Multi-mechanism REYs enrichment in early Cambrian phosphorites within inner-shelf: Constraints from the geochemistry characteristics of francolite in Kunyang, Yangtze Block

Abstract

Middle and heavy rare earth elements and yttrium (MHREYs) are critical to the high-tech and green-energy industries, generating tremendous supply risk in recent decades. Recently, sedimentary phosphorites have been identified as a new resource for MHREYs. The Early Cambrian is considered one of the critical large-scale phosphorus-forming periods in China and globally. During this period, widespread large-scale phosphorites mainly occurred at shelf, while small-scale phosphate concretions predominantly existed in near-slope settings on the Yangtze Block, South China. However, multi controls for ore-forming mechanism of extraordinary REYs enrichment in phosphorites have not been well constrained, limiting the understanding of the formation of large-scale phosphorites at shelf. To better understand the REYs enrichment in phosphorite, the REYs-rich Kunyang phosphorite, the largest phosphorite in shelf environment on the Yangtze Block, was investigated through mineralogy, in-situ geochemistry of major and trace elements in francolites. Based on the mineral characteristics, four types of phosphorite and four types of wall rocks were divided, which can be grouped into high-REYs (435–717 ppm) and low-REYs (224–282 ppm) categories. Comparing the geochemical characteristics of high-REYs and low-REYs groups, multi-mechanisms of REYs enrichment can be supposed. Frequent Fe redox cycling and related suboxic conditions may be responsible for the extraordinary REYs enrichment in phosphorites. In high-energy hydrodynamic systems with a low sedimentation rate, prolonged deposition of francolite enhanced the effects of adsorption and substitution in the early and late diagenetic stages, respectively, significantly increasing REYs uptake. Moreover, in the inner-shelf environment, wave fluctuations and storm effects are significant, leading to extensive diagenetic reworking, which form extremely high REYs contents in the altered rims of francolite. The negative Eu anomalies in the high-REYs group also indicate a positive influence of porewater on REYs enrichment. Regarding the sources of REYs, both seawater and terrigenous detrital material contribute to REYs in phosphorite at the shelf environment. The sedimentary high-REYs group indicate that terrigenous detrital material may enhance REYs content in seawater, which is then transferred to francolite during diagenesis. Consequently, multiple mechanisms controlled the REYs enrichment, leading to the formation of large-scale phosphorite in the inner-shelf.