Metallogeny of the southern Sierras Pampeanas, Argentina: geological, 40Ar– 39Ar dating and stable isotope evidence for Devonian Au, Ag–Pb–Zn and W ore formation

Abstract

Early to mid-Paleozoic metamorphic and igneous rocks of the southern Sierras Pampeanas tectono-stratigraphic terrane, northwestern Argentina, host diverse metallic mineral deposits of Au, Ni–Cu, Ag–Pb–Zn, W, Li, Be, Nb, LREE and Th. Integration of deposit geology, 40Ar– 39Ar geochronology and stable isotope studies with regional geological mapping and U–Pb zircon dating, has led to the resolution of three principal metallogenic epochs. Ordovician intrusion-hosted Ni–Cu sulfide deposits and rare-metal-enriched pegmatites were succeeded by a province-wide metallogenic epoch and concurrent tectonic processes during the Devonian. This epoch was characterized by mesothermal lode Au deposits, high-level Ag–Pb–Zn veins, W±Cu-bearing veins and replacements, and various pegmatite- and granite-related lithophile element deposits. Finally, Miocene–Pliocene epithermal Au–Ag mineralization developed during Andean tectonic events. Sericitic alteration associated with mesothermal lode Au (±Cu±Ag) in the Sierra de Las Minas and Sierras de Córdoba yields Devonian 40Ar– 39Ar step heating ages in the ranges 382–393 and 351–378 Ma, respectively. These ages span the timing of Ag–Pb–Zn sulfide–quartz veining in the El Guaico district (386±4 Ma, sericite), and W-bearing quartz vein mineralization in the Aguas de Ramón and El Morro districts which yielded muscovite ages of 366±1 and 363±1 Ma, respectively. Oxygen isotope δ 18O SMOW compositions of the Au, Ag–Pb–Zn and W ore-forming fluids, calculated from quartz, white mica and chlorite isotope ratios, range from 5.2‰ to 10.8‰ ( n=32), whereas calculated δD SMOW compositions range from −61‰ in the La Rioja Au systems to −141‰ in the W veins ( n=9). Sulfide δ 34S values range from 1.5‰ to 10.0‰, averaging 6.5‰ ( n=15). The oxygen and hydrogen stable isotope data suggest contributions to the ore fluids of either D-depleted meteoric waters that have reacted extensively with metasedimentary rocks, or fluids equilibrated with degassed felsic magmas, or mixtures of both. This mineralization formed during and up to 20–30 Ma after widespread emplacement of high-level peraluminous to metaluminous granites, which yielded U–Pb zircon ages of 404±6–382±6 Ma. The Devonian Au deposits are localised in transcurrent and reverse fault and shear zones that may be related to collision of the Chilenia terrane with the western margin of Gondwana during the newly defined Achalian orogeny. These deposits are considered to be members of the mesothermal lode Au family of systems found worldwide.