Contrasting archean-proterozoic-hosted gold deposit types and associated gold-bearing fluids

The study of fluids from four contrasting Archean-Proterozoic-hosted Brazilian gold deposits shows their very distinctive composition, significative of contrasting physicalchemical conditions during gold deposition. At the Pontal mine, f 0 2 was slightly above and below the QFh4 buffer at the time of gold precipitation, and above the hematite-magnetite buffer at Costa Sena occurrences. The H20-NaCl-KCl fluids found in the Crixás Mine III deposit indicate the trapping of a probable very high salinity magmatic fluid which mixed with a cooler CO2-rich aqueous fluid. At Santa Rita occurrences, the highly saline fluids result from the leaching of evaporitic beds hosted in the Paranoa sequence by H2O-CO2-Nz-rich fluids.