Morfologia e composição de rutilo como guia prospectivo para depósitos de Au: o exemplo do depósito São Jorge, Província Mineral do Tapajós

Abstract

The Tapajós Mineral Province (PMT), inserted in the context of the Tapajós-Parima or Ventuari-Tapajós geochronological province, is recognized as the largest gold-bearing province in Brazil. The São Jorge Jovem Granite (GSJJ), located east of the PMT in a heavily fractured and hydrothermalized area, hosts gold mineralization. The GSJJ exhibits calcium-alkaline affiliation K-rich, composed of rocks of monzogranitic and leucomonzogranitic composition, with amphibole and biotite as the main mafic minerals. Pb-Pb dating of zircon in leucomonzogranites revealed a crystallization age of 1891±3 Ma. TiO2 polymorphs are common accessory phases in several types of rocks. In nature, it occurs in three main polymorphs: anatase and brookite, which are representatives of low temperatures and pressures, and rutile, at medium to high and ultra-high pressures and temperatures. Of these, rutile is the most common polymorph in the Earth's crust. The samples and polished blades used in this research come from drilling holes carried out by Rio Tinto Desenvolvimento Minerais (RTZ) in GSJJ. TiO2 polymorphs are represented by thin (10-100μm) anhedral crystals in or surrounding biotite. Samples from the mineralized zone show varied hydrothermal alteration, with rocks in the propylitic and phyllic alteration stages. TiO2 polymorphs form acicular aggregates or aggregates with a skeletal texture, predominantly replacing titanite. Spot electron microprobe analyses conducted on individual crystals and aggregates of rutile crystals revealed significant compositional variations, particularly concerning the contents of Nb, Al, Zr, V, and W. Rutile crystals from the non-mineralized zone (NMZ) exhibited higher levels of Nb (> 5,000 ppm) compared to crystals from the mineralized zone (MZ; predominantly < 3,000 ppm). Rutile crystals from the MZ display a trend of Al enrichment and V. Crystals from the NMZ sometimes show enrichment in Fe but are always accompanied by levels considered to be Nb. ZM crystals exhibit a trend of enrichment in W, V, and Sb and are always depleted in Nb. Analysis of rutiles demonstrates that their chemical composition can be used to distinguish between mineralized rocks and other types of non-mineralized rocks. The results indicate that the trace element composition of rutile, mainly the V content, and other elements such as Al, Zr, Sb, W, offer the best indications of mineralization in gold deposits.