Rochas ricas em turmalina da formação Morro da Pedra Preta: relações com os turmalinitos portadores de ouro do Grupo Serra do Itaberaba, SP

Abstract

Tourmaline is a common mineral in the Serra do Itaberaba Group (Guarulhos-Brazil), especially in its basal, volcanic-sedimentary sequence, the Morro da Pedra Preta Formation. Tourmalinites, basic metatuffs, intermediate to acid metavolcaniclastic rocks, metapelites and quartz veins are among the tourmaline-rich rocks. The presence of gold disseminated in a tourmalinite of the Tapera Grande prospect led to the investigation of the relation between the environments of tourmalinite genesis and mineralizations in the Serra do Itaberaba Group. The investigation was extended to tourmaline-rich basic metatuffs, intermediate to acid metavolcaniclastic rocks, and metapelites that also show some silicification, carbonatization, potassification, chloritization and sulfidation. The tourmaline chemistry showed that even in the regions proximal to the exhalative center, the fluid/rock ratio is low because the tourmaline preserves most of the host rock's characteristics rather than those of the boron-rich fluids themselves. Such hydrothermal-metamorphic fluids promoted the redistribution of synsedimentary or syngenetic gold. The composition (aqueous-carbonic, low-salinity) of these fluids, however, did not favor the formation of massive sulfide deposits.