Hydrothermal alteration, mineralization and fluid inclusions in the Pista and Fofão prospects: implications for the genetic model of the Coringa polymetallic deposit, SE Tapajós Mineral Province, Amazonian Craton, Brazil

Abstract

Pista and Fofão are prospects located near the main orebodies of the Coringa polymetallic deposit, in southeastern Tapajós Mineral Province of the Amazonian Craton. In both prospects, the mineralization is hosted in an Orosirian alkali feldspar granite, which is associated with coeval volcanic rocks at Pista. The paleo-hydrothermal system comprises an early post-magmatic stage, characterized by albite and K-feldspar alterations. Silicified and brecciated zones, sericitization of wall-rocks, both with adularia, as well as carbonatization (siderite and Mn-siderite rich stage) are the proximal ore alteration zones. Chlorite – hematite – rutile – epidote veinlets represent weak and distal zones of hydrothermal alteration. The mineralization is polymetallic (Au and subordinate Ag, Zn, Pb, and Cu) and occurs as dissemination and filling fissure veinlets. Gold occurs predominantly as Au-Ag-tellurides included in pyrite, and as free gold in association with pyrite, sphalerite and hematite within quartz veinlets. Fluid inclusions trapped in quartz veinlets show H2O, H2O-CO2-NaCl and CO2-only inclusions. Microthermometric data indicate an H2O-CO2-NaCl fluid, likely of magmatic derivation, with low-salinity (5.4 ± 1.3 wt. % NaCl equiv.) and moderate density (0.36-0.94 g/cm3) trapped between 264 and 370ºC, and a low-salinity aqueous fluid (≤11.2 wt % NaCl equiv.) trapped at 280 to 360ºC, possibly evolved from effervescence followed by isothermal mixing with an external heated H2O-NaCl fluid, probably meteoric. After this, the fluids were continuously diluted and cooled. This stage is represented by the liquid-rich H2O-NaCl fluid inclusions of low salinity (up to 5.3 wt. % NaCl equiv.) and relatively low trapping temperature (177 to 113°C). The petrographic and fluid inclusions data suggest that the Au, Ag and base metals precipitation resulted from fluid mixing and dilution, which led to the breakdown of Au(HS)2−. We interpret the Pista and Fofão prospects as part of a magmatic-hydrothermal mineralized system, with some characteristics compa-tible with those of low- to intermediate-sulfidation epithermal systems, despite the absence of some typical textures from these systems.