A New Model for the Optimal Structural Context for Giant Porphyry Copper

https://doi.org/10.1130/G48287.1

Manuscript received 17 August 2020 Revised manuscript received 5 November 2020 Manuscript accepted 16 November 2020

A new model for the optimal structural context for giant porphyry copper deposit formation José Piquer1, Pablo Sanchez-Alfaro1,2 and Pamela Pérez-Flores3 1Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Avenida Eduardo Morales Miranda, Edificio Emilio Pugín, Valdivia, Chile 2Andean Geothermal Center of Excellence (CEGA-FONDAP), Plaza Ercilla 803, Santiago, Chile 3Consultoría e Investigación Geológico Ambiental Ltda., Manuel Rojas 23, Huasco, Chile

ABSTRACT mation of a porphyry-type deposit (Sillitoe, Porphyry-type deposits are the main global source of copper and molybdenum. An im- 2010; Richards, 2018). Therefore, we propose proved understanding of the most favorable structural settings for the emplacement of these that faults misoriented for activation, commonly deposits is necessary for successful exploration, particularly considering that most future under compression or transpression and prone discoveries will be made under cover based on conceptual target generation. A common view to fault-valve behavior (Sibson, 1990), provide is that porphyry deposits are preferentially emplaced in pull-apart basins within strike-slip the most favorable structural control for por- fault systems that favor local extension within a regional compressive to transpressive tectonic phyry deposits. This optimal structural context regime. However, the role of such a structural context in magma storage and evolution in the is a necessary condition for the formation of upper crust remains unclear. In this work, we propose a new model based on the integration of porphyry deposits, but it is not sufficient on its structural data and the geometry of magmatic-hydrothermal systems from the main Andean own; other parameters such as crustal thickness porphyry Cu-Mo metallogenic belts and from the active volcanic arc of southern Chile. We (Chiaradia, 2013), oxygen fugacity, magmatic suggest that the magma differentiation and volatile accumulation required for the formation water content, or the presence of reactive wall of a porphyry deposit is best achieved when the fault system controlling magma ascent is rocks (Richards, 2013) need to be optimized to strongly misoriented for reactivation with respect to the prevailing stress field. When magmas provide the ideal combination of factors that and fluids are channeled by faults favorably oriented for extension (approximately normal allow porphyry deposit formation.

to σ3), they form sets of parallel, subvertical dikes and veins, which are common both during We tested this hypothesis through an integra- the late stages of the evolution of porphyry systems and in the epithermal environment. This tion of all the relevant data available from the new model has direct implications for conceptual mineral exploration. main Andean porphyry copper deposits. This region was selected because it constitutes by far INTRODUCTION 2005; Cloos and Sapiie, 2013). This combina- the largest known Cu concentration in the planet Most of the world’s Cu and Mo are mined tion is thought to be most commonly achieved (985 Mt of Cu; Sillitoe, 2012) and individual from porphyry deposits, which also supply an in pull-apart basins formed at releasing bends deposits of the belts include the largest known important amount of Au and other metals (e.g., in strike-slip faults, a model widely used by concentrations of Cu and Mo in the Earth’s crust Ag, Pd, Zn). Such deposits are a typical product exploration geologists. However, this model (Sillitoe, 2010). Relevant data compiled include of subduction-related magmatic belts (Sillitoe, does not explain satisfactorily the evolution fault geometry and kinematics; the geometry 2010). The most prospective porphyry belts have of magmatic-hydrothermal systems at differ- of porphyritic intrusions related to mineraliza- been explored extensively during the last cen- ent upper-crustal depths. Here we propose that tion, dikes, hydrothermal breccias, and vein sys- tury, and increasingly, the exploration endeavors localized extension in pull-apart basins would tems; and the orientation of the prevailing stress are focusing on the search for porphyry sys- produce a very efficient vertical conduit for tensor during the evolution of the magmatic- tems not exposed at the current surface. Con- magmas and hydrothermal fluids, which would hydrothermal systems. Faults were categorized ceptual models that account for the structural form sheeted dikes and veins (instead of the as syn-mineralization faults, which control the controls on the emplacement of porphyry-type stockworks and hydrothermal breccias typical emplacement of mineralized porphyritic intru- deposits will be a key element in the generation of porphyry deposits), inhibiting magma stor- sions and hydrothermal breccias, or late-min- of under-cover exploration targets. Prevailing age in upper-crustal chambers, as indicated by eralization faults, which cross-cut earlier intru- models of the large-scale structural controls on petrologic data, rock and fluid geochemistry, sions and might control the emplacement of late the emplacement of porphyry deposits agree and numerical simulations (e.g., Spinks et al., dikes and veins. Similarly, the distinction was that an environment of local extension within 2005; Cembrano and Lara, 2009; Tardani et al., made regarding the temporality of the prevail- a regional context dominated by compression 2016; Cabaniss et al., 2018). Magma storage and ing stress field with respect to mineralization: is the most favorable because it allows focused volatile accumulation are fundamental for pro- pre-mineralization stress predominated during magma ascent from the MASH (melting, assimi- ducing the differentiated magmas and the high early stages of the magmatic-hydrothermal sys- lation, storage, homogenization) zone (e.g., Tos- concentrations of volatiles that, when released tem; syn-mineralization stress dominated dur- dal and Richards, 2001; Richards, 2003; Drew, in sudden, catastrophic events, lead to the for- ing formation of the main mineralized body and

CITATION: Piquer, J., Sanchez-Alfaro, P., and Pérez-Flores, P., 2021, A new model for the optimal structural context for giant porphyry copper deposit formation: Geology, v. 49, p. 597–601, https://doi.org/10.1130/G48287.1

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Figure 2. Schematic maps of Neogene porphyry Cu-Mo and epithermal Au- Ag-Cu deposits of central Chile. Various sources used to compile the geol-

ogy and approximate σ1 orientation for each individual deposit, including C D the methodology used for its determination, are all detailed Table S1 (see footnote 1). Figure 1. Map of central and northern Chile showing metallogenic belts (orange—Paleo- gene; pink—Neogene) and mineral deposit cases presented in Figures 2 and 3.

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 E directions qualitatively inferred from the kinematics of the main brittle fault systems and ductile syn-magmatic shear zones. Main stress striking, high-angle reverse fault, whereas indi- (Fig. 2E). Magnetic foliation is subvertical and orientations estimated only from the preferred vidual mineralized centers and intrusive bodies north-northwest striking, compatible with east- orientation of stocks and dikes were not con- are aligned in a northwest trend (Fig. 2B; Perelló northeast shortening during granite emplace- sidered as these estimations can be highly mis- et al., 2012). Late phyllic and advanced argillic ment, which occurred coeval (6.4–6.2 Ma; leading in deformed continental crust containing alteration, in turn, are controlled by northeast- Garibaldi et al., 2018) to the emplacement of deep, preexisting faults, which can act as magma striking faults. In the Río Blanco–Los Bronces the giant porphyry deposits of Río Blanco–Los paths (e.g., Tibaldi et al., 2017). cluster, differentiated porphyries, diatremes, and Bronces and El Teniente. The granite is cut by a A summary of all the geologic and struc- hydrothermal breccias were emplaced along the set of east-northeast–striking faults and dikes of tural data considered for this study is provided misoriented, sinistral-reverse, north-northwest– andesitic composition (Fig. 2). Finally, the late as Table S1 in the Supplemental Material1. striking Río Blanco–Los Bronces fault system Miocene El Indio high-sulfidation epithermal (Fig. 2C; Piquer et al., 2015). On the other hand, deposit (Fig. 2A) is located to the north of the CASE 1: NEOGENE METALLOGENIC the emplacement of andesitic dikes and late veins late Miocene–early Pliocene porphyries (Fig. 1). BELT OF CENTRAL CHILE was controlled by dextral, northeast-striking Cu- and Au-rich epithermal veins were emplaced In the late Miocene–early Pliocene belt of branches of the El Salto fault system (Piquer along a dextral, northeast-striking fault system. central Chile (Figs. 1 and 2), an east-west to east- et al., 2015). At the El Teniente deposit, the main Overall, felsic porphyritic intrusions and

northeast pre- and syn-mineralization σ1 direction dacite porphyry body has a north to northwest large hydrothermal breccia complexes of this is well established (Piquer et al., 2016; Giam- elongation, while the late alteration stages and belt were emplaced along high-angle, north- biagi et al., 2017). The northernmost porphyry the emplacement of the primitive lamprophyre northwest– to north-striking faults, strongly cluster of this belt is Los Pelambres, in which dikes were controlled by the east-northeast–strik- misoriented under the east- to east-northeast–

the main intrusion was emplaced along a north- ing Teniente fault zone (Fig. 2D; McKinnon and trending σ1 that prevailed during mineral deposit Garrido, 2003). To the south of El Teniente, no formation. Late dikes and veins, in turn, were economic porphyry deposit has been discov- emplaced along favorably oriented, northeast- to 1Supplemental Material. Tables S1 and S2, ered, but the geometry of Neogene magmatic- east-northeast–striking high-angle faults. a summary of all the relevant data from mineral hydrothermal systems displays similar patterns. deposits and active volcanic systems compiled for The well-studied Risco Bayo–Huemul multi- CASE 2: PALEOGENE METALLOGENIC testing the model presented in this work. Please visit https://doi.org/10.1130/GEOL.S.13530653 to access phase plutonic complex (Garibaldi et al., 2018; BELTS OF NORTHERN CHILE the supplemental material, and contact editing@ Schaen et al., 2018) includes a late granitic body In northern Chile, a predominantly north- geosociety.org with any questions. that shows a strong north-northwest elongation west-trending σ1 has been proposed for the

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/597/5273394/597.pdf by guest on 24 September 2021 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 evalu- ate our hypothesis is provided by the active A volcanic centers of southern Chile, where B C 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 gen- eral, volcanic complexes controlled by approximately northwest-striking fault systems (e.g., Puyehue–Cordón Caulle, Villarrica-Quetrupil- lán-Lanín), strongly misoriented for activation D E under the prevailing stress field (east-north- F east–trending σ1), show differentiated volcanic products (dacites, rhyolites). Geophysical data (magnetotelluric, ambient seismic noise Rayleigh-wave tomography, and interferomet- ric 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 I developed along dextral, north- to north-northeast–striking segments of the intra-arc Liquiñe- G H Ofqui fault system, the volcanic products are more primitive (Cembrano and Lara, 2009). Figure 3. Schematic maps of Paleogene porphyry Cu-Mo deposits of northern Chile. Various Furthermore, recent structural observations, sources used to compile geology and approximate σ1 orientation for each individual deposit, including methodology used for its determination, are all detailed in Table S1 (see footnote 1). GPS data, and mechanical modeling suggest Legend is as in Figure 2. that megathrust earthquakes impose transient stress variations in the volcanic arc, reactivat- ing northwest-striking inherited crustal struc- Eocene–early Oligocene (Cornejo et al., 1997; the older Spence deposit), perpendicular to the tures upon which the volcanic edifices devel-

Padilla Garza et al., 2001; Niemeyer and Urrutia, predominant direction of σ1 during the Paleo- oped (Stanton-Yonge et al., 2016). Noteworthy 2009; Mpodozis and Cornejo, 2012), although gene (Fig. 3). Potrerillos (Fig. 3I) is a special is that the fluid chemistry and noble gas iso- periods of stress reversals have been postulated case, involving the emplacement of elongated topic signature of the hydrothermal features (Lindsay et al., 1995). In the porphyry Cu-Mo porphyritic intrusions controlled by east-north- also reflect the contrasting structural settings: deposits emplaced during that time span (Fig. 3), east–trending faults (Marsh et al., 1997; Nie- data from hydrothermal fluids related to north- felsic porphyritic stocks and dikes ubiquitously meyer and Munizaga, 2008) but also comprising west-striking faults indicate the involvement show northeast-trending elongation and align- a large stock emplaced in the axis of an anticline of a crustal component in addition to mantle ments (e.g., Quebrada Blanca, Chuquicamata, (Niemeyer and Munizaga, 2008); the latter is volatiles; contrastingly, hydrothermal fluids Toki cluster, Centinela cluster, Gaby cluster, interpreted as a fault-propagation fold associ- related to northeast- to north-northeast–strik- Escondida, El Salvador, and Potrerillos, and also ated with north-northeast–striking reverse faults. ing faults either reflect the volatiles related to

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/5/597/5273394/597.pdf by guest on 24 September 2021 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 Trac- ing Along Subduction”. We thank Daniel Cox, Ryan Mathur, and an anonymous reviewer, whose construc- tive comments and suggestions helped us to improve the quality of this manuscript.

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