The Southwestern North America Porphyry Copper Province
Richard A. Leveille, Ralph J. Stegen, 2012. "The Southwestern North America Porphyry Copper Province", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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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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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.