Fluid inclusion studies on Caxias and Areal gold mineralizations, São Luı́s Craton, northern Brazil

Abstract

Caxias and Areal are minor gold deposits in the São Luı́s Craton, northern Brazil. The geological setting comprises dominant Paleoproterozoic granitoids of the Tromaı́ Suite, with minor occurrences of coeval greenstone belt-like sequences of the Aurizona Group, both correlatives to Eburnean–Birimian rocks of the West African Craton. Deformation of the lithologies is weak, almost restricted to small-sized shear zones. Gold mineralization at Areal is hosted by a monzogranite and associated with a subparallel quartz vein system and its hydrothermalized haloes. Caxias gold mineralization is associated with a narrow, subvertical, NE-trending shear zone and its hydrothermal envelope. In the northern sector of this deposit (CX-N), mineralization is hosted by a small fine-grained tonalitic intrusion with strong chloritic (+carbonate, epidote) alteration and pyrite+sphalerite dissemination, while in the southern sector (CX-S) it is hosted by pyritized quartz–sericite schists and chlorite–biotite schists with sharp subvertical foliation. In both sectors, widespread multidirectional quartz veining is observed. Petrographic, microthermometric and Raman spectroscopic studies of fluid inclusions from quartz veins associated with the mineralizations revealed the presence of early carbonic/aqueous–carbonic and late aqueous fluid inclusions. The CO 2-bearing fluids exhibit three different gaseous compositions: CO 2+moderate CH 4; CO 2+low CH 4+moderate to high N 2 (both restricted to CX-S); and CO 2+low N 2 fluids (occurring in all deposits). The CO 2–N 2 fluid has been defined as the mineralizing fluid having moderate bulk density (0.7–1.0 g/cm 3), low-salinity (mean 5 wt% NaCl equiv.), containing 55–95 mol% H 2O, 6–45 mol% CO 2 and up to 2.5 mol% N 2. Only at CX-S this fluid was modified by introduction of CH 4 and enrichment in N 2 by reaction with carbonaceous wall rocks. The wide range of composition, microthermometric properties and CO 2/H 2O ratio displayed by the CO 2-bearing inclusions were interpreted as product of heterogeneous trapping (by phase separation) of two partially immiscible fluids. Bulk composition isochores combined with chlorite geothermometer bracketed T– P conditions between 262 and 307°C and between 1.3 and 3.5 kbar for Areal, 1.6–3.7 kbar for CX-N and 2.4–4.6 kbar for CX-S. These data indicate depth of 5–15 km for the formation of the deposits, compatible with crustal conditions of greenschist metamorphism and the brittle–ductile transition. The compositional and P– T features of the mineralizing fluid, together with its reduced character (log fO 2 between −29.8 and −34.2), as well as the pyrite abundance in the host rocks, suggest that gold was transported as a sulfur reduced complex and that its deposition occurred in response to phase separation, reduction of H 2S activity by sulfidation of the wall-rocks, and lowering of fO 2 due to fluid–rock interactions. Collectively, the described characteristics favor a metamorphic source for the ore-bearing fluid, without ruling out a magmatic contribution. Aspects such as lithological association, geological setting, hydrothermalism, mineralogy, structural style, chemical and physical behavior and provenance of the auriferous fluid of the Caxias and Areal deposits are similar to those described for mesothermal or epizonal to mesozonal lode-gold deposits found in Archean to Cenozoic cratons and metamorphic belts worldwide.