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Assessment of the Effect of Organic Matter on Rare Earth Elements and Yttrium Using the Zhijin Early Cambrian Phosphorite as an Example

by 

Shengwei Wu

 ^1,2,†,

Haiying Yang

 ^3,†,

Haifeng Fan

 ¹,

Yong Xia

 ^1,*,

Qingtian Meng

 ⁴,

Shan He

 ^1,2 and

Xingxiang Gong

 ^5,6

¹

State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China

²

University of Chinese Academy of Sciences, Beijing 100049, China

³

School of Earth Sciences, Yunnan University, Kunming 650500, China

⁴

No. 104 Geological Team, Guizhou Bureau of Geology and Mineral Exploration & Development, Guiyang 550081, China

⁵

Reserve Bureau of Land and Mineral Resources of Guizhou Province, Guiyang 550081, China

⁶

Technology Innovation Center of Mineral Resources Explorations in Bedrock Zones, Ministry of Natural Resources, Guiyang 550081, China

^*

Author to whom correspondence should be addressed.

^†

These authors contributed equally to this work.

Minerals 2022, 12(7), 876; https://doi.org/10.3390/min12070876

Received: 10 May 2022 / Revised: 5 July 2022 / Accepted: 6 July 2022 / Published: 12 July 2022

(This article belongs to the Section Mineral Geochemistry and Geochronology)

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Abstract

The geochemistry of rare earth elements and yttrium (REY) in phosphorite has been widely studied. However, the effect of organic matter on REY enrichment has not been well determined. We utilized paired inorganic (δ¹³C_carb) and organic (δ¹³C_carb) carbon isotopes, total organic carbon (TOC), and REY content (∑REY) of the Zhijin Motianchong (MTC) phosphorite and compared them with those of Meishucun (MSC) phosphorite to reveal the effect of organic matter on REY. The δ¹³C_carb of the MTC area (≈0‰) is heavier than that of the MSC area (−5.23‰ to −1.13‰), whereas δ¹³C_org is lighter (−33.85‰ to −26.34‰) in MTC than in MSC (−32.95‰ to −25.50‰). Decoupled δ¹³C_carb and δ¹³C_org in MTC indicate the contribution of chemoautotrophic organisms or methanotrophic bacteria. Compared to the MSC phosphorite, the MTC phosphorite has higher ∑REY and TOC, and these parameters have a positive relationship. MTC phosphorite has REY patterns resembling those of contemporary organic matter. Furthermore, dolomite cement has a higher ∑REY than dolomite in the phosphorus-bearing dolostone. Additionally, pyrites are located on the surface of fluorapatite in the Zhijin phosphorites. It is reasonable to suggest that the REY was released into the pore water owing to the anaerobic oxidation of organic matter at the interface between seawater and sediment, resulting in the REY enrichment of Zhijin phosphorites.