The Tocantinzinho Paleoproterozoic Porphyry-Style Gold Deposit, Tapajós Mineral Province (Brazil): Geology, Petrology and Fluid Inclusion Evidence for Ore-Forming Processes

Abstract

The Tocantinzinho gold deposit, located in the Tapajós Mineral Province, Amazonia, Brazil, is considered the largest gold deposit in the region. It is a stockwork-disseminated gold deposit, hosted in a 1982 ± 8 Ma hydrothermalized monzogranite of the Creporizão Intrusive Suite, with petrographic and geochemical characteristics of volcanic arc to post-collisional granites. Gold is mainly associated with phyllic alteration. Primary fluid inclusions trapped in the mineralization stages are H2O–NaCl and unsaturated and homogenize either to the vapor or to the liquid with Th(t) of 300–430 °C, salinity of 2–16 wt % NaCl eq. and density from 0.43 to 0.94 g/cm3. At these conditions, Au is expected to be transported as Au(HS)2− complexes and ore is deposited as the result of boiling in the first mineralizing stages and of mixing between magmatic fluid and meteoric water during the phyllic alteration. Compared with other deposits, Tocantinzinho has similarities with magmatic-hydrothermal oxidized calc-alkaline granite-related gold deposits classified as porphyry gold deposits but we classify as a porphyry-style gold deposit, as it lacks some characteristics of the Phanerozoic porphyry-type deposits. The results from this study can be used to elaborate and guide prospection models in Amazonia and in similar Proterozoic terrains.