Geochemistry of Laramide granitoids and associated porphyry copper mineralization in northwest Mexico

Abstract

The present study investigates the effects of basement variations on the geochemical composition of the Laramide plutonic belt of northwestern Mexico, and implications for the development of the associated porphyry copper mineralization. In the north part, the belt intruded rocks of the North America and Caborca terranes representing cratonic basements juxtaposed by Mid-Jurassic strike-slip faulting. The central part of the belt was emplaced in the Cortes terrane which contains Paleozoic eugeoclinal sequences accreted to North America between Mid-Permian and Late Triassic times. The southern part of the belt intruded island arc-related sequences of the Guerrero terrane, accreted to North America during the Late Cretaceous. A suite of 30 samples of granitoids were studied to characterize the geochemical composition of the belt. The samples range from 56% to 75% SiO₂ and have mid to high-K calc-alkaline and mostly metaluminous compositions. REE results show more evolved chondrite-normalized plots in the north part of the belt, characterized by higher ΣREE and La(N)/Yb(N) ratios, and more pronounced negative Eu anomalies. Southward, the samples show a progressive flattening, eventually with almost no Eu anomalies in the Guerrero terrane. ⁸⁷Sr/⁸⁶Sr initial ratios higher than 0.7062 and initial εNd below -4 characterize the granitoids from the north part of the belt. Slightly less evolved isotope signatures occur in the central part, whereas ⁸⁷Sr/⁸⁶Sr ratios below 0.7063 and more positive εNd values characterize the samples from the south. The data suggest a garnet-bearing source for the granitoids emplaced in the north and central parts of the belt, whereas much less garnet fractionation is needed in the southern part. The effect of different crustal domains on the porphyry copper mineralization is less obvious. Hypogene Cu grades do not vary much along the belt, but the associated metals such as Mo, W, and Au show a certain degree of spatial control. Higher Mo grades are associated with the North America and Caborca terranes. Tungsten deposits are not terrane-constrained, but the more conspicuous mineralization occurs in central Sonora, mainly associated to the Caborca terrane. Higher Au values seem to occur in porphyry copper and associated breccia deposits in the Guerrero terrane. In general, the associations Cu-Mo-WO₃ in terranes of North America affinity, and Cu-Au in the Guerrero terrane suggest basement control.