Reconnaissance Stable Isotope (C, O, H, S) Study of Paleoproterozoic Gold Deposits of the São Luis Craton and Country Rocks, Northern Brazil: Implications for Gold Metallogeny

Abstract

Caxias, Areal, and Pedra de Fogo are structurally controlled syn- to late tectonic and post-metamorphic gold deposits of Paleoproterozoic age located in the São Luis craton, northern Brazil. Previous fluid inclusion studies indicated reduced, low-salinity CO2-H2O (±CH4-N2) fluids trapped at 260°-300°C (Caxias, Areal) to 330°-400°C (Pedra de Fogo), and at 2 kbar as responsible for the mineralization. Oxygen isotope ratios of quartz (+10.4 to +16.2‰), chlorite (+7.6‰), and sericite (+5.1‰), and hydrogen isotope ratios of chlorite (-46‰), sericite (-58‰), and fluid inclusion water (-32 to -70‰) indicate fluid Δ18O values of +2.6 to +5.6 per mil (Caxias), +0.6 to +3.5 per mil (Areal), and +10.3 to +12.1 per mil (Pedra de Fogo) and fluid ΔD composition of -24 to -53 per mil (Caxias), -49 to -62 per mil (Areal), and -70 per mil (Pedra de Fogo) at the assumed temperatures. These estimated fluid compositions are consistent with metamorphic sources. Calcite and fluid inclusion CO2 Δ13C values between -3.1 and -10.9 per mil are not diagnostic of a particular origin, but a more negative value (-20.2‰) found at Caxias indicates a greater organic component, at least locally. Sulfide Δ34S values of -2.8‰ to -11.0‰ reflect possible achievement of more oxidized conditions. Hydrogen and oxygen isotope analyses of the country rocks suggest minor isotope disequilibrium and resetting of oxygen isotope geothermometers. This might indicate subsolidus post-crystallization isotopic exchange, linked with metamorphism and/or hydrothermal alteration.