Geología de un cuerpo cuarzoso en el área de la Estación Pablo Acosta, partido de Azul, basamento del Sistema de Tandilia

Among the several outcrops of the mainly Palaeoproterozoic igneous-metamorphic basement of the Tandilia System exposed near the Estación Pablo Acosta, a quartz body, associated to a NE-SO shear zone, was identified. Field work, petrologic and geochemical studies revealed that it was formed from a siliceous fluid generated by metamorphic differentiation during the Transamazonian orogeny. On the other hand, the high degree of quartz purity together with the estimated outcropping volume points out a potential importance for industrial commercialization of the mineralization. In addition, this research provides valuable information to the geological-genetic model for the Tandilia System of the Buenos Aires Province

Significance of graphite inclusion occurrence in the minerals of the San Miguel skarn for the Palaeoproterozoic basement of Tandilia Belt (Argentina) and for the Río de la Plata Craton

Graphite in Archaean-Palaeoproterozoic rocks has been a subject of interest since it could represent an evidence of early life on Earth. In the Palaeoproterozoic basement of the Tandilia Belt, graphite was found both in fluid inclusions (FI) hosted in the San Miguel skarn calc-silicate minerals, and as solid inclusions in calcite crystals from the protolithic marble (a13C enriched carbonate from the “Lomagundi-Jatuli event”). FI microthermometry and oxygen stable isotope ratios indicated the skarn minerals formation within the range of 630–650 °C (at ∼5 kbars) and ∼642–654 °C, respectively. Also, the characterisation of the metasomatic fluid (of a low salinity< 7 wt% NaCl eq. NaCleH2O/NaCleKCleH2O aqueous system) pointed out that the zonal crystallisation pattern shown by the skarn minerals (wollastonite-vesuvianite, grossular-diopside-calcite and diopside-calcite zones in the exoskarn, and grossular-diopside and diopside-calcic plagioclase zones in the endoskarn) responds to the increase of the involved cation activity gradients (Ca2+-Si4+-Mg2+-Fe2+/3+-Al3+) and not to significant changes in the temperature or concentration of CO2 in the system. Variation in the crystallinity degree of the graphite hosted in the skarn minerals and in marble calcite, shown by Raman spectroscopy, would indicate that the graphite could have been formed from the ripening of organic matter present in the sedimentary rocks during the metamorphic-metasomatic event (Transamazonian Orogeny). In this sense, the increase of the organic carbon productivity in the oceans during the Palaeoproterozoic, represented by the “Lomagundi-Jatuli event”, would support this graphite origin and also the possible existence of a marine sedimentary basin in the previous stages of the Río de la Plata amalgamation (Siderian-Rhyacian), in the San Miguel area of the Tandilia Belt