Roger G. Skirrow

U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin

Abstract New SHRIMP zircon and monazite 206Pb/238U and 208Pb/232Th ages on structurally controlled units and 40Ar-39Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.

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, 40 Ar– 39 Ar 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 Cordoba yields Devonian 40 Ar– 39 Ar 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 Ramon and El Morro districts which yielded muscovite ages of 366±1 and 363±1 Ma, respectively. Oxygen isotope δ 18 O 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 δ D SMOW compositions range from −61‰ in the La Rioja Au systems to −141‰ in the W veins ( n =9). Sulfide δ 34 S 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.