Widespread hydrothermal alteration overprinting epizonal Ordovician rocks in the Puna region of Argentina (Salta and Jujuy provinces)

Abstract

X-ray-diffraction and textural studies under scanning electron microscopy (SEM) using backscattered electron images were employed to outline the thermal post-depositional evolution of Ordovician slates and metavolcanic rocks from four representative localities in southern Puna (the Sierra de Guayaos, the southern zone of the Sierra de Cobres, Cordón de Pozuelos, and Cordón del Gallego) and the Sierra de Cochinoca-Escaya in northern Puna of Argentina. In the case of a meta-andesite that in a previous study had displayed complex clay mineralogy, additional XRD analysis along with a detailed TEM study was performed. Clay mineralogy, Kübler index and chlorite geothermometry indicated that the rocks had undergone prograde metamorphic evolution attaining the epizone. Textural evidence proves that the metamorphic event was syntectonic with the folding that affected this succession, producing a conspicuous slate cleavage in sedimentary and volcanic rocks. Afterwards, the rocks were affected by hydrothermal alteration that produced retrograde changes in mineralogy at centimeter, micron and nanometer scales. The hydrothermal fluid partially dissolved the metamorphic minerals and enhanced the crystallization of clays, kaolinite being the most frequent clay in the altered rocks. Alunite-jarosite was identified in one slate and also evidences of metal transport, suggesting an acid hydrothermal fluid and temperatures between 100 and ~ 300 °C. These data confirm that the Ordovician successions of southern Puna were affected by widespread hydrothermal alteration, as the equivalent rocks in northern Puna. The hydrothermal event was probably associated with the intrusion of the Eastern Puna Magmatic Belt. In the meta-andesite outcropping in the Sierra de Cochinoca-Escaya, HRTEM images and compositional mapping reveal the steps in the paragenetic evolution of the hydrothermal event that produced the alteration of mafic phenocrysts and the formation of intergrowths between high-charge corrensite, trioctahedral vermiculite, and kaolinite.