Petrografia, geoquímica e química mineral das rochas encaixantes do depósito São Chico, Domínio Tapajós, sudoeste do Pará

Abstract

The São Chico gold deposit is located in the homonym Village, in the southwest of Pará state, situated about 220 km south of the Itaituba city. Its enclosing rocks initially were mapped as belonging to Parauari Intrusive Suite (PIS) and the surrounding rocks as Cuiú-Cuiú Complex (CC) and Creporizão Intrusive Suite (CIS). However, petrographic, geochemical and chemical mineral studies in this work revealed that the rocks of the SIC are the enclosing of the deposit itself. In addition, this study allowed the individualization of six petrographic facies to the rocks of the CC: amphibole-biotita-granodiorite (ABGRN), pyroxene-quartz-monzonite (PQMZN), amphibole-biotite-monzogranite (ABMZG), biotite-leucogranodiorito (BLGRN), biotite-leucomonzogranito (BLMZG) and biotite-leucosienogranito (BLSG). The first three facies are located in the northwest portion of the area, while the last three are concentrated in the southeastern portion. In the CIS were distinguished biotite-amphibole-monzogranite (BAMZG), dominant and host mineralization, and of subordinated way, biotite-amphibolegranodiorite (BAGRN). In the PIS were recognized amphibole-granodiorite (AGRN), amphibole-monzogranite (AMZG) and amphibole-syenogranite (ASG). The rocks of the CIS, PIS and that one of the northwestern portion of the CC are isotropic, while the rocks of the southeastern portion have mild deformation. In general, the rocks are medium to coarse grained and show granular hypidiomorphic, granophiric and microporphyritic textures. The separation of the units has been reinforced by the composition of the main mafic phases, biotite and amphibole. Amphiboles of the ABGRN facies of the CC are Mg-hornblende, while in the rocks of the CIS and PIS have Fe-hornblende composition. Biotites of the ABGRN of the CC and CIS were characterized as primary biotites, while that ones of the BLSG of the CC are reequilibrated biotites. This set is similar in composition to biotites of the calc-alkaline granites. The geochemical study allowed the definition of calc-alkaline high-K signature, metaluminous to peraluminous character, relatively similar multielementary standard, as well as enrichment of light rare earth elements (LREE) in relation to heavy rare earth elements (HREE). The geochemical characteristics and correlation studies indicate geodynamic evolution involving an orogenic context related to a subduction environment where the rocks of the CC were generated, followed by a transitional environment between a convergent context for intracontinental extensional context, marked by the generation of the rocks of the CIS and PIS. Comparative geochemical studies were performed with units already studied and that occur near the São Chico gold deposit. Among them, Palito Granite Parauari Granodiorite and representative rocks of CC, CIS and PIS described by CPRM exhibt good correction with the studied units in this work. On the other hand, the Old São Jorge Granite, the Fofoquinha Granodiorite and Jardim do Ouro Granite showed a different geochemical pattern with the rocks of this study.