ABSTRACT The Laramide continental arc formed in southwestern North America at about the same time the Sierra Nevadan arc was shutting down, and the Laramide arc was active concurrent with the progress of the Laramide orogeny, from ca. 80 Ma to ca. 45 Ma. East-central Arizona offers an excellent opportunity to explore aspects of tectonics, structural geology, magmatism, and hydrothermal systems in a segment of the Laramide arc that is exceptionally well endowed with porphyry copper deposits. The structure of this region is especially complicated, with multiple generations of normal faults commonly superimposed on originally moderate-angle reverse faults with associated fault-propagation folds. A large new porphyry copper deposit, Resolution, was discovered near Superior in the mid-1990s. The discovery started a new round of development in the mining life cycle at the Resolution deposit; in the region, it contributed to copper exploration again becoming vigorous in the last decade. In the years since discovery of Resolution, important new scientific insights have been gained, including at the regional scale. Post-ore crustal extension exposed multiple levels of Laramide and older igneous and hydrothermal systems at the surface where they can be more easily mapped and sampled, and palinspastic reconstructions of post-mineral normal faulting permit the exposures to be restored to their original positions. The porphyry-related products that are observed at higher levels include local advanced argillic alteration and Cordilleran-style veins and associated mantos, such as at the Magma mine, Resolution deposit, and Old Dominion mine in the shallowest levels of the Superior-Globe-Miami area. Most porphyry copper ore bodies were developed at intermediate depths, where porphyry intrusions exhibit sericitic and potassic alteration and carbonate rocks were converted to skarn, such as in the heart of the Miami-Inspiration, Resolution, Ray, and Christmas deposits. Plutonic rocks are exposed at deeper paleodepths, where pegmatites, quartz veins, and greisen muscovite are locally observed, especially directly beneath porphyry copper orebodies, as in the Schultze and Granite Mountain plutons. Likewise, sodic-calcic alteration may be developed on the deep flanks of porphyry systems, such as adjacent to the Tea Cup pluton. Subsequent Cenozoic extension variously buried or exhumed the hypogene portions of these hydrothermal systems, leading to the development of various supergene products, both in situ and exotic.

Abundant pumice fragments occur in the Apache Leap Tuff of east-central Arizona, an ash-flow sheet with a maximum thickness of 600 m and a K-Ar age of 20 m.y. The amount of flattening of pumice fragments is widely variable at any particular locality, but systematic measurements show that the mean degree of flattening, defined as the “flattening ratio,” steadily increases from the top downward into the body of the sheet. Ultimately the fragments are so compacted that they lose their identity. On a logarithmic scale the plot of flattening ratios is approximately linear relative to depth of burial. The uniform downward increase in flattening combines with evidence obtained from zoning and specific gravity characteristics to show that most of the deposit is a single cooling unit. Because of the uniform trend, flattening also provides a guide to the original thickness of overlying tuff at localities at which fragments can be measured. This permits the development of stratigraphy for the seemingly uniform deposit and provides a means to estimate pre-erosion thickness of the ash-flow sheet and the amount of stratigraphic throw on faults. A mining company used flattening ratios to predict successfully the ash-flow thickness cut by a new shaft. Postemplacement crystallization and diagenetic processes have greatly reduced the initial porosity of the deposit, and present porosity values erroneously indicate a considerably higher degree of welding than is inferred from deformation of the pumice fragments. It seems that in deposits where crystallization and diagenesis have been significant, flattening ratios of pumice fragments may be a better guide than porosity to the degree of welding that occurred during cooling of the deposit. The change of flattening ratio with depth can also serve as an approximate guide to the relative viscosity of pumice during emplacement. Viscosity is determined chiefly by temperature, chemical composition, volatile content, and crystallinity. The downward change in flattening ratio in the Apache Leap Tuff is gradual, indicating a relatively high viscosity. By assuming high volatile content and low groundmass crystallinity at the time of emplacement, the high viscosity can be ascribed to the combined result of nonperalkalic chemical composition and relatively low temperature.