Neoproterozoic Tin-Niobium-Tantalum Metallogenesis In The Western Part Of Jiangnan Orogen

Highly evolved granites are often closely related to the mineralization of Sn,W,Nb,and Ta.The potential of these rocks to bear mineralization and the type of mineralization are largely controlled by magmatic and hydrothermal processes,including metal mobilization by partial melting of the source,metal enrichment during fractional crystallization,redistribution during fluid-melt separation,and metal precipitation during fluid-rock interaction.The factors leading to mineralization change with intrusions,which makes the ore-forming process complex and variable.The South China Block is well-known for its episodic(Proterozoic,Paleozoic,Early and Late Mesozoic)granite and Sn-W-Nb-Ta deposits,which account for more than50%of the world's W and 20%of Sn reserves,and contain considerable Nb and Ta Resources.However,covered by the light of massive metal resources of the Late Mesozoic,the metallogenesis in the pre-Yanshanian period gets little attention.There are few studies on the relationship between mineralization in different eras,and the leading factors controlling episodic mineralization remains unclear.The western margin of the Jiangnan orogenic belt is the oldest Sn metallogenic area in South C hina.Some important scientific issues,such as the potential of granites to bear Nb-Ta deposit,the mineralization age,the ore-forming process,and the metal and fluid sources of Sn polymetallic deposits,remains unknown or controversory.Based on the previous studies,this article used various methods,such as thin-section observation,scanning electron microscope and electronic microprobe analysis,U-Pb isotope and trace element analysis for the ore minerals(cassiterite,columbite,wolframite,wolframite)and accessory minerals(zircon,titanite),whole rock major,trace element and Sr-Nd-Pb-Li-B isotopic analysis,tourmaline Li-B isotopic analysis,and detailed studied the Sn-W-Nb-Ta-bearing granite and Sn polymetallic(tungsten,copper,etc.)deposits,and further discussed similarities and differences of episodic Sn-W-Nb-Ta mineralization in South C hina,control of the source process on the mineralization,separation of metals and fractionation of Li-B isotopes during magma-hydrothermal transition.This study yielded the following results and understandings:1.Age of emplacement and mineralization:The U-Pb ages of zircon,wolframoixiolite,and columbite indicate that the crystallization of Neoproterozoic tin-specialized granites in the western segment of the Jiangnan Orogen occurred at?819–832 Ma.The U-Pb ages of cassiterite and wolframite from the greisen,tourmaline-quartz vein,schist-type and amphibolite-type ores indicate that the tin polymetallic mineralization occurred at?823–831 Ma,which implies that magma emplacement and hydrothermal mineralization are almost simultaneous.The U-Pb age of titanite from schist-type ore indicates that the regional metamorphism of?420 Ma altered the granites and tin deposits,resulting in recrystallization and Pb-loss of minerals(e.g.wolframoixiolite).There is no evidence indicating that the alteration contributes to mineralization.In addition,the detrital-sedimentary tourmaline from the Sibao-Group schist may come from older(>?850Ma)basement rocks of the South C hina Block.B isotopic data(?¹¹B=–13.1 to+15.4‰)indicates that these tourmalines have multiple sources(old granite,metapelites,and seawater/marine clastic sediments).However,no evidence indicating there is tin enrichment before emplacement of granite.2.Sources of metals and fluids:The Sn-mineralization related magma and hydrothermal tourmaline are generally enriched in the elemental association of Sn,Zn,Li,F that is typical for granite.The tourmaline hosted by the wall rocks and those hosted in the granite have almost the same?¹¹B range(?–12 to–9‰),further indicating that the Sn-bearing fluid originated from the granite.The main and trace element compositions of cassiterite and titanite from the ores of Jiumao tin mine(greisen,schist-and amphibolite-type ores)indicate that Sn,W,Nb,Ta,U,Zn come from the F-B-rich Yuanbaoshan granite instead of the ultramafic rocks.Considering the following evidence:(a)Cassiterite from the amphibolite-type ore is enriched with elements such as Cr,V,Ni;(b)Cassiterite is associated with Ni-rich minerals such as rammelsbergite([NiFeCo]As₂)and breithauptite(NiSb);(C)Cr-rich nigenianite occurred in schist ore near the ultramafic rocks;(4)Cu ore bodies of tin polymetallic deposits(e.g.Jiumao)are mainly distributed near the ultramafic rocks,we inferred that the ultramafic rocks may contributed metals such as Cr,Ni,V,and Cu to polymetallic mineralization.3.Ore-forming potential and controlling fractors of granite:Crystallization of Neoproterozoic wolframoixiolite,and columbite indicates that the Neoproterozoic granite is highly-evolved granite.It remains possible that there are concealed Neoproterozoic rare-metal deposits in the western part of Jiangnan Orogen.This is the earliest W-Nb-Ta mineralization event in South China,and then the prelude to episodic mineralization was opened.Whole-rock Nd isotope data indicate that the remelting of ancient crustal materials(?1.6–1.9 Ga)formed these Neoproterozoic Sn-W-Nb-Ta-bearing tourmaline leucogranites.Episodic W-Nb-Ta mineralization may have involved comparable source rocks,while the Mesozoic tin-bearing granites obviously have higher contribution of juvenile material.Sn-rich protoliths and/or high melting temperatures may be the key factor for the large-scale tin mineralization of the Mesozoic granite in the Nanling area.The higher availability of volatile components(e.g.B,H₂O)in the western part of Jiangnan Orogen than in the eastern part may have promoted more extensive melting and fractional crystallization,eventually leading to Neoproterozoic granites with Sn(-W-Nb-Ta)mineralization in the west and barren granites in the east.4.Magmatic-hydrothermal transition:The obvious tetrad effect in whole-rock rare-earth pattern,and secondary texture of mica,zircon and niobium-tantalite group minerals indicate intensive fluid activities at late stages of magma development.Tin,W,Nb,and Ta fractionated during the separation of fluid and melt,e.g.Nb-Ta rich melt and Sn-and W-rich fluids in the Yuanbaoshan area.Exsolved fluids altered early-crystallized columbite group minerals,eventually leading to the dissolution and reprecipitation of Nb minerals,while Sn-rich fluids metasomatized biotite granite and surrounding rocks to form tin deposits.In the Fanjingshan area,scheelite and cassiterite closely coexist,and the mineralization of Sn and W is almost in a same period.Whether Sn and W are separated or not may be closely related to magma oxygen fugacity and fluid composition.In addition,during the magmatic hydrothermal transformation,the Li isotopic system is controlled by a variety of silicate minerals,while the B isotopic system is mainly controlled by tourmaline.This constrasting control leads to the decoupling of the Li and B isotopic systems during magmaic and hydrothermal processes.The?¹¹B variation range of magma and hydrothermal tourmaline from Yuanbaoshan area is relatively small(–12.5 to–9.3‰),reflecting the characteristics of metasedimentary source.In contrast,the?⁷Li of magma and hydrothermal tourmaline shows three trends,corresponding to three individual processes,i.e.fractional crystallization,magma-hydrothermal transition,and fluid-rock interaction.Tourmaline Li isotope shows a good potential for tracing the magmatic-hydrothermal transition.5.Fluid-rock reaction and ore-fromation process:The schist-type and amphibolite-type ores formed at comparable temperature(the silicate-and sulfide-stage ores formed at?570–350 and 350–170°C,respectively)and oxygen fugacity(?NNO).The differences in Sn grade(The schist-type ore commonly contains<1.4%Sn while the Sn grade of amphibolite-type ores can reach 29%)and mineral assemblage(schist-type ore is Fe-rich while the amphibolite-type ore is Mg-rich)may be controlled by constrasting processes of fluid-rock reaction.The chemical composition and Li-B isotopic composition of tourmaline also indicate that extensive material exchange(HREE,Mg,Li,B,Sn)occurs between the wall rocks and the ore-forming fluid.The fluid-rock reaction controlled the mineral and elemental association,affected the grade and isotopic composition of ore,which is critical to the formation of tin polymetallic deposits.