Discovery History of a Giant, High-Grade, Hypogene Porphyry Copper-Molybdenum Deposit at Los Sulfatos, Los Bronces-Río Blanco District, Central Chile
Vicente Irarrazaval, Richard H. Sillitoe, Alan J. Wilson, Juan Carlos Toro, William Robles, Graeme D. Lyall, 2010. "Discovery History of a Giant, High-Grade, Hypogene Porphyry Copper-Molybdenum Deposit at Los Sulfatos, Los Bronces-Río Blanco District, Central Chile", 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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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.
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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.