Influence of weathering and hydrothermal alteration on the REE and δ56Fe composition of iron formation, Cauê Formation, Iron Quadrangle, Brazil

Abstract

Continental margin (Superior-type) iron formation of the Cauê Formation, Iron Quadrangle, Brazil, is composed of a laminated, magnetite-rich chemical meta-mudstone that was altered during hydrothermal circulation of basinal fluids, greenschist to amphibolite facies metamorphism, and subsequent supergene enrichment of Fe. The absence of traction current deposits and storm-generated structures suggests that deposition occurred on the distal shelf, possibly as deep as the continental slope. Paragenesis-based lithogeochemical analyses of two drill cores with contrasting alteration styles highlight the importance of hydrothermal, metamorphic, and supergene processes on the rare-earth element plus yttrium (REE + Y) and δ56Fe compositions of iron formation. The first drill core penetrates to a depth of 540 m in the Ouro Fino Syncline (OFS) where lithofacies have experienced intense hydrothermal and supergene alteration. The second core is from the Gandarela Syncline (GS) and contains 450 m of metamorphosed iron formation that has undergone later supergene enrichment of Fe.Petrographically, euhedral to subhedral magnetite in the OFS and GS is fabric destructive, and depending on the style and degree of alteration, can co-occur with dolomite, siderite, goethite, and/or martite. Synsedimentary magnetite and other primary Fe-oxide phases are absent. The effect of supergene enrichment and weathering is best captured along the depth profile of each core by the inverse relationship between SiO2 and Fe2O3; SiO2 values are lowest and Fe2O3 values highest near the surface. The presence of goethite to 500 m records the maximum depth of these processes in the subsurface. The REE + Y, and Y anomalies from the deepest and least altered lithofacies in both cores are typical of deposition in a redox-stratified water column. The weathered lithofacies show several changes in the REE + Y patterns, Ce anomalies and absence of Y anomalies, caused mainly by fluid percolation.The change in Fe isotopic composition with depth is more complex. In drill core from the OFS, negative δ56Fe values shift from −0.91‰ at depth to less negative values of −0.6‰ near the surface. The most altered samples do not lie on this trend because they are characterized by positive values of 0.30‰ and 1.33‰. The δ56Fe values from the GS increase linearly from −0.11‰ to 1.23‰ towards the surface with the most negative values −0.7‰ and −1.16‰ near the top. The most negative values from the OFS could reflect synsedimentary microbial dissimilatory Fe reduction, and the positive values from the GS may be related to partial Fe2+ oxidation between the hydrothermal Fe source and Fe-(oxyhydr)oxides. However, when interpreted together with mineralogical and REE data, these data likely record the various alteration processes that affected the Cauê Iron Formation. Such findings highlight the importance of coupling sedimentologic analysis with elemental and isotopic chemistry to understand iron formation deposition and Fe ore-forming processes.