ABSTRACT

Barite deposits consisting of chimneys and crusts were recovered and documented from hydrocarbon seep sites in Garden Banks Block 382 and Mississippi Canyon Block 929 on the Gulf of Mexico slope offshore Louisiana at 510 and 640 m water depth, respectively. The chimneys are dominated by barite (BaSO₄) whereas the crusts are composed of barite coexisting with calcite. Compared with barites from hydrothermal and normal marine settings, the Gulf of Mexico barites are anomalously enriched in Sr and Ca replacing Ba in the crystal lattice.

Sulfur and oxygen isotope compositions of the chimneys are similar to those of modern seawater sulfate. In contrast, the crusts are enriched in both ³⁴S and ¹⁸O relative to the ambient seawater sulfate. On the basis of the isotope results, we propose that these barites formed by mixing of sulfate-rich seawater with Ba, Sr, and Ca-rich formation fluids advecting with the hydrocarbons. The chimneys formed at or above the sediment/water interface whereas the crusts formed below the sediment/water interface. The crusts precipitated from residual seawater-derived sulfate left over from a pool consumed by sulfate-reducing bacteria in a closed or semi-closed system.