Mineralogical Study On Metallogenic Processes Of Rare(Earth)Metals From Jiepailing Porphyry Deposit,Hunan Province

Jiepailing tin polymetallic deposit in Hunan Province is one of the typical granite porphyry deposits in Nanling area.Different degrees of hydrothermal alteration occurred between the granitic porphyry and the limestone in the deposit,resulting in abundant rare metal minerals and sulfides in the altered rocks.In this study,we selected the altered granite porphyry and altered wall rock from the Jiepailing deposit as the research objects,analysed the minerals in various rocks by EPMA,BSE images and mapping images,and detected the trace elements of K-feldspar and Li-bearing micas in altered granite porphyry by LA-ICP-MS,with the aims to discuss the alteration processes of granite porphyry,and to reveal the concentration,migration and mineralization of rare and rare-earth metals during the alteration processes of granite porphyry.The results are as follows:(1)K-feldspar,quartz and fluorite occur as phenocrysts in the altered granite porphyry.In the early stage of alteration,K-feldspar phenocrysts were dolomitized,sericitized,albitization and silicified on different degrees.With alteration ongoing,the amount of K-feldspar phenocrysts disappear gradually.In the middle stage of alteration,a large number of muscovite and sericite appeared.K-feldspar phenocrysts were completely replaced by secondary micas,and zinnwaldite was partially altered into muscovite.Large-scale fluoritization and micacization happened in the late stage of alteration.Fine-grained fluorite and micas were widely spread in the altered rocks,and lots of mica-fluorite(calcite)veins developed.(2)Amounts of rare and rare-earth minerals formed in altered granite porphyry,mainly including Nb-Ta-bearing oxides(columbite,manganocolumbite,ilmenorutile),rare earth minerals(xenotime,fluocerite,bastnaesite,florencite-Ce),tin minerals(cassiterite,stannite),tungsten minerals(wolframite),lithium minerals and beryllium minerals such as zinnwaldite and helvite.Cassiterite and wolframite were also developed in the altered wall rock,where large-scale mineralization of lithium and beryllium occurred,resulting in the formation of large amounts of ephesite and bertrandite.(3)The enrichment and mineralization of rare earth metals mainly occurred in magmatic stage.Their mineralization was in close relation to the crystallization of K-feldspar and fluorite matrix.Nb-Ta oxides such as ilmenorutile,columbite,manganocolumbite and rare-earth minerals including xenotime and fluocerite were first crystallized.Then tin,tungsten,lithium and some other elements were gradually enriched and precipitated into rare(earth)metal minerals.Some rare earth minerals from the magmatic stage were altered into secondary minerals by hydrothermal fluids in the late stage of alteration.Amounts of ephesite and bertrandite occurred in the late stage of alteration.(4)In the altered surrounding rock,the chalcopyrite dots were formed when sphalerite was replaced by hydrothermal fluid rich in copper and iron,while in the altered granite porphyry chalcopyrite dots mainly exsolved from sphalerite crystals.The exsolution genesis of chalcopyrite dots indicated that the Jiepailing deposit experienced high temperature and the change of physicochemical conditions during its formation,which were conducive to the migration,concentration and mineralization of rare and rare-earth metals.