Geological Characteristics And Genesis Of The Muluozhai REE Deposit In Sichuan Province

Carbonate-type rare earth?REE?deposit is an important type of rare earth deposits in the world.The main metallogenic surrounding rocks of this type of deposit are syenite and carbonate rocks,and the influence of other wall-rocks on mineralization remains to be studied.The Mianning-Dechang REE belt is located in the southwest of Sichuan Province and mainly includes Maoniuping,Daluchao,Muluozhai and Lizhuang REE deposits.The Muluozhai deposit is located in the northern part of thebelt,it has a harsh environment,and it's a deposit that is mined by cave mining.Therefore,there were few predecessors studying it.The ore characteristics,and the relationship between wall-rocks and mineralization are still not clear.Based on two-year field geological survey,a detailed study was conducted on the Diaoloushan and zhengjialianhgzi section of the mining area.The Diaoloushan section is controlled by a single extension fault.The ore bodies are mainly produced in the syenite fissures,and a small part of them are in the contact surfaces of syenite and marble,green schist,metamorphic diabase.The ore type is massive,and the symbiotic combination of minerals is:bastnasite+fluorite+quartz+gypsum+barite+calcite+pyrite+galena etc.The Zhengjialiangzi section is controlled by 4⁶ pressure-torsional and torsional faults.The ore body is mainly produced in the marble fractures,and a small part is in the contact surface of marble and metamorphic greenstone.The ore type is disseminated.Ore broken seriously.X-ray diffraction analysis showed that this type of ore is mainly composed of minerals such as barite?4 to 56%?and calcite?32 to 89%?,and the total REE content reaches 2221 to 9774 ppm.The mineral symbiotic combination is:bastnasite+fluorite+quartz+barite+calcite+pyrite+phlogopite etc.Ore specimens,microscopic and BSE maps of the two sections all show that the bastnasiteite superimposed on gangue minerals such as fluorite and calcite,and the ore deposits mainly form in the late hydrothermal stage.The wall-rocks mineralization is not obvious,and the alteration is less.It shows that the Muluozhai ore-forming mineralization is dominated by filling and the metasomatism is secondary.The results of LA-ICP-MS zircon U-Pb dating show that the ages of alkaline granite,monzogranite,granite porphyry and syenite are 25.92±0.45 Ma,26.98±0.64 Ma,26.22±0.37 Ma and 25.8±0.18 Ma respectively.The ages of marble and green schist are more than 230 Ma.The ages of three types of granites and syenites are basically the same as those of the Arnica Ar-Ar age?26.64±0.62 Ma?,which represented the ore-forming age.The marble and green schist are at least 200 Ma earlier than the metallogenic age.The results of the whole rock analysis show that the total REE contents of these three types of granite,syenite,marble,and green schist are 277³78 ppm,630³233 ppm,4²1 ppm and 86⁹7 ppm respectively,Compared with the REE content?630⁴0807 ppm?in the syenite-carbonate complex in the metallogenic belt,the syenite content is similar to it,while the content of other surrounding rocks are too low,so syenite has a closer relationship with mineralization.The C-O isotope shows that the marble outside the mining area is formed by the metamorphism of marine carbonates.The marble adjacent to the veins is affected by the magmatic activities in the later period.Marble may provide carbonate ions for mineralization under the action of magma or hydrothermal fluids.The Hf-O isotope shows that the syenites in the four major deposits have similar mantle source regions and are subject to varying degrees of contamination of the crust source material.The characteristics of inclusions and sulfur isotopes indicate that the ore-forming fluid of the Muluozhai REE deposit comes from carbonate-syenite magma,immiscibility provides CO₂ for mineralization,and atmospheric precipitation results in a decrease in the temperature and pressure of the mixed ore-forming fluid,resulting in decomposition and precipitation of the REE complex,this is the main mechanism of mineralization.The metallogenic characteristics of the Muluozhai deposit are basically similar to those of other deposits in Mianning-Dechang REE belt.