Mineral assemblages of very low-grade metamorphism related to hydrothermal alteration, southwestern of Trevelin, Chubut, Argentina

Abstract

Mesozoic volcanic rocks outcrop in North Patagonian Andes, near Trevelin locality, Chubut province, Argentina. Basaltic andesites and basalts with porphyritic to aphyric textures with calcium plagioclase, clinopyroxene, and olivine phenocrysts are the most common rocks. Their groundmass texture is felty to intergranular. These units are affected by a hydrothermal alteration that replaced their phenocrystals and groundmass and filled veins, veinlets and amygdales. Minerals were analyzed by conventional microscopy, scanning electron microscopy-energy dispersive scan, X- ray diffraction, infrared spectroscopy, and fluid inclusions microthermometry. Albite, prehnite, pumpellyite, epidote, clinozoisite, smectite/chlorite, laumontite, wairakite, mordenite, titanite, quartz, and calcite are the secondary minerals. Textural relationship between secondary minerals and their chemical compositions evidence a very low-grade metamorphism in the high-zeolite facies, transitional to prehnite-pumpellyite facies. P-T conditions of the metamorphic event were <260°C and <1.5 kb. Moreover, feathery-flamboyant quartz, adularia, gold, and electrum fill other veinlets as the result of a low sulphidation epithermal system. Volcanism, emplacement of granitoids and extensional events took place in the volcanic arc located in the western sector. These processes favoured fluid flow and gave a high temperature geothermal gradient. These conditions would be favourable for formation of a low-sulphidation epithermal assemblage and typical mineral assemblages of very low-grade metamorphism, as an alkaline hydrothermal alteration.