Superposed Magmatic and Hydrothermal Systems, and the Evolution of the Laramide Arc and Porphyry Copper Province, Southwestern North America

Abstract

Southwestern North America is unusually rich in Cu resources (>330 Mt) that are hosted in a cluster of porphyry copper deposits. These formed in part of a magmatic arc along the southern margin of the Laramide tectonic province. An unequal distribution of Cu also occurs within the area: individual igneous centers have very different Cu endowments and are variably clustered in space and time. Tilted exposures of the Laramide arc’s upper crust provided by post-mineral extensional deformation expose links between deeper plutonic and shallow volcanic levels, and associated hydrothermal systems. Superposition of contrasting magmatic and hydrothermal systems is common on multiple scales: in individual igneous centers, mining districts, and regionally. Studies in the Tucson area highlight distinctive patterns in these systems, which are then extended to a regional synthesis of the evolution of the porphyry Cu province.The Tucson Mountains contain a late Cretaceous caldera on which both late Cretaceous and Paleocene igneous centers are superposed. The older (~73-72 Ma) Amole center contains a compositionally diverse pluton and voluminous cogenetic volcanic rocks, which are associated with numerous episodes of hydrothermal activity related to circulation of non-magmatic fluids (forming Na-Ca alteration and early IOCG-type epithermal veins and syngenetic hematite-silica), as well as weak porphyry-type magmatic-hydrothermal features. Several small, younger (60-57 Ma) centers contain compositionally homogeneous felsic igneous rocks, relatively minor volcanism, and are associated with better-developed but not economically significant porphyry Cu hydrothermal systems. These are similar in age and style to large porphyry Cu systems in the nearby mountain ranges (e.g., Sierrita, Pima, Rosemont) which have been extensionally dismembered. Extension here is unusual in that it occurred simultaneously in multiple directions. It is reconstructed to show that the porphyry deposits belong to a large, diffuse cluster, superimposed on older calderas and igneous centers resembling those in the Tucson Mountains. Another, still younger (latest Paleocene to Eocene) phase of strongly peraluminous magmatism, apparent mainly in deeper exposures, represents the denouement of arc magmatism. This pattern and geochemical evidence suggest a simple model in which increasing crustal contributions gradually filtered out ascending mafic magmas, and thereby contributed to a shift in magmatic style that is correlated with a decrease in caldera-forming eruptions and increase in porphyry Cu mineralization. A regional synthesis demonstrates coherent patterns in the style of magmatism and Cu mineralization which are analogous to those in the Tucson area. Magmatic patterns adhere to a magmatic facies-like spatial pattern which migrated southeastward over time (10s of m.y.), resulting in superposition of contrasting styles. Generally, along and away from the arc axis, magmatism progressed from northwest to southeast from (1) peraluminous magmatism to (2) felsic calc-alkaline magmatism, to (3) calderas associated with intermediate magmatism, to (4) intermediate magmatism without calderas, to (5) mafic magmatism located on the southern, landward, and locally trenchward periphery of the arc. The migration of magmatic facies is correlated with (1) the evolving geometry of subduction zone, controlled by a buoyant slab segment commonly implicated in the Laramide orogeny, and (2) patterns in exhumation (crustal thickening?). These correlations suggest a variety of tectono-magmatic controls. The large porphyry Cu-Mo deposits which dominate the metal endowment are characteristically located along the main part of the arc, associated with intermediate or felsic magmatism near the transition to peraluminous magmatism. More Au-rich deposits are associated with more mafic magmas and thus typically older or peripheral. Porphyry occurrences are characteristically clustered, perhaps increasingly so with time. This study highlights controls on metallogeny at multiple scales, and provides a framework which may help predict some key characteristics of undiscovered igneous centers and related ore deposits.