Abstract Los Bronces-Río Blanco copper-molybdenum district, located in the late Miocene to early Pliocene magmatic arc of central Chile, has been the focus of exploration and mining activities since the discovery of highgrade copper ore in 1864. From humble beginnings as a source of small tonnages of direct-shipping ore, the district has grown to be one of the world’s premier copper producers, with 2009 production exceeding 448,000 metric tons (t) of copper from open-pit and underground mines owned and operated by Anglo American (Los Bronces) and Codelco (Río Blanco). Anglo American acquired Los Bronces in 2002 as part of its purchase of Compañía Minera Disputada de Las Condes from an Exxon Mobil Corporation affiliate. Near-mine exploration conducted by Anglo American Exploration Division since the acquisition resulted in addition of at least 65 million metric tons (Mt) of copper to the district mineral inventory in two world-class porphyry deposits, Los Sulfatos and San Enrique-Monolito. Exploration activities at Los Sulfatos were undertaken by previous owners of Los Bronces in the late 1960s and early 1990s, but it was not until Anglo American’s third drilling campaign, in 2006 to 2007, that large volumes of high-grade, hypogene porphyry- and breccia-style mineralization with clear economic potential were discovered beneath a previously untested part of the prospect area. Copper-molybdenum mineralization at Los Sulfatos is related to at least two discrete porphyry centers and a large, composite magmatic-hydrothermal breccia complex. The current inferred mineral resource at Los Sulfatos is 1,200 Mt averaging 1.46 percent Cu and 0.02 percent Mo, although the overall potential is envisioned to be 4,000 to 5,000 Mt at 0.8 to 1.0 percent Cu. Location of the deposit in precipitous high Cordilleran terrain, brevity of the summer field season, and rigorous environmental restrictions combined to complicate the exploration, and mandated helicopter support for the program. Key discovery tools at Los Sulfatos are considered to have been high-quality geologic mapping, innovative geologic thinking, and deep diamond drilling. Additional important factors include the early decision to appraise and rank all district prospects prior to commencement of exploration and to assign the program to Anglo American Exploration Division rather than to the Los Bronces mine geology team, thereby ensuring that timely evaluation of Los Sulfatos was conducted by personnel unencumbered with the daily responsibilities of an operating mine.

Abstract The Río Blanco-Los Bronces copper-molybdenum porphyry district in the late Miocene to early Pliocene magmatic arc of central Chile is currently being mined by state mining company CODELCO (Río Blanco) and Anglo American Sur (Los Bronces). Combined annual production in 2011 was nearly 450,000 metric tons (t) of copper plus by-product molybdenum. With Anglo American's recent high-grade discoveries in the district (3.7 billion tons (Bt) grading 0.7% Cu at San Enrique-Monolito, and 4.5 Bt grading 0.9% Cu at Los Sulfatos) adding more than 65 Mt of fine copper to the mineral inventory, the district now ranks as the world's largest by contained metal, with more than 200 Mt of copper. Volcanic and volcaniclastic rocks of the Abanico and Farellones Formations represent premineralization host rocks ranging in age between 22.7 ± 0.4 and 16.8 ± 0.3 Ma (U-Pb dating of zircons). The bulk of the copper-molybdenum porphyry endowment is related to evolution of the San Francisco batholith, a large (200 km 2 ) granodiorite-dominated complex with U-Pb zircon ages between 16.4 ± 0.2 to 8.4 ± 0.2 Ma. Three geologic domains are defined in the district on the basis of rock types, structural breaks, and age determinations: the Los Piches-Ortiga block in the west, the San Manuel-El Plomo block in the center, and the Río Blanco-Los Bronces-Los Sulfatos block in the east. These geologic domains are younger progressively to the east, with most of the known copper endowment on the easternmost (Río Blanco-Los Bronces-Los Sulfatos) block. Intrusive and hydrothermal activity in the Los Piches-Ortiga block spanned ∼ 2.5 m.y., from 14.8 ± 0.1 to 12.3 ± 0.1 Ma. Although these events apparently did not produce high-grade copper deposits, silver-bearing veins associated with a high sulfidation hydrothermal system are present in the block. To the east, in the San Manuel-El Plomo block, a series of magmatic-hydrothermal systems developed during a ∼ 3-m.y. period between 10.8 ± 0.1 and 7.7 ± 0.1 Ma. These events also apparently failed to generate high-grade copper systems. Magmatic-hydrothermal activity in the eastern Río Blanco-Los Bronces-Los Sulfatos block, hosting virtually all of the copper endowment recognized in the district, spanned a ∼ 4-m.y. period from 8.2 ± 0.5 to 4.31 ± 0.05 Ma. Copper endowment in the district is associated with vertically continuous breccia bodies and quartz-veinstockworked porphyries. Hydrothermal assemblages follow a characteristic vertical and lateral zonation pattern. Remnants of high sulfidation and/or advanced argillic assemblages (quartz-enargite-tennantite-galenasphalerite-gypsum-anhydrite with dumortierite-pyrophyllite-alunite) and peripheral sericite-illite reflect preservation of shallow levels, whereas K-silicate alteration assemblages (biotite-K-feldspar-albite) are present in association with chalcopyrite-bornite and chalcopyrite-pyrite at depth. Between the mineralized bodies, a distal assemblage of hydrothermal chlorite-epidote-specularite-pyrite predominates. At Río Blanco-Los Bronces, igneous and/or hydrothermal- and hydrothermal-cemented breccias developed in intimate association with porphyry phases. At shallow depths, the hydrothermal breccia cement generally comprises quartz-sericitetourmaline, and contains pyrite > chalcopyrite/molybdenite. At deeper levels the breccia cement is predominantly biotite-K-feldspar containing bornite-chalcopyrite-molybdenite. These progressively grade outward into chalcopyrite-pyrite-dominated zones and ultimately to pyrite-dominated zones. Within the Río Blanco-Los Bronces-Los Sulfatos block, the upper presence of the K-silicate assemblage varies from ∼ 3,000 m above sea level (a.s.l.) in the poorly telescoped northern area (Río Blanco), to ∼ 4,000 m a.s.l in the highly telescoped southern area (Los Sulfatos). These differences may reflect varying rates of synmineral structural exhumation, or varying depth of porphyry emplacement along the Río Blanco-Los Bronces-Los Sulfatos structural corridor. Key factors contributing to the copper productivity in the district are considered to reflect both far-field tectonic conditions, and district-scale structural controls. Following the last significant phase of volcanism documented in the district (∼ 16.8 Ma), a temporally discrete period of peak compression and rapid exhumation, between ∼ 6 to 3 Ma, affected the central Chilean Andes. This period of uplift relates to flat-slab subduction of sea floor containing the Juan Fernandez Ridge into the Chile Trench and overlaps part of the emplacement history of the Río Blanco-Los Bronces-Los Sulfatos block (8.2–4.31 Ma). The lack of contemporaneous volcanism and concomitant tectonic uplift are interpreted to reflect a state of increased horizontal crustal compression due to shear coupling of the downgoing slab. By suppressing volcanism, these conditions are considered to promote the retention of magma in the deep crustal environment, where higher pressures promote greater solubility of magmatic volatiles and higher temperatures may promote longer lived magma chambers by slowing fractionation processes. Under such conditions, the potential is enhanced for increased amounts of metals and volatiles by addition of fresh batches of magma to the deep magma chamber. At the district scale, closely spaced (2 km) structures that control the position of the porphyry and breccia bodies in the Río Blanco-Los Bronces-Los Sulfatos block appear to have focused long-lived, multistage magmatic-hydrothermal activity within a narrow structural corridor, contributing to the development of large, high-grade porphyry/breccia systems.

Abstract A review of the record of copper discovery in the Andes over the past four decades reveals that discoveries peaked in the 1990s when 14 discoveries were made. The 1980s and 1990s were the most important decades in terms of amount of copper discovered, accounting for 115 million metric tons (Mt) of copper. During the most recent decade, discoveries have dropped by 57 percent due to exploration maturity in well-exposed terrain, lack of effective exploration methods in concealed terrain, and a greater focus of exploration expenditures on known resources rather than new discovery. The first concealed deposit was recognized in 1986 at Candelaria, and since then nine additional concealed deposits have been discovered, accounting for 47.8 Mt of copper. Geochemistry played an important role in several of these copper discoveries, but unfortunately the published record on these case histories is sparse. In the late 1960s, recently developed geochemical exploration methods using field-based analytical techniques were applied for the first time in South America. Although crude, these early applications were effective in aiding discoveries at Santa Clara, Argentina, and Los Pelambres, Chile. At Los Pelambres, the rugged topography and extensive talus deposits led to testing the use of finegrain talus as a sample medium. A subsequent talus survey effectively outlined the extent of mineralization. Reconnaissance stream-sediment geochemistry and leached cap geochemistry were instrumental in the discovery of Escondida and later Zaldivar-Escondida Norte. Although the discovery of the completely gravelcovered Spence deposit is mainly attributed to systematic grid-based drilling of vast covered regions, base-ofgravel copper geochemistry did provide a vector to nearby mineralization. The use of panned-concentrate geochemistry in southern Ecuador, initially designed for gold exploration, highlighted several areas of base metal mineralization, leading to the discovery of the San Carlos deposit. The discovery of the Haquira oxide mineralization in southern Peru, and subsequent discovery of primary mineralization, resulted from follow-up of anomalous molybdenum and copper in stream sediments. The Haquira deposit is hosted in nonreactive siliciclastic rocks and does not show visual signs of alteration, despite the fact that the deposit subcrops. The use of unconventional geochemical methods, such as partial extraction geochemistry and ground-water geochemistry, has not yet resulted in a copper discovery in the Andes. Future discoveries in the Andes are likely to be in covered regions, deeper settings in outcropping areas where subtle signs of mineralization and alteration are present, and in poorly explored regions due to remoteness or political, social, or security concerns. An improved understanding of supergene enrichment processes in northern Chile, involving leaching and enrichment under semiarid conditions, and subsequent saline metasomatism under hyperarid conditions, help clarify secondary geochemical dispersion processes. Geochemical methods optimized to detect this dispersion should lead to greater success in exploring covered areas in this environment. The discovery of deep, high-grade hypogene mineralization at Los Sulfatos demonstrates the importance of this target type. Improved three-dimensional vectoring methods and zoning models are needed to aid in defining these targets. Other methods, such as ground-water geochemistry and the use of porphyry copper indicator minerals, are exciting developments that should contribute to future copper discoveries in the Andes.

Abstract The discovery of ore deposits, particularly those of world-class stature, is a rare and noteworthy event, not only for individual companies but for the minerals industry at large. Furthermore, the role of exploration leading to discovery in major companies has been greatly debated in recent times. The details of each discovery are unique, typically complex, and always of general interest, not only to fellow practitioners of the art and science of exploration but also to other members of the industry, including senior management. In particular, given that the majority of the mining industry’s wealth is captured in a few world-class, or Tier 1 mines, it is these rare deposits that provide the industry with the best opportunity to make a positive and lasting contribution to society. The singular most important contribution geoscientists can make to society is the discovery of new deposits; it has been suggested that recently, junior companies are more successful and cost-efficient. However, over the decade spanning 1999 to 2010, Anglo American Exploration (AAE) discovered 15 base metals deposits and has been recognized as one of the most successful base metals explorers. AAE’s discovery odds and costs are in the lower quartile of the industry benchmark, whereas the resource tons and grade for the discovered deposits are in the top quartile. In the past, the company’s priority has been competing with others in finding and building Tier 1 mines, but today the focus has changed to include building partnerships on all levels of society. Leadership, expertise, and a shared understanding of value creation are today the major differentiators for successful exploration and discovery. At Anglo American, effective management of the complex interplays of innovation and social licence, while at the same time encouraging a discovery-driven team culture, has resulted in delivery of growth options and distinguishes Anglo American from its peers. Independent analysis of nine of the 15 AAE discoveries during the1999 to 2010 period indicates they were largely the result of tried-and-tested, although innovative field-based, activities carried out at carefully selected greenfield and brownfield sites, using novel, in-house technology where appropriate. For example, development of the low-temperature superconducting quantum interference device (LT-SQUID) allowed precise electromagnetic definition of massive sulfide bodies at three deposit sites. All AAE discoveries were made by small, exploration-focused teams that were both highly motivated and suitably experienced. Importantly, some of the teams contained talented geoscientists who do not fit the usual corporate mold. For AAE, sustainable exploration today and success in the future therefore means delivery on advancing new exploration frontiers, maintaining access to land, quantifying mineral resources, and developing talent to provide options for growth through discovery, acquisition, and innovation.