Chapter 1 Copper Provinces

© 2012 Society of Economic Geologists, Inc. Special Publication 16, pp. 1–18 Chapter 1 Copper Provinces RICHARD H. SILLITOE† 27 West Hill Park, Highgate Village, London N6 6ND, England Abstract 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 sulfida- tion-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 thicken- ing 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 associ- ation 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 particu- lar the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwith- standing 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 pre- disposed 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. Introduction numbers of exceptionally endowed deposits, and copper is no THE GLOBAL INVENTORY of metals is critically dependent on exception (Singer, 1995; Laznicka, 1999). Indeed, approxi- the inordinately large contributions made by relatively limited mately one-third of the world’s defined copper resources are contributed by just seven districts (Fig. 1), and approximately † E-mail: [email protected] 2.5% of producing mines currently supply 25% of total copper 1 Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3811772/9781629490410_ch01.pdf by guest on 05 August 2020 2 RICHARD H. SILLITOE FIG. 1. The world’s supergiant copper deposits and districts (defined as those containing ≥24 Mt [Singer, 1995] to ≥25 Mt [Laznicka, 1999] Cu in resources and past production) and preeminent provinces, keyed to deposit types. The newly dis- covered Kamoa deposit in the Central African Copperbelt (Broughton and Rogers, 2010) contains 22 Mt Cu, but is also considered as a supergiant because of the likelihood of further growth. Data compiled from numerous published and unpub- lished sources, including company press releases. output (M. Harris, Rio Tinto, unpub. comp., 2012). Further- are not specifically discussed, although because of the impor- more, large proportions of most major metals, particularly tance of Noril’sk, Russia (Fig. 1), the first of these sources has well exemplified by copper, are concentrated in areally re- a paper devoted to it (Burrows and Lesher, 2012). stricted provinces (Fig. 1), which were typically assembled The copper endowment considered herein (≈2,500 million during several discrete metallogenic epochs. At least in the metric tons [Mt]; Figs. 1, 2) exceeds the global inventory of case of intrusion-related deposits, individual epochs com- 1,900 Mt determined by Kesler and Wilkinson (2008), and is monly have durations of ≤10 m.y. (e.g., Sillitoe, 1988). This more than four times larger than some other recent estimates spatial and temporal confinement of copper and other metal (e.g., ~570 Mt; U.S. Geological Survey, 2011, p. 48–49). Al- deposits was appreciated by Lindgren (1909) and subsequent though only formal resources plus past production are taken pioneers, as reviewed by Turneaure (1955), but has become into account, the greater copper tonnage may be attributed to much better defined over the ensuing century as a result of major recent expansions of hypogene resources, particularly numerous discoveries and geologic advances, particularly in the central Andes (e.g., Sillitoe, 2010a), and application of direct isotopic dating of ore-related minerals. lower cutoff grades. If probabilistic methodologies employed Although the fundamental reasons for the development of by Cunningham et al. (2007) and Kesler and Wilkinson (2008) the world’s largest copper deposits and premier copper belts are followed, then at least twice the number of copper de- and provinces are not well understood, this introductory paper posits exist (most at greater depths), albeit probably mainly explores some of the more plausible possibilities. The principal confined to the currently defined belts and provinces. contributors to the global copper inventory, namely porphyry and any associated skarn deposits (~70%), sediment-hosted Deposit-Scale Contributions to Large Size stratiform deposits (~15%), and, a distant third, iron oxide and High Grade copper-gold (IOCG) deposits, are emphasized both herein For the sake of brevity, only copper deposits and districts (Fig. 1) and throughout the rest of this volume. Other relatively that attain supergiant status by containing ≥24 Mt (Singer, minor copper sources, including magmatic nickel-copper, vol- 1995) or ≥25 Mt (Laznicka, 1999) of copper metal are dis- canogenic massive sulfide (VMS), nonporphyry-related skarn, cussed and individually plotted in Figure 1; most exceed the vein, Chilean manto-type, and carbonatite-hosted deposits 31.1-Mt Cu threshold used to define behemothian deposits 0361-0128/98/000/000-00 $6.00 2 Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/3811772/9781629490410_ch01.pdf by guest on 05 August 2020 COPPER PROVINCES 3 FIG. 2. Total copper endowment (resources and past production) of different segments of Phanerozoic circum-Pacific magmatic arcs. Only Paleozoic arc terranes, potentially somewhat more deeply eroded because of their greater antiquity,

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