Metallogenic characterization of the Aguinaldo structurally-controlled Epithermal gold occurrence, Alta Floresta Gold Province (Brazil)

Abstract

In the eastern-southeast sector of Alta Floresta Gold Province (Amazon Craton), the 1.78 - 1.77 Ga calc- -alkaline magmatism is interpreted as answerable for ore-forming processes responsible for its disseminated, vein-type, and filonar gold occurrences hosted in 2.05 - 1.77 Ga units. The Aguinaldo structurally-controlled gold occurrence exhibits ore zones hosted in 0.5 - 1.5m thick quartz + sulfide ± carbonate veins crosscutting the porphyroblastic mylonite orthogneiss, with SHRIMP U-Pb-zircon concordia crystallization age of 1,984.9 ± 5.2 Ma. Distal to ore zones are observed high temperature, intensive, and pervasive (1) albite-rich alteration, followed by (2) K-Feldspar alteration, telescoped by lower temperature pervasive alterations: (3) sericite-muscovite alteration, (4) silicification and silica infills; (5) carbonate alteration, (6) chlorite alteration, and (7) late veinlet-type quartz. The silica injection represents one of the major hydrothermal stages due to generating barren quartz veins and quartz ± carbonate vein with chalcopyrite + pyrite ± gold ± silver ± sphalerite ± galena ± bismuthinite veins. Although the veins are partially deformed, local comb-quartz texture remains preserved. Gold occurs as small inclusions or in contact with pyrite and chalcopyrite, and subordinately, in sphalerite. Electron microprobe analysis indicates chlorite of brunsvigite composition formed between 242° and 420° C, whereas white mica shows low phengite contents. The polymetallic veins signature, the high chalcopyrite contents with galena and sphalerite inclusions, the comb-texture quartz veins, together with the hydrothermal alteration sequence, suggest low to moderate temperature, neutral to slightly alkaline pH, and moderate O2 and S2 conditions, possibly triggered by boiling process. This scenario indicates intermediate-sulfidation epithermal ore-forming processes as the responsible for the genesis of Aguinaldo gold occurrence.