Quantitative mineral resource assessment of copper, molybdenum, gold, and silver in undiscovered porphyry copper deposits in the Andes Mountains of South America

Abstract

Quantitative information on the general locations and amounts of undiscovered porphyry copperresources of the world is important to exploration managers, land-use and environmental planners, economists, and policymakers. This publication contains the results of probabilistic estimates of theamounts of copper (Cu), molybdenum (Mo), silver (Ag), and gold (Au) in undiscovered porphyry copperdeposits in the Andes Mountains of South America. The methodology used to make these estimates is called the "Three-Part Form.” It was developed to explicitly express estimates of undiscovered resourcesand associated uncertainty in a form that allows economic analysis and is useful to decisionmakers. The three-part form of assessment includes: (1) delineation of tracts of land where the geology ispermissive for porphyry copper deposits to form; (2) selection of grade and tonnage models appropriatefor estimating grades and tonnages of the undiscovered porphyry copper deposits in each tract; and (3)estimation of the number of undiscovered porphyry copper deposits in each tract consistent with the gradeand tonnage model. A Monte Carlo simulation computer program (EMINERS) was used to combine theprobability distributions of the estimated number of undiscovered deposits, the grades, and the tonnages ofthe selected model to obtain the probability distributions for undiscovered metals in each tract. Thesedistributions of grades and tonnages then can be used to conduct economic evaluations of undiscovered resources in a format usable by decisionmakers. Economic evaluations are not part of this report. The results of this assessment are presented in two principal parts. The first part identifies 26 regional tracts of land where the geology is permissive for the occurrence of undiscovered porphyrycopper deposits of Phanerozoic age to a depth of 1 km below the Earth’s surface. These tracts are believed to contain most of South America's undiscovered resources of copper. The second part presentsprobabilistic estimates of the amounts of copper, molybdenum, gold, and silver in undiscovered porphyry copper deposits in each tract. The study also provides tables showing the location, tract number, and age (if available) of discovered deposits and prospects. For each of the 26 permissive tracts delineated in thisstudy, summary information is provided on: (1) the rationale for delineating the tract; (2) the rational for choosing the mineral deposit model used to assess the tract; (3) discovered deposits and prospects; (4) exploration history; and (5) the distribution of undiscovered deposits in the tract. The scale used toevaluate geologic information and draw tracts is 1:1,000,000. There are 69 known porphyry copper deposits in the Andes using criteria adopted herein to definea deposit. This assessment estimates that about 145 undiscovered deposits remain. In other words, abouttwice as many new deposits might be found as have already been found. Not only have large quantities ofcopper been discovered, but also even larger quantities of undiscovered copper remain. About 590 million metric tons of copper have been discovered. About 190 million tons of this amount are in 57 deposits of the general porphyry copper type whereas nearly 400 million metric tons are in an additional 12 giantdeposits. This study estimates that there are approximately 750 million metric tons of copper in undiscovered deposits of these two types in the Andes. This undiscovered copper resource is the sum ofthe mean estimated copper in each of 26 tracts. About 470 million metric tons of this copper are estimated to occur in 137 undiscovered deposits in 24 tracts of the general porphyry copper type, plus another 280 million metric tons estimated in 8 undiscovered deposits in 2 tracts of the giant porphyry copper type. Thetotal discovered, mined, and undiscovered copper amounts to an endowment, or grand total, of about 1.3 billion metric tons. The estimates suggest there may be almost 1.3 times as much copper to be found in porphyry copper deposits of the Andes as has already been found. In addition to copper, the undiscovered deposits also contain large estimated amounts ofmolybdenum (20,000,000 metric tons), gold (13,000 tons), and silver (250,000 tons). The estimated undiscovered copper is equivalent to about 80 percent of the world reserve base; molybdenum, 105 percent; gold, 14 percent; and silver, 44 percent. Not all of these undiscovered mineral resources are likely to be available for discovery anddevelopment. Commonly, a significant amount of land delineated as permissive or even favorable for theoccurrence of undiscovered mineral deposits is not available for mineral exploration, discovery, and development because it consists of urban areas, transportation corridors, forest and wildlife preserves,sensitive ecosystems, protected biodiversity areas, sensitive and threatened surface and groundwatersupplies, wilderness areas, national parks, private land where mining is not desired, and many other reasons. Global inventory and analysis are needed of such restrictions and their probable impact on futuremineral supply and costs. The porphyry copper resources reported herein are not evenly distributed throughout the Andesregion. The Chuquicamata tract (SA10a,bPC) and the El Teniente tract (SA14bPC) are remarkable incontaining exceptionally large deposits of discovered, mined, and estimated undiscovered copper that do not conform to grade and tonnage characteristics of the general porphyry copper model appropriate forother Andean tracts. The deposits in these two tracts being significantly larger in tonnage and higher in grade necessitated a new giant model that is more representative of their characteristics. The greatest premining endowment of copper is in Eocene-Oligocene tract SA10a,bPC with about 460 million metric tons, followed by late Miocene-early Pliocene tract SA14bPC with about 220 million metric tons. Thenext largest endowments are in tract SA8PC (a Paleocene–Eocene tract of Chile-southern Peru) containing an endowment of about 98 million metric tons and in tract SA6PC (the middle–late Miocene tract of Peru-Ecuador) with an endowment of about 96 million metric tons. The undiscovered copper remaining to be found in these four tracts is estimated to be: tract SA10a,bPC, 210 million metric tons; tract SA14bPC, 69 million; tract SA6PC, 49 million; and tract SA8PC, 43 million. Density of estimated undiscovered metal in tracts was examined as mean metric tons of metal per km2 in undiscovered deposits. Giant tracts SA10a,bPC and SA14bPC have the highest copper densitieswith about 8,200 and 7,400 metric tons per km2, respectively. The next richest tracts, with just over 2,000 metric tons per km2, are tracts SA12PC and SA14dPC. About 90 percent of the total discovered and estimated undiscovered copper resources in Andean porphyry copper deposits is of Cenozoic age; the remainder is Cretaceous (4%), Jurassic (5%), andPermian (2%) in age. The Cenozoic resources are in tracts with host rock ages of Eocene–Oligocene (39%), Miocene–Pliocene (29%), Paleocene–Eocene (12%), and Miocene (11%). The Eocene–Oligocene and Miocene–Pliocene tracts are dominated by giant tracts SA10a,bPC (includes Chuquicamata deposit) and SA14bPC (includes El Teniente deposit). There appear to have been three approximately 16-million-year-long episodes of porphyry copper deposit formation in the Andes during Cenozoic time: Paleocene–early Eocene, late Eocene–early Oligocene; and middle Miocene–early Pliocene. These three episodes appear to be separated by twointervening quiescent periods of 5 to 9 million years duration in the middle Eocene (44-49 Ma) and late Oligocene–early Miocene (20-29 Ma). These two periods correspond to times of change in Pacific plateboundaries, geometries, and directions and rates of spreading and convergence. There are variations in the geographic trends of magmatic arcs in the southern Andes. Tract SA14cPC, for example, follows a transverse SE-trending band of late Miocene–early Pliocene volcanicrocks and associated upper Miocene porphyry copper deposits, prospects, and other areas of altered and mineralized rocks. The parent magmatic source is approximately perpendicular to the grain of the fault-controlled outcrops of metamorphic basement rocks that reflect the surface geology and could becontrolled by a tear fault in a subducted plate. Southeast-trending tract SA19PC, which extends from theAndes south southeasterly across the Patagonia of Argentina to the Atlantic Ocean, and contains the Bajo de la Leona prospect, also shows that Mesozoic porphyry copper ore-forming processes occurred outside of the present-day trend of the Andes. The results of this assessment study are likely to spur research and exploration activities in theAndes and adjacent regions, which in turn will enable the evaluation and update of regional assessmentsof porphyry copper resources in the future.