ABUNDANCE AND BEHAVIOR OF ORE AND OTHER METALS IN LATEST CRETACEOUS TO EARLY TERTIARY ("LARAMIDE") METALUMINOUS AND PERALUMINOUS GRANITOIDS, SOUTH-CENTRAL ARIZONA AND NORTH-CENTRAL SONORA

The well-known association of numerous porphyry Cu deposits in the southern Arizona region with granitoids of a single characteristic age, Late Cretaceous to early Tertiary ("Laramide"), suggests that these plutonic rocks might be compositionally special. However, our new data corroborate several previous studies in showing that the Laramide granitoids constitute an ordinary metaluminous, compositionally expanded, high-K calcalkaline suite. Our database comprises some 85 analyses of unmineralized and minimally altered Laramide metaluminous-suite granitoids (MAG) from south-central Arizona and north-central Sonora (southwest of Tucson, north of Caborca). For comparison, we also have about 80 analyses of early Tertiary (late Laramide) peraluminous leucogranites (PALG), generally not associated with any ore deposits.

Despite their common association with porphyry Cu deposits or prospects, the MAG are unexceptional in their Cu concentrations. At the high-silica end of their compositional range, the MAG contain little or no more Cu than the PALG. The MAG as a whole do have higher Cu contents, but only because they are more mafic. However, Cu abundances (median 11 µg/g, geometric mean 16 µg/g) in the MAG are normal for rocks of their composition, and similar to average abundances in the upper continental crust (published values 14 – 25 µg/g). Any special endowment in Cu is subtle or, more likely, absent. The MAG also have unremarkable concentrations of the other nine period-4 transition metals (PFTM; Sc – Zn), Mo, W, Sn, Pb, Sb, Nb, Ta, Zr, Hf, REE, Th, and U. In both their major and trace element compositions, the MAG are quite ordinary granitic rocks.

The PALG are characterized by very low concentrations of Cu, other PFTM, Mo, W, and Sb; average levels of Sn and Pb; significant enrichment (3 – 4 times average upper crust) in Nb and Ta; and low Nb/Ta. Only in the case of Nb and Ta in the PALG do the Laramide granitoids of south-central Arizona and north-central Sonora have any special endowment of any of the metals examined.

Published Nd, Sr, and Pb isotopic data show that both the MAG and PALG are largely or entirely melts or assimilates of lower crust. The nominal abundance of Cu in the MAG and low abundance of Cu in the PALG give no support to the idea that the continental crust of the southern Arizona region is specially enriched in Cu.