TRACING PALEOSALINITY CONDITIONS IN THE MINAS SUPERGROUP USING MULTI-PROXY GEOCHEMISTRY, QUADRILÁTERO FERRÍFERO, BRAZIL

The Quadrilátero Ferrífero (QF), Brazil, is a world-class iron deposit with Banded Iron Formation (BIF)-hosted iron systems. The Minas Supergroup, exposed in this region, comprises over 3000 meters of Paleoproterozoic metasedimentary successions, including clastic and chemical rocks. Among them, the Cauê Formation represents a Lake Superior-type BIF (~2.4 Ga), with thicknesses reaching up to 300 meters. The region underwent greenschist-facies metamorphism during two major tectonic events, around 2.0 and 0.5 Ga. The depositional evolution of this basin is critical for reconstructing paleoenvironmental conditions that preceded and succeeded large-scale iron accumulation (Cauê Formation), as well as for providing insights into the biogeochemical processes active during the Paleoproterozoic. In this study, we analyzed 27 bulk-rock samples from outcrops and drill cores in the Gandarela Syncline. The sampled intervals included fine-grained sedimentary rocks from the Caraça Group (pre-BIF sedimentation) to the Piracicaba Group (post-BIF). To reconstruct paleosalinity conditions, we applied geochemical proxies based on elemental ratios (B/Ga, Sr/Ba, S/TOC), which are sensitive to temporal variations in water salinity. In addition, we evaluated the enrichment factor (EF) for redox-sensitive elements (RSE), including: Cr, Cu, Mo, Ni, U, and Zn. These elements may provide evidence of the redox dynamics and microbial activity in the surface water. Paleosalinity indicators display an upward increase in values throughout the stratigraphy: B/Ga ranges from 0.7 to 5.6, Sr/Ba from 0.04 to 0.62, and S/TOC from 0.01 to 0.4. RSE concentrations are higher in the northeastern sector of the study area, particularly within carbonaceous phyllites (Batatal Formation), where the maximum values reached up Cr_EF (3.0), Cu_EF (12.4), Mo_EF (6.4), Ni_EF (10.9), Zn_EF (105), and a Mo_EF/U_EF ratio (1.4). Despite the effects of regional metamorphism, consistent trends in geochemical proxies suggest a coherent environmental evolution. The paleosalinity data indicate a shift from freshwater to low-brackish conditions during deposition of the Batatal Formation, evolving to predominantly marine environments during the accumulation of the Cauê, Gandarela and Cercadinho formations. This interpretation revises previous models that proposed marine conditions were already established during Batatal Formation sedimentation. Furthermore, the enrichment factors of redox-sensitive elements (e.g., Mo, U, Cu, and Ni), along with the Mo_EF/U_EF ratio, point to predominantly suboxic to anoxic conditions in the Minas basin. The relatively high concentrations of Cu, Ni, and Zn in carbonaceous phyllites support the occurrence of microbial activity and elevated bioproductivity preceding BIF deposition. Our results help to refine the paleoenvironmental model for the Minas Basin, demonstrating the importance of multi-proxy paleosalinity studies for tracking environmental gradients in metamorphosed sedimentary basins.