Geochemistry of apatite individuals in Zhijin phosphorites, South China: Insight into the REY sources and diagenetic enrichment

Abstract

Marine sedimentary phosphorites are rich in rare earth elements (REE) and yttrium (REY) which primarily hosted in collophanite composed of closely-packed apatite nanocrystals. Limited by small particle diameters, the fine metallogenic process of apatite crystals and REY enrichment remains unknown. To address this, using early Cambrian REY-rich phosphorite from Zhijin, South China as an example, mineralogical and in situ geochemical characteristics of apatite individuals were analyzed and compared with collophanites. Our study found that: (1) the apatite individuals, with diameters between 40 and 100 μm, were divided into authigenic/diagenetic apatites, terrigenous clastic apatites, and hydrothermally altered apatites; (2) Terrigenous clastic apatites have highest ∑REY levels (4673–12232 ppm) and lowest Y/Ho (23–28), in addition, Y/Ho showed negative relationships with ∑REY in all apatite monominerals, as well as bulk phosphorites; (3) Compared with collophanites and old phosphorite, authigenic/diagenetic apatite individuals showed similar REY patterns, in addition, diagenetic apatites have higher ∑REY, Ce/Ce* and Eu/Eu* ratios but lower (La/Yb)N and (La/Sm)N (>0.5) ratios relative to authigenic apatite individuals and collophanites. This dataset indicated that weathering of terrestrial magmatic rocks provided not only clastic apatites but also REY into the South China ocean during the early Cambrian, mixed with REY derived from deep seawater. Authigenic apatite individuals form similarly to apatite nanocrystals in collophanites, with diagenetic modification playing vital role. During this process, apatite individuals grew up and underwent stronger diagenesis than apatite nanocrystals in collophanites, absorbing REY from porewater, which resulted in higher REY concentrations in apatite individuals. This study provides new data and insight into the formation of authigenic/diagenetic apatite individuals and improves our understanding of REY sources and diagenetic effects on REY enrichment.