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

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 minerali

-zation 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. The gold occurs as inclusions in with silver-tellurides, inclu

-sions in pyrite and free gold in 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.

Journal of the Geological Survey of Brazil