Textural features and in situ trace element analysis of fluorite from the Wujianfang fluorite deposit, Inner Mongolia (NE China): Insights into fluid metasomatism and ore-forming process

Abstract

The Wujianfang fluorite deposit shows a four-stage mineralization history: (1) pre-ore stage of disseminated fluorites, (2) early-ore stage of fluorite-muscovite-pyrophyllite, (3) main-ore stage of fluorite-pyrophyllite, and (4) late-ore stage of fluorite-albite-quartz. Detailed cathodoluminescence imaging, fluid inclusion study, and in-situ LA-ICP-MS trace element analysis of fluorite in different stages or mineral assemblages were applied to reveal the fluorite mineralization and metasomatic history of this deposit. These data show that water-rock reaction between hydrothermal fluids and wall rocks not only provides the dominant ore-forming materials for fluorite mineralization, but also promotes the precipitation of fluorite by changing the pH of ore-forming fluids. Temperature change also plays an important role in fluorite precipitation. Fluorite grains from the pre-, early-, and main-ore stages have been variably metasomatized by a coupled dissolution-reprecipitation-recrystallization mechanism, which is characterized by complex petrographic and chemical zones. During this process, many trace elements were leached from the primary fluorite grains or other accompanied minerals, and parts of these elements were immediately reprecipitated into the new phases. Then, the recrystallized phases were formed from the leachate solutions of primary minerals. Our study highlights that fluorite generally has a complex growth history due to metasomatic alteration by hydrothermal fluids during the ore-forming process. Thus, in-situ microanalysis is essential and necessary to obtain effective and accurate constraints on hydrothermal evolution, and most of the interpretations based on bulk analysis of fluorites crystals should be cautioned.