Evolução metalogenética e correlações regionais da ocorrência mineral Porcão, província aurífera Alta Floresta, Matupá-MT

Abstract

This study is focused on one of the highest gold productions in Mato Grosso State, in the context of the Alta Floresta Gold Province (PAAF) in the Amazon Craton, encompassing portions of the geochronological provinces Ventuari-Tapajós and Rio Negro-Juruena. The genesis of the PAAF rocks and mineralizations is related to the evolution of magmatic arcs during the Paleoproterozoic, generating concentrations that are currently exploited in the region, mainly in secondary deposits. Thus, the object of the present study is the Porcão mineral occurrence, an area with mineralized granitic plutons, mined in secondary material in the surroundings. It seeks to understand the relationship between the metallogenic processes responsible that generated the primary mineralization and correlate them with tectonic and regional structural evolution. To achieve this, different methods were applied, including lithostructural mapping, lithogeochemical analysis, satellite image analysis, geophysical methods such as aeromagnetometry, terrestrial magnetometry, microprobe analysis, scanning electron microscope and U-Pb LA-ICP-MS geochronology. The granite bodies are configured in two main ellipsoidal hills surrounded by small outcrops. The granitoids were defined as biotitemonzogranites and individualized into two facies, according to petrographic characterization and age. Thus, the greyish pink facies biotite-monzogranite, restricted to a small area of mineral occurrence, appears as the oldest rocky body, with 2035 ±10 Ma and characterized by fine to medium granulation, magnetic, isotropic, and equigranular. Predominant on the ground, the pinkish facies biotite-monzogranite occurs in two generations, the first being 1948 ±17 Ma and the second 1873 ±27 Ma, characterized by medium to coarse granulation, magnetic, isotropic, equigranular and higher incidence of hydrothermal alterations. In addition, there is the occurrence of a 1915 ±16 Ma gabbro dyke crossing the older granitoids of the terrain, with fine to medium grain and ophitic texture. Structurally, the mineral occurrence is affected throughout its extension by a shear zone that generated three families of penetrative foliations, with medium attitudes NW-SE (N40W/78NE), WNW-ESSE (N80W/80NE) and NE-SW (N45E/68NW), defined in corroboration with magnetometry analysis. These structures are interpreted as conduits for hydrothermal fluids, which resulted in quartz veins (medium attitude V1=N54E/60SE), which are sometimes mineralized, as well as gabbro dykes (average attitude N42W/87NE), in addition to alterations in the host rocks. The hydrothermal alterations resulted from processes of chloritization, sericitization, propylitization, oxidation, potassification and silicification, moreover to zones with disseminated sulfides. The sulfide zones predominate where the hydrothermal alterations of sericitization are more intense and there is the occurrence of pyrite, magnetite and, less commonly, chalcopyrite, pyrrhotite and gold. Gold occurs, with high Ag grades, as inclusions in sulfides, magnetite, and quartz. Lithogeochemical analyzes indicate syn-collisional magmatic arc granites. The Au contents are mainly compatible with those of S and, secondarily, with SiO2, Al2O3 and K2O, related to the hydrothermal alteration zones. The tectonic-structural evolution of the terrain starts from a context of plate convergence and island arc formation, with a first moment of crustal thickening during plane subduction at 2035 ±10 Ma, followed by crust extension between 1948 ±17 and 1915 ±16 Ma and, finally, the resumption of the flat subduction crustal thickening dynamics at 1873 ±27 Ma. The N40W transcurrent shear zone, derived from the advance and rotation of the continental crust, is suggested as the main control structure of the alterations, and identified mineralizations, coming from a meso to epithermal hydrothermalism.