Geochemical variations of precursor and ore-related intrusive rocks associated with porphyry copper deposits in Sonora, northwestern Mexico

The present study analyzes the geochemical composition of samples of both, pre- and ore-related intrusive rocks collected from five porphyry copper deposits of Sonora, Mexico. These deposits are included in the porphyry copper belt of the western North American Cordillera, and particularly in the great porphyry copper cluster of Arizona, New Mexico and Sonora, recognized as one of the most important copper provinces at world level. The ore systems were emplaced during the Late Cretaceous-Eocene, under very particular geotectonic conditions. The sampled sites are El Pilar, Buenavista del Cobre, La Caridad, Suaqui Verde and Piedras Verdes. This work focuses on an evaluation of major and trace element changes in the interphase of the late stage cooling of the main porphyry copper associated pluton and the extraction of the mineralizing porphyritic stock. The age variation between each pair of samples are normally less than 2 Ma and of ~5 Ma in Buenavista del Cobre, the largest deposit in Sonora. The results indicate that both intrusive pulses are very similar in most senses, suggesting a calc-alkaline source linked to a magmatic arc emplaced in continental crust above the North America/Farallon subduction zone. Major oxide elements and most trace elements suggest that the ore-related stocks are relatively more differentiated than the precursor plutons. The most remarkable findings indicate that the REE were differentially fractionated, showing a regular decrease of these elements in the porphyritic phase. This is coupled with an inversion of the Eu anomaly, from clearly negative in the precursor plutons to positive in the ore-related stocks. Moreover, there is a relatively more concave shape in the HREE segment of the chondrite-normalized patterns. Although preliminary, we conclude that all these features suggest that the conditions of the magma during the stage that generated the mineralized porphyritic stocks occurred in equilibrium with a mineral association including hornblende-clinopyroxene-sphene, and a change in the oxidation conditions leading to a more reducing environment. This conclusion appears to be also applicable for the porphyry copper deposits of Arizona, in which the elemental spectra have a similar behavior.