Coexistence of Algoma- and Superior-type BIFs in back-arc systems: Evidence from Xincai, North China Craton

Abstract

Superior-type and Algoma-type banded iron formations (BIFs) generally developed in distinct tectonic environments and have unique geological characteristics. Superior-type BIFs were deposited on passive continental shelves or intracratonic basins and are laterally extensive, whereas Algoma-type BIFs were formed near volcanic arcs and spreading centers. Here, we present a case where late Archean (∼2.5 Ga) Superior- and Algoma-type BIFs co-developed in a back-arc setting at Xincai, along the southern margin of the North China Craton. The Algoma-type BIFs at Xincai exhibit high Ge/Si ratios (4.84 to 8.84) and a broad range of εNd(t) values (−7.2 to +1.2). These geochemical signatures, along with similar εNd(t) values (+1.2) in contemporaneous amphibolites, indicate a significant influence of hydrothermal fluids associated with submarine volcanism. In contrast, the Superior-type BIFs at Xincai are characterized by positive La anomalies, low Ge/Si ratios (0.21 to 4.60) and narrow range of εNd(t) values (−6.1 to −1.0) suggesting a major contribution from terrigenous sources. The coexistence of Algoma- and Superior-type BIF types at Xincai is related to the interplay of multiple fluid systems in a back-arc environment. The Algoma-type BIFs formed from the confluence of hydrothermal fluids originating from enriched subcontinental lithospheric mantle (high Ge/Si and low εNd(t)) with those emanating from the adjacent back-arc spreading centers (depleted mantle like—high Ge/Si and εNd(t)). On the other hand, the Superior-type BIFs formed near the continental margins of the back-arc basins, where surface seawater carrying terrigenous substances (low Ge/Si and εNd(t)) mixed with hydrothermal fluids from the back-arc spreading centers. The distinct co-existence of Algoma- and Superior-type BIFs at Xincai underscores the dynamic nature of back-arc environments as crucibles for diverse BIF mineralization processes, offering new perspectives on the contributions of submarine volcanic activity and terrigenous inputs to the BIFs.