A preliminary study of the rare earth element enrichment characteristics and ore formation environment of the Early Cambrian Yangchang phosphorite deposit in the northeastern Yunnan

<p indent="0mm"> A wide range of early Cambrian phosphorite deposits were developed in South China. With the extensive attention paid to phosphorite-type rare earth deposits in recent years, the formation environment and rare earth element and yttrium (REY) enrichment characteristics of the Yangchang super-large phosphorite deposit, discovered in recent years, need to be revealed urgently. Thus, the mineralogy, elemental geochemistry, and in-situ geochemical analysis of Yangchang phosphorite deposit have been carried out in this paper in order to identify the REY compositional characteristics of phosphorites and to reveal the formation environment and its constrain to the REY enrichment. The study shows that the average ∑REY content of the whole rock of the Yangchang phosphate deposit is 454.12×10^–6, with a maximum of 1274.08×10^–6, while the average ∑REY of the collophane is 1081.30×10^–6, with a maximum of 3175.70×10^–6. This indicates that there is a considerable potential for the REY resources, especially for the Y element which accounts for 1/3 of the ∑REY in the whole rock and 0.45 in the collophane. The petrographic characteristics revealed that the sedimentary facies of the Yangchang phosphorite deposit are characterized with ″gradually progressive + multi-cycle″ in the vertical direction, and the overall transition from subtidal flat to intertidal flat to supratidal flat, showing a typical evolutionary trend of sea regression. The geochemical indicators, including Ce/Ce^* ratios (0.18~0.32), Sr/Ba ratios (0.84~6.33), and Sr/Ca ratios (19.67×10^–4~95.88×10^–4) of whole rocks reflect that the Yangchang phosphorite deposits was formed under an arid and oxidized condition. The degree of climate aridity increased in the vertical direction, which is consistent with the evolution trend of sea regression. The positive correlations between ∑REY contents and ratios of Sr/Ba and Sr/Ca of whole rocks show that relatively arid conditions were beneficial to the REY enrichment. The oxidized seawater provided the possibility for the adsorption and release of REY by organic matters and/or iron and manganese oxides. The REY was adsorbed by organic matters and/or iron and manganese oxides from seawater and was released from them under reduced conditions, resulting in the increase of ∑REY contents in the pore water and the subsequent enrichment of REY in apatite. In this process, the Xiaotan-Yangchang Basin provided a suitable paleogeographical location, permitting the enrichment of REY in apatite in the early diagenetic process.