Mineralogical and Geochemical Constraints on the Mobilization and Mineralization of Rare Earth Elements in the Lala Fe-Cu-(Mo, REE) Deposit, SW China
Wei Terry Chen, University of Hong Kong, Department of Earth Sciences, Hong Kong, China and Mei-Fu Zhou, The University of Hong Kong, Department of Earth Sciences, Hong Kong, China
Contact First Author: Wei Terry Chen; chenweifly1@163.com
Previously Published Material: these findings are submitted to a scientific journal, currently under review.
English Abstract:
The Lala Fe-Cu-(Mo, REE) deposit in Sichuan province, SW China, has a paragenetic sequence of Stage I of pre-ore Na-alteration, Stage II of magnetite and minor apatite, and Stage III of Cu-(Mo) sulfides and rare earth element (REE) minerals. REE minerals are monazite, parisite and bastnäsite with minor xenotime. They are generally associated with biotite, muscovite and calcite of Stage III, accompanying with K-Ca-carbonate alteration. In places 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 monazite inclusions with minor bastnäsite, calcite and sulfide minerals. These monazite-bearing apatite grains are similar to the metasomatized apatite reported in experiments, and thus are suggested to have formed from the Stage II apatite via a dissolution-precipitation process during of the Stage III fluids. Such a fluid metasomatism has mobilized and leached REEs from the primary Stage II apatite. For country rocks altered by the Stage III fluids (i.e. K-Ca-carbonate alteration), mass balance analysis indicates that up to 70% of their light REEs are also mobilized and leached out during alteration, and thus the country rocks are inferred to be important sources for REE mineralization in the Lala deposit.
Together with mineral assemblages of Stages II and III and previous fluid inclusion data, compositions of amphibole and biotite demonstrate that Stage III fluids have higher K, CO2 (or HCO3- and CO32-)and HF/HCl fugacity but lower salinity and Na activity than Stage II fluids. We suggest that elevated K and CO2 in Stage III fluids or associated K-carbonate alteration are possibly important for REE mobilization. The mobilized REEs from country rocks were inferred to be transported as chlorite-complexing before depositing as REE minerals at certain levels. Deposition of REE minerals was likely triggered by mixing with relatively low temperature, F-rich, and oxidized basinal brines and/or local interaction with carbonate hosts. We propose that the common association of REEs with Cu mineralization in many IOCG deposits are possibly ascribed to the specific nature of K-CO2-rich Cu mineralizing fluids.
