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Paleomagnetic Dating of Sulfide Mineralization and Cap Rock Formation in Gulf Coast Salt Domes

KYLE, J. RICHARD, and WULF GOSE, University of Texas at Austin, Austin, TX

Remnant magnetization of stratiform sulfides within salt dome cap rocks provides a means of dating the timing of mineralization and its relationship with tectonic, sedimentation, and fluid evolution events within the local basin. The Gulf Coast basin contains several areas of metal-rich formation waters that have been widely accepted as modern analogs of the mineralizing solutions for ore deposits that occur in older geologic terranes. This type of metalliferous brines is also responsible for geologically young Zn-Pb-Ag sulfide mineralization in Jurassic carbonate rocks and in salt dome cap rocks. Some cap rock sulfide layers accreted in an orderly sequence as the cap rock formed by underplating at the salt/cap rock contact and can be evaluated in terms of a progressive paragenesis.

The Winnfield salt dome in northern Louisiana hosts stratiform laminae of pyrrhotite and other sulfides in anhydrite cap rock. Detailed paleomagnetic analyses indicate the presence of numerous magnetic reversals that can be correlated with the sea-floor magnetic anomaly sequence, thus providing the first direct determination of a salt dome cap rock formation age. The sampled section that represents about two thirds of the total anhydrite thickness formed between 157 and 145 Ma (latest Jurassic). Accumulation rates calculated from these data indicate that the average rate for the oldest cap rock sampled is 5.7 m/m.y. and decreases to 2.8 m/m.y. for the younger strata. The Winnfield diapiric salt contains about 3% anhydrite, indicating the diapir growth rate was at least 30 times faster a figure that is compatible with independent geologic estimates.

The best documented cap rock-hosted sulfide concentrations occur at the Hockley salt dome in south-central Texas. Hockley is a relatively young diapir with the cap rock believed to have developed within the last 45 Ma. Sulfides occur throughout the 285-m thick cap rock, but major metal concentrations occur within a 20-m zone within the central cap rock stratigraphy. If this zone formed at comparable rates to the Winnfield cap rock, then the main pulse of Hockley mineralization lasted only a few million years. Paleomagnetic studies are in progress to constrain timing of mineralization of Hockley.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)