Elemental Geochemistry and Pb Isotopic Compositions of the Thick No. 7 Coal Seam in the Datun Mining Area, China

Abstract

Thick coal seams recorded abundant petrological, geochemical, and mineralogical information regarding their formation, which in turn can reflect the characteristics of the coal-forming environments, provenance attributes, paleoclimate, and so on. In order to explore the geochemical and lead isotope characteristics of thick coal seams, the No. 7 coal seam in the Datun mining area, Jiangsu Province of China, was selected as the research object. In this work, 29 samples (including coal, roof, and floor rock samples) were collected from three coal mines in the Datun mining area. Through an analysis of the mineral composition and element geochemical characteristics in the coal samples, the enrichment degree of trace elements and modes of rare earth elements were determined. The genetic mechanism of abnormal enrichment of enriched elements is discussed, especially the modes of occurrence and isotope characteristics of Pb. The results showed the following: (1) The main minerals in the coal samples include quartz, potassium feldspar, plagioclase, calcite, dolomite, pyrite, gypsum, and clay minerals, with clay minerals, calcite, quartz, and dolomite being the most common. (2) The major element oxides in coal mainly include SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, P2O5, and FeO. In the vertical direction, the variation of SiO2, Al2O3, Fe2O3, MgO, K2O, and FeO in coal samples from the three coal mines is consistent. The average value of Al2O3/TiO2 in the samples of Kongzhuang, Longdong, and Yaoqiao coal mines is 28.09–50.52, which basically locates the samples in the felsic source area, such that the sediment source is considered to be felsic source rock. (3) Elements U, La, Pb, and other elements are more enriched in Kongzhuang coal mine samples; elements Th, U, La, Pb, and other elements are more enriched in the Longdong coal mine samples; and elements Th, U, La, Pb, and other elements are more enriched in the Yaoqiao coal mine samples. Furthermore, W is enriched in Yaoqiao mine samples and is highly enriched in Longdong mine samples. The mining area is generally rich in the elements U, La, and Pb. The distribution curves of rare earth elements in the three mines are inclined to the right, with negative Eu anomalies. The enrichment is of the light rare earth enrichment type. (4) Pb isotope data show that the samples from the three mines are mainly distributed in the orogenic belt and the subduction zone lead source areas, where the upper crust and the mantle are mixed, with individual sample points distributed in the mantle and upper crust lead source areas.