Abstract

Re-Os dating of two molybdenite samples from the Squaw Peak porphyry copper-molybdenum prospect in central Arizona returned essentially identical ages of 1,729 ± 7 and 1,738 ± 7 Ma. Therefore the prospect is not a component of the Laramide (Late Cretaceous-early Tertiary) porphyry copper province of southwestern North America as previously presumed. These Paleoproterozoic ages are similar to that of 1,740 ± 15 Ma for the I-type, magnetite-series Cherry batholith, within which the Squaw Peak porphyry stock and associated mineralization are located. Squaw Peak cannot be more than a few million years younger than volcanogenic massive sulfide (VMS) copper deposits in the nearby Jerome district, which are part of the Yavapai Supergroup, host to the Cherry batholith. These volcanic and intrusive rocks and their associated copper mineralization were formed in a juvenile island-arc setting and are now part of the Yavapai province, which was assembled and accreted to the Archean nucleus of North America by ~1.68 Ga.

The Paleoproterozoic age for Squaw Peak in conjunction with the existence of the slightly older VMS deposits shows that the Laramide province of southwestern North America first developed its copper metallogenic signature >1,700 m.y. ago. The presence of this Paleoproterozoic copper mineralization may be taken as further support for recently proposed metasomatism of the mantle lithosphere during Paleoproterozoic subduction as a precursor to formation of at least part of the Laramide porphyry copper province. Similar spatial associations between inferred metal sources in Proterozoic mantle lithosphere and lowermost crust, relatively minor copper and/or molybdenum mineralization in Proterozoic magmatic arcs, and important post-Paleozoic porphyry copper and/or molybdenum provinces have recently been documented elsewhere, particularly in eastern China, and could prove to be of more general exploration significance.

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