Abstract Fe‐ and Mn‐oxides are common secondary minerals in faults, fractures, and veins and potentially record information about the timing of fluid movement through their host rocks. These phases are difficult to date by most radioisotopic techniques, but relatively high concentrations of U and Th make the (U‐Th)/He system a promising approach. We present new petrographic, geochronologic and thermochronologic analyses of secondary oxides and associated minerals from fault zones and fractures in southeastern Arizona. We use these phases in attempt to constrain the timing of fluid flow and their relationship to magmatic, tectonic, or other regional processes. In the shallowly exhumed Galiuro Mountains, Fe‐oxide (U‐Th)/He dates correspond to host‐rock crystallization and magmatic intrusions from ca. 1.6 to 1.1 Ga. Step‐heating 4He/3He experiments and polydomain diffusion modeling of 3He release spectra on these samples are consistent with a crystallite size control on He diffusivity, and little fractional loss of radiogenic He since formation in coarse‐grained hematite, but large losses from fine‐grained Mn‐oxide. In contrast to Proterozoic dates, Fe‐ and Mn‐oxides from the Catalina‐Rincon and Pinaleño metamorphic core complexes are exclusively Cenozoic, with dates clustering at ca. 24, 15, and 9 Ma, which represent distinct cooling or fluid‐flow episodes during punctuated periods of normal faulting. Finally, a subset of Fe‐oxides yield dates of ca. 5 Ma to 6 ka and display either pseudomorphic cubic forms consistent with oxidative retrogression of original pyrite or magnetite, or fine‐grained botryoidal morphologies that we interpret to represent approximate ages of recrystallization or pseudomorphic replacement at shallow depths.

Sodic-calcic alteration has affected numerous districts with Laramide porphyry systems across Arizona and New Mexico. Previously undocumented Na-Ca alteration has been identified at Charleston, Sycamore Canyon, Silver Bell, and the Ninetysix Hills in Arizona and the Central mining district in New Mexico. These new occurrences are compared to sodic-calcic alteration at the Sierrita-Esperanza deposit (Pima district), the Ajo mining district, the Tea Cup pluton (Kelvin-Riverside district), and the Eagle Pass dike swarm in Arizona. Spatial positions, crosscutting relationships, and mineral assemblages and compositions allow for interpretation of the geochemical conditions of formation and implications for associated hydrothermal processes. Diopside- or andradite-grossular solid solution (Ad(68)-Ad(72))-stable assemblages representative of calcic alteration typically occur in deep and/or distal positions but are less commonly exposed. The sodic-calcic and sodic alteration types, however, occur more proximally to the porphyry orebody. Sodic-calcic assemblages containing Na plagioclase (An(10)-An(33))-Ca amphibole +/- epidote are focused along granite cupolas near the base of orebodies, and these grade into shallower sodic assemblages containing albite (An(00)-An(09))-epidote-chlorite. Mineral composition and fluid inclusion data suggest that Na-Ca plagioclase-bearing assemblages form at higher temperatures (360 degrees-470 degrees C+) than later, lower-temperature albite-epidote assemblages (>250 degrees C). Calculated delta D isotope compositions of fluids in equilibrium with Na-Ca alteration minerals span a broad range (-46 to -1%) and are consistently less negative than magmatic fluids. Where members of the Na-Ca family of alteration assemblages are present in porphyry systems, many lines of evidence-including isotopic compositions of minerals and the geologic alteration zoning patterns-are consistent with an origin by incursion of saline external fluids. Where Na-Ca alteration is present, it is best developed at relatively deep levels of porphyry systems. Several Laramide systems that are well exposed at comparably deep levels lack Na-Ca alteration, probably because sources of saline fluid were absent or hydrologic conditions were unfavorable for the incursion of saline external fluids. Though similar in temperatures, isotopic compositions, mineral assemblages, and zoning patterns, Na-Ca alteration in Laramide systems is volumetrically smaller than in Jurassic systems documented in the western United States, probably because saline external fluids were much less abundant during the Laramide after a tectonically driven excursion in paleoclimate from arid to humid conditions.

This study combines results from reconnaissance-scale mapping of hydrothermal alteration, rock types, and structures to provide a district-scale cross section and associated palinspastic reconstruction of an area with two previously undescribed Laramide (similar to 70 Ma) porphyry systems at Sheep Mountain and Copper Basin (Crown King). Extension at the district scale is placed in an arc-scale context using an original compilation of strike and dip measurements on Tertiary rocks to reconstruct the Laramide porphyry belt prior to extension.The study area contains five sequential, partially superimposed sets of normal faults that are nearly planar where exposed. Dips of all normal faults initiated at 60 to 70 and rotated during slip to angles as gentle as 20. A palinspastic reconstruction reveals that two, spatially distinct hydrothermal systems overlie different cupolas of a Late Cretaceous pluton. Hydrothermal alteration is zoned from grcisen to potassic to transitional greisenpotassic assemblages from deep to shallow structural levels. The reconstruction is used to identify two covered exploration targets. The prospects may be porphyry molybdenum systems of the quartz monzonitic-granitic porphyry Mo-Cu subclass, joining others in an arc that is best known for porphyry copper deposits.The Laramide porphyry belt prior to extension displays a variably well-defined axis, similar to 100 km wide, with gaps and clusters of deposits along its 700-km strike length. The majority of deposits lie along the axis, but others lie in fore- or rear-arc positions. The interpreted preextensional geometry of the Laramide porphyry belt resembles other porphyry belts and the distribution of active volcanoes at convergent margins.

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.