Perovskite-Hosted LREE-Nb Mineralization in Pyroxenites and Magnetite-Perovskite Cumulates in Jashora Ultrabasic Alkaline Complex, India: Insights From Magmatic Segregation and Carbonatite Fluid Interaction

Abstract

This study provides the first comprehensive assessment of rare earth element (REE)—niobium (Nb) mineralization in the Jashora Ultrabasic Alkaline Complex (JUAC), northeastern India, addressing a critical knowledge gap in understanding the genesis and economic potential of enriched ultrabasic rocks. The JUAC hosts pyroxenites and magnetite-perovskite cumulates with exceptionally high concentrations of light REE (LREE; up to 20,000 ppm) and Nb (up to 1700 ppm). This study integrates field observations, petrography, whole-rock geochemistry, and in situ mineral chemistry to unravel the formation mechanism and mineralogical controls on REE–Nb enrichment in JUAC rocks. Perovskite (CaTiO3) is identified as the principal host mineral, contributing 58%–66% of total LREE and 48%–63% of Nb in pyroxenites, and 74%–94% (LREE) and 84%–94% (Nb) in magnetite-perovskite cumulates. This enrichment is explained by coupled substitution mechanisms (2Ca2+ ⇌ Na+ + REE3+ and 2Ti ↔ Nb5+ + Fe3+) on the perovskite A and B sites during magmatic crystallization. Accessory minerals such as ancylite, bastnäsite, strontianite, apatite, and pyrochlore precipitated at late-stage carbonatite vein boundaries, collectively contributing 35%–40% of LREE in pyroxenites and 10%–25% in cumulates, and up to 50% of total Nb despite their low modal abundance (< 1 vol%). The REE–Nb mineralization in JUAC results from magmatic segregation and gravity settling of magnetite-perovskite cumulates crystallized from a Ca-Ti-REE-enriched, silica-undersaturated magma under moderately oxidizing conditions, followed by metasomatic alteration linked to carbonatite intrusion. Geochemical signatures and mineralogy of JUAC closely resemble global alkaline-carbonatite complexes such as the Kola Province (Russia), indicating similar magmatic differentiation and metasomatic processes. Given the rising global demand for REE and Nb in technology and clean energy sectors, JUAC stands out as an important and unexplored resource for critical minerals. Our findings provide new insights into REE–Nb enrichment mechanisms in alkaline ultrabasic systems and also highlight the need for further exploration and resource evaluation in the alkaline complexes of north-east India.