Geologia, alteração hidrotermal e gênese do depósito IOGG Cristalino, Província Mineral de Carajás, Brasil.

Abstract

The Archean Cu-Au Cristalino deposit is located 40 km east from Sossego mine in the eastern end of the regional WNW-ESW Carajás shear zone, in the Serra do Rabo region. Its host rocks are mainly the mafic volcanic rocks of the Parauapebas Formation, and subordinately the BIF of the Carajás Formation. Field-work, petrographic data seconded by SEM-EDS, in addition to microprobe analysis and fluid inclusion and stable isotope (O, H, C and S) systematics, allowed characterizing a hydrothermal system that was responsible for the development of successive alteration zones in the wall rocks. Sodic metasomatism (650°C ≥ T > 400°C, and P > 1.8 Kbar) formed near pure albite, schorlitic tourmaline, REE-rich minerals (allanite-Ce, monazite) and minor calcite and quartz. It was followed by a more pervasive calcic-ferric alteration which produced abundant actinolite (XMg = 0.9-0.7 and up to 0.6 wt. % Cl), allanite-Ce (up to 0.6 wt. % Cl) and magnetite, associated with sulfide disseminations and replacement breccia-like bodies composed of chalcopyrite-pyrite-magnetite-Au (early ore association). Locally, Fe-edenite (XMg=0.7-0.4, Cl up to 2.9 wt. %) replaced calcic-ferric assemblages within restrict sodic-calcic alteration halos. From 410o down to 220o C and 1.8 Kbar > P > 0.6 Kbar, the previous alteration assemblages were overprinted by potassic (Kfeldspar, minor biotite) and propylitic/carbonatic (epidote, chlorite, calcite) alterations, in this order. K-feldspar is practically stoichiometric, but with high contents of BaO (up to 1.2 wt. %). Chlorite shows the greatest compositional variation among all minerals and seems to have been controlled by the type of host rock, chemistry of the hydrothermal fluid and temperature. Both chamosite and clinochlore (XFe=0.4-0.8) are present, the former being more common. Chlorine contents are in general < 0.02 wt. % and a little more significant in chlorites that replaced chessboard albite (up to 0.06 wt. %). The late ore association (chalcopyrite±Au±pyrite±hematite) is contemporaneous with the potassic and propylitic alterations and bears evidence that the Cristalino system evolved to the final stages with increase in oxygen fugacity. The ore fluid was hot (> 550°C), hypersaline and chemically approached by the system H2O-NaCl-CaCl2-CO2±MgCl2±FeCl2. Salinity exceeded 55.1 wt. % NaCl equiv. in the early stages but decreased progressively to 7.9 wt. % NaCl equiv. from 250o C on, after incursion of surficial water into the system. Initially 18O-enriched/D-depleted (δ18Ovsmow =+9.7 to +6.5 ‰; δDvsmow= -30.8 to -40.2 ‰) and most likely derived from magmatic sources, the fluid became relatively 18O-depleted/D-enriched (δ18Ovsmow =+5.57 to - 0.28‰; δDvsmow= -19.15 to -22.24‰) as result of dilution caused by mixing with meteoric water. δ13CVPDB values for vein and breccia calcite (-6.5 to -3.8‰) are consistent with a deep source for CO2, which was probably released from an underlying magma chamber. The δ34SVCDT values for chalcopyrite show narrow variation (+1.6 to +3.5 ‰) and indicate a homogeneous reservoir for sulfur, which was likely of igneous origin. Although most data point to a magmatic affiliation, a few samples reveal significant influence of sedimentary rocks on their isotope composition. Mostly transported as chloride complexes (>350oC), Cu and Au precipitated in response to decrease in temperature and Cl- activity and increase in pH. An aqueous, colder (200-150o C) and less saline (21-3.1 wt. % NaCl equiv.) fluid appears to have circulated in the Cristalino deposit area, being trapped as secondary fluid inclusions. The origin is unknown, but it could be related to a nearby Paleoproterozoic granitic intrusion. The data presented here support previous interpretations that consider Cristalino as of IOCG typology. In comparison with other Archean Carajás IOCG deposits, particularly those that lie in the southern sector of the Carajás Domain, the Cristalino deposit shows many similarities regarding ore fluid composition and evolution, as well as the isotopic signature of sulfides and carbonates.