XU Jian-bin1,2, XIAO Jia-fei1*, YANG Hai-ying1,2, XIA Yong1, WU Sheng-wei1,2, XIE Zhuo-jun1
(1.State Key Laboratory of Ore Deposits Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;
2. University of the Chinese Academy of Sciences, Beijing 100049, China)
Abstract: The REE contents of phosphorites in the Gezhongwu Formation of the Motianchong ore block in the Zhijin phosphorite-type REE deposit are high with average ∑REE and Y contents of approximately 650×10-6 and 200×10-6, respectively. Through petrological, mineralogical and major and trace elements geochemical studies of phosphorites, it is believed that the REE enrichment characteristics and constraints of phosphorites are given below. 1) The variation of REE contents along the profile section is greatly affected by rock types, with the subsequently gradual increase of REE and Y contents in the following order from dolomite, phosphorous-bearing dolomite, dolomitic phosphorite, phosphorites, to black (carbonaceous) phosphorites. 2) The contents of ∑REE and Y are positively correlated with P2O5 contents of various rocks and phosphorites, reflecting that ∑REE and Y contents may be closely related to phosphorus chemical compositions and mineral components of rocks. 3) The phosphorites of middle and lower parts of the Gezhongwu Formation have mainly sandy structure with columnar and strip-shaped mineral grains and a certain amount of biological debris. The phosphorites of upper section of the Gezhongwu Formation have mainly silty structure with elliptical or round mineral grains. P2O5 and REE contents of rocks in the middle and lower sections are relatively high but those of rocks in the upper section are relatively low. Therefore, it is inferred that the REE enrichment of those rocks may be related to the mineral structure and biological activity, and biological bodies are helpful to the enrichment of REE, especially LREE. 4) The Post-Archean Australian Shale (PAAS) standardized REE patterns of the phosphorites show a "hat type" distribution mode with the MREE enrichment, and obviously negative Ce anomalies. The LREE/HREE ratios are greater than 5, showing the enrichment of LREE. The Ce/Ce* ratios of phosphorites in the Gezhongwu Formation are all lower than -0.1, indicating an oxidized environment during their deposition time. The oxidized depositional environment may be in favor of the enrichment of phosphorus and REE. The negative correlation between Ce/Ce* and LaN/NdN values indicates that the enrichment of LREE may be related to the redox condition of ancient seawater. The more oxidation of redox condition is the more enrichment of LREE.
Keywords: REE enrichment; constraints; phosphorus deposit; Gezhongwu Formation; Zhijin
ACTA MINERALOGICA SINICA Vol. 39, No.4, 2019, page 371-379
