COMPOSIÇÃO E EVOLUÇÃO DOS FLUIDOS NO DEPÓSITO AURÍFERO SÃO FRANCISCO, FAIXA SERIDÓ, PROVÍNCIA BORBOREMA, NORDESTE DO BRASIL

Abstract

The Sao Francisco gold deposit is located within the Serido Belt, Borborema Province, Northeast Brazil. The deposit area consists predominantly of mica schists and minor calc-silicate lenses (Serido Formation), that are metamorphosed to mid-upper amphibolite facies conditions and cut by pegmatite intrusions. The primary mineralization is considered to have occurred synchronously with the peak metamorphism and is hosted by the sillimanite-muscovite zone confmed to a sin-S 3 medium to high angle shear zone, ali considered to be Neoproterozoic. Free gold, or associated with Fe ± Cu ± Pb sulfides (especially pyrite), occurs within the veins or host mica schists as interstitial fine grains (primary mineralization) or segmented in millimeter- to centimeter-sized fractures (secondary mineralization). This study, accomplished in auriferous and barren quartz veins, consisted of the investigation in the mode of occurrence of the fluid inclusion populations, their textural relationships and analyses by microtherrnometry and Raman microspectroscopy. Four types of fluid inclusions have been identified, in decreasing order of abundance: type l - CO 2 , with subordinate amounts of CH. and N 2 ; type 2 - H 2 O, could contain a complex mixture of solutes; type 3 - CH 4 -rich; and type 4 - H 2 O-CO 2 , with low-salinity. Most of the fluid inclusion populations occur along inter- and intragranular healed microfractures. Types l, 3 and 4 are texturally the earliest and fill low- to moderate-angle microfractures, whereas type 2 inclusions are later and delineate mostly moderate- to high-angle microfractures, ali referred to the XZ surface of finite strain ellipsoid. The microfractures host only one fluid inclusion type, except in one barren vein where carbonic (type 1) and aqueous-carbonic (type 4) inclusions were found along the same micro fracture. In general, the auriferous veins contain fluid inclusions of types l, 2 and 3, whereas the barren veins show types l and 2 or l, 2 and 4. Dissolved salts in the aqueous fluid inclusions (type 2) pf the barren veins are dominated by NaCl (± KC1), but in the auriferous veins NaCl, CaCL 2 and probably KC1, LiCl, MgCl 2 and FeCL 2 appear as important solutes. The results of this study, combined with structural, petrographic and metamorphic data, demonstrate that the circulation of aqueous-carbonic (low salinity) and aqueous fluids (varied salinity) occurred coevally during peak metamorphism and formation of the Sao Francisco gold deposit. Most of the inclusions of the aqueous-carbonic fluids suffered dehydration post-entrapment (the current carbonic inclusions) and the variable salinity of the aqueous fluids is consequence of the chemical mobility during the metamorphism. The primary mineralizing fluids were probably low-salinity aqueous- carbonic and both chlorine- (possibly dominant) and sulfur-complexes may have had an important role in the transport of the gold. It is possible that aqueous fluids with complex salts played a prominent role in the remobilization of the primary gold (sulfides) to the fractures. Although further studies are necessary for a better assessment of the processes responsible in the deposition of the primary gold, mixing of fluids, sulfidation and immiscibility within the fluid conduits (veins) appear to have been particularly important.