Abstract: Petrography and Geochemistry of Barite Chimneys Associated with Hydrocarbon Vents on the Gulf of Mexico Slope

Baoshun Fu

Barite chimneys up to 30 cm high were recently documented and recovered from hydrocarbon venting areas on the Louisiana Slope in the Gulf of Mexico in water depths of 510-520 m. The chimneys are dominated by barite (BaSO₄) associated with minor amounts of pyrite, iron oxide, Mg-calcite, and detrital silicates. The barites display distinct string-like and dendritic-like morphologies assembled from rosette assemblages that are typically 20 to 40 µm in diameter. The interiors of chimneys exhibit macroscopic growth layers 1 to 5 mm thick which alternate between dark-gray and light-yellow colors.

Compared with barites from hydrothermal, marine, and continental settings, the Gulf of Mexico (GOM) barites are more enriched in Sr (average 15.5 mol% and maximum 30 mol%) and Ca (average 2.8 mol% and maximum 4.6 mol%). Backscatter images and electron microprobe traverse analyses indicate that most barite crystals exhibit rhythmic chemical zonations because of the variation of concentrations of Sr and Ca. The ^dgr³⁴S (from 20.30 to 28.87^pmil) and ^dgr¹⁸O (from 9.5 to 13.6^pmil) of GOM barites suggest that the barite chimneys may form at or above the sediment-water interface from Ba-, Sr-, and Ca-rich formation fluids dissolving the underlying Jurassic-age salt and mixing with sulfate-rich seawater. Bacterial reduction of sulfate took place in the formation of so e barite chimneys.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana