The Relationship between Hydrocarbon and Stratabound Metal Sulfide Deposits: The Upper Smackover as an Analog

DENHAM, MILES E., Texas A&M University, College Station, TX

A genetic link between hydrocarbon and stratabound metal sulfide deposits has often been suggested. Both are thought to result from similar processes operating during the evolution of sedimentary basins, yet exploitable hydrocarbon and metal sulfide deposits are not found together. Consequently, the nature of their genetic relationship remains unclear. The Upper Jurassic Smackover Formation of the North Louisiana Salt Basin--a prolific hydrocarbon producer--contains disseminated authigenic sulfide minerals reminiscent of stratabound metal sulfide deposits. The close association of these sulfide minerals with hydrocarbon deposits provides an opportunity to examine the relation between the two.

The mineralogy and chemistry of late-stage authigenic phases in the Upper Smackover are similar to ore and "gangue" minerals of Mississippi Valley-type sulfide deposits. The sulfide minerals consist of replacement or pore-filling sphalerite, galena, pyrite, marcasite, and chalcopyrite. The associated "gangue" minerals are dolomite, calcite, anhydrite, barite, celestite, and fluorite. Sphalerite is cadmium-rich and relatively iron-poor and its cathodoluminescence reveals compositional zoning. Analysis of fluid inclusions in "gangue" calcite suggests precipitation temperatures (90 degrees-120 degrees C) similar to those for Mississippi Valley-type deposits. The mineralogy and chemistry of the sulfides and their related minerals vary spatially throughout the basin.

These variations reflect local processes and the sources of the sulfide minerals' constituents. The same source rocks from which hydrocarbons are derived are likely sources of base metal ions. Likewise, reduced sulfur is related to hydrocarbons by either a common source or by thermochemical sulfate reduction. Thus, spatial variations in chemistry and mineralogy of the late-stage authigenic sulfides of the Upper Smackover may be the key to understanding the relationship between hydrocarbons and stratabound metal sulfide deposits.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)