ADVANCES IN GEOLOGY OF PORPHYRY COPPER DEPOSITS OF THE LARAMIDE ARC, SOUTHWESTERN NORTH AMERICA (Invited Presentation)

The Laramide porphyry province is one of the world’s major copper belts, with 6 of the largest 25 deposits in the world, a total endowment of >300 M tonnes Cu, and continued discoveries. Many huge Laramide deposits remain open at depth with the potential to be mined for another century. Here we provide an update since publication of books by Titley (1982) and AGS Bootprints (1995) and the review paper by Leveille and Stegen (2012). Felsic systems with Cu-(Mo) mineralization are most common, but less silicic systems with important Au credits (Ajo, Dos Pobres) also are present. The mapped relations and geochronology of deformation, intrusion, and alteration now are more detailed and robust. Porphyry deposits were formed during crustal shortening that occurred across the arc from at least ~80-60 Ma during low-angle subduction, forming basement-cored uplifts bounded by moderate-angle reverse faults with fault-propagation folds; nonetheless, at most porphyry deposits ore formed several m.y. after local shortening ended. Many deposits subsequently were tilted and dismembered by multiple generations of mid- to late Cenozoic normal faults. Tilted fault blocks and deep drilling reveal relations of ore formation to construction and evolution of upper crustal batholiths with large, homogeneous phases. In some districts, gaps of several m.y. occur between distinct mineralization events with major reversals in crosscutting vein relations, perhaps indicating magmatic recharge. Sodic-calcic and/or greisen muscovite is exposed in the roots and on the deep flanks of systems. All systems with deep exposures contain greisen muscovite, which is best developed in the Schultze Granite in Globe-Miami. Sodic-calcic alteration formed on the deep flanks of a few systems, such as Kelvin-Riverside and cyclically in the Pima district, where the geology points to the availability of external brines as a fluid source as the probable cause. A few systems expose extensive advanced argillic alteration and high-sulfidation mineral assemblages at shallow levels, as at Resolution and in the Patagonia Mountains. Primary Cu grades vary widely from 0.1-2%, and certain deposits have major chalcocite resources associated with weathering of sericitically altered, pyrite>chalcopyrite protoliths, whereas other systems have leachable oxide copper resources, generally in rocks with potassic alteration.