Fluid evolution in the Pedra Preta wolframite ore deposit, Paleoproterozoic Musa granite, eastern Amazon craton, Brazil

The Pedra Preta wolframite ore deposit is hosted by a vein system that cuts rocks of the Archean Andorinhas supergroup and the cupola of the 1.88 Ga Musa granite. Several hydrothermal events have been recognized in the deposit area. The first is represented by pre-ore-stage veins that formed prior to the emplacement of the Musa granite. The second involved F-poor aqueous fluids (probably with CO2) exsolved from the crystallizing magma, which mixed with the host rock pore fluids, which contained CH4 and N2, and accounted for the ore-stage veins that formed along fracture planes at 2 Kbar pressures. The third event resulted from the reopening of older sealed fractures and served as a channelway for a reducing, metamorphic, CH4-bearing fluid. The oxidizing environment generated by that intrusion led to a substantial transformation of CH4 into CO2 at the site of emplacement. Late in the circulation of this fluid, wolframite crystallized at 330 °C. The fourth event was an F metasomatism (topaz and fluorite) related to H2O and CO2 fluids. The fifth and final hydrothermal event resulted from tectonic relaxation and formation of post-ore-stage veins by highly saline fluids (42 wt% CaCl2 equivalent) at the beginning and ≈30 wt% NaCl equivalent toward the end at 1.0 Kbar and less than 220 °C conditions. Most evidence drawn from the fluid inclusion studies points to the metamorphic fluids as the main W mineralizing solutions, not the Musa magmatic solutions.