Abstract
Pista and Fofao are prospects located near the main orebodies of the Coringa polymetallic deposit, in southeastern Tapajos 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 trappingtemperature (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 Fofao prospects as part of a magmatic-hydrothermal mineralized system, with some characteristics compatible with those of low- to intermediate-sulfidation epithermal systems, despite the absence of some typical textures from these systems.
Highlights
The Tapajós Mineral Province (TMP), south-central portion of the Amazonian Craton (Fig. 1A), is an important metallogenic province of Brazil
Questions about the physical and chemical conditions under which this hydrothermal paleosystem evolved remain uncertain. These include: (i) have the textures been obliterated by erosive or supergene processes, which were very intense in the Amazon region? (ii) has the crustal depth of formation of Pista and Fofão targets been deeper when compared to that of the classic epithermal deposits, which could have prevented the formation of classic epithermal textures, and, the fact that the fluid paleosystem be richer in CO2 than most epithermal systems (Bodnar et al 2014), which indicates a higher depth of formation? These questions indicate the need for further studies to investigate and answer them
The geological characteristics observed in the Pista and Fofão targets, such as the: (i) shallow plutono-volcanic association of host rock; (ii) Au-Ag mineralization and metalbase (Zn, Pb, and Cu) sulfide mineral assemblages associated mainly with quartz veins in sericite rich hydrothermal alteration zones; (iii) quartz - adularia - pyrite ± sericite hydrothermal association; (iv) abundant hydrothermal alteration zones, in which, quartz - sericite - pyrite - carbonates are the main gold-related alteration zone, with distal and late and propylitic alteration indicate that the targets studied are part of a low sulfidation epithermal system
Summary
The Tapajós Mineral Province (TMP), south-central portion of the Amazonian Craton (Fig. 1A), is an important metallogenic province of Brazil. - JGSB 2020, 3 (1) 33-59 as well as the nature and evolution of the ore fluids and the proposition of genetic models for individual deposits (Dreher et al 1998; Jacobi 1999; Juliani et al 2002; Klein et al 2004; Juliani et al 2005; Borges et al 2009; Souza 2009; Misas 2010; Veloso and Santos 2013; Santiago et al 2013; Villas et al 2013; Santos et al 2013; Assunção and Klein 2014; Juliani et al 2014; Guimarães et al 2015; Tokashiki et al 2015; Silva Júnior and Klein 2015; Queiroz and Villas 2015; Borgo et al 2017; Cassini et al 2017; Queiroz and Klein 2018; Biondi et al 2018; Lopes and Moura 2019; Oliveira et al 2019) In this context, the Coringa deposit (845.000 t @ 7.95 g/t Au; 216 koz of Au; indicated resources – Gunnesh et al 2019) and the southeastern part of the TMP became widely studied (e.g., Guimarães et al 2015; Tokashiki et al 2015; Corrêa Lima et al 2014; Guimarães et al 2019). We present new information about hydrothermal fluids based on a microthermometry study on fluid inclusions trapped in mineralized quartz veins
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