The Role of Geochemistry in Andean Copper Discoveries
David L. Kelley, Chris Benn, K. Brock Riedell, Teresa Johnson, 2010. "The Role of Geochemistry in Andean Copper Discoveries", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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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.
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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
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 oreseeable 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, andpredict 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 for the massive increases in metal consumption that world population growth, rising living standards, and rapid industrialization and urbanization in China, India, and other emerging markets appear to portend. For example, Rio Tinto's projections suggest that "by 2030 the additional supplyrequired will be equivalent to replicating the iron ore output of the Pilbara region of Australia every five years, adding another aluminium production complex the size of Canada's Saguenay every nine months, and developing another copper mine the size of Escondida in Chile each year. Future energrequirements are such that an entire Hunter Valley coal supply chain needs to be created each year plus a uranium mine the size of Ranger every four years" (Albanese, 2010, p. 7). Clearly, the exploration business has to become increasingly effective if it is to rise to the challenge of finding mineral resources of the right caliber to assure that this burgeoning demand can be adequately satisfied.