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Integrated Hydrothermal Model for Proposed Deep Crustal Borehole on Texas Gulf Coast--Origins of Geopressured Brines and Lead-Zinc, Uranium, Hydrocarbon, and Cap-Rock Deposits

Malcolm P. R. Light, Harry H. Posey

Sediment accumulation over Jurassic salt in the Gulf Coast has resulted in an interrelated sequential development of salt domes and diagenetic, hydrothermal, and hydrocarbon generation zones. Primary anhydrites within the salt with high 87Sr/86Sr ratios suggest early generation of underlying fluids rich in radiogenic strontium that were incorporated in the salt during its diapiric rise to the surface. Subsequently, late-stage, hydrocarbon-rich, saline hydrothermal fluids migrated up the margins of the salt domes, and caused precipitation of several generations of calcite cements, followed by uranium and Mississippi Valley-type lead-zinc-barite deposits near or at salt dome rims.

Present fluids in the lower Frio (deeper than 4,270 m or 14,000 ft) at the Pleasant Bayou geopressured-geothermal test well (Brazoria County, Texas) are highly saline and enriched in iron, manganese, lead, zinc, and carbon dioxide, and are saturated in methane. These lower Frio waters must have migrated into the area recently because they are not in isotopic equilibrium with diagenetic albite cements formed at temperatures greater than 120°C (248°F) less than 7.5 million years ago. Isotopic and geochemical data suggest that the fluids trapped by geopressure in the lower Frio at the Pleasant Bayou well are the parent fluids of those causing salt dome cap-rock mineralization.

A deep crustal borehole proposed for the Gulf Coast will investigate the geochemical and isotopic nature of the entire diagenetic succession and should put constraints on the model proposed above. Among the many other problems to be explored using data from this well are the nature of the underlying crust, timing of rifting, and depositional and thermal history of the Gulf basin sediments.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.