SR, ND, PB ISOTOPES AND AR-AR DATING OF THE “EL ARCO” PORPHYRY COPPER DEPOSIT, BAJA CALIFORNIA: EVIDENCE FOR CU MINERALIZATION WITHIN AN OCEANIC ISLAND ARC

The El Arco porphyry copper deposit (EAPC) is located in central Baja California. Its geographic position makes the EAPC different from other porphyry copper deposits known from mainland Mexico, Arizona, and New Mexico. The EAPC is embedded in meta-volcanic rocks of an Early Cretaceous volcanic arc that was intruded by the Peninsular Ranges batholith (PRB). Primitive island arc-related intrusions to the west are older (140-105 Ma) than more evolved igneous rocks from the eastern zone of the PRB (105-80 Ma). Previously reported K-Ar ages from the EAPC (~93-107 Ma) have indicated a relation to the eastern PRB.

Cu mineralization is concentrated in a core of potassic alteration in shallow plutonic rocks surrounded by propylitic alteration in andesitic lavas. Argilic alteration is absent. Mafic dikes that intruded the EAPC are not mineralized but they are affected by low-grade metamorphism. The dikes are the most primitive, while andesites and intrusives display typical volcanic arc-like characteristics. Sr and Nd isotopes show that the magmas evolved from a depleted mantle reservoir. The Pb isotope data (whole rock, quartz, feldspar, sulfides) indicate: (1) the intrusive body and the surrounding andesites evolved from a similar source with an average µ-value of 9.4; (2) no external lead was added during mineralization; (3) Pb isotope compositions were slightly disturbed by later low grade metamorphism. ⁴⁰Ar-³⁹Ar laser dating of single pyrite crystals yielded an isochron at 137.3 ± 1.4 Ma (1s), similar as a Rb-Sr whole rock isochron of 134 ± 10 Ma (95% c.l.). A non-mineralized dike gives an ⁴⁰Ar-³⁹Ar age of 95.2 ± 0.8 Ma (1s). Our data favor a model in which the EAPC developed during the Early Cretaceous within an outboard island arc terrane that was accreted in the late Early Cretaceous, forming the Yuma composite terrane of Baja California.