Geologia e alteração hidrotermal do depósito aurífero Tucano, NE do Cráton Amazônico

Abstract

The Tucano gold deposit (1.8 Moz at 1.7 g/t; Great Panther Mining public NI 43-101 report 2022) in the southeastern portion of the Guiana Shield, is a shear-zone hosted orogenic gold deposit equilibrated under amphibolite facies conditions at mid crustal levels. Main host rocks are marble and banded iron formation (BIF), belonging to the chemical metasedimentary interval of the NW-SE trending, paleoproterozoic Serra do Navio greenstone belt. Peak metamorphic conditions are estimated at temperatures of 570-640 °C and pressures of 4.1 ± 0.6 kbars. Hydrothermal alteration is characterized by skarn-type calc-silicate assemblages replacing metamorphic minerals and overprinting precursors’ fabric. The onset of alteration (550-600 °C) is given by a quartz-clinopyroxene-garnet ± biotite, tourmaline, ilmenite, manifested as veins, lenses and patchy aggregates. The common observation of boudinaged or assymetrically folded veins places the early alteration stage as pre- to early kinematic. The gold-bearing main alteration stage is characterized by the dominant amphibole-phlogopite-magnetite-pyrrhotite ± calcite assemblage, with widespread replacement of peak metamorphic and early hydrothermal assemblages. Enhanced fluid flow at this stage resulted in large volumes of altered rocks assembled into a distal zone of variable width (from few mm in marble up until 10 m in BIF) and a typically narrower proximal zone (usually < 2 m), with BIF alteration assemblage comparatively less varied, usually lacking garnet and phyllosilicates. The ore assemblage at the studied central TAP C orebody is largely dominated by pyrrhotite, with trace chalcopyrite. Additional species, such as loellingite, arsenopyrite and sphalerite are exclusive to the marble host rock and indicate reduced fluid conditions. Ore textures constrain the main stage alteration as syn- to late-tectonic. Visible gold is found in equilibrium with sulfide-arsenide assemblages as well as associated with silicate-oxide minerals with no direct contact with sulfides. This suggests that gold deposition was not only due to sulfidation of the host rocks, but possibly also as a result of small fO2 fluctuations with no immediate sulfur destabilization in the ore fluid. Garnet-bearing muscovite leucogranite dikes and stocks are unmineralized and did not take up shear strain, thus interpreted as postdating the mineralization event. The Au-Ag-S-Te-Na ± W-Bi-Se-V-Cu-P fluid geochemical signature is largely compatible with hypozonal orogenic gold deposits worldwide, except for Se, V and P. Se has a very strong positive correlation with Te (R = 0.91), having possibly been sourced alongside S and Te from sulfidic metapelitic rocks. V gain is reflected in the abundance of V-bearing hydrothermal magnetite, of which high abundances in proximal alteration zone are a unique feature of this deposit. The strong positive correlation between P and W (R = 0.72) also suggests a common source for both elements. The Tucano gold deposit is thus a hypozonal orogenic gold system featuring sodic alteration and developed under relatively reduced conditions and low sulfur activity, as attested by the persistence of locally abundant hydrothermal magnetite in the proximal zone.