Mineralogy Study Of Rare-metal Granites In South China

Rare metals,including niobium(Nb),tantalum(Ta),tungsten(W)and tin(Sn),are defined as "strategic resource" or a "critical material" in terms of the economic importance and supply risk for high technology applications.Detailed study of mineralogical investigation is of importance for constraining the magmatic fractionation,magmatic-hydrothermal transition of rare-metal minerals and related rock-forming minerals.In this study,we take the Huangshan Nb deposit and nearby Songshugang Ta deposita,in Lingshan complex,Jiangxi Province,South China,as examples to study the rare-metal minerals and rock-forming minerals(especially micas)by using optical microscope,scanning electron microscope(SEM),electron microprobe(EPMA),laser ablation inductively coupled plasma(LA-ICP-MS)to evaluate the rare-metal behavior during magmatic-hydrothermal evolution.Meanwhile,the secondary ion mass spectroscopy(SIMS)and laser ablation inductively coupled plasma(LA-ICP-MS)are also used to obtain the in situ U-Pb dating on zircon and columbite.Several small Nb-Ta deposits are distributed around the major Lingshan complex,east of Qin-Hang belt,South China,among which Huangshan suite located in the west is well known for several decades.Two parts are classified based on the occurrence of Nb-rich mica and amount of columbite minerals:medium-grained granites with Nb-rich mica and fine-grained granite and pegmatite with quantity of columbite.Columbite in general Nb-rich and Mn-poor with Ta/(Ta+Nb)? 0.5 and Mn/(Mn+Fe)lower than 0.2,featured by complex zoning with various "go and back" processes.Such reverse internal variation in each columbite crystal is uncommon in normal rare metal granites and related-pegmatite,and not fit for fractional crystallization as interpretation.Thus it inferred that at least two different magma source have donated multi-mixing processes.The medium-grained granites in Huangshan suite are differed by their mica compositions:lithian(lithium-rich)annite(MA granite)and "protolithionite"(MP granite).They are characterized by elevated Nb contents(average 144 ppm in MP and 158 ppm in MA)and very high Nb/Ta ratios(average 15.3 in MP and 31.2 in MA).Niobium is mainly hosted in the micas,with an average Nb content of 1,347 ppm in the lithian annite,and 884 ppm in the "protolithionite",being the highest ever reported in granite micas.With an estimated content of?80 kt Nb,the Huangshan granites represent a new style of potential Nb resource.The Songshugang Ta granite is found in the west of Lingshan suite but enriched in Ta.The major Songshugang albite-topaz granite is buried and covered by layers of K-feldspar-topaz granite,greisens and pegmatite.All the granites are strongly peraluminous.The columbite minerals combined with zircons and cassiterites are constant with each type of rocks and display the similar two-stage textures.For columbite,the primary subhedral-shaped columbite-(Fe)(donated as CGM-I)with low Mn/(Mn+Fe)value(0.16-0.37)and Ta/(Ta+Nb)value(0.05-0.29),is penetrated by the later-stage tantalite veinlets(CGM-?)or surrounded by complex Nb-Ta-Sn-W mineral assemblages,including tantalite-(Fe),wodginite(sl),cassiterite and ferberite.The tantalite combined with wodginite also enriched in Ta made up of the edge of columbite,regarded as CGM-?.Similar to Nb-Ta oxides the early-stage zircon is commonly included by the later-stage zircon with sharply boundary.They have contrasting Hf contents,and HfO2 of the latter-stage zircon is up to 28.13 wt%.Petrographic features indicate that the early-stage of columbite and zircon were formed in magmatic environment.However,the later-stage of rare-element minerals were influenced by fluxes-enriched fluids at the magmatic-hydrothermal transitional stage.Micas as the one of the rock-forming minerals also show two-stage texture,with large amounte of columbites included.The micas in Songshugang granite contains high SiO2(44.94-47.57 wt%)5 FeO(8.97-11.81 wt%)and Li2O(estimated 3.08-4.09 wt%),referring to the zinnwaldite.The enrichment of Rb in the margin(Znw-?)is the result of magmatic fractionation,which could also been testified by the decrease of Nb(16.3-108 ppm compare with core of 109-313 ppm).However the invariable low value of Ta,W and Sn demonstrate that the residual melt has no influence of later post-magmatic,contrary to the columbite minerals.Finally,after using the in situ zircon and columbite U-Pb dating by SIMS and LA-ICP-MS,the result reveals that both Huangshan and Songshugang granite were emplaced at ca.130 Ma,yielding at Later Yanshanian(or Cretaceous),contemporary with the big Lingshan complex.This indicates that the Later Yanshanian deposit exist,which enriched the time frame of rare-metal mineralization.In summary,the texture and chemistry of columbite is a good tool for indicating the magmatic-hydrothermal stage,as well as the variation of melt composition.The micas could act as the potential target for Nb extraction,on the other hand is the ideal indicator the crystallization system of columbite mineralization,should be paid more attention in the following rare-metal studies.In situ columbite U-Pb dating yields accurate mineralization age of rare-metal granites,contributing to build the framework of rare-metal granite in South China and proved that the existence of Later Yanshanian(Early Cretaceous)rare-metal granites.