Geologia e mineralizações auríferas da área do Garimpo do Manelão, região do Bacajá (Pa)

Abstract

The Manelão area is part of the Itacaiunas Shear Belt located in the center-west of the State of Pará. The Archean-Proterozoic lithostratigraphic units recognized in this area are Xingu Complex granites and gneisses, amphibolites, micaschists and quartzites from the São Manoel volcano-sedimentary sequence and the Felicio Turvo intrusive stratoid monzogranite. Mesozoic gabbro and diabase dykes, Cenozoic laterites and recent colluvial and alluvial deposits complete the lithostratigraphic picture. The main structures identified in the area were originated by a sinistral transcurrent deformation of a ductile to ductile-ruptile nature that involved a rock mass transport from ESE to WNW. Such tectonism caused low to medium easy amphibolite metamorphism, although locally more intense hydrothermalism caused alteration of high greenschist facies. Gold occurs both in veins lodged in amphibolites and schists of the São Manoel volcano-sedimentary sequence, as well as in lateritic cover and in alluvial/colluvial deposits. Two systems of veins were identified, one with direction N70E and dip of 80° to NW, the other with direction N23E and dip equal to the previous one. In these bodies the gold is essentially in the free form, in particles of low purity (about 870) that fill fractures in the quartz or quartz-albitic gangue. Furthermore, it is believed that the pyrite disseminated both in the veins and in the mylonitized and heavily altered host rocks also contains sub-microscopic gold. The hydrothermal alteration resulted in a rough zoning in the host rocks. Upon contact with the veins, a philic alteration is normally observed, while the intermediate zone exhibits propylitization. The outer zone is marked above all by an intense epidotization of the rocks. The metallic minerals, pyrite (two generations), pyrrhotite, chalcopyrite, ilmenite and rutile are found in the form of crystals or small scattered aggregates and are more abundant in the host rocks than in the veins themselves. The fluid inclusion study revealed that the mineralizing fluids were essentially low-temperature (minimum temperature 160-180°C) aqueous solutions of low to moderate salinity (probably H2O-KCl-CaCl2 and H2O-NaCl-CaCl2) and low density. (0.9 - 1.1 g/cm 3 ). Rare three-phase inclusions suggest, however, that high temperature and high salinity solutions equally participated in the mineralizing process. Although CO2 was not detected in this preliminary assessment, its presence in subordinate amounts cannot be ruled out. Considering the predominance in the sulfide system and low temperatures of fluid trapping, the gold seems to have migrated mainly in the form of thio-complexes. In the lateritic cover developed over the São Manoel sequence, gold occurs in fine particles or small nuggets of high purity (about 985) with frequencies intergrown with iron oxyhydroxide both in the mottled zone and in relict fragments of the ferruginous crust. The high degree of purity of lateritic gold suggests that the mobilization of the metal occurred mainly after complexation with organic and thiosulfate ligands. Silver leaching into primary gold particles may also have accounted for the significant difference in purity. Gold particles and nuggets are also dispersed in the clayey matrix of colluvial deposits. Finally, gold forms concentrations of relevant economic value in the lower gravel horizon of alluvial accumulations.