Mapeamento geológico de detalhe e controles estruturais e hidrotermais da mineralização aurífera da Mina do Paiol, Almas (TO)

Abstract

Gold deposits hosted in greenstone belt sequences require integrated approaches for the proper understanding of their lithological, structural, and hydrothermal controls. In the municipality of Almas, southeastern Tocantins State, the Paiol gold deposit is located at the northernmost portion of the Brasília Belt, at the base of the Riachão do Ouro Group, within a greenstone belt–related geological setting. The area is characterized by rocks metamorphosed under greenschist facies conditions and subsequently affected by hydrothermal events. In this context, this study aimed to produce a detailed lithological–structural map of the Paiol Mine, focusing on the identification and characterization of lithologies, the interpretation of the main structural controls, and the analysis of the relationship between Au grades, lithotypes, and hydrothermal alteration styles. The methodology integrated open-pit geological mapping, description of reverse circulation (RC) drill holes, geochemical analyses of RC samples and grab rock samples by fire assay with MP-AES determination, as well as data interpretation and statistical treatment of the geochemical results. The mapping allowed the individualization of five main lithotypes, derived from a mafic volcanic protolith of the Córrego do Paiol Formation, which, according to regional data and previous petrographic studies, was subjected to amphibolite facies metamorphism and subsequently retrogressed to greenschist facies conditions. In the Paiol Mine area, these lithotypes are intensely reworked by ductile transcurrent shear and hydrothermal alteration processes, macroscopically recognized by mineral assemblages involving chloritization, sericitization, carbonation, silicification, and sulfidation. Shear zones with a preferential NNE–SSW orientation were recognized, responsible for the development of penetrative mylonitic foliation and for the geometric control of mineralization. Lithology-based statistical analysis indicates that the highest average gold grades are associated with the most deformed and most intensely silicified lithotypes, highlighting the direct relationship between structural permeability, fluid–rock interaction, and metal precipitation efficiency. The first detailed geological map of the Paiol Mine enabled the representation of dextral ductile shear zones that control the deposit, influence the direction and continuity of mineralization, and define the internal architecture of the host lithologies.