 286 samples analyzed for molybdenum (Mo), 12 samples had Mo concentrations above 150 mg/kg. The concentrations ranged from 12 mg/kg to 3474 mg/kg, with a mean of 61 mg/kg. Calcium and magnesium concentrations from 7913 mg/kg to 437850 mg/kg and from 2045 mg/kg to 132412 mg/kg were observed with mean concentrations of 287064 mg/kg and 66457 mg/kg, respectively. Four samples with elevated Mo concentrations were selected for detailed mineralogical analyses. The mineral powellite (CaMoO₄) was the main Mo phase in two samples, with 80 % and 44 % of the total Mo present in powellite. The mineral occurred in close association with the mineral celestine (SrSO₄). Scanning electron microscope analyses indicated the late formation of powellite even after the celestine, which excluded powellite as the primary source of Mo. Instead, the occurrence of sulfurized organic matter was inferred based on energy-dispersive X-ray spectroscopy measurements and a linear correlation of total organic carbon and total sulfur in 24 samples from the aquifer matrix. The contents of total organic carbon and total sulfur ranged from 0.2 to 28 % and from 0.1 to 8 %, respectively. Comparing the total amount of Mo in a given sample to the total amount of Mo in pyrite indicated that less than 4 % of the total Mo were present in pyrite. Hence, pyrite was excluded as a significant source of Mo. This left sulfurized organic matter as the main Mo phase before the incorporation into powellite. A close association of powellite, celestine and sulfurized organic matter was assumed to result from the percolation of oxygenated water through abundant cracks and fissures in the carbonate matrix during diagenesis. The reaction of oxygen with Mo-containing sulfurized organic matter likely resulted in the release of sulfate and molybdate and hence the supersaturation of celestine and powellite. Thus, the co-occurrence of celestine and powellite seems to be an indicator of intermittent oxic conditions during diagenesis.