Fluid evolution of Triassic and Jurassic W mineralization in the Xitian ore field, South China: Constraints from scheelite geochemistry and microthermometry

Abstract

Rare earth element (REE)-enriched scheelite is widely distributed in many ore deposits, and has been utilized in a broad range of geochronological and geochemical studies. In an effort to decipher skarn and quartz vein-hosted W mineralization process, scheelite from Triassic and Jurassic granites, skarn ores and quartz vein-hosted ores in the Xitian ore field (South China) were analyzed for major and minor element content and fluid inclusion microthermometry. All scheelite grains show homogeneous backscatter electron and cathodoluminescence response and uniform W and Ca distribution. The chondrite-normalized REE patterns of scheelite from Triassic granites to skarn ores show similar Pr–Sm-enrichments and negative Eu anomalies but different HREE contents. In contrast, the chondrite-normalized REE patterns of scheelite from Jurassic granites to quartz vein-hosted ores changes from flat with slight LREE enrichment, to concave with remarkable LREE depletion. The homogenization temperatures and salinities of fluid inclusions from the scheelite in Triassic granites and skarn ores are similar, both reduce significantly from the Jurassic granites to quartz vein-hosted ores. Combined with pressure, pH and mineral assemblage analysis, we propose that the REE signatures in the ore-forming fluids of skarn-type W deposits are mainly controlled by crystallization of REE-enriched skarn minerals (e.g., garnet, diopside, epidote). The similarity in LREE patterns and Y/Ho ratios between the Triassic granite and skarn ore scheelite indicates that the skarn ore-forming fluid was derived from the Triassic granite, while the distinct REE patterns and variable Y/Ho ratios in scheelite from Jurassic granites and quartz vein-hosted ores imply possible fluid-mixing for Jurassic W mineralization. In light of these results, REE-rich scheelite could serve to discriminate fluid source and document mineralization processes in a complex ore-forming system.