Detrital material controlling the enrichment of critical element Li in No. 9 coal seam of the Ningwu Coalfield, northeastern Shanxi Province, China: Heavy mineral and detrital zircon constraints

Abstract

As a critical element, Li is currently in high demand due to rapid technological development. Anomalous Li enrichment in Pennsylvanian coals, such as those in Shanxi Province, has been discovered in China. Previous studies have shown that Li enrichment in coal and coal-bearing strata in Shanxi Province is generally evident in clay minerals and is related to mineral matter originating from nearby granite or bauxite and, in some cases, it is associated with hydrothermal fluid. Determining the exact sources of Li responsible for the Li enrichment in these coals is essential. This study investigated the spatiotemporal provenance of mineral matter evolution and source-to-sink system of No. 9 coal seam in the Anjialing Mine, Ningwu Coalfield, northeastern Shanxi Province, China. In total, 17 coal samples, six parting samples, one roof sample, and one floor sample from No. 9 coal seam were collected. Geochemical, mineralogical, and geochronological analyses were conducted using X-ray powder diffraction (XRD) and Raman spectroscopy for minerals, inductively coupled plasma–optical emission spectroscopy (ICP–OES) for major-element oxides, inductively coupled plasma–mass spectrometry (ICP–MS) for trace elements, and laser ablation–ICP–MS (LA–ICP–MS) for geochronology. The mineral matter in the coal samples consists mainly of kaolinite, boehmite, quartz, with varying proportions of calcite, pyrite, nacrite, anatase and goyazite, whereas in non-coal samples, the mineral matter is dominated by kaolinite, quartz, with minor amounts of anatase and pyrite. There are two heavy mineral assemblages: titanite–biotite–zircon–apatite and titanite–biotite–anatase–apatite. Relative to the elemental composition of the World hard coal, the coal benches are enriched in Li and Sr and slightly enriched in Ga, Zr, Hf, and Th. Relative to the elemental composition of the World clays, the parting samples in No. 9 coal seam are enriched in Li and slightly enriched in Mo, the roof sample is slightly enriched in Hf, and the floor sample is slightly enriched in Li and Hf. Detrital zircon ages in the roof and floor samples can be divided into two main ages: 2500–1700 and 326–293 Ma. The youngest UPb ages of zircon grain in the roof and floor samples are 292.7 ± 7.1 and 295.5 ± 9.7 Ma, respectively, indicating a well-constrained Early Asselian–Sakmarian stage. Detrital zircons, with ages of 326–293 Ma, in No. 9 coal seam are mainly derived from granitic intrusions and volcanic rocks in the Inner Mongolia Paleo-uplift (IMPU) rather than bauxite deposits. Based on mineralogical, geochemical, and geochronological evidence, the high Li enrichment in the studied samples is mainly caused by detrital material input. The dominant detrital materials in the coal and non-coal samples originate from felsic-intermediate igneous rocks in the IMPU.