A refined model for the mechanisms of Precambrian phosphorite formation

Abstract

Despite the economic and scientific importance of Precambrian phosphorites, our understanding of the mechanism leading to their formation remains limited, including for the largest phosphogenic episode in the late Neoproterozoic. To improve our understanding of Precambrian phosphorite formation, we combined sedimentology, petrography, and elemental, and Fe-C isotopic analyses to study the two main phosphorite beds (the lower and upper phosphorite beds) in the Ediacaran Doushantuo Formation, Zhangcunping area, South China. The phosphorites consist mainly of granular textures characterized by densely packed grains, some of which are coated with secondary phosphate growth. However, there are notable differences in the mineralogy, microfossil assemblages, and elemental contents of the two beds. The lower phosphorites have no Ce anomaly, and relatively low Y/Ho ratios and positive δ⁵⁶Fe values (0.04–0.30 ‰, average of 0.19 ‰). In contrast, the upper phosphorites have negative Ce anomalies and higher Y/Ho ratios and near-zero δ⁵⁶Fe values (−0.29–0.19 ‰ (average of −0.01 ‰). These observations suggest that the lower phosphorites formed in anoxic-suboxic environments, whereas the upper phosphorites formed in relatively oxygenated environments. The δ¹³C_carb values of the phosphorites range from −3.97 ‰ to 1.71 ‰ (average of −1.56 ‰), and are lighter than values in dolostones (−0.52 ‰ to 4.39 ‰, average of 2.02 ‰). This suggesting that formation of the Doushantuo phosphorites was influenced by degradation of organic matter in an ocean with high primary productivity. The lower phosphorites, which were also regulated by Fe redox pumping, have positive δ⁵⁶Fe values, along with the presence of pyrite framboids and iron oxides, suggesting deposition near the Fe-redox boundary where extensive Fe cycling. The upper phosphorites show positive correlations between Mn and Fe, and Mn/Fe and P₂O₅, suggesting formation near the Mn boundary with extensive Mn cycling, primarily mediated by Mn redox pumping. Sedimentological observation indicate that primary phosphates were concentrated into granular phosphorites by winnowing processes following primary precipitation. Accordingly, we propose a refined model for Precambrian phosphorite formation in which degradation of organic matter, Fe and Mn pumping, and physical reworking of deposits co-evolve and interact within a dynamic Precambrian redox environment. Our model provides a reasonable explanation for the distribution of global phosphorite deposits throughout geological history.