Quartz veins in the south of Santiago del Estero

Abstract

A study of the mineralogy, textural features and geometry of large quartz deposits, in Ambargasta and Sumampa ranges, allow us to divide them into two different groups: small pegmatites and large hydrothermal veins. The former were related to wall-rock emplacement, whereas the latter were controlled by regional lineaments that affected the Proterozoic basement. Hydrothermal veins are mainly composed of quartz with minor amount of opal, chalcedony and iron oxides. The most important hydrothermal vein, called Cantamampa, is approximately 3 km long, up to 8 m wide and 70 m high. The limited alteration produced by the hydrothermal fluids suggests that their precipitation temperatures must have been low. Only epidote, sericite and chlorite occur as wall-rock alteration. Pegmatites bodies are smaller and, although quartz and alkaline feldspars are the main components, muscovite is also present in minor amounts. Quartz from the most important pegmatite (Piedra Blanca) contains about 99.9% of SiO₂. However, in the hydrothermal veins, where Si is partially replaced by Al, the SiO₂ amount in the quartz decreases to 93%.