Porphyry-type deposits are the main global source of copper and molybdenum. An improved understanding of the most favorable structural settings for the emplacement of these deposits is necessary for successful exploration, particularly considering that most future discoveries will be made under cover based on conceptual target generation. A common view is that porphyry deposits are preferentially emplaced in pull-apart basins within strike-slip fault systems that favor local extension within a regional compressive to transpressive tectonic regime. However, the role of such a structural context in magma storage and evolution in the upper crust remains unclear. In this work, we propose a new model based on the integration of structural data and the geometry of magmatic-hydrothermal systems from the main Andean porphyry Cu-Mo metallogenic belts and from the active volcanic arc of southern Chile. We suggest that the magma differentiation and volatile accumulation required for the formation of a porphyry deposit is best achieved when the fault system controlling magma ascent is strongly misoriented for reactivation with respect to the prevailing stress field. When magmas and fluids are channeled by faults favorably oriented for extension (approximately normal to σ3), they form sets of parallel, subvertical dikes and veins, which are common both during the late stages of the evolution of porphyry systems and in the epithermal environment. This new model has direct implications for conceptual mineral exploration.
Most of the world’s Cu and Mo are mined from porphyry deposits, which also supply an important amount of Au and other metals (e.g., Ag, Pd, Zn). Such deposits are a typical product of subduction-related magmatic belts (Sillitoe, 2010). The most prospective porphyry belts have been explored extensively during the last century, and increasingly, the exploration endeavors are focusing on the search for porphyry systems not exposed at the current surface. Conceptual models that account for the structural controls on the emplacement of porphyry-type deposits will be a key element in the generation of under-cover exploration targets. Prevailing models of the large-scale structural controls on the emplacement of porphyry deposits agree that an environment of local extension within a regional context dominated by compression is the most favorable because it allows focused magma ascent from the MASH (melting, assimilation, storage, homogenization) zone (e.g., Tosdal and Richards, 2001; Richards, 2003; Drew, 2005; Cloos and Sapiie, 2013). This combination is thought to be most commonly achieved in pull-apart basins formed at releasing bends in strike-slip faults, a model widely used by exploration geologists. However, this model does not explain satisfactorily the evolution of magmatic-hydrothermal systems at different upper-crustal depths. Here we propose that localized extension in pull-apart basins would produce a very efficient vertical conduit for magmas and hydrothermal fluids, which would form sheeted dikes and veins (instead of the stockworks and hydrothermal breccias typical of porphyry deposits), inhibiting magma storage in upper-crustal chambers, as indicated by petrologic data, rock and fluid geochemistry, and numerical simulations (e.g., Spinks et al., 2005; Cembrano and Lara, 2009; Tardani et al., 2016; Cabaniss et al., 2018). Magma storage and volatile accumulation are fundamental for producing the differentiated magmas and the high concentrations of volatiles that, when released in sudden, catastrophic events, lead to the formation of a porphyry-type deposit (Sillitoe, 2010; Richards, 2018). Therefore, we propose that faults misoriented for activation, commonly under compression or transpression and prone to fault-valve behavior (Sibson, 1990), provide the most favorable structural control for porphyry deposits. This optimal structural context is a necessary condition for the formation of porphyry deposits, but it is not sufficient on its own; other parameters such as crustal thickness (Chiaradia, 2013), oxygen fugacity, magmatic water content, or the presence of reactive wall rocks (Richards, 2013) need to be optimized to provide the ideal combination of factors that allow porphyry deposit formation.
We tested this hypothesis through an integration of all the relevant data available from the main Andean porphyry copper deposits. This region was selected because it constitutes by far the largest known Cu concentration in the planet (985 Mt of Cu; Sillitoe, 2012) and individual deposits of the belts include the largest known concentrations of Cu and Mo in the Earth’s crust (Sillitoe, 2010). Relevant data compiled include fault geometry and kinematics; the geometry of porphyritic intrusions related to mineralization, dikes, hydrothermal breccias, and vein systems; and the orientation of the prevailing stress tensor during the evolution of the magmatic-hydrothermal systems. Faults were categorized as syn-mineralization faults, which control the emplacement of mineralized porphyritic intrusions and hydrothermal breccias, or late-mineralization faults, which cross-cut earlier intrusions and might control the emplacement of late dikes and veins. Similarly, the distinction was made regarding the temporality of the prevailing stress field with respect to mineralization: pre-mineralization stress predominated during early stages of the magmatic-hydrothermal system; syn-mineralization stress dominated during formation of the main mineralized body and emplacement of coeval porphyritic intrusions and hydrothermal breccias; and late-mineralization stress dominated during the emplacement of late dikes and veins. For our study, we considered paleo-stress fields calculated from the inversion of fault slip data and paleo-stress directions qualitatively inferred from the kinematics of the main brittle fault systems and ductile syn-magmatic shear zones. Main stress orientations estimated only from the preferred orientation of stocks and dikes were not considered as these estimations can be highly misleading in deformed continental crust containing deep, preexisting faults, which can act as magma paths (e.g., Tibaldi et al., 2017).
A summary of all the geologic and structural data considered for this study is provided as Table S1 in the Supplemental Material1.
CASE 1: NEOGENE METALLOGENIC BELT OF CENTRAL CHILE
In the late Miocene–early Pliocene belt of central Chile (Figs. 1 and 2), an east-west to east-northeast pre- and syn-mineralization σ1 direction is well established (Piquer et al., 2016; Giambiagi et al., 2017). The northernmost porphyry cluster of this belt is Los Pelambres, in which the main intrusion was emplaced along a north-striking, high-angle reverse fault, whereas individual mineralized centers and intrusive bodies are aligned in a northwest trend (Fig. 2B; Perelló et al., 2012). Late phyllic and advanced argillic alteration, in turn, are controlled by northeast-striking faults. In the Río Blanco–Los Bronces cluster, differentiated porphyries, diatremes, and hydrothermal breccias were emplaced along the misoriented, sinistral-reverse, north-northwest–striking Río Blanco–Los Bronces fault system (Fig. 2C; Piquer et al., 2015). On the other hand, the emplacement of andesitic dikes and late veins was controlled by dextral, northeast-striking branches of the El Salto fault system (Piquer et al., 2015). At the El Teniente deposit, the main dacite porphyry body has a north to northwest elongation, while the late alteration stages and the emplacement of the primitive lamprophyre dikes were controlled by the east-northeast–striking Teniente fault zone (Fig. 2D; McKinnon and Garrido, 2003). To the south of El Teniente, no economic porphyry deposit has been discovered, but the geometry of Neogene magmatic-hydrothermal systems displays similar patterns. The well-studied Risco Bayo–Huemul multi-phase plutonic complex (Garibaldi et al., 2018; Schaen et al., 2018) includes a late granitic body that shows a strong north-northwest elongation (Fig. 2E). Magnetic foliation is subvertical and north-northwest striking, compatible with east-northeast shortening during granite emplacement, which occurred coeval (6.4–6.2 Ma; Garibaldi et al., 2018) to the emplacement of the giant porphyry deposits of Río Blanco–Los Bronces and El Teniente. The granite is cut by a set of east-northeast–striking faults and dikes of andesitic composition (Fig. 2). Finally, the late Miocene El Indio high-sulfidation epithermal deposit (Fig. 2A) is located to the north of the late Miocene–early Pliocene porphyries (Fig. 1). Cu- and Au-rich epithermal veins were emplaced along a dextral, northeast-striking fault system.
Overall, felsic porphyritic intrusions and large hydrothermal breccia complexes of this belt were emplaced along high-angle, north-northwest– to north-striking faults, strongly misoriented under the east- to east-northeast–trending σ1 that prevailed during mineral deposit formation. Late dikes and veins, in turn, were emplaced along favorably oriented, northeast- to east-northeast–striking high-angle faults.
CASE 2: PALEOGENE METALLOGENIC BELTS OF NORTHERN CHILE
In northern Chile, a predominantly northwest-trending σ1 has been proposed for the Eocene–early Oligocene (Cornejo et al., 1997; Padilla Garza et al., 2001; Niemeyer and Urrutia, 2009; Mpodozis and Cornejo, 2012), although periods of stress reversals have been postulated (Lindsay et al., 1995). In the porphyry Cu-Mo deposits emplaced during that time span (Fig. 3), felsic porphyritic stocks and dikes ubiquitously show northeast-trending elongation and alignments (e.g., Quebrada Blanca, Chuquicamata, Toki cluster, Centinela cluster, Gaby cluster, Escondida, El Salvador, and Potrerillos, and also the older Spence deposit), perpendicular to the predominant direction of σ1 during the Paleogene (Fig. 3). Potrerillos (Fig. 3I) is a special case, involving the emplacement of elongated porphyritic intrusions controlled by east-northeast–trending faults (Marsh et al., 1997; Niemeyer and Munizaga, 2008) but also comprising a large stock emplaced in the axis of an anticline (Niemeyer and Munizaga, 2008); the latter is interpreted as a fault-propagation fold associated with north-northeast–striking reverse faults. In turn, late veins related to advanced argillic alteration and high-sulfidation mineralization and post-mineralization dikes (more primitive than the northeast-trending intrusions) show a strong preferred northwest strike in all the deposits cited above (Fig. 3).
ANALOGIES WITH ACTIVE MAGMATIC-HYDROTHERMAL SYSTEMS
A modern analog that can be used to evaluate our hypothesis is provided by the active volcanic centers of southern Chile, where structural controls on volcanism have been extensively studied (e.g., Cembrano and Lara, 2009; Pérez-Flores et al., 2016). Although there are no porphyry deposits related to this recent magmatic arc, it is an ideal case for testing the particular processes of interest for our model because the interaction between magmatism, hydrothermal activity, and fault systems with different orientations under a well-constrained stress regime can be observed directly (e.g., Tardani et al., 2016; Cox et al., 2020). In general, volcanic complexes controlled by approximately northwest-striking fault systems (e.g., Puyehue–Cordón Caulle, Villarrica-Quetrupillán-Lanín), strongly misoriented for activation under the prevailing stress field (east-northeast–trending σ1), show differentiated volcanic products (dacites, rhyolites). Geophysical data (magnetotelluric, ambient seismic noise Rayleigh-wave tomography, and interferometric synthetic aperture radar [InSAR] inversion data) support these observations and suggest that magmatic plumbing systems controlled by northwest-striking faults promote the development of long-lived, large-scale crustal magma reservoirs at shallow (<10 km) depths (Table S2). In contrast, when the controlling structures of volcanism strike approximately northeast (e.g., Lonquimay, Carrán–Los Venados), in some cases associated with pull-apart basins developed along dextral, north- to north-northeast–striking segments of the intra-arc Liquiñe-Ofqui fault system, the volcanic products are more primitive (Cembrano and Lara, 2009). Furthermore, recent structural observations, GPS data, and mechanical modeling suggest that megathrust earthquakes impose transient stress variations in the volcanic arc, reactivating northwest-striking inherited crustal structures upon which the volcanic edifices developed (Stanton-Yonge et al., 2016). Noteworthy is that the fluid chemistry and noble gas isotopic signature of the hydrothermal features also reflect the contrasting structural settings: data from hydrothermal fluids related to northwest-striking faults indicate the involvement of a crustal component in addition to mantle volatiles; contrastingly, hydrothermal fluids related to northeast- to north-northeast–striking faults either reflect the volatiles related to primitive mantle asthenospheric upwelling or do not form high-enthalpy systems (Tardani et al., 2016; Table S2). As shown above, the similarities to Andean porphyry Cu-Mo deposits are remarkable.
We conclude that the most favorable conditions for the emplacement of a porphyry copper deposit are met when structural control is provided by deep-seated faults severely misoriented for activation (Fig. 4). They could correspond to restraining (not releasing) bends in segments of major strike-slip faults. These faults or fault segments would normally be under compression (i.e., during megathrust interseismic periods), and at least some of those associated with the studied Andean deposits correspond to long-lived, arc-transverse faults. The origin of these deep, long-lived fault systems oblique to the present-day continental margin is still a matter of debate, although the predominant hypothesis is that they are the result of a complex pre-Andean history of continental terrane accretion and collision and rifting episodes occurring between the Neoproterozoic and the Triassic (Yáñez and Rivera, 2019). Misoriented faults would inhibit the ascent of magma and hydrothermal fluids, allowing the magma to achieve the residence times required for it to differentiate and also for a large amount of volatiles to accumulate at the apex of the magma chamber. Differentiated magmas and volatiles could then be violently released and channeled by the misoriented faults during transient, catastrophic events related to local or regional stress reversals, for which different triggers have been proposed: major subduction earthquakes (Mpodozis and Cornejo, 2012; Stanton-Yonge et al., 2016) that produce a transient inversion of the stress field and fault kinematics; temporary relaxation of the misoriented faults produced by their interaction with conjugate strike-slip faults striking at low angles with respect to σ1 (Piquer et al., 2015); or localized, transient extension at the core of anticlines in the hanging wall of high-angle reverse faults (Amilibia and Skármeta, 2003; Niemeyer and Munizaga, 2008). During later stages and also in the more distal halos of the system, fluid pressures would be smaller, and veins and dikes would be emplaced along favorably oriented faults, which would cross-cut earlier, misoriented faults and the felsic porphyries (Fig. 4). This model is remarkably consistent for Cenozoic porphyry deposits of northern and central Chile (the Earth’s largest deposits of this type), considering all the current knowledge about the geometry of the magmatic-hydrothermal systems and the prevailing stress fields before, during, and after mineralization. This commonality highlights the role of fault systems at different structural levels and times during the formation of porphyry deposits. Conceptual exploration for these deposits should therefore include misoriented, deep-seated faults as a key targeting criterion.
The main motivation for this work arose from observations made by the authors at various porphyry deposits and volcanic systems over several years, and we are grateful to all the colleagues who gave us the opportunity to work with them, both in academia and in industry. J. Cembrano and D.R. Cooke are deeply acknowledged for their careful reviews of an early version of the manuscript. Piquer acknowledges support from Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT; Chile) grant 11181048 and Amira (Perth, Australia) project P1202. Sanchez-Alfaro acknowledges support from FONDECYT grant 1201219, Agencia Nacional de Investigación y Desarrollo–Fondo de Financiamiento de Centros de Investigación en Áreas Prioritarias (ANID-FONDAP) project 15090013, and Iniciativa Milenio grant NC130065 “Millennium Nucleus for Metal Tracing Along Subduction”. We thank Daniel Cox, Ryan Mathur, and an anonymous reviewer, whose constructive comments and suggestions helped us to improve the quality of this manuscript.