Rare Earth and Rare Metal Elements Mobility and Mineralization During Magmatic and Fluid Evolution in Alkaline Granite System: Evidence from Fluid and Melt Inclusions in Baerzhe Granite, China

Abstract

AbstractBaerzhe Be–Nb–Zr–REE deposit is hosted in alkaline granite (125 Ma) which intrudes in the late Jurassic Baiyingaolao Formation in the middle of the Great Hinggan Metallogenic Belt in China. The ore‐forming granite consists of three lithological facies: arfvedsonite‐bearing alkaline granite at the bottom, aegirine‐bearing albite aplite in the middle and pegmatite crust on the top. The albite aplite is the main orebody. We recognized three magmatic‐hydrothermal stages: orthomagmatic stage, late‐magmatic stage and hydrothermal stage, with the late‐magmatic stage being divided into two substages, the pegmatite substage and the aplite substage. Petrographic study on the granite, the microthermometric study on fluid inclusions and in situ laser‐ablation inductively coupled plasma mass spectrometry analysis for quartz‐hosted melt inclusions reveal the process of magmatic‐hydrothermal evolution. The finding indicates that primary magma evolved to more peralkaline by fractional crystallization, with synchronously increasing high field strength elements. An extremely high content of Zr and Nb are in the melt inclusions from last stage albite aplite (Zr, min 52 548 ppm, and Nb, min 4104 ppm). This implies that the residual magma directly formed the orebody of rare metal elements. Meanwhile, volatility was increasing during the magma evolution process and F‐bearing aqueous fluid was oversaturated at temperatures higher than 800°C. The separation of fluid from magma caused Li‐REE enrichment in F‐bearing fluid and depletion in residual melt, and led to the difference of the Y/Ho ratio between whole rock compositions and melt inclusion data. Fluid separated into a high‐salinity liquid and a low density vapor phase above 697°C, and enriched REE in the high‐salinity liquid. The oxygen isotope data shows mixing between primary magmatic‐hydrothermal fluid and meteoric water. The ubiquitous pseudo‐secondary fluid inclusions have a wide range of salinity below 462°C, which is similar to the melting temperatures of REE‐bearing daughter minerals. A model involving the mixing by meteoric water could be a mechanism for precipitation of REE minerals.