The Giant El Teniente Breccia Deposit: Hypogene Copper Distribution and Emplacement
M. Alexandra Skewes, Alejandra Arévalo, Ricardo Floody, Patricio H. Zuñiga, Charles R. Stern, 2002. "The Giant El Teniente Breccia Deposit: Hypogene Copper Distribution and Emplacement", Integrated Methods for Discovery: Global Exploration in the Twenty-First Century, Richard J. Goldfarb, Richard L. Nielsen
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The giant El Teniente copper-molybdenum deposit, located in the Andes of central Chile, is one of the world's largest known copper deposits, containing estimated resources of >75 × 106 t of fine copper in ore with grades greater than 0.67 percent. El Teniente has been described in the past as a porphyry deposit developed around a Pliocene dacite porphyry stock, with 80 percent of its copper mineralization hosted in Miocene andesites. However, new mapping—both regional and in underground mine workings—along with petrological studies, indicates that El Teniente, like the other giant Miocene and Pliocene copper deposits in central Chile, is actually best classified as a breccia deposit. Most of the high-grade hypogene copper at El Teniente occurs in and surrounding multiple magmatic-hydrothermal breccia pipes. Mineralized breccia complexes, with copper content >1 percent, have vertical extents of >1.5 km, and their roots are as yet unknown. These breccias are hosted in a pervasively biotite-altered and mineralized mafic intrusive complex composed of gabbros, diabases, and porhyritic basalts and basaltic andesites, and not in andesite extrusive rocks. The multiple breccias in El Teniente include copper- and sulfide-rich biotite, igneous, tourmaline, and anhydrite breccias, generated by the exsolution of magmatic fluids from cooling plutons, and also magnetite and rock-flour breccias. Surrounding biotite breccias, a dense stockwork of biotite-dominated veins has produced pervasive biotite alteration, and copper mineralization characterized by chalcopyrite >> bornite + pyrite. Later veins, with various proportions of quartz, anhydrite, sericite, chlorite, tourmaline, feldspars, and copper sulfide minerals, formed in association with the emplacement of younger breccias and felsic porphyry intrusions. These generated sericitic alteration in the upper levels of the deposit, and in some cases contributed more copper to the deposit, but in other cases eliminated or redistributed preexisting mineralization. Both the Teniente Dacite porphyry and the central rock-flour breccia of the Braden pipe, the dominant lithostructural unit in the deposit, are copper poor. Their emplacement at a late stage in the development of the deposit created a relatively barren core, surrounded by a thin (~150-m) zone of bornite > chalcopyrite, in the larger main area of chalcopyrite-rich, biotitealtered mafic rocks and mineralized breccias.
The multistage development of breccia emplacement, alteration and copper mineralization at El Teniente occurred over a time span that was greater than 2 m.y., between >6.4 and 4.4 Ma, at the end of a more than 10-m.y. episode of Miocene and Pliocene magmatic activity, and just prior to the eastward migration of the Andean magmatic arc as a consequence of decreasing subduction angle. Decreasing subduction angle also caused crustal thickening, uplift and erosion, resulting in telescoping of the various breccias and felsic intrusions in the deposit. El Teniente is located at the intersection of major north-south, northwest-southeast, and northeast-southwest Andean structures, but what actually focused magmatic activity and mineralization at this one locality for so long remains an unsolved problem, the solution of which would provide an important tool for exploration of similar giant deposits.
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Integrated Methods for Discovery: Global Exploration in the Twenty-First Century
The dominant forces that affect the mining industry in today's economy are globalization, entrepreneurship, and unprecedented minerals availability. The current malaise and diminished economic importance of the base and precious metal mining industry in the world economy are the result of ongoing, longterm trends. Shifting market forces within a changed geopolitical landscape have resulted in excess supplies and reduced demands for high-unit-value mineral products and for geologists and engineers in the exploration and mining industries. Although these changes are still evolving, professionals and the mining companies that employ them must realize that these changes are irreversible.
Mining has five dominant characteristics: (1) it is essential to society, (2) it is effective in keeping society supplied with abundant, low-cost mineral products, (3) historically, it has been wealth destructive for investors, (4) it has been subsidized, and (5) it is shaped by social and political forces. What is different now is that instead of being subsidized, the industry is being handicapped by governments and abandoned by investors. The industry has reacted to a marketing problem with an inappropriate strategy of cost reduction and increased production. The increased supply has resulted in decreased prices. Mining companies are earning low or negative rates of return.
Remedies for the larger mining companies and the high-unit-value metals businesses include: Exemplary behavior to regain the trust of the public, governments and investors; Continued consolidation to become sector leaders; Profit through value-added vertical integration and direct marketing to consumers Development of innovative consumer-based financing mechanisms, especially for exploration ventures Individuals and small groups that have been displaced from the mainstream must become entrepreneurs. They must engineer their own survival by shifting careers or by finding, developing, and capitalizing on exploration and production opportunities that are unrecognized by, or are too small for, the major companies.
The mining industry is here to stay. It is too necessary to society to be abandoned; however, the future size and shape of the industry will probably be unrecognizable to most of us. It will be leaner and more opportunistic. It will be characterized by entrepreneurial corporations of all sizes that dominate niches and sectors and that anticipate and profit from the changing needs of society