William N. Agee Jr.
Fe-sulfides occur with lesser quantities of Zn-, Pb-, and Ag-sulfides within anhydrite and calcite cap rocks at the Hockley salt dome near Houston, Texas. Metal zonation patterns for Ag, Cd, Pb, and Zn in the anhydrite cap rock were evaluated to determine the nature and evolution of the metal reservoir. Gulf Coast formation waters of different ages and host formations were examined as source fluids for cap-rock mineralization.
The assay data for the "H" fence of drill holes on the southern perimeter of the dome indicate an overall increase of Zn relative to Pb downward in the anhydrite cap rock. Metal concentrations, expressed as Pb/(Pb + Zn) ratios, exhibit values that commonly range from 0.25 to 0.40 in the upper part of the holes; ratios less than 0.20 and commonly less than 0.10 are typical of the lower sections. Anhydrite cap rocks form by sequential underplating of anhydrite residue as the salt diapir is dissolved. Textural evidence indicates that some of the metal sulfides are stratiform deposits that precipitated at the salt/cap-rock contact as the anhydrite zone accumulated. Thus, the metal ratio data suggest that the sulfide-precipitating fluid became more Zn-rich during anhydrite cap-rock accumul tion.
Metal-bearing brines have been identified in Gulf Coast oil wells and are believed to be the source of the metals in the salt dome-cap rock deposits. Brines move updip out of the overpressured deep Gulf of Mexico basin via formational aquifers and major fault systems that serve as vertical pathways. ^dgr34S composition of sulfides in core hole HH2 suggests that brines from deeply buried Mesozoic carbonate formations may have been driven upward along growth faults and/or salt-dome margins into the cap-rock-forming environment. The metalliferous formation waters responsible for cap-rock mineralization likely originate from the Lower Cretaceous carbonate reservoirs. The apparent large-scale metal zonation suggests either variation in the metal source and/or evolution of a meta reservoir via rock-water interactions.
AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.