Geochemistry Of High Differentiation Granite And Rare Metal Mineralization

Since the beginning of the 21st century,the strategic importance of critical metals in both social development and national security has been continuously rising.Highly fractionated granites,which are closely related to the mineralization of tungsten,tin,niobium,and tantalum,have become important targets for mineral exploration and breakthroughs in critical metal prospecting.Therefore,the study of these granites has significant economic value.The genesis of highly fractionated granites and the enrichment and mineralization mechanisms of rare metal elements are urgent scientific problems to be solved in the study of rare metal deposits.It is essential to determine whether the mineralization of different rare metal elements is related to magmatic processes or hydrothermal processes.In this regard,this paper focuses on the Baishaziling Sn deposit and Shangbao Nb-Ta deposit,Hunan Province,which are important rare metal deposits in the Nanling Range of China,as well as the recently discovered Duxiushan hidden granite,Anhui Province,which is located in the Middle and Lower Yangtze River copper-polymetallic mineralization belt.We have carried out researches on the geology,diagenetic and metallogenic ages,rock geochemistry and genetic mineralogy of ore deposits.In order to discuss the formation age of rare metal deposits and their genetic relationship with granite,the formation mechanism of granite in the area and evaluated its rare metal mineralization ability,and described the metallogenic mechanism of these rare metals in detail.The metallogenic regularity and metallogenic dynamic background are summarized systematically,which provides theoretical basis for future prospecting and exploration.The main achievements and insights gained are summarized as follows:The Baishaziling greisen-type tin deposit is located in the central part of the Dayishan tin-polymetallic mineralization district in Southern Hunan,China.The age of the granite is 153～154 Ma,which is consistent with the U-Pb age of the cassiterite in this deposit,indicating that the mineralization occurred during the Middle-Late Jurassic period and is a product of the W-Sn polymetallic mineralization peak in the Nanling Range.The granite is a weakly peraluminous,high-K calc-alkaline,and highly fractionated A-type granite.It is characterized by high fluorine content and low oxygen fugacity.In combination with the coupling relationship between the granite and mineralization,it is believed that the granite is the mineralized rock,which partially melted in the Precambrian basement of the Cathaysia Block under an extensional tectonic background,with the involvement of a small amount of mantle-derived material.The granite then underwent intense fractional crystallization to form the current product.Four types of tourmaline were found in the area.Almost all tourmalines have a high Fe/(Fe+Mg)ratio and belong to the alkali-group schorl.The disseminated tourmaline in the magmatic granite has high Li,Zn,Sc,V and Ga contents.In the late magmatic-hydrothermal transition stage,tourmaline appears in the form of tourmalinequartz veinlets,and its geochemical characteristics are similar to those of the disseminated tourmaline in the granite.The early quartz tourmaline veins have low tin contents,indicating that the fluid exsolution did not cause initial Sn enrichment.In the greisen sample,the Sn content in the tourmaline core can be as high as 1157 ppm,and it has a high content of heavy rare earth elements.This may due to the interaction between magmatic-hydrothermal fluids and the host granite,which resulted in silicate mineral alteration and the release of a large amount of Sn ions.On the other hand,the on the edge of the tourmaline,consisting with cassiterite,exhibits low Sn,Li and HREE contents,which may be related to co-crystallization with cassiterite and associated minerals.The major element exchange vectors of tourmaline indicate that the relative oxidizing environment and high salinity during the precipitation process may be important factors leading to cassiterite precipitation.Based on the calculated δ11B values,the primary magma,early hydrothermal fluids,and mineralizing fluids haveδ11B values of-15.5‰～-14.5 ‰,-12.63‰～-12.22 ‰,and-11.90 ‰～-10.01 ‰,respectively,indicating that they have the same boron source.The process of greisenization is the key to tin mineralization,and the enrichment of ore-forming elements is mainly controlled by the hydrothermal stage.The Shangbao Nb-Ta deposit located in the southern part of Hunan Province,and situated to the east part of the Dayishan pluton.The intrusive rock in this area is monzogranite with an age of 87.3±0.9 Ma.The ore-forming age of the Nb mineralization is consistent with the formation age of the granite,indicating a close relationship between magmatism and mineralization during the Late Cretaceous in the Nanling Range.Geochemical studies reveal that the Shangbao pluton is a highly differentiated A-type granite with low oxygen fugacity and high volatility.The pluton intruded into the Shangbao anticline during the Late Cretaceous with lithospheric extension background in the Nanling Range,and the magma was mainly derived from the partially melting of the Mesoproterozoic basement of the Cathaysia block with about 20%mantle-derived material added.Mineralogical studies of the columbite group minerals show that the initial enrichment of Nb due to the crystal fractionation of the magma,resulting in the formation of columbite and Nb-enriched rutile,followed by Ta enrichment during the magmatic-hydrothermal transition stage,which led to the formation of tantalite and Ta-enriched rutile,and finally,the addition of fluids resulted in the re-enrichment of Nb and W.Three types of tourmaline were identified,including the disseminated-type tourmaline in the late-stage granite,the nodule-type tourmaline in the magmatic-hydrothermal transition stage,and the vein-type tourmaline.All of the tourmalines are alkali-group schorl with high Fe/(Fe+Mg)ratios.The δ11B values of the tourmalines range from-17.16‰ to-14.81 ‰,indicating that the boron source was consistent a.The disseminated-type and vein-type tourmalines have high Nb content,while the nodule-type tourmaline is enriched in Ta,revealing that the high differentiation of the magma is the main factor causing Nb enrichment,and the magmatic-hydrothermal transition stage is critical for Ta enrichment.Compared with Dayishan magma,Shangbao magma has a higher content of F,which may be an important factor contributing to the long-term crystallization differentiation of Shangbao magmatic rocks,thus promoting niobium mineralization.A suite of highly fractionated granites and associated pegmatite,were revealed through drill hole in the Anqing ore-cluster region in the Lower Yangtze River belt,Eastern China,for the first time.Through the research of the petrogenesis age of the hidden two-mica granite,its magma has an approximate formative age of 137.0±2.1 Ma,indicating that the ore-forming age was Late Jurassic,which is the same as the first stage magmatic ore-forming age of the adjacent areas,indicating that it may be the lower limit of the first magma activity of the region.The age of Duxiushan granitepegmatite is slightly later than or the same as the granite age,indicating that they were formed synchronously in the early Cretaceous period.The U-Pb age of cassserite in pegmatite is 136.6±1.9 Ma,indicating a close relationship between diagenesis and mineralization.The hidden granites in Duxiushan have high SiO2,P2O5,and Rb content,high Rb/Sr ratio,low Zr+Nb+Ce+Y content,and low Zr/Hf and Nb/Ta ratios,indicating that they are highly differentiated S-type granites.The consistent isotopic characteristics show that the granites,granite-pegmatite,and pegmatite of Duxiushan have similar sources,which are derived from the partial melting of Jiangnan basement strata,and were contaminated by a very small amount of mantle material,and have undergone different degrees of crystal differentiation.The magmatic system of Duxiushan has a low oxygen fugacity and an environment rich in volatiles.Moreover,in Duxiushan pegmatite,the contents of Sn,Rb and Nb in Muscovite and tourmalin are significantly increased,indicating the metallogenic potential of rare metals in this area.Combining with the latest research progress,it is found that at around 160 Ma,the Pacific Plate started to undergo clockwise rotation,resulting in a change in the subduction angle from positive to oblique.With the retreat of the subducting plate,a window was opened,allowing asthenosphere material to upwell to the lower crust,forming high-potassium,high-magnesium,and highly differentiated granitic rocks of crust-mantle source,which in turn contributed to the formation of the Nanling W-Sn deposit.Between 145 and 135 Ma,in the Middle and lower Yangtze River belt,the change in subduction angle and the tearing of the slab during subduction triggered mantle material upwelling and crustal remelting,leading to the formation of highly differentiated S-type granites.The Late Cretaceous mineralization of Nanling belt may be related to the late-stage magmatic activity and mineralization of the the Youjiang region,which is associated with the retreat of the Neotethys subducting plate.Trace elements of tourmaline and mica can indicate ore-forming potential,but the trace elements of different stages magmatic rocks and minerals may reflect different ore-forming potentials.Therefore,Analyses should systematically be conducted to determine the magmatic ore-bearing potential of granites,whether in terms of mineralogy or in terms of whole rock geochemistry.