Abstract The global shortage of conventional rare earth elements (REE) deposits has prompted an increased interest in non-conventional sources, such as coal deposits. However, the underlying mechanisms governing the enrichment and occurrence of REE in such sources remain poorly understood globally. The present investigation examines the geochemical behavior and mode of occurrence of REE in Oligocene sub-bituminous coals from the Makum coalfield, northeast India. Through the integration of a sequential extraction procedure combined with FE-SEM-EDS, FTIR, ICP-MS, XRD, and XRF analysis, we characterized the distribution, mineral association, and mode of occurrence of REE across three coal seams. Vertical variability in REE concentrations is significant, with mean values increasing from Seam III (bottom,15.5 ppm) through Seam II (middle, 26.4 ppm) to Seam I (top, 61.9 ppm). Felsic source of REE is evident from high Al2O3/TiO2 ratios (average 25.8) and mineralogical composition dominated by quartz, kaolinite, and feldspar. Weak positive europium anomalies (mean EuN/EuN* = 1.44) are present across all seams. Geochemical anomalies, particularly weak positive gadolinium (GdN/GdN* = 1.31) and weak negative cerium (CeN/CeN* = 0.96) anomalies, along with ubiquitous framboidal pyrite and elevated sulfur content (2.81–4.63%), indicate substantial marine influence during peat formation. Sequential leaching reveals that REE predominantly occur in organic-bound fraction (58–95%), likely associated with hydroxyl and carboxyl functional groups identified by FTIR spectroscopy, followed by decreasing abundance in silicate and aluminosilicate, acid-soluble, sulfide, ion-exchangeable, and water-soluble fractions. Organic association is more pronounced in HREE (82–87% for Lu) than LREE (35–44% for Ce). This integrated investigation contributes to understanding fundamental processes controlling REE distribution in coal systems, with implications for similar organic-rich sequences. Research highlights <OrderedList> <ListItem> <ItemNumber>1.</ItemNumber> <ItemContent> Organic phase serves as the predominant REE host (58–95%) in Makum coalfield, with HREE exhibiting stronger organic affinity than LREE. </ItemContent> </ListItem> <ListItem> <ItemNumber>2.</ItemNumber> <ItemContent> REE concentrations increase systematically four-fold vertically upward through successive coal seams, influenced by periodic marine incursions. </ItemContent> </ListItem> <ListItem> <ItemNumber>3.</ItemNumber> <ItemContent> Strong negative correlations between REE and aluminosilicate components (SiO2: r=-0.80; Al2O3: r=-0.80) demonstrate REE enrichment occurs independently of mineral phases. </ItemContent> </ListItem> <ListItem> <ItemNumber>4.</ItemNumber> <ItemContent> Progressive decrease in organic-bound REE despite increasing total REE concentrations indicates enhanced mineral-phase contribution in upper seams, reflecting transition from passive to active continental margin settings. </ItemContent> </ListItem> </OrderedList>

Geochemical behavior and mode of occurrence of Rare Earth Elements in Oligocene coals from the Makum Coalfield, NE India

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发表日期：

2026年5月18日

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Shivam Sharma,
Prakash Tiwari,
Ramkrishna Mondal,
Ritu Bala,
Parveen Goyal,
Debabrata Das

摘要

Abstract

The global shortage of conventional rare earth elements (REE) deposits has prompted an increased interest in non-conventional sources, such as coal deposits. However, the underlying mechanisms governing the enrichment and occurrence of REE in such sources remain poorly understood globally. The present investigation examines the geochemical behavior and mode of occurrence of REE in Oligocene sub-bituminous coals from the Makum coalfield, northeast India. Through the integration of a sequential extraction procedure combined with FE-SEM-EDS, FTIR, ICP-MS, XRD, and XRF analysis, we characterized the distribution, mineral association, and mode of occurrence of REE across three coal seams. Vertical variability in REE concentrations is significant, with mean values increasing from Seam III (bottom,15.5 ppm) through Seam II (middle, 26.4 ppm) to Seam I (top, 61.9 ppm). Felsic source of REE is evident from high Al₂O₃/TiO₂ ratios (average 25.8) and mineralogical composition dominated by quartz, kaolinite, and feldspar. Weak positive europium anomalies (mean Eu_N/Eu_N* = 1.44) are present across all seams. Geochemical anomalies, particularly weak positive gadolinium (Gd_N/Gd_N* = 1.31) and weak negative cerium (Ce_N/Ce_N* = 0.96) anomalies, along with ubiquitous framboidal pyrite and elevated sulfur content (2.81–4.63%), indicate substantial marine influence during peat formation. Sequential leaching reveals that REE predominantly occur in organic-bound fraction (58–95%), likely associated with hydroxyl and carboxyl functional groups identified by FTIR spectroscopy, followed by decreasing abundance in silicate and aluminosilicate, acid-soluble, sulfide, ion-exchangeable, and water-soluble fractions. Organic association is more pronounced in HREE (82–87% for Lu) than LREE (35–44% for Ce). This integrated investigation contributes to understanding fundamental processes controlling REE distribution in coal systems, with implications for similar organic-rich sequences.

Research highlights

Organic phase serves as the predominant REE host (58–95%) in Makum coalfield, with HREE exhibiting stronger organic affinity than LREE.

REE concentrations increase systematically four-fold vertically upward through successive coal seams, influenced by periodic marine incursions.

Strong negative correlations between REE and aluminosilicate components (SiO₂: r=-0.80; Al₂O₃: r=-0.80) demonstrate REE enrichment occurs independently of mineral phases.

Progressive decrease in organic-bound REE despite increasing total REE concentrations indicates enhanced mineral-phase contribution in upper seams, reflecting transition from passive to active continental margin settings.