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Abstract

Some salt dome cap rocks contain stratiform laminae of sulfides, which accreted in an orderly sequence as the cap rock formed by underplating at the salt/cap-rock interface. The occurrence of the magnetic mineral pyrrhotite makes it possible to use the remanent magnetization to date the timing of mineralization and its relationship with tectonic, sedimentary, and fluid evolution events within the local basin. Detailed paleomagnetic analyses of the cap rock of the Winnfield salt dome in northern Louisiana delineate a magnetic reversal pattern that can be correlated with the geomagnetic time scale, thus providing the first direct determination of the age of cap-rock formation. The sampled section, which represents about two-thirds of the total anhydrite thickness, formed between 157 and 145 Ma (latest Jurassic). Anhydrite accumulation rates calculated from these data decrease from 5.5 m/my to 2.6 m/my for the younger strata. Because the mother salt at Winnfield contains about 3 percent anhydrite, the growth rate of the salt diapir must have been at least 30 times faster, yielding values similar to other geologic estimates.

The best documented cap rock-hosted sulfide concentrations occur at the Hockley salt dome in south-central Texas. The Hockley cap rock is believed to have developed within the last 45 Ma. Several magnetic reversals have been observed, but the data set is too incomplete at this time to further constrain the time of mineralization. The major metal concentrations are found within a 20-m zone within the central cap-rock stratigraphy. If this zone formed at rates comparable to the Winnfield cap rock, then the main pulse of mineralization at Hockley lasted only a few million years.

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