Trace elements and iron speciation analysis of Paleoproterozoic phyllite from the Gandarela Syncline: Implications for salinity and redox conditions in the Quadrilátero Ferrífero, Brazil

Abstract

The Quadrilátero Ferrífero, Brazil, is among the most significant iron ore provinces. This study utilizes geochemical proxies, including elemental paleosalinity ratios (B/Ga, Sr/Ba, S/TOC), redox-sensitive elements (RSE), and iron speciation, to investigate water mass evolution. Samples were analyzed from three drill cores and four outcrops in the Gandarela Syncline, providing new insights into the Paleoproterozoic phyllites associated with the Cauê BIF. Although metamorphism of these rocks might affect the application of the proxy, consistent directional trends in proxy results indicate a refined and/or robust paleoenvironmental reconstruction of the Minas Basin. Paleosalinity data indicate a transition from freshwater to low-brackish conditions during early sedimentation (Caraça Formation), followed by predominantly marine conditions during the deposition of the Cauê, Gandarela, and Cercadinho Formations. This model revises earlier interpretations suggesting an initial dominance of marine facies during fine sediment deposition in the Batatal Formation. Elevated concentrations of trace elements (Cu, Ni, and Zn) in carbonaceous phyllites highlight the role of microbial activity in bioproductivity before BIF deposition. Additionally, the decreasing enrichment factor _(EF) of Mo_EF/U_EF ratio and RSE concentrations toward younger rocks, coupled with variations in Fe_HR (highly reactive iron), suggest that the early basin waters were characterized by anoxic to suboxic conditions, with free O₂ likely present in surface waters. These results point to a stratified water column, featuring an oxic surface layer overlying ferruginous, anoxic water. This study advances understanding of the paleoenvironmental evolution of the Minas Basin and the depositional processes that facilitated the formation of the Quadrilátero Ferrífero's giant iron deposits. It also underscores the value of paleosalinity proxies for reconstructing environmental conditions in Paleoproterozoic systems, while also highlighting the need for more studies of how paleoenvironmental proxies behave during metamorphism.