Geologic Setting and Hydrothermal Alteration at the Tucano Gold Deposit in Northern Brazil: Evidence for a Hypozonal Orogenic Gold System in the Guiana Shield

Abstract The Tucano gold deposit, located in the southeastern portion of the Guiana Shield, is structurally controlled and hosted in amphibolite facies metasedimentary rocks, mainly marble and banded iron formation (BIF), of the Paleoproterozoic Serra do Navio greenstone belt. The distinct high-pressure-temperature (P-T) hydrothermal alteration and relative timing of gold mineralization with respect to regional metamorphism and emplacement of leucogranite dikes make this deposit an ideal field laboratory to test the hypothesis that orogenic gold systems extend to hypozonal crustal levels. Combined detailed mapping, diamond drill core logging, petrography, mineral chemistry, and thermodynamic studies revealed that gold mineralization at Tucano is coeval with high-P-T replacement of peak metamorphic minerals at amphibolite facies conditions. Local preservation of open space-filling textures further supports the true high-temperature nature of hydrothermal alteration and gold mineralization. An early alteration stage (550°–600°C) is characterized by quartz-clinopyroxene-garnet and expressed as attenuated, boudinaged, or disrupted veins (pre- to early-kinematic). The gold-bearing main alteration stage (480°–590°C) has a dominant amphibole-phlogopite-magnetite-pyrrhotite ± calcite assemblage, with widespread replacement of metamorphic and early hydrothermal minerals. Sulfide precipitation increases toward the proximal alteration zone and is dominated by pyrrhotite with trace chalcopyrite. The assemblage pyrrhotite-loellingite-arsenopyrite is restricted to the proximal alteration zone in the marble host rock and indicates reduced fluid conditions. Gold-related hydrothermal textures, comprising both aligned alteration minerals within the shear zone fabric and poorly oriented euhedral to subhedral platy and prismatic alteration minerals, constrain the mineralization as a syn- to late-kinematic protracted event. Pressure estimates at 4.9 ± 1.2 kbar using the garnet geobarometer place this hydrothermal event at a hypozonal crustal level. Leucogranite dikes and stocks within the deposit are undeformed and are hence interpreted as postdating the gold mineralization event. Visible gold is in equilibrium with sulfide-arsenide assemblages and silicate-oxide minerals. Mass balance calculations indicate hydrothermal Na addition without concomitant K gain. Tucano is the first documented high-P-T orogenic gold deposit in the Guiana Shield, thus opening significant new exploration search space for orogenic-gold-style mineralization at deeper-crustal-level amphibolite facies terrains.