Estudo de inclusões fluidas e química mineral do depósito aurífero do alvo Jerimum de Baixo, campo mineralizado do Cuiú-Cuiú, província aurífera do Tapajós, Pará

Abstract

The Jerimum de Baixo gold target is located in the Cuiú-Cuiú golfield, central region of the Tapajós Gold Province, Amazonian Craton. The target comprises monzogranitic rocks, essentially isotropic, that were weak to strongly hydrothermal and carriers of Fe-rich biotite. Chloritization, sericitization, sulfidation, silicification and carbonatization are the most important types of alteration. The produced chlorite is enriched in Fe of the chamosite type and was formed mainly between 280 and 315°C, whereas the white mica assumes muscovitic compositions. The mineralization is represented by quartz veinlets with low sulfide content (pyrite + pyrrhotite ± chalcopyrite ± galena ± sphalerite) in which gold occurs as free-milling particles and in more fragil and altered zones, usually associated with pyrrhotite. The petrographic and microtermometric study of fluid inclusions hosted in quartz veinlets defined aqueou-carbonic, carbonic and aqueous inclusions. The fluids with CO2 represent the probable mineralizing fluid and were generated by phase separation processes between 280 and 380°C, mainly. Further infiltration and mixing processes are indicated for the later aqueous fluids. Temperatures <400°C and the reduced character of the environment (pyrrhotite compounding the ore) point to H2S as the major ligand in the mineralizing fluid and Au (HS)-2 as the primary gold transporting complex. Phase separation, changes in pH conditions, and fluid/rock interaction were the important mechanisms for Au precipitation, which occurred at the brittle to locally brittle-ductile level of the crust (between 2 and 6 km). In general terms, Jerimum de Baixo presents similarities among the other deposits/targets previously studied in terms of hydrothermal alteration, fluid types and mineralization. The features observed in Jerimum de Baixo do not allow a classificatory framework absolutely adequate to any of the classical metalogenetic typological models. Characteristics such as type and style of hydrothermal alteration, type and low content of sulfides, types of fluids involved, estimated depth for mineralization, metallic association (e.g., S, Bi, Te), together with the good correspondence between the data collected in other deposits/targets in the Cuiú-Cuiú goldfield indicate for the Jerimum de Baixo target a gold deposit with magmatic-hydrothermal affiliation, presenting greater similarity to those deposits classified as belonging to Reduced Intrusion-Related Gold Systems (RIRGS).