Celestine (SrSO4) is a common minor diagenetic mineral phase in marine carbonate sediments, where it is thought to form during the transformation of Sr-bearing aragonite or biogenic calcite to low-Sr calcite or dolomite. The largest known sedimentary celestine occurrences, however, contain several million metric tons of SrSO4 and cannot be as readily explained. These large deposits occur as epigenetic replacements and cavity-fill in coastal marine carbonate and evaporite sequences. Host rocks range in age from Silurian to Pliocene. There is variable evidence of associated hydrothermal activity, and the Sr typically has a radiogenic signature. One of the keys to explaining the origin of these massive celestine deposits is determining why celestine and not barite (BaSO4), which is orders of magnitude less soluble, is the replacement phase.
Although precipitation of massive celestine beds during the early stages of evaporation of sea water has been invoked by some, this hypothesis is not supported by either experimental or field observations, which indicate most sea-water Sr is removed as dispersed sulfate phases during the halite stage of evaporation. The association of large celestine deposits with coastal marine carbonate-evaporite sequences more likely reflects in part the significant difference in the geochemical behavior of Sr and Ba during burial diagenesis. The concentration of Sr in subsurface sedimentary fluids is buffered by silicate-carbonate mineral assemblages, and the concentration of Sr increases significantly with increasing salinity. The concentration of Ba, in contrast, is controlled by equilibrium with barite and is inversely related to the concentration of dissolved sulfate. Basinal fluids having the highest Sr/Ba ratios, and thus those most likely to form celestine rather than barite, have high salinities and moderately high levels of sulfate. The precursors to such waters can be produced in coastal marine settings by the evaporation of sea water. As these brines reflux into underlying or laterally adjacent sediments, they can leach substantial amounts of Sr as they attempt to achieve chemical equilibrium with their host sediments. Barium concentrations are kept low by the presence of dissolved sulfate. If these diagenetically altered fluids are then discharged into beds containing calcium sulfate minerals and/or sulfate-rich waters, the precipitation of celestine would result. Porosity produced by uplift and karstification of associated carbonates would help focus fluid flow.