Lead, Zinc, and Strontium in Limestone Cap Rock from Tatum Salt Dome, Mississippi

James A. Saunders

Limestone cap rock at Tatum salt dome, Mississippi, contains disseminated pyrite, sphalerite, and galena, and disseminated to massive amounts of strontianite (SrCO₃) and celestite (SrSO₄). Sulfide minerals are locally present in bitumen-rich areas of the upper, massive portion of the limestone cap rock, whereas strontium minerals are disseminated throughout this zone. However, sulfide and strontium minerals are most abundant in the lower banded portion of the limestone cap rock, which consists of alternating subhorizontal light and dark-colored bands. The dark bands are composed of calcite of variable grain size, sulfides, quartz, dolomite, albite, and up to 1% bitumen that apparently formed by the biodegradation of crude oil. Lighter bands are compo ed of variable amounts of coarsely crystalline, euhedral calcite, strontianite, and celestite resulting in strontium (Sr) contents of up to 30% locally.

Banded limestone cap rock at Tatum dome formed at the top of the actively dissolving anhydrite zone by a combination of sulfate reduction and oxidation of liquid hydrocarbons by bacteria to cause the precipitation of calcite and sulfide minerals and the accumulation of insoluble residue from the anhydrite (quartz, albite, dolomite). Lead and zinc in the sulfide minerals could have been derived from the dissolving anhydrite, but the abundance of Sr minerals present requires an external source. Present-day oil field brines in central Mississippi contain up to 3,000 ppm Sr, and basin brines of similar composition apparently contributed Sr to the cap-rock environment during formation.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.