Mineralogic and isotopic constraints on the origin of strontium-rich cap rock, Tatum Dome, Mississippi, U.S.A.

Abstract

The limestone portion of the salt dome cap rock at Tatum dome, Mississippi, is composed of an upper massive zone and a lower banded zone. The upper zone consists of equigranular fine-crystalline calcite with veinlets and disseminations of carbonaceous matter associated with minor amounts of detrital and authigenic quartz, sulfides and Sr minerals. The lower zone is composed of alternating light and dark calcite bands. The dark bands are composed of fine-grained and peloidal calcite, quartz, bitumen and disseminated sulfide minerals. The lighter bands consist of variable proportions of generally coarse-crystalline euhedral calcite, celestite and strontianite resulting in Sr contents of up to 30% locally. Solubility data for celestite, strontianite, calcite and anhydrite suggest that a decrease in temperature favors the replacement of Ca minerals by Sr minerals, which is consistent with the observed mineral textures and paragenesis. However, the source of cap rock Sr is difficult to determine. Anhydrite at Tatum dome contains 800 ppm Sr, but the abundance of Sr minerals in the limestone cap rock and the 87 Sr 86 Sr ratios of limestone cap rock minerals require a Sr source other than local anhydrite. Sr released by the replacement of anhydrite by calcite is capable of producing a molar ratio of celestite to calcite of only ∼ 0.001, yet locally this ratio is ∼ 3. A likely source of additional Sr is oilfield brines, such as those in central Mississippi that contain up to 3000 ppm Sr along with significant Pb and Zn. Episodic introduction of brines into the cap rock in combination with the action of sulfate-reducing bacteria probably caused the sequential production of the dark bands. The lighter bands probably reflect introduction of a later Sr-enriched fluid or evolution of the original fluid in combination with changes in the chemical parameters controlling mineral precipitation. Calcite was replaced by Sr minerals during this later Sr-rich event.