Revisiting Dolomitization in the Isla de Mona Isolated Carbonate Platform: Marine, Mixing, Reflux, or Biogenic?

Predicting the origin of dolomite is critical to understanding porosity and permeability in conventional oil and gas reservoirs. Studies evaluating the mechanism for dolomitization can be applied to those reservoirs as models for predicting porosity distribution. This study provides a test of the mixing-zone dolomitization model and explores alternative explanations in an area where geochemical data suggest a mixing-zone origin. The purpose of this study is to determine the origin of dolomite in Miocene and Pliocene strata on Isla de Mona, a small island off the western coast of Puerto Rico. The island is composed of two formations; the upper is the Lirio Limestone and the lower is the partially dolomitized Isla de Mona Limestone. Given the geologic setting of Isla de Mona, four dolomitization models were considered plausible and consistent with published stable isotope data—marine, mixing zone, reflux, or biogenic. The petrography and cathodoluminescence show at least two recrystallized dolomite phases. Later luminescent phases do not show signs of recrystallization. Fluid inclusion data range from 35 to 63 ppt salinity. Isotope data show a linear trend between fresh and evaporated seawater end members. These results exclude a marine or biogenic origin for most of the dolomite because the salinity values are higher than expected for those models. Mixing of fresh and evaporated marine fluids could not be ruled out, but petrography and fluid inclusion ranges make this explanation unlikely. The most likely explanation for the isotope data is a physical mixture between: dolomite that was recrystallized in fresh water; and a non-recrystallized dolomite, which formed in seawater and evaporated seawater. When the island was exposed, fresh water recrystallized the early dolomite phases. When sea level was high, evaporation and reflux occurred, which led to precipitation of later dolomite phases.