Crustal shortening and porphyry copper mineralization in the Laramide arc and superimposed extension: Introduction and themes

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.

Cenozoic structural evolution of the Catalina metamorphic core complex and reassembly of Laramide reverse faults, southeastern Arizona, USA

“…2), Fajardo (2015) demonstrated that multiple generations of originally steeply west-dipping curviplanar normal faults progressively tilted the area ~65° E. A similar style of extension has been documented in the Tortilla Mountains (Fig. 1, surrounding areas 1-3) around Romero Wash (Favorito and Seedorff, 2017), Hackberry Wash (Maher et al, 2004;Seedorff et al, 2019a), Kelvin-Riverside (Nickerson et al, 2010), Ray (Barton et al, 2005), and west of Ray near Walnut Canyon (Favorito and Seedorff, 2020). In each of these locales, detailed documentation of crosscutting relations between Cenozoic normal faults and related synextensional strata demonstrate well-defined magnitudes of tilting associated with specific generations of normal faults.…”

“…In several areas along the San Pedro Valley, authors have interpreted extension in terms of domino-style block faulting (Barton et al, 2005;Nickerson et al, 2010;Fajardo, 2015;Favorito and Seedorff, 2017;Favorito and Seedorff, 2020). In this interpretation, multiple generations of originally high-angle normal faults have rotated to lower angles and progressively tilted and extended blocks of the upper crust (e.g., Proffett, 1977;Gans and Miller, 1983;Mandl, 1987;Richardson et al, 2019;Seedorff et al, 2019a). Conversely, variations on low-angle detachment and rolling hinge models (e.g., Wernicke, 1981;Buck, 1988;Wernicke and Axen, 1988;Whitney et al, 2013) typically have been used in recent decades to explain intense crustal extension and related ductile tectonic fabrics in the Santa Catalina and Rincon Mountains (Dickinson, 1991; (Favorito and Seedorff, 2017).…”

The role of crustal anatexis in porphyry copper ore formation during flat-slab subduction: Insights from the Laramide Belt, SW USA

“…This is consistent with paleo-crustal thickness estimates derived from Sr/Y and La/Yb ratios of Laramide granitic rocks 65 and indicates the crust was thinned by a factor of two during subsequent Basin and Range extension. Notably, these findings argue against the traditional premise that Laramide shortening did not result in significant crustal thickening in Arizona 14,15 . We suggest this is because the thrust belt was significantly dismembered by Basin and Range extension and that many of the Laramide thrusts are not easily recognised or appreciated.…”

“…73-53 Ma 7,8,13 (Table S1); and (3) contractional structures active between ca. 80-60 Ma 14,15,16 . All these features are concentrated in a series of NW-SE trending belts (the Colorado Plateau Transition Zone, CPTZ; Fig.…”