Relation of metal sulfide mineralization to anhydrite cap rock formation at Hockley Salt Dome, Harris County, Texas

The Gulf Coast basin is a dynamic hydrologic environment for which mixing of multisource fluids is an attractive and plausible mechanism to explain metal and isotopic variations of sulfide concentrations in the cap rock of the Hockley salt dome. Textural evidence indicates that most metal concentrations are stratiform sulfides that were precipitated at the salt/cap rock contact as the anhydrite zone accumulated by sequential underplating. Because sulfides occur throughout the 935-ft cap rock and show vertical (paragenetic) trends with regard to metal ratios and sulfur isotope composition, the evolution of rock-fluid interactions in the cap rock environment can be evaluated. Reservoirs of reduced sulfur and metals evolved or mixed during the accumulation history of the anhydrite cap rock. Extensive core drilling at Hockley Dome has defined an annular zone of metal sulfides in both the calcite and anhydrite cap rock zones. Marcasite is the dominant sulfide and is associated with lesser amounts of pyrite, sphalerite, galena, and rare acanthite. Significant concentrations of sulfides occur within a 150-ft zone in the central cap rock "stratigraphy". These zones may be related to decreased cap rock accumulation rates and increased supply of metalliferous fluids. The general metal trend in the Hockley cap rock is an overall increase of Zn relative to Pb downward in the anhydrite cap rock. A high concentration of Ag appears in the lower part of the anhydrite cap rock. Metal assay data suggest that the sulfide-precipitating fluid became more Zn- and Ag-rich during cap rock accumulation. δ³⁴S values of sulfide minerals within the Hockley cap rock range from +4 to -35 °/oo (CDT) (96 analyses). A single drill hole (HH2) profile of δ³⁴S pyrite values indicates that sulfide minerals become progressively heavier with depth to the approximate middle of the cap rock where the isotopic trend is reversed. The inflection point corresponds to a local highly mineralized biogenic calcite zone. The complementary metal and δ³⁴S data for HH2 indicate two isotopically distinct sulfur components. A mixing model involving relative contributions of isotopically heavy and light sulfur end-members is offered as an explanation for the observed isotopic trends. The isotopically light H₂S component is believed to be related to biogenic reduction of aqueous sulfate derived from anhydrite cap rock. The correlation between intensity of sulfide mineralization and sulfide δ³⁴S indicates a genetic relationship between the generation of heavy H₂S and the supply of metals by relatively hot, deep-sourced formation waters. A possible source of heavy H₂S is local thermochemical sulfate reduction of cap rock sulfate. Conversely, heavy H₂S could have been supplied along with metals in the formation waters. If these sulfides originated during anhydrite underplating, then metals from basinally derived fluids appear to have increased during the early stages of anhydrite cap rock formation. These events resulted in a greater contribution of heavy H₂S to the mixed sulfur reservoir, either through extrensic supply or by local thermochemical production. However, in younger cap rock, biogenic H₂S began to dominate the mixed sulfur reservoir resulting in a -15 °/oo shift and relatively lighter δ³⁴S values of pyrite to the present cap rock/salt contact. It is proposed that fluid mixing and rock-water interactions produced the complex δ³⁴S isotopic and metal patterns within the anhydrite cap rock. The sulfide-depositing system at Hockley Dome resulted from interactions of cool acetate-type water with warmer metalliferous formation water that migrated up the salt dome flanks. Brines moved updip from the overpressured deep Gulf of Mexico basin via formational aquifers and major fault systems. The stratigraphic relationship of sulfide δ³⁴S values and metal concentrations suggests that basin-derived fluids were continuously leaked into the cap rock environment, but it appears that only prolonged episodes of fluid flow were maintained during the Eocene (Claiborne and Jackson time) and to a lesser extent the Oligocene (Vicksburg and Frio time). The metalliferous formation waters responsible for cap rock mineralization likely originate from deeply buried Mesozoic (carbonate) reservoirs.