Abstract Some sediment-hosted base metal deposits, specifically, the clastic-dominated Zn-Pb deposits, carbonatehosted Mississippi Valley-type (MVT) deposits, sedimentary rock-hosted stratiform copper deposits, and carbonate-hosted polymetallic (“Kipushi-type”) deposits, are or have been important sources of critical elements including Co, Ga, Ge, PGEs, and Re. Cobalt is noted in only a few clastic-dominated and MVT deposits, whereas sedimentary rock-hosted stratiform copper deposits are major producers. Gallium occurs in sphalerite from clastic-dominated and MVT deposits. Little is reported of germanium in clastic-dominated deposits; it is more commonly noted in MVT deposits (up to 4,900 ppm within sphalerite) and has been produced from carbonate-hosted polymetallic deposits (Kipushi, Tsumeb). Indium is known to be elevated in sphalerite and zinc concentrates from some MVT and clastic-dominated deposits, produced from Rammelsberg and reported from Sullivan, Red Dog, Tri-State, Viburnum Trend, Lisheen, San Vincente, and Shalipayco. Platinum and palladium have been produced from sedimentary rock-hosted stratiform copper deposits in the Polish Kupferschiefer. Sedimentary rock-hosted stratiform copper deposits in the Chu-Sarysu basin are known to have produced rhenium. Although trace element concentrations in these types of sediment-hosted ores are poorly characterized in general, available data suggest that there may be economically important concentrations of critical elements yet to be recognized.

Abstract There seems to be general consensus throughout much of the global mining industry that the supply of base and precious metals and some other commodities (e.g., ferrous metals, uranium) is reasonably well assured into the foreseeable future because increases in total resources continue to keep pace with or outstrip global consumption. The basic assumption is that market forces and technological advances will combine to promote and perpetuate this trend (e.g., Tilton, 2003 ; Crowson, 2008 ). Others disagree, however, and predict that shortages are inevitable if metal consumption continues to escalate ( Beaty, 2010 ). It is already becoming clear that many known resources seem unlikely to be mined, irrespective of commodity prices, because of their low grade and/or quality. Hence, many mineral resources that were uneconomic in the early 2000s are likely to remain so, both today and into the foreseeable future because of increases in both the direct (e.g., energy, labor) and indirect (e.g., environmental, social) production costs. This situation is being further exacerbated by the perceived decrease, over at least the past decade, in the discovery rate of base and precious metal resources measured in terms of both the number of major discoveries made and the exploration dollars spent per discovery (e.g., Dummett, 2000 ; Horn, 2002 ; Schodde, 2004 ). There is also a suggestion that the discoveries made are, on average, becoming both smaller and lower grade. Therefore, it seems reasonable to ask whether current exploration practices and success rates are going to be adequate to provide

Abstract We have estimated the magnitude of resources available to support world production of gold into the next millennium. The estimate was made using the tectonic-diffusion computational model in which ore deposits move through depth-time space in response to global tectonism, and it is based on a global compilation of ages and gold contents for each type of deposit. The method was applied to the most important hydrothermal deposits that yield gold, including Carlin-type, epithermal, iron oxide-copper-gold, orogenic, porphyry copper, skarn, and volcanogenic massive sulfide. As production from the Witwatersrand deposits has declined, these types of hydrothermal deposits have supplied a growing fraction of global gold production, and it is likely that this pattern will continue. Estimates were made for gold resources to crustal depths of 1 and 3 km, which are likely depth limits for most mineral exploration and production. Our results indicate that porphyry copper and epithermal deposits will be the most important hosts of gold produced in the future. The contribution to future production from orogenic gold deposits is likely to decrease relative to other types of deposits because orogenic gold deposits do not increase in abundance as rapidly downward through the uppermost crust as do epithermal and porphyry copper deposits, which form at much shallower crustal depths. Although the gold resource estimated here, about 1 million metric tons (Mt) to a depth of 1 km and almost 5 Mt to a depth of 3 km, is large relative to current estimates of gold reserves, recoverable gold will probably be much smaller, possibly by as much as 50 percent, because of cultural, geologic, and mining-processing factors. Recoverable gold resources to a depth of about 3 km in the crust could supply current world mine production of gold for about 1,000 yr. Although this is a long period of time, it is short relative to the ~7,000–yr history of gold mining. These estimates highlight the fact that a growing fraction of world gold supply will have to come from buried deposits, many below postore cover, and from deposits in which gold is a co- or by-product.

Abstract In May 2003, AngloGold Ashanti began greenfields exploration in Colombia with a team of four geologists. By 2007, the program employed 127 field geologists covering about 10.5 million hectares (Mha) with systematic reconnaissance exploration. To date, the result of this work is the discovery of several gold deposits, the most important being La Colosa, containing an initial resource of 381.4 million metric tons (Mt), grading 1.00 g/t Au or 381.4 t Au, using a 0.3 g/t cut-off grade. The La Colosa deposit is a gold-only porphyry system related to a late Miocene multiphase porphyritic diorite-granodiorite complex. Gold grades exceeding 1 g/t are associated with early dioritic phases that are altered to potassic and sodic-calcic mineral assemblages. Potassic and sodic-calcic alteration also affects later diorite porphyries, but gold grades are, on average, <0.4 g/t. A late granodiorite porphyry is mostly barren, with only erratic anomalous gold grades, which are all <0.4 g/t, and weak to moderate propylitic and intermediate argillic alteration. The deposit contains >5 vol percent magnetite. Pyrite content varies between 3 and 5 vol percent. Gold is mainly contained within pyrite. Copper and molybdenum contents are generally at background values for diorite. The La Colosa deposit is a grassroots discovery. It was made by AngloGold Ashanti geologists only 18 months after the initiation of a regional exploration program in the defined Mariquita target region. Discovery is the result of systematic regional data synthesis, conceptual target generation, and disciplined, multiphase, field-based, follow-up, which included stream sediment geochemistry, prospecting, rock chip sampling, and drilling. Early target generation work by AngloGold Ashanti, undertaken at a northern Andean scale between 2000 and 2003, focused field activities into the most prospective regions of Colombia, based not only on geology and mineral potential but also upon factors that would lead to the discovery and eventual development and operation of a successful, socially, and environmentally responsible mining operation. The exploration strategy has maintained a systematic methodology that includes conceptualization, reconnaissance stream sediment surveys and related prospecting, target definition, target drilling and conceptual economic study, and finally prefeasibility and feasibility studies. Clear decision points were established at the end of each work phase, always keeping in mind the company’s minimum economic target criteria. The key factors that led to the discovery of La Colosa included the execution of a well-planned business and exploration model, with recognition of the “first mover” advantage; the acquisition of a large land position with respect to legal exploration tenure, covering essentially all of the deemed prospective areas; implementation of exploration from the regional scale, working down to the prospect scale, instead of vice versa; maintaining a disciplined and systematic field-focused approach; use of a skilled exploration team; and maintaining a long-term (>5 yrs), adequately funded view to exploring in frontier mineral exploration regions.

Abstract Several gold deposits discovered since 1990 in the central Pequop Mountains of Elko County, northeastern Nevada, make up the new Pequop mining district. The most advanced projects, including Long Canyon and West Pequop, have a combined resource exceeding 42.5 tonnes Au and growing. Favorable open-pit mining economics are generated by high-grade, oxidized gold deposits above the water table. The deposits exhibit characteristics typical of Carlin-type gold deposits, including limestone and calcareous siliciclastic host rocks, collapse breccias, and <5 micron gold grains in rims of oxidized arsenian pyrite grains. Host rocks are decalcified, argillized, and locally silicified (jasperoid). Some gold mineralization, particularly at Long Canyon, occurs along the margins of competent blocks of Cambrian Notch Peak dolomite in contact with limestone. The Pequop mining district lies outside the well-known Nevada gold trends. In contrast to many Carlin-type deposits, mineralization is hosted by the Cambrian and Ordovician miogeoclinal sequence of interbedded platform carbonate and siliciclastic rocks. The degree of penetrative deformation and metamorphism is unusually high due to extensive crustal thickening and deep burial during the Jurassic Elko and Cretaceous Sevier orogenies. Zircon U-Pb dates show that the Pequop Mountains were the site of Jurassic (162–154 Ma), Cretaceous (85–70 Ma), and Eocene (41–39 Ma) intrusive activity, which is observed in other Carlin-type districts. Jurassic mafic to felsic dikes and sills, particularly lamprophyres, form passive hosts to mineralization. Eocene felsic dikes on the western side of the Pequop Mountains are unaltered and unmineralized, they lie within a northeast-trending corridor of gold anomalies, older dikes, and positive aeromagnetic anomalies, which is permissive evidence for an Eocene age of mineralization. Geophysical anomalies suggest the Pequop district may lie above a prominent break in the continental crust. It is near a west- to northwest-trending conductor, defined by magnetotelluric surveys that may mark the transition between rocks of the Archean Wyoming Province and the Paleoproterozoic Mojave Province. Aeromagnetic data suggest the district is astride a northeastern alignment of intrusions that extends from the Bald Mountain district, located to the southwest, and can be traced northeast to the Tecoma district. Low-frequency filtering of gravity data reveals a distinct northwest-trending boundary that coincides with a similarly oriented trend of barite vein occurrences. These data, along with the ages of intrusions, suggest the district may be underlain by a deep magmatic plumbing system.

Abstract The Money Knob deposit is a major new, bulk-tonnage, gold discovery located 110 km northwest of Fairbanks, Alaska, within the Tintina gold belt. The deposit was discovered in 2007 and by the end of 2009 contained a combined indicated and inferred resource of 389 metric tons (t) of gold at a grade of 0.85 g/t gold using a 0.5 g/t gold cutoff grade. Gold mineralization is hosted within a fold and thrust sequence of Cambrian and Devonian rocks intruded by 90 Ma dikes and sills, which are contemporaneous with the main-stage gold event. Gold mineralization occurs as shallowly dipping, east-west–trending, tabular bodies within permeable sedimentary and volcanic rocks, with higher grade zones related to north-northwest–trending crosscutting structural zones. The discovery of the Money Knob deposit evolved from a series of exploration programs conducted by eight different companies during a 30-year-long period. An examination of the successes and failures during the exploration history outlines three key concepts that were important in the discovery: (1) the need to operate with multiple, working exploration models that are driven by high-quality data and observations; (2) recognition that large mineral systems are rare and should be fully evaluated in light of potential long-term changes in commodity price, technology, and deposit characteristics; and (3) successful exploration requires a champion and a talented team with vision and perseverance.

Abstract Gold mineralization at the Damang deposit is unique among known deposits in Ghana, comprising two distinct styles of mineralization. These include a stratigraphically controlled auriferous quartz-pebble metaconglomerate that is overprinted by later gold contained in a complex fault-fracture vein array with surrounding hydrothermal alteration. A systematic study using portable, field-based infrared reflectance spectroscopy has proven to be a valuable exploration tool at Damang. Spectral parameters such as the ferrous-iron response, the AlOH/H 2 O absorption depth ratio, and automated mineral identification successfully distinguish metasedimentary and metadoleritic lithologic units at Damang. Systematic variations in these parameters, together with the water/OH absorption depth, both downhole and in three-dimensional models, provide vectors to gold mineralization. The spectral parameters AlOH wavelength and MgOH wavelength are used to define the regolith profile at Damang, throughout which the ferrous-iron response parameter provides a reliable indicator of gold mineralization. All recorded changes in spectral parameters can be linked to sample petrography and are supported by mineral-chemical data. These results show that portable infrared spectroscopy can be used in a variety of roles, including regolith mapping, geologic mapping and logging, and recognition of hydrothermal alteration patterns, as each lithology and alteration style exhibit distinct and identifiable spectral characteristics. These spectrally derived alteration proxies indentify a broader zone of potential gold mineralization than gold grades alone, providing a larger target for exploration. The rapidity of data collection and ease of analysis of spectral data make infrared reflectance spectroscopy a useful methodology that can be readily incorporated into both preexisting and established exploration programs in other tropical terrains.

Abstract Mineral deposits with large inventories or high grades of silver are found in four genetic groups: (1) volcanogenic massive sulfide (VMS), (2) sedimentary exhalative (SEDEX), (3) lithogene, and, (4) magmatichydrothermal. Principal differences between the four groups relate to source rocks and regions, metal associations, process and timing of mineralization, and tectonic setting. These four groups may be subdivided into specific metal associations on ternary diagrams based on relative metal contents. The VMS deposits rarely contain more than 15,600 t Ag (500 Moz). Grades average 33 g/t Ag. Variable Ag-Pb-Zn-Cu-Au ± Sn concentrations are interpreted as having been derived both from shallow plutons and by leaching of the volcanic rock pile in regions of thin or no continental crust and the mineralization is syngenetic. Higher silver grades are associated with areas of abundant felsic volcanic rocks. The SEDEX deposits rarely contain more than 15,600 t Ag (500 Moz). Grades average 46 g/t Ag. Silver, lead, and zinc in relatively consistent proportions are leached from sedimentary rocks filling rift-related basins, where the continental crust is thin, and deposited as syngenetic to diagenetic massive sulfides. Pre-mineral volcanic rocks and their detritus may occur deep within the basin and gold is typically absent. Lithogene silver-rich deposits are epigenetic products of varying combinations of compaction, dewatering, meteoric water recharge, and metamorphism of rift basin-related clastic sedimentary and interbedded volcanic rocks. Individual deposits may contain more than 15,600 t Ag (500 Moz) at high grades. Ores are characterized by four well-defined metal associations, including Ag, Ag-Pb-Zn, Ag-Cu, and Ag-Co-Ni-U. Leaching, transport, and deposition of metals may occur both in specific sedimentary strata and other rock types adjacent to the rift. Multiple mineralizing events lasting 10 to 15 m.y., separated by as much as 1 b.y., may occur in a single basin. Gold is absent at economic levels. The magmatic-hydrothermal silver-rich deposits are epigenetic and related to cordilleran igneous and volcanic suites. Six magmatic-hydrothermal districts each contain more than 31,000 t Ag (1,000 Moz) with grades of veins >600 g/t Ag. Mineralization occurs as veins, massive sulfides in carbonate rocks, and disseminated deposits including porphyry silver deposits, a proposed exploration model. Most deposits are epithermal with low-sulfidation alteration assemblages. Deposits are often telescoped and well-zoned. All large and high-grade magmatic-hydrothermal deposits appear confined to regions of relatively thick continental crust above Cenozoic consuming plate margins on the eastern side of the Pacific Rim. Silver in these deposits may be partly derived by hydrothermal leaching of rocks under or adjacent to the deposits. Specific metal associations in SEDEX and lithogene deposits may reflect confinement of fluid flow to and derivation of metals from specific source rock types. Variable metal associations in VMS and magmatichydrothermal deposits may reflect derivation of metals from a more diverse suite of rocks by convecting hydrothermal systems and processes related to the generation of magma. The discovery rate for silver-rich deposits has accelerated during the past decade, with new deposit types, metal associations, and exploration models being identified that provide numerous exploration and research opportunities.

Abstract The 2003 to 2006 discovery of the important Juanicipio-Valdecañas vein zone in the western part of the famous Fresnillo silver district was the result of 10 years of research and geologic fieldwork predicated on the idea that the Fresnillo district was much larger than conventionally believed. High levels of vein exposure, extensive cover, and weak surface geochemical signatures have historically hampered exploration in the district and arguably resulted in a limited perception of the overall system. Nevertheless, new exploration success was the consequence of a continuation of discovery history in the district, just with a wider application of established concepts and broader scope of vision. Information from published studies and reconnaissance field observations from the district were combined with improved academic understanding of the importance of variations in boiling levels in low-sulfidation epithermal vein systems to generate the concept that the extensive alteration zone, 5 km west of the Fresnillo mine, concealed additional veins. However, detailed geologic mapping, geochemical sampling, and geophysical surveys were required to convince investors that the concept was worth pursuing. The spectacular successes validate the work and demonstrate that several mineralization centers, with different boiling depths, were active in different places at various times in the district. This creates new exploration possibilities for the Fresnillo district and suggests that seeking vertically and laterally shifting mineralization centers in any large epithermal vein camp may be fruitful, particularly where mining and exploration have been confined to relatively limited areas or within vertically restricted zones.

Abstract The La Pitarrilla Ag-Zn-Pb deposit, Mexico, is hosted by Cretaceous, Eocene, and Oligocene strata that record a complex volcano-sedimentary, structural, and hydrothermal history. Deformed Cretaceous rocks form the basement to unconformably overlying Eocene and Oligocene volcanic strata. The Eocene volcaniclastic strata were derived from arc volcanism and from the erosion of subaerial arc volcanoes, with the clastic material transported by sedimentary gravity flows and deposited into a below storm wave base basin that developed within a back-arc extensional setting. Uplift of the arc during the Eocene was accompanied by extension and voluminous silicic pyroclastic volcanism, which is manifested by ignimbrite and pyroclastic surge deposits dated at 49.8 ± 1.0 Ma. Erosion during the Eocene and early Oligocene was accompanied or followed by northeast-and north-northwest–trending faulting, the emplacement of rhyolitic and andesitic sills and dikes, and a 31.59 ± 0.52 Ma rhyolitic dome. The La Pitarrilla Ag-Zn-Pb deposit is characterized by iron oxide- and sulfide-associated mineralization, whichch10 defines a vertically stacked mineralized system centered on rhyolitic dikes and sills that constitute the feeder system for an early Oligocene volcanic center manifest by a rhyolitic dome. The sulfide-associated mineralization is rooted in the basement Cretaceous sedimentary strata and is represented by an areally restricted but vertically extensive zone of disseminated and vein-hosted Ag-Zn-Pb (-Cu-As-Sb) sulfide mineralization and strata-bound replacement mineralization within conglomerates that occur at the Cretaceous-Eocene unconformity. The sulfide mineralization extends upward into the overlying Eocene and Oligocene volcaniclastic strata and rhyolitic sills, where it abruptly grades into a laterally more extensive, supergene zone of disseminated iron oxide-associated mineralization that replaced the sulfides. The main Ag-Zn-Pb mineralization event is interpreted to have occurred during or after emplacement of the early Oligocene rhyolitic dome.

Abstract Corani is a significant recently discovered large silver and base metal deposit situated within the Corani mining district in the central Andes of Peru. The deposit is located 200 km northwest of the city of Puno, between 4,675 and 5,260 m in elevation, and includes 12 mineral claims covering an area of 5.7 km 2 . The silver, lead, and zinc resources represent low- to intermediate-sulfidation–style epithermal mineralization hosted within 23 Ma rhyolitic crystal lithic tuffs. Potentially economic epithermal gold mineralization also occurs within the district and requires further exploration. Previous district production includes small-scale underground antimony mining in the 1940s and selective mining from high-grade silver veins during the 1960s. During the 1990s, limited drilling focused upon an epithermal gold zone at the southern limits of the current Corani deposit. Rio Tinto staked the district in 2003 as a porphyry copper system. The presence of such a system remains a possible deep source for the recognized epithermal mineralization. Bear Creek Mining Corporation acquired the district from Rio Tinto in 2005 and drilled the first discovery holes in 2006. To date, more than 93,640 m of diamond drilling has been completed in conjunction with extensive studies in order to understand the controls of the base and precious metal mineralization. Mineralization occurs as stockwork veins, fracture coatings, and breccias localized within a westerly-dipping listric fault complex resulting from regional extension. These ore-hosting structures cut the rhyolitic tuffs of the Quenamari Formation. Dominant mineral phases include quartz, barite, pyrite, sphalerite, galena, hematite, and freibergite. The total mineable reserve is 139.6 million tons (Mt) averaging 57.5 g/t Ag, 0.94 percent Pb, and 0.46 percent Zn, thus containing 8.03 t Ag (258 million ounces (Moz)), 1.31 t Pb (2.9 billion lbs.), and 0.65 t Zn (1.4 billion lbs.) recovered into concentrates. In addition, 145 Mt of lower grade material is maintained in resources. An understanding of the distribution of mineralization styles, defined by using metallurgical testing, mineragraphic analysis, and detailed core logging, is critical in unlocking the economic potential of the deposit and developing a three-dimensional model for mining. Of particular importance to future development is the overprinting by supergene mineral assemblages, including complex lead and/or barium phosphates and iron and/or manganese oxides.

Abstract The Navidad silver deposits in the Somuncura Massif of Patagonia in the province of Chubut, Argentina, were discovered in 2002 by following up results of a geochemical stream-sediment survey. The multisample Ag-Pb-Zn anomaly was recognized by the company that undertook and interpreted the survey, but it recommended that the anomaly not be staked. The anomaly was rediscovered in data passed to another company on November 28, 2002, and this led to a field examination on December 10, 2002. This quickly confirmed the source of the anomaly in outcropping and subcropping mineralization hosted in Jurassic volcanic and sedimentary rocks. Unlike most other silver occurrences in the region, the Navidad deposits contain no significant gold values and are not vein deposits. Once the initial discovery was made, Navidad was quickly shown to host one of the world’s largest silver resources. It is now recognized to contain 23,359 metric tons (t; 751 Moz) Ag and 1.59 Mt Pb in 201.1Mt of mineralized rock at a grade of 117 g/t Ag and 0.79 percent Pb. Navidad should easily have been found at an earlier date using only simple technology because it is easily accessible and outcropping. However, over the last 1,000 years different groups including indigenous peoples, European settlers, government mineral exploration programs, and finally modern exploration companies, failed to make the discovery due to cultural, not technological, factors. The cultural factors that impeded discovery are varied ranging from the general lack of interest in metallic minerals by early European settlers of Patagonia to the incomplete investigations of the government exploration programs and relatively inaccessible nature of the data they generated. Later corporate explorers emphasized gold, and ignored silver, or focused on remote sensing alteration mineral anomalies which are not well developed at Navidad. Finally, corporate culture including dogmatic model-, and/or commodity specific-, driven exploration instead of a more pragmatic react-to-the-data approach, may have also been a factor.

Abstract The Navidad district, Argentina, is an advanced exploration project with an estimated measured and indicated resource of 19,670 tons (t; 632 million ounces) Ag, 1.32 Mt Pb, and significant contents of Cu and Zn in eight separate to semicontinuous deposits. The mineralization is hosted by volcanic and sedimentary rock units of the Middle to Late Jurassic Cañadón Asfalto Formation and formed in an active continental rift environment. Navidad is an intermediate sulfidation epithermal silver deposit, which is located at the center of a northeast-trending belt of polymetallic deposits within the northern Patagonia Massif. The generally stratiform mineralization is localized within permeable horizons that are abruptly capped by impermeable units. The mineralization is zoned at the district scale about well-defined hydrothermal upflow zones defined by phreatic breccia and greater intensity of intermediate argillic alteration mineral assemblages. Proximal mineralization to the upflow zones is characterized by fine-grained, Ag- and Pb-bearing pyrite, complex sulfide textures, and Ag/Pb ratios as low as 6.6. Proximal mineralization precipitated due to rapid cooling of the hydrothermal solutions and by mixing between acid-sulfate and bicarbonate waters, as evidenced by the presence of hypogene kaolinite, alunite, barite, and abundant calcite. Distal deposits were formed following the lateral migration of hydrothermal fluids for as much as several kilometers from the central upflow zones. Distal mineralization is characterized by fine- to medium-grained sulfide mineralization with acanthite as the most important silverbearing mineral in a medium- to coarse-grained calcite-barite gangue. The Ag/Pb ratio for distal mineralization is as high as 187.8. No gold occurs within the Navidad deposits.

Abstract The Centinela district is a 40-km-long segment of the late Eocene to early Oligocene porphyry copper belt of northern Chile. The main mineralization styles in the district include porphyry copper and associated skarns with gold and molybdenum credits, oxide copper in the form of either exotic deposits or in situ oxidation zones, and supergene copper sulfide enrichment blankets. Structurally controlled, district-wide porphyry copper mineralization formed between 44 and 41 Ma, contemporaneously with transpressional tectonism along the regional-scale Centinela-Limón Verde fault zone, a splay of the major Domeyko fault system. The Domeyko fault system accommodated tectonic uplift of the Cordillera de Domeyko during early stages of the middle Eocene Incaic orogeny. Several productive porphyry copper systems in the district were emplaced syntectonically along reverse- and oblique-slip faults that controlled the shape and attitude of both the dominantly dikelike intrusions and associated alteration mineralization. All the porphyry copper deposits in the district evolved from early potassic alteration with copper mineralization present as chalcopyrite and bornite, through sericite-chlorite alteration with chalcopyrite and pyrite, to late, overprinting pyritic sericitic alteration. Some systems developed advanced argillic lithocaps in their upper parts. Much of the copper was introduced with the potassic alteration, and all the gold-rich deposits display abundant hydrothermal magnetite and a positive correlation between copper and gold. Supergene processes gave rise to oxide copper zones above pyrite-poor, chalcopyrite- and bornite-bearing protore and to chalcocite blankets over less reactive, sericitic zones containing pyrite and chalcopyrite. Coeval regional uplift, erosion, and exhumation resulted in widespread piedmont gravel deposition broadly contemporaneous with the supergene activity, with formation of exotic copper deposits alongside the shallow, actively oxidizing, pyritic parts of porphyry copper deposits. A long exploration history characterizes the Centinela district, from initial prospecting in the 1870s to formal exploration programs by major mining companies approximately 100 years later. However, initial appreciation of the district potential resulted from the discovery of the El Tesoro and Tesoro NE exotic copper deposits between 1990 and 1993 and was confirmed with the discovery of the Esperanza and Telégrafo porphyry copper-gold deposits in 1999 and 2001, respectively. Approximately 18 years of persistent exploration by Antofagasta Minerals resulted in an order-of-magnitude expansion of the district mineral inventory from an original 120,000 metric tons (t) of contained copper at El Tesoro in 1990 to the presently known resources of ~20 Mt of contained copper and ~560 t (18 Moz) gold in the several main deposits. Commercial copper production commenced at the El Tesoro SX-EW plant in 2001, with a cumulative output through 2009 of 790,000 t of cathode copper. With production start-up at Esperanza in late 2010, the district is expected to produce an additional 195,000 t Cu, 7 t (225,000 oz) gold, and 35 t (1,130,000 oz) silver per year in concentrates, whereas Mirador will contribute an additional 95,000 t of cathode copper upon attaining full production in 2012. Early stages of exploration within the district targeted exposed mineralization that had undergone historic, small-scale surface mining. The more recent exploration incorporated geologic mapping and conceptual modeling to test for both extensions to the known mineralization and new deposits as the company’s land holdings expanded. Recent efforts successfully targeted blind mineralization under extensive postmineralization gravel cover through interrogation of comprehensive data sets, use of empirical geologic models, and incorporation of conventional and relatively new geophysical tools to assist with modeling of bed-rock geology. However, the most recent discoveries employed information from nearby drill holes that were completed by previous explorers more than 23 years earlier. Discovery and wealth creation in the Centinela district are considered as products of the company’s long-term commitment to exploration and the exploration team’s view that the district had unfulfilled potential. Persistence overcame at least two cycles of economic turmoil. The most recent discoveries occurred approximately 17 years after initial discovery of El Tesoro and more than 25 years after formal exploration by major mining companies commenced in the district.