Magmatic-Hydrothermal Evolution And Nb-Ta Mineralization In The Lingshan Complex,Southeast China

The Lingshan granitic pluton,located in the middle section of the Ganhang tectonic Belt,southeast China,has produced two famous rare metal deposits,namely the Huangshan Nb-Ta deposit and the Songshugang Ta-Nb-W-Sn deposit and the proven Ta₂O₅ reserves of the Lingshan mining area is the largest one in Asia.In particular,the Huangshan and Songshugang deposits are spatially close to each other,but they exhibit different mineralization characteristics.The Huangshan deposit is extremely enriched in Nb element and belongs to the pegmatite-type Nb-Ta deposit,whereas the Songshugang deposit is dominated by Ta element and belongs to the granite-type Ta-Nb-W-Sn deposit.Up to now,the controlling factors for the metallogenic diversity between the two abovementioned deposits are poorly understood.In the present study,the analyses of U-Pb chronology,whole-rock major and trace elements,Hf-Nd isotopes and mineralogy（e.g.,mica,tourmaline and Nb-Ta oxides）are conducted to effectively（1）constrain the age and genetic relationship of magmatic activities and Nb-Ta-（W-Sn）mineralization in the Lingshan area,（2）decipher the petrogenesis of the Lingshan pluton and the detailed Nb-Ta-（W-Sn）mineralization processes within the Huangshan and Songshugang deposits,and（3）assess the crucial factors responsible for the Nb-Ta differentiation and the generation of giant Ta deposit.Geochronological studies show that the granites in the central and marginal units of the Lingshan pluton are characterized by the constant zircon U-Pb ages between 134-129Ma,which demonstrated that the magmatic activities at Lingshan district took place in Early Cretaceous.Furthermore,the columbite and/or wolframite U-Pb dating show that the Huangshan and Songshugang Nb-Ta-（W-Sn）mineralization occurs uniformly at 129-128 Ma,which within error is also comparable to the magmatic zircon U-Pb ages from the Lingshan granites.The late-stage W-Sn mineralization in the Songshugang deposit reflects the post-magmatism hydrothermal activities as the emplacement of the Lingshan granite.Combined with the regional horizontal-phases of the Lingshan pluton,as well as the identical U-Pb ages of zircon,columbite and wolframite,we therefore propose that both Huangshan and Songshugang ore-mineralized granitic rocks are significant parts of Lingshan pluton.Petrographic results indicate that the Lingshan granites are metaluminous to weakly peraluminous（most A/CNK<1.1）,high alkaline（Na₂O+K₂O）,REE and HFSEs contents as well as high Ga/Al ratios.All these geochemical features reflect that the Lingshan granitic rocks have an affinity to the typical A-type granite worldwide.However,the whole-rock REE contents and Zr/Hf and（La/Yb）_N ratios exhibit an abnormal increase from the central to marginal units.The Li,Rb and Cs elements of biotite as well as the Zr/Hf ratios of zircon do not exhibit a continuous enrichment trend from the central towards marginal granites.All these features suggest that the generation of Lingshan granites is more complex than the fractional crystallization from a single parental magma.Alternatively,a crystal mush model could be applied to better interpret the geochemical and isotopic contrasts between different granite phases.The marginal granites were likely formed by later crystallization events after the rejuvenation and extraction from the crystal mush reservoirs,while the central granites were formed from the residual crystal-rich mush with a relatively low terminal porosity.Mineralogical studies show that the mica and Nb-Ta-Ti oxides from the Huangshan Nb-Ta deposit are characterized by extremely Nb-rich,without any Ta-Sn oxide overgrowths.The Nb-Ta bearing oxides in this deposit are mainly composed of columbite-（Fe）,Nb-rich rutile and mica,without obvious compositional zonations.The compositional analyses of tourmaline from the Huangshan microline pegmatite further indicate that the Nb-Ta mineralization in the Lingshan granite batholith dominantly occurs in the highly-evolved granitic melt rather than during the post-magmatic aqueous fluid.As a whole,we suggest that the Nb-Ta mineralization for the Huangshan deposit is controlled by the extensive fractional crystallization of granitic melts.In contrast with the Huangshan deposit,the Songshugang deposit is characterized by strong metasomatic alteration,particularly albitization and greisenization.The rock-forming minerals（e.g.,mica）and ore minerals（e.g.,columbite-group minerals,cassiterite wolframite and rutile）in the Songshugang deposit display significant compositional zonation.For example,the columbite-group minerals occurring in the unaltered granites are homogeneous,but display obvious core（-mantle）-rim textures in the altered granites.Tantalite,wodginite and microlite occur as overgrowths on the early-formed columbite-（Fe）domains.Consistently,early-formed Ta-poor cassiterite and rutile are partly surrounded by Ta-rich rims from the same samples.All these textural features suggest the occurrence of multiple-stage growth for the Nb-Ta-Sn-Ti oxides.The Ta-rich rims in the zoned CGMs and cassiterite from the altered granites illustrate that the overprint of late-stage Ta-rich melt/fluid on the early-formed granites.The Ta-rich features may be caused by the development of hydrous silicate liquids during the magmatic-hydrothermal transition（deuteric）process.During the magmatic fluid exsolution from the hydrous granitic melts,metasomatism triggers strong albitization and greisenization and the precipitation of Ta-（W-Sn）phases in the upper part of the granitic cupola.Mica among the Songshugang granitic rocks and quartz-veins exhibit compositional zoning in the BSE images,where bright domains are Li-,F-rich zinnwaldite crystallized from highly evolved melts that were relatively enriched in Li,F,Nb and Ta,whereas the dark domains are Li-,F-poor muscovite crystallized from late-stage aqueous fluids that were relatively depleted in Li,F,Nb,and Ta.It is,therefore,evident that the Nb-Ta-W-Sn mineralization in the Songshugang deposit can be divided into three distinctive stages,mainly including（i）magmatic stage for the Nb mineralization,（ii）magmatic-hydrothermal transition stage for the Ta-Sn mineralization,and（iii）hydrothermal stage for the W-Sn mineralization.Based on the above discussion,we propose that the formation of the Lingshan granites,together with the Huangshan and Songshugang deposits,may be the results of multiple extraction from the rejuvenated crystal mush reservoirs:（i）the first extraction formed the relatively less-evolved granites of the Lingshan marginal phase;（i）the second extraction occurred in the Huangshan area,where the more evolved volatile-rich melts underwent differentiation to produce a series of highly-evolved granites,pegmatites;（iii）the third extraction occurred in the Songshugang area,where the highly-evolved,volatile-rich granitic melts transported abundant Nb,Ta,W,and Sn materials,and experienced strong fractionation crystallization and subsequent metasomatism,resulting in large-scale mineralization of rare metal elements at the top of the granitic pluton;（iii）the fourth extraction was relatively smaller in volume and had lower concentrations of Nb and Ta in the granitic melts,which produced the formation of amazonite pegmatite intruding into the surrounding rocks.As for the metallogenic diversity of Nb-and/or Ta-rich deposits,we guess that they are closely associated with the different metallogical processes.The Huangshan deposit is dominantly associated with the magmatic Nb-Ta mineralization,but the Songshugang deposit experienced multiple-stage Ta-Nb-W-Sn mineralization process.Strong metasomatism of hydrous silicate liquids during the magmatic-hydrothermal transitional stage triggered albitization and greisenization and associated Ta-（Sn）mineralization in the apical part of the granitic cupola.It is,therefore,evident that extensive fractional crystallization plays a key role in the pre-enrichment of rare-metal elements（e.g.,Nb,Ta,W,Sn）and the Nb mineralization at the late magmatic evolution.However,to produce an economical large Ta deposit,the strong overprint of deuteric metasomatism is also essential and a crucial process.Albitization and greisenization are two widely hydrothermal alteration within the rare-metal granites and deposits,which are closely associated with the Ta-W-Sn mineralization.Furthermore,several minerals,such as mica,tourmaline and Nb-Ta-Ti oxides,generally display obvious compositional zonations or contains high contents of Nb and Ta in the rare-metal granites,which can be used as potential parameters to explore the Nb-Ta-（W-Sn）mineralization not only in the Lingshan granite and pegmatite but also in similar granite batholith elsewhere in the world.