THE ÁGUAS CLARAS Cu-SULFIDE ± Au DEPOSIT, CARAJÁS REGION, PARA, BRAZIL: GEOLOGICAL SETTING, WALL-ROCK ALTERATION AND MINERALIZING FLUIDS

Abstract

The Águas Claras deposit (ACD) is located in the central part of the Carajás ridge, Pará State, Brazil. Primary sulfide mineralization occurs in quartz veins emplaced along a brittle to brittle-ductile, NE-SW-striking shear zone with subvertical dips. This zone cross-cuts Archean siliciclastic sedimentary rocks of the Águas Claras Formation and basic sills. Supergene enrichment is a common feature even at depths down to 200 m where the sulfides are still partially preserved. Petrographic studies of the wall rocks and quartz veins allowed the identification of an intense hydrothermal alteration, characterized by chloritization, serialization, tourmalinization, silicification, argillization and carbonation, the first being the most widespread process. Texturally the vein quartz shows buck, comb, feather, phantom, breccia and deformation features. Fe-Cu sulfides ± Au are the main ore minerals with minor Zn and W. Pyrite and chalcopyrite are the dominant sulfides with subordinate sphalerite and arsenopyrite. The oxide phases are represented by magnetite, hematite and ferberite. Au was not observed but its occurrence has been reported in close association with arsenopyrite and chalcopyrite (Soares et al. 1994). The equilibrium assemblage pyrite-magnetite-hematite indjcates that relatively oxidizing conditions prevailed during the main mineralization stage with fo2 and fs2 ranges of 10-29 –10-23 atm and l0-9 –10-4 atm, respectively. The microthermometric data showed that the mineralizing fluids were essentially saline aqueous solutions corresponding to the NaCl-CaCl2-MgCl2-H2O system. Homogeneization temperatures were found to be between 360 and 100°C, although the most frequent range was 190-160°C. Two-phase inclusions presented salinity variations from 0.53 to >23.5 wt. % NaCl equiv. whereas the halite-bearing ones showed salinities of 35-45 wt. % NaCl equiv. The changes in salinity at high to moderate temperatures (360-160°C) may represent cyclic events of the mineral deposition. On the other hand, low-salinity, low-temperature (130-100°C) fluid samples may represent later evolving solutions of the system or mixing with meteoric water. The ACD is structurally controlled and was formed at low depths consistent with the thermal regime, vein filling textures and low grade metamorphism of the country rocks. The vein system shows textural features that have been interpreted as the result of repeated pulses with multiple generations of quartz, sulfide assemblages and fluids of different salinities.