THE MANGANESE DEPOSITS OF THE PAMPEAN RANGES, ARGENTINA

Abstract

The example of manganese mineralization studied in this paper makes up the major concentration of this element in Argentina. It comprises an area of 70 by 30 km in the eastern Sierras Pampeanas, located in Cordoba and Santiago del Estero provinces. It is quite unusual since it is hosted in granodioritic and rhyodacitic rocks. Geochemical, petrological and structural analyses demonstrate that these rocks are unrelated to the mineralization process. The geometry and distribution of the veins are controlled by a dextral shear system related to north–south lineaments. Textural and structural analyses of the veins indicate four different stages of mineralization. Ore minerals precipitated during the first three, whereas the gangue was mainly formed during the last stage. X-ray diffraction and electron-microprobe studies show that Mn 4+ oxides (hollandite, pyrolusite, ramsdellite, romanechite and cryptomelane) associated with Fe 3+ oxides (goethite and hematite) are the main ore minerals. Calcite, opal, barite, and lesser amounts of fluorite, quartz and “chalcedony” are the most common gangue minerals. Fluid-inclusion studies show that this mineralization was associated with aqueous solutions of low salinity and a temperature of about 125°C. Isotopic analyses based on several samples of calcite (δ 13 C, δ 18 O) and barite (δ 34 S, δ 18 O) have demonstrated the dominance of meteoric waters in the hydrothermal fluids. Finally, the chemical composition of manganese oxides supports the hypothesis of an epithermal system developed in continental environments.