Characterization and Reconstruction of Multiple Copper-Bearing Hydrothermal Systems in the Tea Cup Porphyry System, Pinal County, Arizona
Phillip A. Nickerson, Mark D. Barton, Eric Seedorff, 2010. "Characterization and Reconstruction of Multiple Copper-Bearing Hydrothermal Systems in the Tea Cup Porphyry System, Pinal County, Arizona", 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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This study exploits a cross-sectional view of the Laramide magmatic arc in the northern Tortilla Mountains, central Arizona, that was created by tilting during severe Tertiary extension of the Basin and Range province. Building upon earlier work, we combine the results of geologic mapping of rock types, structures, and hydrothermal alteration styles, with a palinspastic reconstruction, to provide a system-wide understanding of the evolution of the composite magmatic and hydrothermal Tea Cup porphyry system.
Geologic mapping revealed the presence of at least three, and possibly four, mineralizing hydrothermal systems in the study area that are associated with widespread potassic, sericitic, greisen, sodic (-calcic), and propylitic alteration. The alteration envelops both porphyry copper and porphyry molybdenum (-copper) mineralization. Two areas flanking compositionally distinct units of the composite Tea Cup pluton are characterized by intense potassic and sericitic alteration. Intense alteration and mineralization akin to iron oxide-copper-gold systems was recognized in several areas. The U-Pb dating of zircons from porphyry dikes suggests that hydrothermal activity in the study area was short lived (~73–72 Ma). Subsequently, between ~25 and 15 Ma, the Tea Cup porphyry system was tilted ~90° to the east and extended by >200 percent due to movement on five superimposed sets of nearly planar normal faults. Each fault set was initiated with dips of ~60° to 70°, but modern dips range from 70° to 15° overturned from the youngest to the oldest set. Tertiary normal faulting resulted in the exposure of pieces of the porphyry system from paleodepths of >10 km.
Palinspastic reconstruction of a ~30-km-long cross section reveals that the Tea Cup pluton formed by sequential intrusion of at least four compositionally distinct units. Each major unit generated its own hydrothermal system. The most intense alteration in each hydrothermal system formed above the cupolas of each major phase of the pluton. Potassic alteration dominates the core of each system, whereas feldspar-destructive acid alteration overlaps with the potassic alteration but also extends to higher levels within each system. Deep sodic (-calcic) alteration overlain by iron oxide-rich chlorite-sericite-pyrite alteration flanks these central systems and generally extends 2 to 4 km away from the center of the hydrothermal systems. Greisen-style alteration was recognized 1 to 2 km beneath the potassic alteration in one porphyry copper system but overlaps and extends above the exposed porphyry molybdenum (-copper) system. Propylitic alteration occurs in a distal position and surrounds the other alteration styles. The alteration mapping, combined with the palinspastic reconstruction, revealed two covered exploration targets centered on intense potassic alteration, demonstrating that palinspastic reconstruction represents a powerful exploration technique in a district with more than 100 years of exploration history.
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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.