Abstract

MONTENEGRO, Teresita  and  VATTUONE, María Elena. Mineral assemblages of very low-grade metamorphism related to hydrothermal alteration, southwestern of Trevelin, Chubut, Argentina. Rev. mex. cienc. geol [online]. 2008, vol.25, n.2, pp.302-313. ISSN 2007-2902.

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 felly 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, Metamorfismo de muy bajo grado y alteración hidrotermal, Chubut, Argentina 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 fades, transitional to prehnite-pumpellyite fades. P-T conditions of the metamorphic eventwere <260°C and <1.5kb. Moreover, feathery-flamboyant quartz, adularía, gold, and electrumfill 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.

Keywords : geothermal/hydrothermal alkaline alteration; very low-grade metamorphism; low sulphidation epithermal assemblage; Chubut; Argentina.

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