Turmalinitos vulcanogênicos da formação Morro da Pedra Preta do Grupo Serra do Itaberaba (SP): petrografia, composição química da turmalina e implicações metalogenéticas

Abstract

Tourmalinites associated with amphibolites, metachert, iron formation, metatuffs, metasedimentary, metavolcaniclastic and calc-silicate rocks were found in the Morro da Pedra Preta Formation, basal metavolcano-sedimentary sequence of the Serra do Itaberaba Group (São Paulo State, Brazil). The syn-sedimentary origin of the tourmalinites is attested by the deformation of the alternating tourmaline-rich and quartz-rich layers, the presence of rip-up clasts, and whole-rock chemistry similar to that of surrounding metassedimentary rocks. Microprobe analyses show that the tourmaline compositions are: 1) within the schorl-dravite series, for the examples associated with metatuffs, iron formation, metachert metavolcaniclastic and metasedimentary rocks, and 2) within the dravite-uvite series, for those associated with amphibolites, metatuffs and calc-silicate rocks. The former are characterized by low Ca and high Na and Al contents, with variable Fe and Mg contents (schorl-dravite series), whereas the latter, a F-bearing tourmaline, has low Na and Al and high Mg and Ca contents. The schorl component of the tourmaline of the proximal tourmalinites is indicative of the potential of Morro da Pedra Preta Formation for gold, which contrasts with the conditions required for the formation of volcanic massive sulphide deposits (hypersaline fluids and association with magnesian tourmalinites). The tourmaline of the various tourmalinites of the Morro da Pedra Preta Formation is not typically magnesian and its composition was more strongly influenced by the composition of the neighboring rocks than that of the hydrothermal fluids, especially when the tourmalinite was deposited in an intermediate to distal position in relation to the volcano-exhalative center.