Mineralogy,Geochemistry And Metallogeny Of Rubidium Polymetallic Deposit In Zhengchong,Daoxian County,Hunan Province

Highly fractionated granites and pegmatites have special petrogenesis and potential economic significance due to the enrichment of incompatible elements(Li,Rb,Cs,Nb,Ta,etc.)and flux elements(F,B,Cl,Li,P,Na etc.).The giant Zhengchong Rb polymetallic deposit is located in the Rb metallogenic belt of the Nanling region and is a deposit associated with the highly fractionated granites.The metallogenic geological bodies of the Zhengchong deposit are the quartz-zinnwalditetopaz rocks(QZTR),all of which reach the ore grades and have a high degree of exploration,making them ideal objects for rare metal mineralization research.The QZTR are composed mainly of quartz,zinnwaldite,and topaz with a porphyritic texture,and Rb-Li-Cs metals were concentrated in zinnwaldite.However,the genesis of the QZTR is still highly controversial:are the QZTR magmatic or metasomatic in origin?The genetic link between these rocks and the Jinjiling granite batholith host is unclear,and the magmatic-hydrothermal process involved in the rare-metal mineralization remains enigmatic.Based on field geological investigations,rock and mineral testing and geochemical simulations,the main insights gained in this thesis are as follows:1.We demonstrate that the QZTR at the Zhengchong deposit are magmatic in origin.Main lines of evidence argue against a metasomatic origin for the QZTR.These include:(1)the boundary between the QZTR and granites(or granite porphyries),both macroscopically and microscopically,is marked by an abrupt change in mineral proportions with almost nonexistent contact aureoles,and the existence of granite xenoliths in QZTR;(2)The QZTR show porphyritic textures,and their phenocrysts mostly euhedral.Among them,the growth zones bend of quartz phenocrysts in cathodoluminescence(CL)images show a primary growth feature,and not the result of resorption.In addition,crystallizedmelt inclusions are often present in phenocrysts.(3)The quartz Al content in the QZTR is relatively homogeneous,indicating stable magmatic crystallization conditions with an average Ti content as high as 22.1 μg/g.The crystallization temperatures recorded for quartz in the QZTR(453.3607.2,avg.519.5℃)are equivalent to the low temperature crystallization of the flux element-rich pegmatite systems(～650℃ for the liquidus,<500℃ for the solidus)or the cold magma storage of granites(quartz crystallizes at 474-561℃),but significantly higher than hydrothermal quartz in this study(258.1-365.9℃,avg.311.4℃).(4)The mica in the QZTR is mostly zinnwaldite,and muscovite is rare.The F content in topaz and coexisting zinnwaldite support a magmatic origin.(5)The monazite in the QZTR have high Th content(avg.7.0 wt%),suggesting an igneous origin.The Th/Ce,Y/LREE,Ca/Ce and Si/Ce ratios of monazite with magmatic characteristics,all yielding concordia ages.(6)The presence of continuous compositional variation(e.g.Rb,Cs,and F)from the early stage granites to the QZTR,consistent with the characteristics of differentiation trend of granites rather than magmatichydrothermal alteration features.The Y/Ho ratios of the QZTR are close to the chondritic ratios(24<Y/Ho<34),showing the characteristic CHArgeand-RAdius-Controlled(CHARAC)trace element behavior in pure silicate melts.(7)Additionally,Most of the QZTR samples are characterized differently from the magmatic-hydrothermal alteration by whole-rock Nb/Ta>5 and TE1-3<1.1(TE1-3:the deviation of the first and third tetrad of granite REE patterns from a hypothetical tetrad effectfree REE pattern).2.Based on the textural features and geochronological and chemical data of different types of monazite from Zhengchong,we reconstructed the geochronological framework of the Jinjiling complex,a highly fractionated felsic magmatic system:the Jinjiling granite was formed by the first stage magmatism at 156-153 Ma,the Pangxiemu granite was formed by the second stage magmatism at 152-151 Ma,the granite porphyry was formed by the third stage magmatism at～150 Ma,whilst the QZTR and Rb-Li-Cs orebody was formed by the fourth stage magmatism at 150-149 Ma.The Rb-Li-Cs mineralization at Zhengchong is slightly younger than the Late Jurassic W-Sn-U-Na-Ta-Li-Be mineralization in the Nanling region(concentrated around 155 Ma),which may represent a separate rare metal mineralization event.High[Th/U]N(13-20),[Gd/Lu]N(100-200),and[Eu/Eu*]N(>0.01)monazite crystals represent good pathfinders for magmatic evolution and Rb-Li-Cs mineralization in the Jiuyishan district.3.According to the geochemical simulation results,we exclude equilibrium crystallization and fractional/Rayleigh crystallization as the main mode of crystallization differentiation in the Jinjiling complex,while single in situ crystallization geochemical models require an extreme degree of crystallization(up to 98.3%)to super-enrich the remaining melt with incompatible elements and flux elements.This is inconsistent with the thermodynamic simulation results and is not consistent with the multiple intrusions of magma in the area.Combining the theory of magmatic mush reservoir system,we propose the hypothesis of "multistage in situ crystallization in magmatic distillation columns":in situ crystallization occurs within each intrusion before it solidifies,with more evolved liquid(enriched in magmatic volatile phase and incompatible elements)accumulating at the top of the intrusion,and more depleted crystals accumulating at the bottom,forming crystal-rich mush.After each new intrusion(recharge and remobilization of magma),the composition of reactive melt flow evolved as it passed through,and reacted with,early-formed crystal mush.Eventually,episodic intrusion and invoked multistage in situ crystallization resulted in extreme enrichment of magmatic volatile phase and incompatible elements.Multistage in situ crystallization can superenrich magmatic volatile phase and incompatible elements step by step without the assumption that single extreme crystallization(near 100%)of granitic magma.This geochemical model may have wider applications in illuminating the evolution of magma reservoirs and the concept of magmatic distillation columns.Based on this hypothesis and geochemical data,a three-stage in situ crystallization geochemical model of the Jinjiling complex was established:the Jinjiling granites undergo less than ca.80%in situ crystallization,compositions similar to the Pangxiemu granites and granite porphyries can be achieved by less than ca.70%in situ crystallization of the Jinjiling granites,and compositions similar to the QZTR could result from less than ca.70%in situ crystallization of the granite porphyries.Three-stage in situ crystallization enabled the QZTR super-enrich incompatible elements and Rb-Li-Cs mineralization at Zhengchong.51 figures,22 tables,276 references...