BACKGROUND Monazite is a common rare earth mineral and an important ore mineral in many rare earth deposits, while sulfur-containing monazite is relatively rare and is only found in a few carbonate rocks, kimberlites, and metamorphic rocks.

OBJECTIVES To accurately analyze the chemical composition of S-enriched monazite, and to infer its genesis.

METHODS The main chemical components of S-enriched monazite from the REE ores in the Baiyanggou area of Puchun County, Dabie orogenic belt were determined by polarized light microscopy and electron probe microanalysis.

RESULTS The sulfur-enriched monazite in the rare-earth mineralized amphibole schist in the study area was mostly colloidal and agglomerate, some were plate-like and columnar euhedral crystals, which were granular aggregates. There was mutual interaction, inclusion, and a semi-inclusion spatial relationship between sulfur-enriched monazite and apatite. Crosscut of apatite by network veins was also present. Petrography showed that there was a metasomatism similar to hydrothermal alteration between sulfur-enriched monazite and apatite. The SO₃ content of S-enriched monazite in the rare earth-enriched mineral samples was as high as 14.57%, with an average content of 10.54%, which was the monazite with the highest S content. The S-enriched monazite has a higher CaO content and a low NdO content, which was quite different from the composition of the granite genesis monazite and hydrothermal metasomatic sulfur-deficient monazite. According to the negative correlation between S⁶⁺ and P⁵⁺, as well as the negative correlation between (Sr, Ca)²⁺, S⁶⁺ and REE³+, P⁵⁺, the S-containing monazatite can be explained by the "anhydrite coupling" displacement reaction, that is, (Sr, Ca)²⁺+S⁶⁺↔REE³⁺+P⁵⁺. This indicated that hydrothermal activity related to REE mineralization in the Baiyanggou area was present.

CONCLUSIONS Combined with the geological background of the Baiyanggou area, it is inferred that the source of the hydrothermal fluid may be related to the intense lithospheric extensional movement and the late magmatic activity in the study area since the Cretaceous period. This provides new clues for research on the origin of rare earth polymetallic mineralization in this area.