Nature and origin of the BIF-hosted Sao Bento gold deposit, Quadrilatero Ferrifero, Brazil, with special emphasis on structural controls

摘要

The orogenic banded iron formation (BIF)-hosted Au mineralization at Sao Bento is a structurally-controlled, hydrothermal deposit hosted by Archean rocks of the Rio das Velhas greenstone belt, Quadrilatero Ferrifero region, Brazil. The deposit has reserves of 14.3 t Au and historical (underground) production of 44.6 t Au between 1987 and 2001. The oxide-facies Sao Bento BIF is mineralized at its lower portion, where in contact with carbonaceous, pelitic schists, particularly in the proximity of sulfide-bearing quartz veins. Shear-related Au deposition is associated with the pervasive, hydrothermal sulfidation (mainly arsenopyrite) of the Fe-rich bands of the Sao Bento BIF. Auriferous, sulfide- and quartz-rich zones represent proximal alteration zones. They are enveloped by ankerite-dominated haloes, which reflect progressive substitution of siderite and magnetite within the BIF by ankerite and pyrrhotite, respectively. The Sao Bento BIF was intensely and extensively deformed, first into open, upright folds that evolved into tight, asymmetric, isoclinal folds. The inverse limb of these folds attenuated and gave way to sheath folds and the establishment of ductile thrusts. Mineralized horizons at Sao Bento result from early structural modifications imposed by major transcurrent and thrusts faults, comprising the Conceicao, Barao de Cocais and Sao Bento shear zones. Dextral movement on the SW-NE-directed Conceicao shear zone may have generated splays at a compressional sidestepping zone, such as the Sao Bento shear zone, which is the structural locus for the Sao Bento gold mineralization. Relaxation of the Conceicao shear zone under more brittle conditions resulted in the development of dilatational zones where gold-sulfide-quartz veins formed. These structures are considered to have been generated in the Archean. Geochronological data, are scarce, with Pb-Pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single-stage growth curve at 2.65 Ga. A later compressional, ductile deformation of unknown age overprinted, rotated and flattened the original, N60E-directed structure of the whole rock succession, with development of planar and linear fabrics that appear similar to Proterozoic-aged structures. Fluid inclusion studies indicate low salinity, aqueous fluids, with or without CO_2 and/or CH_4, with extremely variable CO_2/CH_4 ratios, of probable metamorphic origin. Fluid evolution shows a paragenetic decrease in the carbonic phase from 10-15 percent to 5 percent, and increase in the H_2O/(CO_2+CH_4) and CO_2/CH_4 ratios, suggesting important interaction with carbonaceous sediment. Trapping conditions indicate a temperature of 300 deg C at 3.2 kbar.