The mean ⁸⁷Sr/⁸⁶Sr isotopic composition of the dolomite is 0.70887 ±0.00002 (2s, n = 3) which corresponds to the ⁸⁷Sr/⁸⁶Sr ratio of Late Miocene seawater. However, because the dolomite resides in Pliocene strata, it is difficult to invoke unmodified seawater as an agent of dolomitization. Dolomitization therefore requires a source of nonradiogenic strontium. Modern St. Croix groundwater has ⁸⁷Sr/⁸⁶Sr compositions between 0.7076 and 0.7085 (n = 4), well below the ratio of both modern seawater and the dolomites. Mixing calculations show that modern St. Croix groundwater could be a significant source of nonradiogenic strontium in a dolomite formed f om a two-component groundwater-seawater mix. On the basis of strontium-concentration modeling, the groundwater component responsible for the St. Croix dolomites may have ranged from 40% to 80% of the dolomitizing fluids.

Stable isotopic values for the dolomite range from d¹⁸O of +0.7 to +3.8, and d¹³C of +0.6 to +2.4 (PDB), with increasing d¹⁸O and d¹³C values from the margins to the center of the lagoon. The maximum d¹⁸O values in these dolomites are too high to have formed fro unaltered groundwater or seawater, even accounting for ice-volume effects. Therefore the isotopically heaviest dolomite must have precipitated from fluids enriched in ¹⁸O, probably as a result of evaporation.

Dolomitization from fluids produced from a mixture of evaporated seawater and St. Croix groundwater are consistent with the geochemistry and geologic distribution of the dolomite. Calculations show that such a scenario is possible, and may be fairly common, despite the relative complexity of the model. Documented block faulting of the Krause Lagoon area may have provided a stable hydrologic regime for a long enough time for dolomite to form, despite island uplift during the late Tertiary. Other models of dolomitization can be shown to be less likely or untenable on the basis of chemical, lithologic, or hydrologic criteria.