Geologia USP. Série Científica (Jun 2009)
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 ofthe oldest Brazilian gold mines. Gold is associated with stratiform tourmalinites and tourmaline-bearing quartz and carbonateveins and breccias. The highest gold contents (up to 200 g/t) have been recorded in the stratiform tourmalinites, whichbelong 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 fi ne-grained tourmaline that composeshomogeneous tourmalinites. They occur as continuous levels or boudins, and as fragments in quartz-carbonate veinsand 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 fragmentsand 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 and3 tourmalines also contain higher concentrations of LREE, Rb, Sr, Y, Cs, Th and U in relation to type 1. These chemical variations arecoherent with the geochemical conditions that prevail during diagenesis and metamorphism. No evidence was found of the participationof external (e.g., magmatic) fl uids 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 ruledout. The tourmalinites could have evolved during the prograde metamorphism of the beginning of the Transamazonian Event. Type-3tourmaline, grown in the contact between the tourmalinite and quartz-carbonate veins and breccias, could have been generated after themetamorphic peak and during the extensional phase of the Transamazonian Event, with gold remobilization in veins and breccias.
