Zacarias Au–(Ag–barite) deposit, Neoproterozoic Goiás magmatic arc, Brasília Orogen, Central Brazil: constraints on the metallogenesis of a gold-rich quartz–barite exhalite related to volcanogenic massive sulfide mineralization

The Zacarias deposit is a gold-rich volcanogenic mineralization (0.65 Mt @ 4.36 g t −1 Au, 48.86 g t −1 Ag and 10.67% barite) hosted within Neoproterozoic rocks of the Mara Rosa metavolcanic–sedimentary sequence in the Goiás magmatic arc. We investigated the geological and geochemical characteristics of the deposit to determine its origin and the role of barite. The mineralized zone consists of barite–muscovite–quartz–pyrite lenses hosted in mafic and felsic metavolcanic and chemical metasedimentary rocks, all affected by amphibolite facies metamorphism. The aluminous schist (muscovite–phlogopite–quartz and gahnite–phlogopite–quartz) assemblages are interpreted as metamorphosed hydrothermal alterations, reflecting phyllic, chloritic or argillic alteration precursors generated by volcanic–exhalative processes. In this context, the host rocks of the mineralization are interpreted as a gold-rich barite–quartz–muscovite meta-exhalite. We present a comprehensive dataset of mica chemistry that shows significant barium concentrations in muscovite (5.75–6.96 wt% BaO) and phlogopite (2.20–3.36 wt% BaO). Gahnite is common, with a high Cr 2 O 3 content (12.42 wt%). We explore evidence for the sulfur isotopic signatures of pyrite ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>δ</mml:mi> </mml:math> 34 S +2.81 to +7.24‰), which can indicate a mix of sulfur from thermochemical sulfate reduction, with possible contributions from magmatic and/or biogenic sources. Laser ablation inductively coupled mass spectrometry zircon U–Pb dating of the felsic metavolcanic rocks yielded a crystallization age of 908.3 ± 7.4 Ma, indicating that the deposit likely formed by volcanic–exhalative processes during the early stages of the Goiás magmatic arc, as part of the tectonic evolution of the Mara Rosa segment.