Fluid inclusion studies on Caxias and Areal gold mineralizations, Sao LuisCraton, northern Brazil

Caxias and Areal are minor gold deposits in the Sao Luis Craton, northern B razil. The geological setting comprises dominant Paleoproterozoic granitoid s of the Tromai Suite, with minor occurrences of coeval greenstone belt-lik e sequences of the Aurizona Group, both correlatives to Eburnean-Birimian r ocks of the West African Craton. Deformation of the lithologies is weak, al most restricted to small-sized shear zones. Gold mineralization at Areal is hosted by a monzogranite and associated with a subparallel quartz vein sys tem and its hydrothermalized haloes. Caxias gold mineralization is associat ed 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 sc hists and chlorite-biotite schists with sharp subvertical foliation. In bot h sectors, widespread multidirectional quartz veining is observed. Petrogra phic, microthermometric and Raman spectroscopic studies of fluid inclusions from quartz veins associated with the mineralizations revealed the presenc e of early carbonic/aqueous-carbonic and late aqueous fluid inclusions. The CO2-bearing fluids exhibit three different gaseous compositions: CO2 + mod erate CH4; CO2 + low CH4 + moderate to high N-2 (both restricted to CX-S); and CO2 + low N-2 fluids (occurring in all deposits). The CO2-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% CO2 and up to 2.5 mol% N-2. Only at CX-S this fluid was modif ied by introduction of CH4 and enrichment in N-2 by reaction with carbonace ous wall rocks. The wide range of composition, microthermometric properties and CO2/H2O ratio displayed by the CO2-bearing inclusions were interpreted as product of heterogeneous trapping (by phase separation) of two partiall y immiscible fluids. Bulk composition isochores combined with chlorite geot hermometer bracketed T-P conditions between 262 and 307 degreesC and betwee n 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 bri ttle-ductile transition. The compositional and P-T features of the minerali zing fluid, together with its reduced character (logfO(2) between -29.8 and -34.2), as well as the pyrite abundance in the host rooks, suggest that go ld was transported as a sulfur reduced complex and that its deposition occu rred in response to phase separation, reduction of H2S activity by sulfidat ion of the wall-rocks, and lowering of fO(2) due to fluid-rock interactions . Collectively, the described characteristics favor a metamorphic source fo r the ore-bearing fluid, without ruling out a magmatic contribution. Aspect s such as lithological association, geological setting, hydrothermalism, mi neralogy, structural style, chemical and physical behavior and provenance o f the auriferous fluid of the Caxias and Areal deposits are similar to thos e described for mesothermal or epizonal to mesozonal lode-gold deposits fou nd in Archean to Cenozoic cratons and metamorphic belts worldwide. (C) 2000 Elsevier Science B.V. All rights reserved.