RHENIUM-OSMIUM EVIDENCE FOR REGIONAL MINERALIZATION IN SOUTHWESTERN NORTH-AMERICA

More than 40 base metal porphyry ore deposits in southwestern North Am erica are associated with the Laramide orogeny (about 90 million to 50 million years ago). Rhenium-osmium dates on molybdenite, a rhenium-en riched sulfide common in many of the deposits, reveal that in individu al deposits mineralization occurs near the final stages of magmatic ac tivity irrespective of the time of inception, magnitude, or duration o f magmatism. Deposits that differ widely in location and in the extent and timing of magmatism have nearly identical ages for mineralization . Rhenium-osmium ages suggest that mineralization occurred during two distinct intervals from about 74 million to 70 million years ago and f rom 60 million to 55 million years ago. Most deposits that formed in t he oldest interval are within the older Precambrian basement of northw estern Arizona, whereas the younger deposits are restricted to the you nger Precambrian basement in southern Arizona and northern Mexico. Syn chronous, widespread mineralization indicates that similar crust-mantl e interaction occurred on a regional scale for ore deposits once thoug ht to be the product of localized processes.