| Popis: |
The Aguapei Belt in the SW region of the Amazonian Craton is a NW belt of approximately 600 km long and 50 km width. Its is mainly composed of metasedimentary rocks of the Aguapei Group. Basement of this group is represented by basic and ultrabasic rocks, and chemical metassediments of the Rio Alegre Terrain (1,50 a 1,49 Ga) and granitic rocks of the Santa Helena Terrain (1,48 a 1,42 Ga). More than 20 gold deposits were identified in this belt, most of them hosted in the Aguapei Group (1.28 - 0.95 Ga), strongly deformed by the Sunsas-Aguapei Orogeny (1.1-0.9Ga). Here we present studies for three areas along the belt: the northern portion (Sao Vicente Mine); the Central portion (Lavrinha region deposits); and the southern portion (Pau-a-Pique deposit). The gold in these three areas is in quartz veins systems and disseminated into the hosted rocks. Highest gold grades are observed in quartz veins with comb, saccharoidal and replacement textures. Native gold occurs associated with pyrite, pyrrotite, chalcopyrite, native silver, arsenopyrite, galena, hematite, magnetite and martite. Microthermometric studies in fluid inclusions ofquartz veins display three inclusion populations distributed in two systems: trifhasic aquo-carbonic - H 2 O+C0 2 +NaCl (type I) and; biphasic aquous and monophasic aqueous - H 2 +NaCI (type II and III), both with low salinity «8 wt% NaCl eq.). Fluids are 2 related with deep hydrothermal system, and the main gold source is attributed to the process affecting ultrabasic and basic rocks and BIFs from the Rio Alegre and Pontes e Lacerda sequences. Petrographic data and geochemistry studies, associeated with 40 Ar/ 39 Ar ages in sericites from hydrothermal veins of the Pau-a-Pique deposit and from the Lavrinha region, ranges from 908.1 ± 0.9 Ma to 917.8 ± 0.9 Ma for the mineralization processes, indicating a strong fluid circulation at the ending of deformation processes that affected the Aguapei Group during the Sunsas-Aguapei Orogeny, that generated the epigenetic gold deposits. |
