Mineralogical and geochemical constraints on mobilization and mineralization of rare earth elements in the Lala Fe-Cu-(Mo, REE) deposit, SW China

Abstract

The Lala Fe-Cu-(Mo, REE) deposit in the northern Kangdian IOCG metallogenic province, SW China, has a paragenetic sequence that includes pre-ore Na-alteration (Stage I), Fe-(P) mineralization (Stage II), and sulfide mineralization (Stage III). Rare earth element (REE) minerals, which are chiefly monazite, parasite, and bastnäsite with minor xenotime, are associated with sulfide mineralization. These REE minerals, together with biotite, muscovite, and calcite, are accompanied with K-Ca-carbonate alteration of Stage III. Where magnetite and apatite of Stage II are overprinted by minerals of Stage III, the apatite grains are commonly embayed or eroded and contain abundant rounded to subrounded and needle-like inclusions of monazite and minor bastnäsite, calcite, and sulfide minerals. These monazite-bearing apatite grains are texturally similar to metasomatized apatite in the laboratory, and can be attributed to the formation from Stage II apatite via a dissolution-precipitation process during fluid infiltration. The monazite-bearing apatite has REE contents and La/Sm ratios much lower than the original Stage II apatite, suggesting that REEs were leached and mobilized from this Stage II apatite, and that these processes were responsible for the formation of monazite inclusions within the apatite hosts. The monazite inclusions have compositions similar to those of Stage III monazite, suggesting that REEs in Stage II apatite were mobilized by Stage III fluids, consistent with occurrences of sulfide and calcite inclusions within the apatite. Mass balance analysis using bulk compositional data for host rocks that underwent K-Ca-carbonate alteration indicates that REEs of the hosting rocks were also mobilized, and that up to 70 percent of the light REEs were leached out during alteration. Therefore, REE mobilization recorded in apatite grains and country rocks support a model in which the country rocks were important sources for REEs in the Lala deposit. Compositions of amphibole and biotite, together with previously acquired fluid inclusion data, demonstrate that Stage III fluids are dominated by Na+ and Cl- but had higher K, CO2 (or HCO3- and CO32-), and HF/HCl fugacity and lower salinity (NaCl) than Stage II fluids. Elevated K and CO2 in Stage III fluids, or associated K-carbonate alteration, is proposed to be important for extensive REE mobilization on country rocks. The REEs leached from country rocks are inferred to have been transported as chloride complexes prior to being deposited as REE minerals. Deposition of REE minerals from Stage III fluids was likely triggered by mixing with external, relatively low temperature, F-rich, and high-pH fluids and/or by local interaction with carbonate host rocks. We suggest that the common association of REEs with Cu ores in many IOCG deposits is related to the unique nature of the Cu mineralizing fluids.