Melt fluid evolution and ore-forming mechanism of the Murong superlarge single-vein pegmatite-type lithium deposit in western Sichuan, China

Abstract

The Murong superlarge pegmatite-type lithium deposit is located in the Songpan–Ganze orogenic belt in western Sichuan, China, and is now the largest single-vein pegmatite-type lithium deposit in Asia. This paper analyzes the melt fluid evolution process, lithium enrichment, and mineralization mechanism of lithium-rich pegmatite by studying the types of inclusion assemblages, estimated capture temperatures and pressures, and inclusion compositions of spodumene and associated minerals in different zones of the pegmatite. The Murong pegmatite can be laterally divided into three zones: the felsic shell zone (Zone Ⅰ), the coarse-grained spodumene zone (Zone Ⅱ), and the felsic core zone (Zone Ⅲ). Spodumene mainly develops in Zone Ⅱ. This study reveals six types of inclusions in the Murong pegmatite: melt inclusions (T_1-1), melt–fluid inclusions (T_1-2), crystal–rich inclusions (T_1-3), CO₂–NaCl–H₂O system inclusions (T₂), NaCl–H₂O system inclusions (T₃), and monophase CO₂ inclusions (T₄). There are significant differences in the melt/fluid inclusion assemblages and the formation temperatures of inclusions in different zones. The overall trend is from melt inclusions + melt–fluid inclusions to crystal–rich inclusions to CO₂–NaCl–H₂O system inclusions and then to NaCl–H₂O system inclusions, which records the complete process of pegmatite magma evolution from melt fluids to hydrothermal fluids, indicating a high degree of evolution of the pegmatite magma. During evolution from melt fluids to hydrothermal fluids, the composition system evolves from a high temperature (>450 °C) lithium-rich phosphate-lithium-rich carbonate silicate system enriched with volatile components (F₂, CO₂, B) to a medium to high temperature (240–350 °C) lithium-rich phosphate–lithium-containing carbonate silicate system enriched with volatile components (CO₂, B) and then to a medium to low temperature (<300 °C) hydrothermal fluid system enriched with volatile components (CO₂). The ore-forming process mainly occurred during decompression and cooling as melt fluids evolved into hydrothermal fluids, with pressures ranging from approximately 5 kbar to 1.5 kbar during mineralization. The ore-forming fluids evolved from a weakly alkaline environment in the early stage to an acidic environment in the late stage. Immiscibility is an important mechanism for promoting the precipitation of spodumene during the evolution of Murong pegmatite magma fluid. Compared with other pegmatite-type lithium deposits in the same tectonic belt, the extremely thin Zone Ⅰ (magmatic crystallization zone) and the absence of planar zoning in the study area suggest that the original magma, from which the pegmatite-type lithium deposits in the study area formed, may have been highly differentiated. The full-vein mineralization of the pegmatite may be related to the extremely high degree of differentiation of the original magma. Volatile components (F, B, P, and CO₂, etc.) playing an important role in the enrichment and migration of rare alkali metals.