Shearing and fluid evolution of the Porto Nacional orogenic gold district, western Brazil: microstructural, fluid inclusion, and C-O isotopic evidence

Abstract

The Porto Nacional Gold District, in the northern Tocantins Province, Brazil, is one of the few crustal-scale shear systems that contain both hypozonal and mesozonal orogenic gold deposits. It is situated within a Paleoproterozoic terrane, hosted by the crustal-scale Neoproterozoic Transbrasiliano-Kandi Shear System. The Cachimbo Shear Zone (CSZ) hypozonal deposits are hosted by phyllonites characterized by staurolite, garnet, and median to high-crystallinity graphite, consistent with amphibolite facies conditions. The mesozonal deposits, hosted in the Mutum Shear Zone (MSZ) and the Conceição Shear Zone (CoSZ), show a paragenesis that reflects greenschist-facies conditions. All zones host quartz fault-fill veins and adjacent extensional quartz-carbonate veinlet swarms. Two mineralizing stages are recognized. Stage 1 occurs in old quartz porphyroclasts in the fault-fill veins of the CSZ and MSZ deposits, and comprises pyrite, Type-2 graphite, gold, and other sulphides. In the CSZ stage 1, pressure-temperature data derived from mineral thermometry, quartz microstructure, and fluid inclusion isochores are compatible with a ductile deformation regime (433° to 580 °C and 3.5 to 6.3 kb), equivalent to 16 to 24 km depths under lithostatic conditions. Conditions for MSZ stage 1 are 350° to 415 °C and 2.0 to 3.1 kb, where the calculated depth (9 to 12 km) suggests the MSZ developed at shallower levels in the crust than the CSZ. In both shear zones the ore fluids are represented by H2O-CO2 fluid inclusions that are CH4-bearing and low salinity (0.3 to 6 wt% NaCl), and have low ƒO2. Reactivation along the shear zones caused folding and boudinaging of the extensional veinlets and recrystallization by subgrain rotation in the vein quartz, suggesting temperatures of ~ 400 °C. Ore stage 2 (gold-galena) occurs in vug-fill quartz in the veins and in extensional veinlets of the MSZ and CoSZ. It is estimated to have formed at lower temperatures (340° and 390℃) and pressures (1.8 to 4.5 kb) than ore stage 1, at depths of 6 to 18 km. The fluids were H2O-CO2 rich, with salinities of 3 to 8 wt% NaCl. The high and fluctuating pressures, above lithostatic conditions, suggest supralithostatic conditions. The wide fluid pressure variation is consistent with the fault-valve model for both ore stages. Later, higher salinity aqueous fluids (up to 15 wt% NaCl) with no CO2 or CH4 circulated at 300℃ and at 1.0 to 1.7 kb, which is more compatible with hydrostatic conditions at depths of 4 to 7 km. The stable isotopic composition of the fluids (δ18OH2O = 2 to 12‰, δ13CCO2 = −4 to −8‰) and the local geological constraints indicate that the mineralizing fluids were metamorphic. The P-T characteristics of the system, which indicate a decrease in T and P from the CSZ to the MSZ and CoSZ suggests that the PNSZ developed concurrently in the greenschist-amphibolite window. The further development of footprints for the hypozonal-mesozonal orogenic gold deposits of the PNGD could help identify new target areas for exploration in Brazil and elsewhere in this Neoproterozoic crustal-scale shear system.