Edad 40Ar/39Ar del magmatismo y mineralización en el área de El Arriero, Sierra de La Huerta, San Juan, Argentina

CASTRO DE MACHUCA, BRÍGIDA; PÉREZ DE ARCE, CARLOS; MATTHEWS, STEPHEN

La Plata

Congreso; XVI Congreso Geológico Argentino; 2005

Asociación Geológica Argentina - Universidad Nacional de La Plata

Subvolcanic bodies amongst which is El Arriero Dacite (DEA) intrude the crystalline basement of the La Huerta range, province of San Juan. Hydrothermal alteration and ore depositation are associated to this magmatic activity. The 40Ar/39Ar method on biotite is used to date a sample from the DEA. The porphyritic rock shows plagioclase and subordinate hornblende, biotite and quartz phenocrysts in a fine-grained groundmass of similar composition. The whole DEA is altered and mineralized. Propylitization is dominant, but quartz-sericite and potassic alteration (secondary K-feldspar and biotite) also occur. The sulfides content is ~8 percent in volume, pyrite is the main metallic mineral with minor magnetite, chalcopyrite, pyrrothite, molybdenite and native gold. Biotite in dated sample corresponds to slightly chloritized phenocrysts, but secondary one is also present. The former has been reseated to the temperature of formation of the later. A slight decrease in the real age of the intrusion is explained as a consequence of 40Ar loss attributed to the hydrothermal alteration. Since the DEA is considered broadly as a syn-mineralization body, the absolute age of 299.9±3.6 Ma could represents that of the magmatic-hydrothermal-mineralizing event, and confirms the presence of another component of the Gondwanic magmatic-metallogenic cycle in the Western Pampean Ranges.