Petrografia, química mineral e processos hidrotermais associados ao depósito de ouro São Jorge, Província Aurífera do Tapajós, Cráton Amazônico

Abstract

The Paleoproterozoic São Jorge gold deposit is situated in the Tapajós Gold Province of the Amazonian Craton. The deposit is hosted by an amphibole biotite monzogranite, composed of quartz, potassic feldspar, plagioclase, amphibole, biotite, titanite and opaques. Four dominant mineral assemblages were recognized in the deposit. Assemblage 1 was formed during the magmatic stage and is characterized by the presence of amphibole and andesine-oligoclase. Assemblage 2 shows the total replacement of the amphibole and intense saussuritization of the primary plagioclase, epidote is a ubiquitous phase and biotite is partially chloritized. Assemblages 3 and 4 are related to the hydrothermal processes, which generated the sulfide and gold mineralization. Assemblage 3 is dominated by chlorite and albitic plagioclase with subordinate white micas and, sometimes, biotite. Assemblage 4 is dominated by white micas, carbonates and pyrite. It is a result of a phyllic alteration, with associated carbonatization. Chlorite geothermometer suggests temperatures of 300±40 °C for the 3 and 4 assemblages. Al in hornblende geobarometer indicates low pressures, around 1 kbar for the crystallization of the mineralized granites. Oxidizing conditions, above NNO, prevailed during the genesis of the deposit. The hydrothermal assemblages of São Jorge differ from those described at Joel and Davi and are not dominated by epidote minerals as registered in other areas of the Tapajós province. A porphyry model or intrusion-related model are best adapted for the São Jorge deposit. The latter has similarities with the Serrinha deposit in the Juruena province and Batalha in the Tapajós province but it has more strong analogies with the Volta Grande hydrothermal system in southern Brazil.