Isidoro Bernardo Abel Schalamuk

Magmatic characteristics of the Paleocene Cerro Nevazón region and other Late Cretaceous to Early Tertiary calc-alkaline subvolcanic to plutonic units in the Neuquén Andes, Argentina

Abstract Voluminous Late Cretaceous–Early Tertiary calc-alkaline magmatism in the northwest part of the Neuquen province, Argentina, is characterized by a NS-trending belt of olivine basalt-augitic andesite volcanic and amphibole-rich diorite subvolcanic facies that were grouped in the Neuquen-Mendoza volcanic province. Of samples associated with skarn mineralization in the Cerro Nevazon area, two were dated by K/Ar in amphibole and one by Ar/Ar in plagioclase; together, they yield ages between 60.1±1.6 and 56.0±1.7 Ma. The igneous rocks contain normally zoned plagioclase (64–74%), amphibole (magnesiohastingsite and edenite; 15–20%), biotite (5–10%), quartz (4–18%), minor enstatite–ferrosilite (Wo2.5–3En71–73Fs24–25), and augite (Wo42–43En42–43Fs14–15), with accessory magnetite, minor ilmenite, apatite, sphene, and traces of zircon. The fractionation of calcic plagioclase+olivine+pyroxene±magnetite±ilmenite is responsible for the evolution of the parental magmas, which originated in approximately 10% melting of a mantle source contaminated by fluids from the oceanic subducted lithosphere, with sparse or no residual garnet. The gabbro rare earth element patterns are similar to those of the Antuco volcano (37°25′S) and other Quaternary basaltic rocks from the southern volcanic zone (CSVZ) of the Andes (37–41°5′S) emplaced in a relatively thin continental crust (≈30 km). Although the Cerro Nevazon granitoids share some geochemical signatures with the Paleogene Caicayen igneous units, also located in the northwestern region of Neuquen, they have different evolutionary styles, as expressed by the lack of amphibole fractionation and less Cs enrichment in the Caicayen rocks. With similar SiO2 contents, the Upper Cretaceous–Paleocene Campana Mahuida subvolcanic rocks from the same region are richer in incompatible trace elements (K, Rb, Sr, Ba, Nb, La, Ce, Yb, Th, and U) than either the Nevazon or Caicayen units, which suggests that their parental magmas were contaminated with crustal material. The low Fe2O3/FeO ratios in the Nevazon igneous rocks, combined with the presence of ilmenite in most rocks and the low Mg/(Mg+Fe+2) ratios in mafic minerals, suggest that the Nevazon magmas evolved under more reduced conditions than did the Caicayen or Campana Mahuida magmas.

Jurassic magmatism and Au-Ag mineralization in the Deseado Massif (Patagonia Argentina): Lead and sulfur isotopic studies

Lead isotope analyses of volcanic rocks and hydrothermal minerals and sulfide isotope analyses of sulfurs from Au-Ag La Josefina prospect (Deseado Massif geological province, Argentina) are reported. Lead isotope ratios for sphalerite and pyrite have mean values of 18.48, 15.69 and 38.61 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, respectively, and are similar to Pb isotope ratios for the Jurassic volcanic rocks of Bajo Pobre and Chon Aike Formations. δ34S‰ suggesting that the sulfur at La Josefina was primarily of magmatic origin. These results are interpreted to indicate that the Jurassic volcanic rocks are closely related with low sulfidation Au-Ag occurrences.