FLUID EVOLUTION AND GOLD DEPOSITION AT THE CUIABÁ MINE, SE BRAZIL: FLUID INCLUSIONS AND STABLE ISOTOPE GEOCHEMISTRY OF CARBONATES

ROBERTO PEREZ XAVIER, CATARINA LABOURÉ BENFICA TOLEDO, BRUCE TAYLOR, ALFONSO SCHRANK
2000 Revista Brasileira de Geociências  
The gold-bearing quartz-carbonate vein deposits of the Cuiabá Mine, northern sector of the Quadrilátero Ferrífero, State of Minas Gerais, SE Brazil, are hosted by a sequence of banded carbonaceous metasedimentary rocks within tholeiitic basalt flows of the Archean Rio das Velhas Greenstone Belt. A regional greenschist metamorphic grade characterizes rocks in this section of the Belt. Episodes of hydraulic fracturing enhanced the focussing of fluids percolating along the host primary layering,
more » ... ich promoted pervasive carbonatization and sulfidation associated with crosscutting fractures and quartz-carbonate veins/breccias. The gold mineralization is closely related to the sulfiderich zones. Fluid inclusion investigations show that the mineralizing fluids were dominantly aqueous, of low salinity (3-6 wt.% NaCleq.), with variable concentrations of CO 2 (0 -11.4 mol%), CH 4 (1.5 -10.2 mol%), subordinate N 2 (0 -0.7 mol%), and traces of H 2 S/HS -. Carbonates in the carbonaceous host rock display heavier δ 13 C compositions (-4.3 to -0.2 o / oo ) than carbonates in the gold-related, carbonate-sulfide altered zones and associated veins and breccias (-8.1 to -5.5 o / oo ). The range of carbonate δ 18 O values in the carbonaceous host rock (12.8 to 16 o / oo ) is broadly similar to the carbonates in the mineralized zones (10.6 to 15.2 o / oo ), with a slightly larger variation in the latter. Collectively, fluid inclusion and carbon isotope data indicate that: (1) the mineralizing fluid was originally composed of a H 2 O-CO 2 solution, with significant concentrations of CH 4 added via hydrolysis of the carbonaceous matter during vein-fluid/wallrock interaction; (2) sedimentary carbonates, probably of marine origin, are readily distinguished from those carbonates formed by reaction between CO 2 -bearing vein fluids and wallrocks; (3) gold-related vein-fluid CO 2 was relatively homogeneous in isotopic composition, suggesting equilibration with carbonate and reduced carbon reservoir (e.g. mantle-derived magma); (4) gold deposition may have been triggered by a sharp decrease in fO 2 caused by the CH 4 enrichment of the fluid.
doi:10.25249/0375-7536.2000302337341 fatcat:yj6dc5cmhfesnnstdkdrvfkxdq