Geochemical characteristics and genesis of element enrichment of niobium-rich-rare earth metal layer in Longtan Formation of Zhina coalfield

Background With the increasing demand for strategic metals and the gradual depletion of traditional metal deposit resources, coal and coal-bearing rock series are considered to be potential sources of rare metals, and coal series associated has become one of the main directions of current mineral exploration. A set of coal measure strata rich in niobium (Nb) and rare earth elements (REE) has been found in Zhina coalfield, western Guizhou and the contents of Nb and REE are as high as 334.89 μg/g and 1014.51 μg/g, respectively. This discovery is of great significance to the theoretical study of coal measure metal deposits. Methods The Longtan Formation coal measures in Zhina coalfield were selected as the research object. Industrial analysis, Scanning Electron Microscope Energy Dispersive Spectrum (SEM-EDS), X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) were used to analyze the geochemical characteristics of coal-bearing strata rich in Nb and REE in Zhina coalfield, and the enrichment and genesis of key metal elements in coal-bearing strata were discussed in depth. Results and Conclusions (1) Coal measures belong to anthracite with low moisture, low-medium ash, low volatile and medium-high sulfur. The minerals in coal are mainly composed of pyrite, clay minerals and quartz, and sphalerite and anatase are also found. The major elements are mainly SiO₂ and Al₂O₃, and the contents of Fe₂O₃, MgO and TiO₂ are high. (2) Compared with the average trace elements in the world's coals, the coal-bearing strata are enriched in key metal elements such as Nb, Ta (tantalum), Zr (zirconium), Hf (hafnium) and REE. Vertically, the key metal elements of No.9 coal seam are the most enriched in the coal seam. On the plane, the content of key metal elements in the study area has a trend of gradually increasing from south to north. The type of rare earth elements in Zhina coalfield coal is light rare earth enrichment type, and the degree of light and heavy rare earth fractionation is high. At the same time, the negative anomalies of Eu and Ce elements indicate that the terrestrial supply is relatively stable, and the water depth does not change much. (3) Through the analysis of inactive elements and REE provenance tracing, the key metal-enriched materials in the coal measure strata of Longtan Formation in Zhina coalfield are derived from the weathered debris of Emeishan high-titanium basalt and top intermediate-acid rock in Kangdian ancient land, and are also affected by the input of co-deposited alkaline volcanic ash and felsic volcanic ash. At the same time, the key metal elements in coal form Nb(Ta)-Zr(Hf)-REY polymetallic mineralization enrichment layer under the common constraints of material source and sedimentary environment.