Geochemical Characteristics of Rare, Dispersed, and Rare Earth Elements in the Middle Jurassic Yan'an Formation of the Ningdong Coalfield and Their Indication for a Sedimentary Environment

Rare, dispersed, and rare earth elements are important strategic resources because of their special performance and function. At the same time, because of its unique geochemical properties, it is of great significance in the study of paleoclimate, paleo-water and paleo-sedimentary environment, and sediment sources. Previous studies have found that Ga in the Ningdong Coalfield is slightly enriched and rare earth element content is low.

To explore the possibility of enrichment and mineralization of rare, dispersed, and rare earth elements in the coal series of the Yan'an Formation of Middle Jurassic in the Ningdong Coalfield, and discuss the characteristics of paleosedimentary environment and properties of provenance rock.

Systematic sampling was carried out in the core of boreholes from the Lingwu and Jijiajing mining areas of the Ningdong Coalfield. ICP-MS was used to determine the content of rare, dispersed, and rare earth elements in mudstone, sandstone and top (bottom) plate of the Yan'an Formation.

The overall content of rare, dispersed, and rare earth elements in the Yan'an Formation of the Ningdong Coalfield is low, which do not meet the requirements of the industrial indicators of the deposit and cannot be exploited comprehensively. The highest content of Ga is 29.40μg/g, which is close to the lowest industrial grade of 30.00μg/g. Compared with the average content of elements in world coal, rare elements Li, Nb, Ta, Rb, Cs, Zr and dispersed elements Ga, In, Sc, Hf are slightly enriched or enriched. The distribution of rare earth elements shows LREY type, and significant negative anomaly of Eu.

Rb, Sr and REY indicate the humidity of paleoclimate conditions, Li, Sr and Ga represent the freshwater deposition environment, and Ce, Ce/La reflect the anoxic reducing environment. The REY content is not affected by diagenesis and can be used to discriminate the original sediment source.