Isotopic and Geochemical Characteristics of Laramide Magmatic Systems in Arizona and Implications for the Genesis of Porphyry Copper Deposits

Major element, trace element, and Nd-Sm and Rb-Sr isotope data are described from igneous rocks in 11 multiphase magmatic complexes ofLaramide 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 SiO2, 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 f Laramide igneous rocks forms an array from •dT and (87Sr/S6Sr)•n•al values of 0 and 0.704, to-14 and>0.710, respectively. Early volcanic rocks have a more primitive signature consistent •vith retention of a significant mantle component. Subsequent intrusions were derived predominantly fro