Early Neoproterozoic Gold Deposits of the Alto Guapore Province, Southwestern Amazon Craton, Western Brazil

Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at similar to 300 degrees C and similar to 2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of delta O-18 values of quartz (12 +/- 1 parts per thousand) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of delta D for hydrous minerals (-116 to -55 parts per thousand) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at similar to 300 degrees C and similar to 2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of delta O-18 values of quartz (12 +/- 1 parts per thousand) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of delta D for hydrous minerals (-116 to -55 parts per thousand) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores. Fluid inclusion data, coupled with stable isotope geochemistry and geothermometry, indicate that gold precipitated from a low-salinity, CO2-rich fluid at similar to 300 degrees C and similar to 2.5 kbar. The source for the fluid and gold was the interbedded pelites during devolatilization of the Aguapei Group sequence. The aqueous-carbonic fluid inclusions and the narrow range of delta O-18 values of quartz (12 +/- 1 parts per thousand) from many auriferous veins from the central part of the province represent a regional ore-forming fluid. The broad range of delta D for hydrous minerals (-116 to -55 parts per thousand) reflects influx of small amounts of meteoric water into the steeply dipping shear zones during postgold exhumation. The 40Ar/39Ar geochronology from hydrothermal muscovite indicates a widespread hydrothermal event along the belt between 928 and 920 Ma. Collectively, the geological, geochronological, and geochemical data suggest that metamorphic fluids migrated laterally into and then upward along the Aguapei belt and deposited gold in lower-order structures where strain gradients existed between lithounits. The province has many characteristics of large orogenic gold provinces worldwide and represents a highly prospective and underexplored target region for early Neoproterozoic gold, a time period that generally is not well endowed in gold ores.

Automated summary

Fluid inclusion data, stable isotope geochemistry, and geothermometry suggest that gold was formed from a low-salinity, CO2-rich fluid at around 300 degrees C and 2.5 kbar pressure. The fluid and gold originated from interbedded pelites during the devolatilization process of the Aguapei Group. A regional ore-forming fluid is represented by the aqueous-carbonic fluid inclusions and the narrow range of delta O-18 values of quartz from auriferous veins in the central part of the province. The variation in delta D values of hydrous minerals reflects the influx of small amounts of meteoric water into shear zones after gold deposition. The geochronology data indicate a widespread hydrothermal event in the belt between 928 and 920 Ma. The migration of metamorphic fluids along the Aguapei belt and the deposition of gold in lower-order structures suggest the province's potential as an underexplored target region for early Neoproterozoic gold, a time period that generally lacks significant gold deposits.