Mineralogical compositions and distributions of trace and rare earth elements in Eocene carbonaceous sediments of Western India: implications for paleoenvironment during peat accumulation

Abstract

The mineralogical compositions and distributions of trace and rare earth elements (REEs) in carbonaceous sediments provide valuable insights into the paleoenvironmental conditions during peat formation. Given the increasing demand for REEs in modern technologies, understanding their occurrence and economic potential is critical. However, the precise mineral hosts of trace and REEs in these sediments and their implications for the paleoenvironment during the Eocene epoch in India remain less explored. Therefore, this study examines Eocene carbonaceous sediments from the Saurashtra Basin in western India to uncover mineralogical controls on trace and REEs distribution and to infer paleoenvironmental conditions during peat accumulation. A total of 15 samples were collected from two mines (Surkha-lignite and Khadsaliya-shale). X-ray Diffraction (XRD), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), and Inductively Coupled Plasma Optical Emission Spectroscopy ICP-OES analysis were utilized to know the mineralogy, REEs, and trace elements distribution. The total REE concentrations in shale and lignite were low, while for in shale (avg. 195.89 ppm) was relatively higher than lignite (avg. 177.32 ppm), with cerium (Ce) being the most abundant element in both rock types. The concentrations of REEs in the studied lignite samples followed the order Ce˃Nd˃La˃Y˃Sc˃Gd˃Pr˃Sm˃Dy˃Er˃Yb˃Eu˃Ho˃Tb˃Tm˃Lu, while in shale samples the order was Ce˃Y˃Nd˃La˃Sc˃Gd˃Pr˃Dy˃Sm˃Er˃Yb˃Eu˃Ho˃Tb˃Tm˃Lu. The REE concentrations in the studied samples are notably lower than global averages, yet the presence of critical REEs suggests potential economic value. The outlook coefficient (C_outl) values ranging from 0.82 to 2.59 indicate promising REE sources within the basin. XRD studies revealed the presence of various mineral phases in the analyzed samples, including quartz, kaolinite, dickite, zeolite, coesite, anatase, pyrite, gypsum, calcite, biotite, clinopyroxene, montmorillonite, and magnetite. The dominance of kaolinite and quartz indicates that felsic rocks are the primary source of inorganic sediments in the paleomire. The major and trace element ratios suggest that the deposition of the studied lignite and shales occurred under conditions of increased oxygen deficiency, ranging from dysoxic-suboxic to anoxic environments. The transition from lake water to brackish water conditions was also evident with limited terrestrial influx in the basin. Furthermore, the Ce anomalies observed in the samples, ranging from 3.51 to 5.05 in shales and 3.92 to 4.13 in lignite, suggest shales were formed under oxidizing conditions and lignites in more restricted, potentially freshwater environments.