Brief Analysis About Generation Of Dalucao Rare-earth Deposit Of Cryptoexplosive Breccia Type In Western Sichuan Province

Dalucao cryptoexplosive breccia type rare earth deposit is located at the southern end of Mianning-Dechang REE belt in western Sichuan Provinc.In this paper,the Dalucao REE deposit is viewed as research targets,through field investigation and laboratory observation,the lithofacies of the magmatic rocks are divided,mineral characteristics in the different lithofacies of the deposit are described.The metallogenic source area is discussed by using Hf-O isotopes,and the SHRIMP U-Pb dating results of the quartz diorite and syenite are carried out,combining with the characteristics of inclusions,the metallogenic epoch and stage of Dalucao deposit are analyzed,also does the genesis of the deposit.The formation of Dalucao REE deposit is mainly related to carbonatite-syenite complex.The deposit mainly consists of two ore bodies,No.1 and No.3,which both occur in breccia pipes and have similar ore grade containing 1%⁴.5%rare earth oxides.Weathered ore is the mainly type ore in the No.1 ore body,the most typical characteristics of weathered ores are relatively loose.There is a large number of porous among individual mineral and clastic rocks,and parasite are rare.While No.3 ore body is mainly composed of breccia type ore,the crystals of bastn?site are relatively large,and the crystal shape is relatively complete,slightly fractured or crystal fragments,possibly because the No.3 orebody has experienced less breccia and fracture than the No.1 orebody.The SHRIMP U-Pb dating of zircon shows that the age of the syenite of Dalucao deposit is 26.49±0.63Ma,indicating that is the deposit has experienced multiple stages of magmatic activity,and the age of quartz diorite is 769.5±8.7Ma,which is in agreement with the previous results.In situ Hf-O isotope study of the zircons from Dalucao syenite shows that the value of the?Hf?t?is concentrated between-7.1‰to-0.4‰,strong enrichment shows that the source area of enriched mantle is not the only source of syenite.The?¹⁸O of zircon in syenite of Dalucao REE deposit is between5.28‰to 6.66‰,indicating that the main body of the magma source area is the mantle source rather than the crust source,and the crust source material is mixed in the process of magma emplacement.The metallogenic stage of the deposit can be divided into magmatic stage,pegmatite stage,hydrothermal stage and supergene stage.The inclusions are divided into three types:molten inclusions?M?,melt-fluid inclusions?ML?and fluid inclusions?L?.The ore-forming fluid experienced the melting stage,pegmatite stage,high-temperature hydrothermal stage?>800??and low-temperature hydrothermal stage?metallogenic stage??<350??,and the homogenization temperature and the metallogenic pressure of the inclusions gradually decreased during the metallogenic process,from the melting inclusions to the CO₂-rich inclusions containing subcrystal and gas-liquid inclusions,indicating that the ore-forming fluid evolved from the magma into the hydrothermal fluid and the process of reducing the temperature pressure of the ore-forming fluid.The breccia in the No.1 orebody of the Dalucao deposit has experienced multiple periods of tectonic and breccia activity,providing the precipitation space of REE minerals and promoting the circulation of the ore-forming fluid,and because of its formation in the more shallow crust,it has experienced a stronger weathering effect and finally formed a high grade ore.The brecciation and faulting in No.3 ore body promote the reaction between the fluid and the syenite carbonatite complex,and lead to the formation of the breccia ore and fine particles of the gangue minerals and rare earth minerals.