Estudo dos fluidos hidrotermais relacionados às "brechas" mineralizadas com sulfetos de ouro da área Bahia-Carajás

Abstract

The Bahia deposit is hosted by the Igarapé Bahia Group, of Archean age, which occurs in the Mineral Province of Carajás (SE of Pará). This group consists of basic metavolcanic, metapyroclastic and clastic metasedimentary rocks, in addition to banded iron formations and breccias, being cut by dikes of basic composition. All this rocky package is intensely altered by hydrothermalism that generated mineral associations compatible with the greenschist facies. The Bahia deposit is made up of the bodies Acampamento Norte, Acampamento Sul, Furo Trinta and Alemão. In the first three, mineralization is widespread and is mainly located in breccias. The latter, more recently discovered, is formed by massive sulphide lenses. The present work was based on testimonies of seven drill holes that were executed in Corpo Acampamento Sul. The breccias occur in the contact, today verticalized, between the basic metavolcanic rocks and clastic metasedimentary rocks and generally present gradational contacts with the host rocks. The clasts originate from metavolcanic rocks of basic composition and from banded iron formations, constituting angular to subangular fragments with dimensions between 1 and 3 cm most frequently. The matrix consists of chlorite, siderite, chalcopyrite, quartz, magnetite, tourmaline and calcite, with a predominance, in general, of one or two of these minerals. The matrix/clast ratio is variable and some breccias show slight matrix foliation and clast orientation. Sometimes, interspersed with breccias, massive beds of magnetite and sulfides occur. These breccias are suggested to have a phreatic origin, whose fragments were reworked and transported to deeper zones by debris currents. In breccias, mineralization also occurs in veins and pockets where sulphides join quartz and/or siderite and are more enriched in copper and gold than in host rocks. Chalcopyrite and pyrite are the main sulfides in all lithological types, but in the stratiform massive beds of magnetite and sulfides, bornite is also present. Magnetite is an abundant mineral, occurring in fragments of banded iron formation, disseminated in the matrix of breccias or in massive beds. Chloritization, carbonation, magnetization and sulfidation are the most important types of hydrothermal alteration, with silicification and tourmalinization also being recorded in a subordinate manner. Chloritization has affected almost all rocks to a greater or lesser extent, lending them a characteristic greenish tinge. Carbonation, magnetization and sulfidation are represented, respectively, by the precipitation of siderite and calcite, magnetite and chalcopyrite, pyrite and bornite. Studies on quartz crystals revealed biphasic and triphasic aqueous inclusions chemically represented by the H2O-NaCl-CaCl2 system, with variable salinity (5.3 - 41.5 % eq. NaCl). Inclusions with pure CO2 were also found, some with traces of N2. Homogenization temperatures were more frequent in the ranges of 110-140ºC for biphasic inclusions and 150-225ºC for aqueous triphasic inclusions. With the pressure correction made, the prevailing conditions for the formation of the Bahia deposit were estimated at 160-240°C and 1-2 kbar. The aqueous fluids were interpreted as seawater modified as a result of cognitive movement that made it circulate and leach metals from the volcanic sedimentary package for later deposition in the form of sulphides. On the other hand, the carbonic fluids are a probable mantle source and are responsible for the precipitation of siderite. The chlorite geothermometer proved to be inadequate to define the paleotemperatures, given the very different results obtained according to the equation and correction for Al (IV) used. The geological characteristics of the Bahia deposit favor its interpretation as a Besshi-type volcanogenic deposit, which may have incorporated, during later events, uranium and rare earths that occur in it at anomalous levels.