Monazite occurrence and low-temperature alteration in river sediments from contrasting tectonostratigraphic units in southwestern Taiwan

Monazite from placer deposits along the southwestern coast of Taiwan was previously exploited as a source of rare earth elements (REEs). However, the formation mechanisms of the distinct monazite types remain debated and are commonly attributed to fluid-related processes. Due to the extremely high denudation rates of rivers in Taiwan, sediments undergo rapid transport, allowing the protolith characteristics of their provenance to be preserved. In this study, we examine river sediments from the Zengwen, Ailiao, and Laonong Rivers, which drain contrasting tectonostratigraphic units within each catchment. We characterize the occurrence and elemental distributions (e.g., La, Th, Nd) of monazite and examine the REE geochemical behavior of the bulk sediments. This study provides a comprehensive mineralogical and geochemical assessment of monazite associated with specific tectonostratigraphic units, offering constraints on sediment provenance.

Preliminary results summarize the variations in monazite occurrence and alteration across different tectonostratigraphic units. In the sediments of the Zengwen River, which primarily drains the Western Foothills, we observed a predominance of detrital monazite (<10 µm), as well as monazite associated with TiO₂, apatite, and clay minerals. These features suggest an origin primarily from the physical weathering of detritus or minor fluid precipitation, differing significantly from the occurrences of monazite found on the southwest coast.

Sediments from the Ailiao and Laonong rivers, which drain low-grade metamorphic rocks of the Central Range, exhibit evidence for variable degrees of low-temperature alteration affecting primary monazite. This includes inclusion-hosted, morphologically black monazite comparable to that observed along the southwestern coast of Taiwan. In the Laonong River, which additionally drains the Western Foothills, monazite occurs either within quartz grains or within the interlayers of clay minerals, similar to that observed in the Zengwen River. Furthermore, in rivers originating from the Slate Belt, in addition to monazite as a heavy mineral, we identified pyrite spherules comparable to those in the slate host rocks, as well as xenotime associated with thorite. Overall, these observations reveal distinct patterns in monazite occurrence and alteration among different tectonostratigraphic settings, with implications for sediment provenance in high-denudation river systems.