| Mianning-Dechang(MD)rare earth element(REE)belt in southwestern Sichuan Province,southwestern China,is controlled by a series of Cenozoic strike-slip faults.One giant(Maoniuping),one large(Dalucao),one medium(Muluozhai),and one small(Lizhuang)carbonatite(carbonatite-syenite complex)rare earth element deposits are mainly developed in the belt.In field geological surveys in recent years,a detailed study of the deposit geology of the Maonuping deposit has been carried out.Mineralization are mainly developed in the Dagudao main orebodies,and typical vein mineralization patterns is developed.Three vein types have been recognized:veins(with widths of>1 0cm),stringers(with widths of 1?10cm)and stockworks(with widths of<1cm),according to the scale of the development of the veins.Among them,veins can be divided into aegirine-augite,calcite,phlogopite and potassium feldspar veins based on the major minerals they contain.Obvious mineral zones existed in them,from wall rock(syenite)to the center of mineral zones,the regular pattern of the changing of minerals is:phlogopite?aegirine-augite?potassium feldspar?arfvedsonite?barite?fluorite?calcite?quartz?bastnasite.It is worth mentioning that REE enrichment typically appears in veins,stringers and stockworks barely have REE mineralization.These veins are spatially distributed in a regular pattern.The top of develops mainly veins,and the bottom develops mainly stockworks,and the middle part develops some stringers.For this purpose,the "three-layers building"model is proposed to indicate the distribution of these veins.LA-ICPMS results suggest that rare earth elements content gradually increased(?REE:aegirine-augite:345?685 ppm;arfvedsonite:164?2563 ppm;Barite:2074?2228 ppm;Colorless fluorite:610?1114 ppm;Purple fluorite:317?689 ppm),and Mineral enrich of Ba,Sr,Th,U,Pb,and depleted of Zr,Hf,Th of mineral zones.In contrast,ore forming fluid of temperature gradually reduced(homogenization temperature decrease from 441 to-150 'C of fluid inclusions)from the edge to center in the vein.Combined with experimental petrology evidences show that large amounts of SO42-and F-in the late fluid are great importance in transportation and precipitation of the REE.Based on the field geological survey and the above research,the evolution model of the vein-formed pattern was proposed.First,the volatiles of the magmatic fluid interact with the carbonate-sycnite complex through tectonic activities to form phlogopite,potash feldspar,aegirinc-augite and arfvedsonite in the mineral zoning,with no obvious mineralization.With the continuation of tectonic activity in different directions and stages and the incorporation of meteoric waters into ore-forming fluids,the temperature of the ore-forming fluids decreases and fluorite,barite and calcitc are formed and overlay on the earlier-formed minerals,resulting in the formation of the vein system.The veins are controlled by multi-group and multi-phase tectonic activities.With the dissolution of the ore-forming fluid,the mineral zoning continues to form,ranging from phlogopite,potash feldspar,aegirine-augite,and arfvedsonite,to fluorite,barite,calcitc,and bastnasite.Throughout the process,the temperature of the formation of mineral zones gradually decreased and the REE content gradually increased.For stringer and stockworks,tectonic activity is not developed,fluid evolution is not sufficient,only phlogopite,potash feldspar,acgirine-augite and arfvedsonite arc formed.With the evolution of ore-forming fluids,a large amount of barite,fluorite,and calcite are formed in the veins,and relatively enriched with higher REE.The Ar-Ar dating results of phlogopites of fenitization yields a well-defined age of 26.2±2.3 Ma,and this age may constraint the formation age of the main veins and mineral zones of the Maoniuping deposit. |
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