The Paleoarchean Northern Mundo Novo Greenstone Belt, São Francisco Craton: Geochemistry, U-Pb-Hf-O in zircon and pyrite δ³⁴S-Δ³³S-Δ³⁶S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U-Pb-Hf-O isotopes of zircon and multiple sulfur isotopes (³²S, ³³S, ³⁴S, and ³⁶S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ±25 Ma old and has zircon with ε_Hf(t)= -2.47 to -1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ¹⁸O=+3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ±22 Ma. These detrital zircons have ε_Hf(t)=-5.40 to -0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ¹⁸O=+4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ³³S between -1.3‰ to +1.4‰) and a Δ³⁶S/Δ³³S slope of -0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ³⁴S range (0 to +2‰), Δ³³S~0‰, and Δ³⁶S~0‰. A carbonaceous schist shows positive δ³⁴S (2.1‰-3.5‰) and elevated Δ³³S (1.2‰-1.4‰) values, with corresponding negative Δ³⁶S between -1.2‰ to -0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ³⁴S, Δ³³S, and Δ³⁶S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.