Geochemical And Silicon Isotope Systematics Of Rare Metal-rich Geothermal Waters In Southern Tibet-Tengchong Areas

Geothermal systems in southern Tibet and in Yunnan province are two typical high temperature geothermal systems in China.Geothermal systems contain aboundant geothermal resources and mineral resoures?hot spring rare metal deposits?.However,the evolution of grothermal system and elements migration behavious in geothermal fluid still need further study.Besides,the factors and mechanisms affect silicon isotope fractionation during the process of Si O₂ precipitation from geothermal fluid,and diagenetic process occuring after precipitation need further research.Based on these,in this study,we focus on three geothermal areas?Dagejia,Kawu and Semi?in southern Tibet and Rehai geothermal areas in Tengchong,Yunnan,in oreder to research characters of ore-forming fluid,the evoulution of geothermal system,rare metal elements behaviors in geothermal fluid and silicon isotope fractionation mechanism.The concentrations of rare metal elements in host rocks in southern Tibet are higher,providing a prerequisite for the enrichment of rare metal elements,e.g.Cs content can reach thousands of times of the original mantle.Geothermal fluids with abundant rare alkaline elements have a close relationship with the evolution of geothermal fluid.Results show that Semi area has intense water-rock interaction favors accumulation of rare metal elements in geothermal fluids,and rare metal elements and boron were removed early in the alteration.Host rocks enriching rare alkaline elements and variours sinters fomed in Semia area,indicate that Semi area has a great potential in the search for rare metals.Sinter formation can further concentrate Cs element?up to 9000 ppm?and it mainly exists in newly formed sinter.The concent of Cs in sinter will decrese with phase transformation and increasing time,and quartz has the lowest Cs content.Silicon circulation in geothermal systems is a dynamic process,relating processes where silicon is leached from silicate minerals to the sinks where sinter is precipitated around geothermal springs.The rates of water-rock interaction and sinter precipitation follow the order of Semi>Kawu>Dagejia and Dagejia>Kawu>Semi respectively,and it explains the difference of silicon concentration and silicon isotope compositions in these areas.Silicon isotope fractionation during sinter precipitation(?³⁰Si_{precipitate-solution} is<-0.1‰)is less significant than that in water-rock interaction(?³⁰Si_{solution-rocks},at least as high as-0.47‰),which makes it possible to use the?³⁰Si signatures of springs to evaluate the intensity of water-rock interaction.In lamelleted sinter which formed in main spring vent?from opal to chalcedony?,?³⁰Si_{chalcedony-fluid}is-0.19?-0.33‰;?³⁰Si values difference between opal-A and opal-CT(?³⁰Si_opal-A-CT?+0.7‰),and?³⁰Si values difference between modern and fossil opal-CT(?³⁰Si_{modern-fossil opal-CT}?-1.0‰)indicate that Silicon isotope compositions of sinter will change accompanying phase transformation and aging process.Three types geothermal fluid were observed in Rehai area:A type?p H:1.5?2.5?,B type?p H:6.4?,C type?p H:8?10?geothermal fluids.The elements concentration and silicon isotope compositions of three geothermal fluids are clearly different.The degree of evolution of three types of geothermal fluid is C type fluid>B type fluid>A type fluid,leading to elements in geothermal fluid and in sinter have the same trend that rare metals are most abundant in C-type fluids and sediments,which is closely related to the deeper depth of origin and strong water-rock interaction.Strong water-rock reaction occurs in C type fluids leading to rare metal elements leached out from host rocks during geothermal fluid upwater migration.Then,H₂S and Cl react with rare metals to form complex to migrate to the surface,so the deep of studied area has potential for rare metals mineralization.The mechanisms that control silicon isotope fractionation between solid and fluid in three types of fluid are different.At 50?75oC and p H?2 is?³⁰Si_{solid-fluid,eq}=+1.09±0.04‰?+2.78±0.04‰,and?³⁰Si_{solid-fluid,eq} decrease from+2.23±0.07‰to+1.13±0.09‰as temperature increases from 55oC to 70oC,indicating that equilibrium fractionation contols silicon isotope fractionation between solid and fluid.Silicon isotope fractionation between solid and fluid in vent of geothermal system in B type and C type system is from-1.88±0.08‰?+0.55±0.16‰.We compare?³⁰Si_solid-fluid with precipitation rates and temperatre,and results indicate that?³⁰Si_solid-fluid is not only controlled by temperature and precipitation rates,and it is affected by other factors that sinter react with fluid after sinter precipitation leading to isotope exchange between sinter and fluid after precipitation and modify the initial silicon isotope information).