Sillitoe (2010) Porphyry copper system

0361-0128/10/3863/3-39 3 Porphyry Copper Systems* RICHARD H. SILLITOE† 27 West Hill Park, Highgate Village, London N6 6ND, England Abstract Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sediment- hosted Au deposits in increasingly peripheral locations; and superjacent high- and intermediate-sulfidation epi- thermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several mil- lion years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas car- bonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal- or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediate- sulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sul- fides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the litho- caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during por- phyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeabil- ity. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems. Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic- hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper- saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate- salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The pro- gressive thermal decline of the systems combined with synmineral paleosurface degradation results in the char- acteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineral- ization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace. Many features of porphyry Cu systems at all scales need to be taken into account during planning and exe- cution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a pow- erful exploration concept once one or more systems are known. At the deposit scale, particularly in the por- phyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore- bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in por- phyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to † E-mail, aucu@compuserve.com *An Invited Paper ©2010 Society of Economic Geologists, Inc. Economic Geology, v. 105, pp. 3–41 Submitted: April 15, 2009 Accepted: November 18, 2009 Introduction PORPHYRY Cu systems are defined as large volumes (10−>100 km3) of hydrothermally altered rock centered on porphyry Cu stocks that may also contain skarn, carbonate-replacement, sediment-hosted, and high- and intermediate-sulfidation epi- thermal base and precious metal mineralization. Along with calc-alkaline batholiths and volcanic chains, they are the hall- marks of magmatic arcs constructed above active subduction zones at convergent plate margins (Sillitoe, 1972; Richards, 2003), although a minority of such systems occupies postcol- lisional and other tectonic settings that develop after subduc- tion ceases (e.g., Richards, 2009). The deeper parts of por- phyry Cu systems may contain porphyry Cu ± Mo ± Au deposits of various sizes (<10 million metric tons [Mt]-10 bil- lion metric tons [Gt]) as well as Cu, Au, and/or Zn skarns (<1 Mt−>1 Gt), whereas their shallower parts may host high- and intermediate-sulfidation epithermal Au ± Ag ± Cu orebodies (<1 Mt−>1 Gt). Porphyry Cu systems were generated world- wide since the Archean, although Meso-Cenozoic examples are most abundantly preserved (e.g., Singer et al., 2008; Fig. 1), probably because younger arc terranes are normally the least eroded (e.g., Seedorff et al., 2005; Kesler and Wilkinson, 2006; Wilkinson and Kesler, 2009). Porphyry Cu systems presently supply nearly three-quar- ters of the world’s Cu, half the Mo, perhaps one-fifth of the Au, most of the Re, and minor amounts of other metals (Ag, Pd, Te, Se, Bi, Zn, and Pb). The systems also contain major resources of these metals as well as including the world’s largest known exploitable concentrations of Cu (203 Mt: Los Bronces-Río Blanco, central Chile; A.J. Wilson, writ. com- mun., 2009) and Mo (2.5 Mt: El Teniente, central Chile; Camus, 2003), and the second largest of Au (129 Moz: Gras- berg, including contiguous skarn, Indonesia; J. MacPherson, 4 RICHARD H. SILLITOE 0361-0128/98/000/000-00 $6.00 4 search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more promi- nent alteration-mineralization. Butte Bau Cananea Pueblo Viejo Cerro Colorado Collahuasi dist. Chuquicamata dist. Gaby Gaby Recsk Majdanpek Chelopech Rosia Poieni Sar Cheshmeh Saindak Reko Diq Tampakan Boyongan-Bayugo Grasberg-Ertsberg Frieda River dist.(Nena) Wafi-Golpu Dizon Bau Cabang Kiri Batu Hijau Ok Tedi Panguna Cadia Cadia Koloula Ray Northparkes Mamut Lepanto & Guinaoang (Mankayan dist.) Santo Tomas & Nugget Hill II Kounrad Sepon Almalyk Oyu Tolgoi OyuT olgoi Taca Taca Bajo Bajo de la Alumbrera (Farallón Negro dist.) & Agua Rica Nevados del Famatina Pascua-Lama & Veladero Esperanza Escondida & Chimborazo El Salvador Potrerillos Andacollo Marte & Caspiche El Abra Mineral Park Toquepala & Cuajone Ray Globe-Miami Santa Rita (Central dist.) Sierrita- Esperanza Red Mountain Resolution (Superior dist.) Morenci Bingham Galore Creek Mt Polley Tintic Highland Valley dist. Island Copper Yerington Bisbee Bisbee Copper Canyon Yanacocha Choquelimpie Los Pelambres El Teniente Los Bronces- Río Blanco Cerro de Pasco & Colquijirca Cotabambas Antamina Pebble Cu-Mo Porphyry Porphyry + major skarn/ carbonate replacement High-sulfidation epithermal porphyry± Miocene-Pleistocene Principal metals Deposit type Age Cu-Mo-Au Eocene-Oligocene Cu-Au Late Cretaceous-Paleocene Ag-Pb-Zn-Cu Late Triassic-Early Cretaceous No porphyry known Late Devonian-Carboniferous Ordovician FIG. 1. Worldwide locations of porphyry Cu systems cited as examples of features discussed in the text along with five ad- ditional giant examples. The principal deposit type(s), contained metals, and age are also indicated. Data mainly from sources cited in the text. writ. commun., 2009). Typical hypogene porphyry Cu de- posits have average grades of 0.5 to 1.5 percent Cu, <0.01 to 0.04 percent Mo, and 0.0× to 1.5 g/t Au, although a few “Au- only” deposits have Au tenors of 0.9 to 1.5 g/t but little Cu (<0.1 %). The Cu and, in places, Au contents of skarns are typically higher still. In contrast, large high-sulfidation epi- thermal deposits average 1 to 3 g/t Au but have only minor or no recoverable Cu, commonly as a result of supergene removal. This field-oriented article reviews the geology of porphyry Cu systems at regional, district, and deposit scales. The resul- tant geologic model is then used as the basis for a brief syn- thesis of porphyry Cu genesis and discussion of exploration guidelines. The deposits and prospects used as examples throughout the text are located and further characterized in Figure 1. The economically important results of supergene oxidation and enrichment in porphyry Cu systems have been addressed elsewhere (Sillitoe, 2005, and references therein). Regional- and District-Scale Characteristics Belts and provinces Porphyry Cu systems show a marked tendency to occur in linear, typically orogen-parallel belts, which range from a few tens to hundreds and even thousands of kilometers long, as exemplified by the Andes of western South America (Sillitoe and Perelló, 2005; Fig. 2) and the Apuseni-Banat-Timok- Srednogorie belt of Romania, Serbia, and Bulgaria (Jankovic´, 1977; Popov et al., 2002). Deposit densities commonly attain 15 per 100,000 km2 of exposed permissive terrane (Singer et al., 2005). Each belt corresponds to a magmatic arc of broadly similar overall dimensions. One or more subparallel belts constitute porphyry Cu or epithermal Au provinces, several of which give rise to global-scale anomalies for Cu (e.g., north- ern Chile-southern Peru, southwestern North America) or Au (northern Peru; Sillitoe, 2008). Notwithstanding the ubiquity of porphyry Cu belts, major deposits may also occur in isola- tion or at least as distant outliers of coherent belts and provinces (e.g., Pebble in Alaska, Butte in Montana, and Bingham in Utah; Sillitoe, 2008; Fig. 1). Pueblo Viejo in the Dominican Republic (Fig. 1) is the best example of a major, isolated high-sulfidation epithermal Au deposit, albeit with no currently known porphyry Cu counterpart. Porphyry Cu belts developed during well-defined metallo- genic epochs, which isotopic dating shows to have typical du- rations of 10 to 20 m.y. Each porphyry Cu epoch is closely linked to a time-equivalent magmatic event. Again, the Andes (Sillitoe and Perelló, 2005), southwestern North America (Ti- tley, 1993; Barra et al., 2005), and Apuseni-Banat-Timok- Srednogorie belt (Zimmerman et al., 2008) provide prime ex- amples. Individual porphyry Cu belts are commonly spatially separate rather than superimposed on one another, reflecting arc migration as a result of steepening or shallowing of sub- ducted slabs between the individual magmatic-metallogenic epochs (e.g., Sillitoe and Perelló, 2005). The processes of sub- duction erosion and terrane accretion at convergent margins may assist with land- or trenchward migration of the arcs and contained porphyry Cu belts (e.g., von Huene and Scholl, 1991; Kay et al., 2005). Nevertheless, several temporally discrete porphyry Cu-bearing arcs may be superimposed on PORPHYRY COPPER SYSTEMS 5 0361-0128/98/000/000-00 $6.00 5 Lineament Fault Porphyry Cu deposit PERU BOLIVIA ARGENTINA 72° 68° 18° 22° 26° 30° ARICA ANTOFAGASTA COPIAPÓ LA SERENA SANTIAGO Collahuasi ChuquicamataEl Abra Escondida PotrerillosEl Salvador 100km CALAM A-E LT ORO ARCHIBARCA FIG. 2. A preeminent example of spatial and temporal coincidence be- tween a porphyry Cu belt and an intra-arc fault zone: the northern Chile part of the central Andean middle Eocene to early Oligocene porphyry Cu belt and Domeyko fault system (summarized from Sillitoe and Perelló, 2005). The apparent termination of the belt in northernmost Chile is a result of con- cealment beneath Miocene volcanic rocks. Approximate positions of the main arc-transverse lineaments in northern Chile are also shown (after Sal- fity, 1985, in Richards et al., 2001). wbkc 高亮 wbkc 高亮 one another: five since ~45 Ma in the Chagai belt, Pakistan (Perelló et al., 2008). Tectonic settings Porphyry Cu systems are generated mainly in magmatic arc (including backarc) environments subjected to a spectrum of regional-scale stress regimes, apparently ranging from mod- erately extensional through oblique slip to contractional (Tos- dal and Richards, 2001). Strongly extensional settings, typi- fied by compositionally bimodal basalt-rhyolite magmatism, lack significant porphyry Cu systems (Sillitoe, 1999a; Tosdal and Richards, 2001). The stress regime depends, among other factors, on whether there is trench advance or rollback and the degree of obliquity of the plate convergence vector (Dewey, 1980). Nevertheless, there is a prominent empirical relationship between broadly contractional settings, marked by crustal thickening, surface uplift, and rapid exhumation, and large, high-grade hypogene porphyry Cu deposits, as exemplified by the latest Cretaceous to Paleocene (Laramide) province of southwestern North America, middle Eocene to early Oligocene (Fig. 2) and late Miocene to Pliocene belts of the central Andes, mid-Miocene belt of Iran, and Pliocene belts in New Guinea and the Philippines (Fig. 1; Sillitoe, 1998; Hill et al., 2002; Perelló et al., 2003a; Cooke et al., 2005; Rohrlach and Loucks, 2005; Sillitoe and Perelló, 2005; Perelló, 2006). Large, high-sulfidation epithermal Au deposits also form in similar contractional settings at the tops of tectonically thick- ened crustal sections, albeit not together with giant porphyry Cu deposits (Sillitoe and Hedenquist 2003; Sillitoe, 2008). It may be speculated that crustal compression aids development of large mid- to upper-crustal magma chambers (Takada, 1994) capable of efficient fractionation and magmatic fluid generation and release, especially at times of rapid uplift and erosional unroofing (Sillitoe, 1998), events which may presage initiation of stress relaxation (Tosdal and Richards, 2001; Richards, 2003, 2005; Gow and Walshe, 2005). Changes in crustal stress regime are considered by some as especially favorable times for porphyry Cu and high-sulfidation epi- thermal Au deposit generation (e.g., Tosdal and Richards, 2001), with Bingham and Bajo de la Alumbrera, Argentina, for example, both apparently occupying such a tectonic niche (Presnell, 1997; Sasso and Clark, 1998; Halter et al., 2004; Sil- litoe, 2008). Faults and fault intersections are invariably involved, to greater or lesser degrees, in determining the formational sites and geometries of porphyry Cu systems and their constituent parts. Intra-arc fault systems, active before as well as during magmatism and porphyry Cu generation, are particularly im- portant localizers, as exemplified by the Domeyko fault sys- tem during development of the preeminent middle Eocene to early Oligocene belt of northern Chile (Sillitoe and Perelló, 2005, and references therein; Fig. 2). Some investigators em- phasize the importance of intersections between continent- scale transverse fault zones or lineaments and arc-parallel structures for porphyry Cu formation, with the Archibarca and Calama-El Toro lineaments of northern Chile (Richards et al., 2001; Fig. 2), the Lachlan Transverse Zone of New South Wales (Glen and Walshe, 1999), comparable features in New Guinea (Corbett, 1994; Hill et al., 2002), and the much wider (160 km) Texas lineament of southwestern North America (Schmitt, 1966) being oft-quoted examples. These transverse features, possibly reflecting underlying basement structures, may facilitate ascent of the relatively small magma volumes involved in porphyry Cu systems (e.g., Clark, 1993; Richards, 2000). Deposit clusters and alignments At the district scale, porphyry Cu systems and their con- tained deposits tend to occur as clusters or alignments that may attain 5 to 30 km across or in length, respectively. Clus- ters are broadly equidimensional groupings of deposits (e.g., Globe-Miami district, Arizona; Fig. 3a), whereas alignments are linear deposit arrays oriented either parallel or trans- verse to the magmatic arcs and their coincident porphyry Cu belts. Arc-parallel alignments may occur along intra-arc fault zones, as exemplified by the Chuquicamata district, northern Chile (Fig. 3b) whereas cross-arc fault zones or lin- eaments control arc-transverse alignments, as in the Cadia, New South Wales (Fig. 3c) and Oyu Tolgoi, Mongolia dis- tricts (Fig. 3d). Irrespective of whether the porphyry Cu systems and con- tained deposits define clusters or alignments, their surface distributions are taken to reflect the areal extents of either underlying parental plutons or cupolas on their roofs. Within the clusters and alignments, the distance (100s−1,000s m) be- tween individual deposits (e.g., Sillitoe and Gappe, 1984) and even their footprint shapes can vary greatly, as observed in the Chuquicamata and Cadia districts (Fig. 3b, c). Clusters or alignments of porphyry Cu systems can display a spread of formational ages, which attain as much as 5 m.y. in the Chuquicamata (Ballard et al., 2001; Rivera and Pardo, 2004; Campbell et al., 2006) and Yanacocha districts (Longo and Teal, 2005) but could be as much as ~18 m.y. in the Cadia district (Wilson et al., 2007). This situation implies that the underlying parental plutons have protracted life spans, albeit intermittent in some cases, with porphyry Cu formatio