Geologia e mineralização aurífera da área de Chapada-TO

Abstract

The Chapada gold deposit is located in the extreme north of the outer zone of the Brasília Belt, in the central part of the Tocantins Province. The geological framework of the area consists of an orthogneissic basement of tonalitic to granodiorite composition or Manoel Alves Complex, in the Água Suja metavolcanosedimentary Sequence metamorphosed in the high greenschist facies and comprising acidic and basic metavolcanic rocks, and metasedimentary rocks of clastic and chemical origin (older than 2172±5 Ma), in the Xobó intrusive granitic Suite (minimum age of crystallization: 2012±3 Ma) and a folded cover consisting predominantly of quartzites, corresponding to the Natividade Group. In the Chapada Garimpo, there are essentially quartz-biotite schists and metadacites of the Água Suja Sequence cut by veins and pockets of pegmatoid to aplitic, leucocratic granite, belonging to the Xobó Suite. These rocks were affected by intense transcurrent movements of general direction N30E that resulted in a shear zone hosting a complex and anastomosed system of quartz veins. The host rocks of the veins show strong signs of mylonization, eventually cataclase. The biotite shales were, in particular, transformed into muscovite-biotite mylonites. Finely disseminated metallic minerals are found in both quartz veins and mylonites in the contact zones, with pyrrhotite and pyrite being the earliest, followed by chalcopyrite and sphalerite, and finally by arsenopyrite and free gold or included in the latter, all formed at the end of the deformational/hydrothermal event. The main types of alteration observed are muscovitization, epidotization and silicification. The hydrothermal process involved several phases and was initially marked by the formation of very hot aqua-carbon solutions (<330°C). Subsequently, with the increasing participation of shallower waters, essentially lower temperature aqueous fluids acted (from 300/250 °C to about 150 °C), basically belonging to the systems CaCl2-NaCl-H2O and KCl-NaCl-H2O. The mineralized quartz and sulphide + gold vein system of the Chapada area fits into the category of orogenic lode type deposits. The isotopic analysis by the Rb-Sr method of the micas of the host mylonites of the veins allowed to determine at 517±40 Ma the age of the regional shear and, consequently, of the gold mineralization, which would be, therefore, directly related to the formation of the intracontinental megastructure referred to as Lineament. Late Neoproterozoic Transbrasilian.