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Thermal Model for Salt Dome Cap Rocks and Mineral Deposits

Harry H. Posey, Malcolm P. R. Light, J. Richard Kyle, Peter E. Price

The salt dome class of sedimentary mineral deposits offers important clues about fluid products of thermal diagenesis in subsiding basins. Isotopes of carbon, oxygen, sulfur, and strontium are used to identify the sources of these fluids.

The d13C values from calcite (-5 to -51 ^pmil, PDB) indicate a mixture of biogenic methane and enriched thermogenic methane or marine carbonate. Calcite d18O values (-4 to -11 ^pmil, PDB) indicate marine, formation, and meteoric fluid mixtures for precipitation between 30° and 70°C. d34S values from caprock pyrite (+15 to -35 ^pmil, CDT) indicate that caprock sulfate or oil-hosted sulfur was reduced biogenically. The d34S values in pyrite from dome-flank sediments (+25 to +15 ^pmil) indicate derivation either from hotter hydrocarbon sources, or from thoroughly reduced caprock anhydrite. The 87Sr/86Sr values of all strontium-bearing phases (0.7068-0.7103) require mixed marine and clastic source rocks.

A simple model for caprock formation and mineralization is as follows. Sub-salt fluids invaded the salt section, aiding diapirism and recrystallizing some of the anhydrite. Salt dissolved to form anhydrite cap rock, and both continued to move upsection. Smectite converted to illite, releasing radiogenic strontium and, perhaps, iron to solution. Organic acids and CO2, produced during hydrocarbon maturation, dissolved either limestone adjacent to the diapir, or marine false cap rocks. These fluids plus sulfate-reducing bacteria produced calcite cap rocks and associated pyrite. Albitization released lead and barium, which combined with iron and zinc from shale and possibly carbonate rocks, and produced sulfides and barite in the cap rocks. Deep-basin fluids migrated into the f ank sediments to produce roll-front type uranium and pyrite. Except for the flank sediment deposits, all of the materials can be explained as derivatives of mineral reactions cooler than zeolite facies metamorphism.

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