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The Southwestern North America Porphyry Copper Province

By
Richard A. Leveille
Richard A. Leveille
1
Freeport-McMoRan Exploration Corporation, 333 North Central Ave., Phoenix, Arizona 85004
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Ralph J. Stegen
Ralph J. Stegen
2
Freeport-McMoRan Exploration Corporation, 10861 N Mavinee Dr #141, Oro Valley, Arizona 85737
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Published:
January 01, 2012

Abstract

The southwestern North America porphyry copper province comprises adjacent parts of the U.S. states of Arizona, New Mexico, and Texas, and the Mexican states of Sonora, Chihuahua, northern Sinaloa, and Baja California. The province has an estimated total endowment (production + reserves + resources) of 295 million tonnes of copper metal, 96% of which is in deposits of Laramide (∼80–45 Ma) age. This is one of the major accumulations of the element known in the Earth's crust. In addition to having played a major historical role with regard to the discovery, development, mining, and scientific study of porphyry copper deposits, large remaining reserves and resources assure that the this porphyry province will continue to be a factor in world copper supply for many years to come.

The southwestern North America province is largely underlain by mid-Proterozoic continental crust covered by upper Proterozoic to mid-Paleozoic miogeoclinal sedimentary rocks. Arc magmatism commenced in the area in the Triassic and continued until extinguished by ridge subduction that started in early Miocene time; this resulted in its conversion from a convergent to a transform margin. A long-lived series of NW-trending sedimentary basins evolved parallel to the arc from Jurassic through mid-Cretaceous time.

The Laramide porphyry deposits were emplaced during a continental-scale episode of flat subduction and strong compression. This was accompanied by metaluminous to weakly peraluminous calc-alkaline I-type magmatism. Hypogene porphyry copper mineralization is dominated by chalcopyrite (± bornite), accompanied by K-silicate, transitional K-silicate-sericitic, and sericitic alteration. Subsequent to porphyry emplacement, much of the region was subjected to an episode of weathering and erosion, followed in the mid-Tertiary by a switchover to an extensional tectonic regime and the eruption of silicic volcanic rocks that blanketed much of the region. Two episodes of weathering-related oxidation, leaching, and enrichment, one prior to mid-Tertiary volcanisim (Eocene-Oligocene) and the second of Miocene age, produced rich supergene copper ores that were the mainstay of production in the province through the 1970s.

Southwestern North America Laramide porphyry copper deposits are very near the median of worldwide porphyry deposit distributions in regard to size, contained Cu metal, median Cu grade, and median Mo grade. Median Au grades are significantly lower than the global median.

The depth of erosion of pre-Laramide arcs in southwestern North America probably accounts for the relative paucity of porphyry deposits, and there are segments of the Laramide arc that have been tectonically denuded during mid-Tertiary extension, also resulting in a lack of deposits. There are, however, segments of the Laramide arc with similar, apparently appropriate, levels of erosion and preservation that have widely different deposit densities and/or copper endowment that is as yet unexplained. The apparent lack of Eocene to Oligocene porphyry deposits is also difficult to account for; hypabyssal rocks of this age are quite common and copper-bearing polymetallic skarn and high-temperature replacement and vein deposits are locally associated with them, but there is only one small, low-grade porphyry deposit of this age documented in southwestern North America.

Although the region has been heavily explored for exposed porphyry copper deposits, there remain excellent opportunities for discovery, given that an estimated 54% of the porphyry trend is covered by post-Laramide rocks or unconsolidated sediments. Exploration targets include undrilled or poorly drilled extensions of known deposits—both laterally and, especially, at depth; deposits or dismembered parts of deposits under structural cover in areas of post-Laramide extension; and deposits under postore volcanic and alluvial cover. Future production will increasingly be dominated by open pit low-grade and underground high-grade hypogene milling ores, with an important molybdenum by-product credit.

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Contents

Special Publications of the Society of Economic Geologists

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
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Michael Harris
Michael Harris
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Francisco Camus
Francisco Camus
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Society of Economic Geologists
Volume
16
ISBN electronic:
9781629490410
Publication date:
January 01, 2012

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