Chemical Composition of the Tourmaline from Stratiform Tourmalinites of the Passagem de Mariana Gold Mine, Southeastern Quadrilátero Ferrífero (Minas Gerais, Brazil)

Abstract

The Passagem de Mariana Mine in the southeastern part of the Quadrilátero Ferrífero, Minas Gerais, Brazil, is one of the oldest Brazilian gold mines. Gold is associated with stratiform tourmalinites and tourmaline-bearing quartz and carbonate veins and breccias. The highest gold contents (up to 200 g/t) have been recorded in the stratiform tourmalinites, which belong to the Batatal Formation (Caraça Group, Minas Supergroup). Three petrographic types of tourmaline have been defined and analyzed by electron microprobe and ICP-MS. Type 1 corresponds to the very fine-grained tourmaline that composes homogeneous tourmalinites. They occur as continuous levels or boudins, and as fragments in quartz-carbonate veins and breccias. Type 2 zoned tourmaline is coarser-grained and composes both homogeneous and banded tourmalinites. Type 3 coarse-grained, zoned tourmaline grows perpendicularly to the contact between the tourmalinite or its fragments and the quartz-carbonate veins and breccias. The main chemical variations observed in the three tourmaline types are decrease of SiO2 and Al2O3 and increase of MgO, Na2O and F contents with increasing CaO and TiO2 contents. Additionally, types 2 and 3 tourmalines also contain higher concentrations of LREE, Rb, Sr, Y, Cs, Th and U in relation to type 1. These chemical variations are coherent with the geochemical conditions that prevail during diagenesis and metamorphism. No evidence was found of the participation of external (e.g., magmatic) fluids in the generation of the stratiform tourmalinites, for which the main source for boron could have been the Batatal Formation carbonaceous metapelites. Therefore, the syngenetic origin of the tourmalinites cannot be completely ruled out. The tourmalinites could have evolved during the prograde metamorphism of the beginning of the Transamazonian Event. Type-3 tourmaline, grown in the contact between the tourmalinite and quartz-carbonate veins and breccias, could have been generated after the metamorphic peak and during the extensional phase of the Transamazonian Event, with gold remobilization in veins and breccias