Regional setting and magmatic evolution of Laramide porphyry copper systems in western Mexico

摘要

This study investigates the contrasting tectonic settings in western Mexico and the controls they exert on porphyry copper deposits. Igneous rock affinities are deduced by integrating existing lithologic, geophysical, and geochemical information with new major-, trace-, rare earth-element, radiogenic isotope, and mineral phase compositional data. Regional findings are compared with theGeology of porphyry complexes in cratonic (Cuatro Hermanos) and accreted settings (Tameapa). Geochemical results identify four tectonic domains. Lower Cretaceous volcano-sedimentary successions are underlain by oceanic crust represented by the Bacurato Ophiolite. This domain is bordered on the south by the Lower Cretaceous San Pablo island arc Complex, on the west by much thicker crust (>30 km) and Jurassic and/or older sedimentary assemblages of Precambrian provenance, and on the north by the Upper Triassic Sonobari back-arc Complex. Plutonic rocks intruded the four domains. Upper Cretaceous anatectic plutons from a collisional setting have S- or I-type characteristics depending on the host rock they intrude. Combined Sr, Nd, and O isotope results from Upper Cretaceous and Laramide granodiorites show a primitive-to-evolved (εNd(I) = +4.6 to -3.8) gradient from central Sinaloa toward the north, west, and south, identifying the presence of Lower Cretaceous oceanic crust and volcanogenic materials well into southeastern Sonora. At Cuatro Hermanos, batholith (≈59 Ma) and porphyry (56 Ma) suites are broadly cogenetic and contaminated with significant radiogenic crust (εNd(I) = -3.3 to -4.0). Relative to batholith, porphyry magmas assimilated lesser sedimentary material, and are not well homogenized. At Tameapa, batholith (57 Ma) and porphyry (53 Ma) suites were derived from a mantle source, volcanogenic materials, and/or oceanic crust. Adjacent, coeval, and compositionally comparable batholithic intrusions exhibit disparate radiogenic signatures (εNd(I) = +3.1 and -2.5), indicating magma or source heterogeneity. Geochemistry of porphyries points to very high assimilation to fractionation ratios, extraction from a residual melt, heterogeneity, and magma chamber recharge. Faults have substantially rotated and dismembered both porphyry systems, as previously recognized at Cuatro Hermanos. However, Tameapa is the first deposit-scale documentation of large-magnitude extension observed in Sinaloa. Both lack of consanguinity of porphyries with well-homogenized host batholith and amount of extension have exploration significance in the region.