The role of microstructural and fluid processes in the genesis of gold-bearing shear zones : Fazenda Maria Preta mine, Rio Itapicuru greenstone belt, Bahia, Brazil

Xavier, Roberto Perez (1991) The role of microstructural and fluid processes in the genesis of gold-bearing shear zones : Fazenda Maria Preta mine, Rio Itapicuru greenstone belt, Bahia, Brazil. University of Southampton, Doctoral Thesis.

Abstract

Gold mineralization at the Fazenda Maria Preta mine is confined to two regional-scale brittle-ductile shear zones: (i) shear zone I characterized by left-lateral movement coupled with normal-oblique faulting located mainly along lithological contacts (intermediate metavolcanics and volcanogenic metagreywackes or sub-volcanic intrusives); and (ii) shear zone II also exhibiting left-lateral movement but with a component of reverse-oblique thrusting, and transecting the intermediate metavolcanics. Hydrothermal alteration denoted by carbonization, sericitization, and subordinate silicification and albitization, together with the occurrence of several shear, tension and breccia vein systems are ubiquitous throughout the shear zone domains. The gold mineralization occurs in highly deformed carbonaceous phyllonites, silicified metarhyodacites, and in shear veins sub-parallel to the foliation. The gold-bearing vein systems were emplaced syn- to late tectonic during crack-seal episodes promoted by cyclic fluctuations of fluid pressure. The gold (2 to 80μm in size) is closely related to arsenopyrite, pyrite, pyrrhotite, sphalerite, and chlorite, and this gold-related mineral assemblage shows an intrinsic association with carbon-rich shear bands in the carbonaceous lithotypes and stylolites in the shear veins. Alignment of these microstructural domains with an extensional direction during progressive shearing caused the formation of dilatant sites for the subsequent infiltration of gold-bearing fluids. Other anisotropic microstructural planes, such as transgranular microfractures, veinlet/host rock contacts and sub-grain boundaries, and pressure shadows around iron sulphides were also sites of precipitation of the gold. Accordingly, peak gold deposition coincided with the latest deformation events within the shear zone domains, particularly during the reactivation of microstructures. Collectively, fluid chemistry and the isotope signature of the fluids (δ18O= 9.7 to 12.6%o; δD= = -37 to -82%o) favour a dominantly metamorphic provenance for the auriferous fluids of the Fazenda Maria Preta gold mine. The fluids were released during the prograde metamorphism of subducted oceanic crust and sedimentary cover and the granitic basement, as well as contact metamorphism of the volcano-sedimentary sequence caused by the emplacement of the syn-tectonic granitic domes. As magmatic and metamorphic processes are interrelated in the deep crust, an input of magmatic or mantle-derived components for the auriferous fluids, particularly CO₂ (δ13C_CO2= -6.5 to -100.2%o) and S(δ34S_{sulphide =} 2.5 ± 0.4%o), cannot be ruled out.

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