Geology, petrography and geochemistry of igneous rocks related to mineralized skarns in the NW Neuquen basin, Argentina: Implications for Cordilleran skarn exploration

摘要

Five mineralized Cu, Au, and Fe skarns and related igneous rocks of the Andes Cordillera of NW Neuquen (Paleogene Campana Mahuida, Caicayén and Cerro Nevazón) and SW Mendoza (Neogene Vegas Peladas and Hierro Indio) provinces of Argentina (34–38°S) are reviewed to demonstrate that geochemical signatures of igneous rocks can be used to predict the metal potential of skarn prospects of the Andes Cordillera of Argentina. These igneous rocks are calc-alkaline, metaluminous, and derived from a sub-arc mantle source without residual garnet. They were emplaced at shallow depths. The Vegas Peladas, Hierro Indio, and Cerro Nevazón igneous rocks have similar major and trace element contents, and are typical of primitive, I-type plutons associated with Fe skarns worldwide. The Vegas Peladas and Hierro Indio plutons are, however, less reduced and they have lower Ni concentration than the Cerro Nevazon rocks, whose skarns have higher Au concentrations. In the Caicayen district, small skarns with sub-economic Cu concentrations and abundant pyrite are associated with porphyry copper style alteration in igneous rocks that underwent amphibole fractionation. Their chemical compositions are intermediate between the most primitive plutons associated with Fe skarns and the most evolved plutons associated with Cu skarns, both with high fO_2. At Campana Mahuida a small porphyry copper deposit and associated Cu skarns are linked to andesite dikes and pluton with similar SiO2 contents but richer in incompatible trace elements (K, Rb, Sr, Ba, La, Ce, and Th) than the Mendoza, Cerro Nevazon and Caicayen igneous rocks. This suggests that their parental magmas evolved in an open system and were contaminated with crustal material, thus resulting in the strongest fractionation of amphibole and accessories phases. These intrusions share many features with typical Cu skarns-related plutons.