The La Pitarrilla Silver-Zinc-Lead Deposit, Sierra Madre Occidental, Mexico: A Description of the Mineralization and a Reconstruction of Its Volcano-Sedimentary Environment
Claire M. J. Somers, Harold L. Gibson, Ron Burk, 2010. "The La Pitarrilla Silver-Zinc-Lead Deposit, Sierra Madre Occidental, Mexico: A Description of the Mineralization and a Reconstruction of Its Volcano-Sedimentary Environment", 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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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.
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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.