Controls of REY enrichment in the early Cambrian phosphorites

Abstract

Rare earth elements and Yttrium (REYs) are critical to the emerging high-tech and green-energy industries, generating tremendous REY demand in recent decades. Recently, many sedimentary phosphorites have been reported to have extraordinary REY enrichment (> 1000 ppm) and may become new REY resources. However, the controls of REY enrichment in phosphorites have not been well constrained. To better understand the discrepant REY enrichment in phosphorites, the early Cambrian high-REY Zhijin (ZJ) phosphorites (∼500–2000 ppm) and the relatively low-REY Meishucun (MSC, ∼200–400 ppm) and Xia’an (X’A, mostly < 200 ppm) phosphorites on the Yangtze Block of South China were investigated with mineralogy, bulk-rock elements, total organic carbon, in-situ elements, and Zn-Fe isotopes. The mineral characteristics, REY indexes, and in-situ REY mapping indicate that the X’A phosphorites may represent pristine phosphorites, whereas the ZJ and MSC phosphorites may have experienced intensive diagenetic alteration. Diagenetic alteration can only explain the REY enrichment in the MSC phosphorites compared to the pristine X’A phosphorites, but it does not sufficiently explain the extraordinary REY enrichment in the ZJ phosphorites. Additionally, the lower δ66Zn values of the ZJ and X’A phosphorites (δ66Znaverage = 0.16‰ and 0.14‰, respectively) than those of the MSC phosphorites (δ66Znaverage = 0.75‰) indicate higher productivity levels in the ZJ and X’A areas. However, the high-productivity X’A phosphorites yielded very low REY concentrations, indicating that the extraordinary REY enrichment in the ZJ phosphorites cannot be ascribed to high productivity levels. Notably, the ZJ phosphorites may have experienced more intensive Fe redox cycling under fluctuating oxic–suboxic deposition conditions (∼0.0‰–0.45‰ δ56Fe values in near-pure phosphorites) than the MSC and X’A phosphorites with completely oxic deposition conditions (∼0.0‰ δ56Fe values). Frequent Fe redox cycling can greatly enrich REYs in porewater, which can be subsequently transferred into francolites during its formation and early diagenesis. Therefore, frequent Fe redox cycling driven by fluctuating oxic–suboxic seawater conditions may be responsible for the extraordinary REY enrichment in the ZJ phosphorites. If this is the case, we propose that phosphorites deposited near the oxic–suboxic redox chemocline are favorable for extraordinary REY enrichment, such as the coeval near-slope phosphate concretions on the Yangtze Block and other phosphorites or phosphatic rocks formed in different basins at different geological times.