Classification and Provenance of Monazite in the Zhuoshui River System, Taiwan

Monazite is a phosphate mineral that contains Rare-Earth Elements (REEs), whose micro-textures and chemical compositions can serve as effective indicators of sediment provenance. In Taiwan, monazite in fluvial and coastal sediments may reflect contributions from both external sources and orogenic recycling; however, a systematic classification framework and provenance constraints for monazite in major river systems remain limited. This study focuses on the Zhuoshui River catchment and establishes a monazite classification scheme based on integrated microtextural and geochemical characteristics, which is then applied to assess the provenance of the monazite.

    Bedrock and riverbed sediment samples were collected from the upper reaches and along the Zhuoshui River system during both wet and dry seasons. Scanning Electron Microscope (SEM) backscattered electron (BSE) imaging and semi-quantitative Energy Dispersive Spectrometer (EDS) analyses were used to characterize grain morphology, inclusion features, and REE–Th–Y elemental systematics. The results show that monazite populations and La/Ce systematics are consistent between wet- and dry-season samples, indicating that the provenance signal is stable and not significantly affected by seasonal hydrological variability.

  Based on REE proportions, grain morphology, and inclusion characteristics, monazite grains can be classified into three types. Detrital monazite is generally larger, inclusion-free, relatively enriched in Th, and commonly displays rounded grain boundaries. Hydrothermal altered monazite is typically Th-depleted and LREE-dominated, commonly containing quartz–feldspar inclusions and occurring in association with hydrothermal minerals. Inclusion-hosted monazite shows distinct compositional boundaries and characteristic REE signatures, with relatively elevated middle-REE signals, suggesting early encapsulation rather than late-stage replacement. Similar micro textures and comparable La/Ce ratios observed in both upstream bedrock and downstream sediments support an orogen-derived provenance for monazite in the Zhuoshui River system. Two compositional clusters in La/Ce further imply at least two source regions, tentatively linked to metamorphic source rocks in the Central Range and the Hsuehshan Range. Ongoing U–Th–Pb geochronology using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) will provide independent temporal constraints. These data will strengthen the proposed classification and provenance interpretations, clarifying sediment transport pathways in the Zhuoshui River.