Amethyst occurrences in basalts from Triângulo Mineiro region (Minas Gerais-Brazil): Description and comparisons with similar deposits from Rio Grande do Sul

In southern Brazil and surrounding areas, especially in Ametista do Sul (RS) and Artigas (Uruguay) regions, global-class amethyst deposits occur associated to Serra Geral Formation (Lower Cretaceous). They extend to the north up to Triângulo Mineiro region (MG), where two amethyst deposits and one occurrence (Barreiro Farm, Incopol Quarry and Tembezinho) with small proportions, along with their host basalts, have been studied. The main objective of this work was to characterize geologically and mineralogically such occurrences, as well as to compare them to the deposits which occur in akin basalt flows in the south. Petrographically these basalts present plagioclase, pyroxene, volcanic glass and opaque minerals, besides devitrification and alteration of glass and silicate minerals from the rock mesostasis to clay minerals, as well as secondary fillings of chalcedony, smectite, micro and macrocrystalline quartz and calcite. Geochemical analyses indicated they are high Ti (2.67-3.98% TiO₂); only one sample has been characterized as basaltic andesite according to TAS diagram. When compared to their southern counterparts, they are rocks chemically similar to mineralized high Ti basalts from Ametista do Sul. Studied amethyst occurrences are associated to basaltic flows with apparent thickness less than 50 m, filling vertical fractures or semi-spherical to conical geodes. Calcite and celadonite, which are common and abundant minerals associated with amethyst from RS, were only observed at Incopol Quarry. Based on observed elevations, possibly at least three different lava flows can be considered for the Serra Geral Formation at Triângulo Mineiro region. (author)