Evolution of metal ratios and δ34S composition of sulfide mineralization during anhydrite cap rock formation, Hockley Dome, Texas, U.S.A.

Abstract

Metal zonation and sulfur isotope patterns of cap rock-hosted sulfides reveal a complex evolution of the sulfide-depositing system at the Hockley salt dome. The general metal trend observed is an overall increase of Zn relative to Pb downward in the anhydrite cap rock. In the upper cap rock section, the Pb ( Pb+Zn) ratios commonly range from 0.25 to 0.40. Ratios of <0.20 and commonly <0.10 are typical of the lower cap rock section. The concentration of Ag also appears to increase in the lower part of the anhydrite cap rock. Textural evidence indicates that some of the metal concentrations represent stratiform sulfides that were precipitated at the salt-cap rock contact as the anhydrite zone accumulated by sequential underplating. The metal ratio data suggest that the sulfide-precipitating fluid became more Zn-, and possibly Ag-rich, during anhydrite cap rock accumulation. δ 34S-values of sulfide minerals within the Hockley Dome cap rock range from −30 to +4‰ (CDT) (59 analyses). A profile of δ 34S-values from a single drill hole indicates that sulfide minerals become progressively heavier with depth to the approximate middle of the cap rock where the isotopic trend reverses. The inflection point corresponds with a local highly mineralized biogenic calcite zone. A mixing model involving variable contributions of isotopically heavy-and light-sulfur end-members appears to best account for the observed isotopic trends. Fault-leaked H 2S from mature oil and gas reservoirs is a possible extrinsic source of heavy sulfur, but local thermochemical sulfate reduction of cap rock sulfate cannot be eliminated by existing data. Bacterial reduction of cap rock sulfate is believed to be the source of light H 2S. The relative abundance of deep-sourced metalliferous brines appears to have increased with time during the accretion of the upper part of the anhydrite cap rock. Biogenic H 2S subsequently began to dominate the mixed sulfur reservoir resulting in a −15‰ shift and relatively light δ 34S-values of pyrite to the present cap rock-salt contact.