Isotopic and geochemical characteristics of Laramide magmatic systems in Arizona and implications for the genesis of porphyry copper deposits

University of British Columbia, Department of Earth and Ocean Sciences, Vancouver, BC, Canada
University of Arizona, United States

Major element, trace element, and Nd-Sm and Rb-Sr isotope data are described from igneous rocks in 11 multiphase magmatic complexes of Laramide age in Arizona. Eight complexes contain barren stocks as well as productive plutons associated with the formation of porphyry base metal deposits, whereas in three complexes porphyry orebodies have not been recognized. Most of the complexes evolved through similar stages. Early, dominantly intermediate volcanic rocks were intruded by multiphase complexes temporally characterized by increases in SiO ₂ , changes in the ferromagnesian mineral assemblage, increasingly porphyritic textures, and a progression from barren to subproductive to productive intrusions. Within complexes the sequence is independent of both the absolute age of initiation of local magmatism and its duration. Analytical data and field relationships are combined to show elements of the petrogenetic history of these complexes, potential relationships between productive and barren magmas, and factors which may have influenced formation of the characteristic magmatic hydrothermal sequence. The isotopic composition of Laramide igneous rocks forms an array from epsilon _NdT and ( ⁸⁷ Sr/ ⁸⁶ Sr) _initial values of 0 and 0.704. to -14 and >0.710, respectively. Early volcanic rocks have a more primitive signature consistent with retention of a significant mantle component. Subsequent intrusions were derived predominantly from isotopically heterogeneous. Precambrian lower crust but probably also contain an undefined amount of mantle component. Individual complexes do not exhibit any consistent isotopic trends over time and unique types of source rocks are not indicated for productive magmas. In contrast, rare earth elements (REE) in several complexes display temporally systematic behavior. Progressively younger intrusions in a given complex follow a path of decreasing concentration of REE, steepening profiles, greater upward concavity in heavy REE profiles, and changes in the Eu anomaly from negative to either markedly less negative or positive. The REE data from individual complexes suggest an increasing petrogenetic involvement of amphibole, either in the restite of the source region or as a fractionating phase, and similar paths of petrogenetic evolution in spite of chronologic and trace element evidence which argues against completely comagmatic development. Productive and barren intrusions cannot be clearly distinguished by the isotopic data, but the systematic REE trends parallel increasing productivity of magmas in individual complexes. Models which reconcile isotopic variation, systematic REE behavior, protracted local but largely noncomagmatic magmatism and the increasing development of porphyry mineralization include: (1) progressive metasomatism of the crustal component, (2) increasing assimilation of amphibolitic crust by mantle basalts, and (3) migration of crustal anatexis from deep anhydrous rocks to shallower amphibolites. The relative importance of these processes cannot yet be clearly assessed from the data, but a plausible metallogenic impact of each would be upon the concentration, and possibly the composition, of magmatic volatiles in the primary melts and/or during subsequent fractionation. In this light, existing models which propose that the principal control upon intrusion productivity is a balance between crystal fractionation and the timing of saturation and release of magmatic hydrothermal fluids may be strongly influenced by processes operative either in their source regions or during their interaction with the crust. Initiation of such processes may be a response to abrupt changes in tectonic regime in the volcanic-are environment.

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