Tin Mineralization Events And Fertility Of Granitoids In The Youjiang Basin,South China

The Youjiang Basin in the west of South China is one of the most important tin clusters in China,and hosts a series of large–super-large Sn-polymetallic deposits along its margin.The world-class Gejiu district in Yunnan and the Dachang district in Guangxi are the two largest ones in the cluster.Although abundant researches have been conducted on geological features and genesis of Sn-polymetallic deposits since the1980s,the genesis of these deposits still remains disputed,with arguments focusing on whether they are of SEDEX origin or of granite-related magmatic hydrothermal origin.To address this issue,the Gejiu and Dachang Sn-polymetallic districts are selected for the study of metallogeny of tin.Systematic cassiterite U–Pb ages of tin deposits,mineral chemistry,bulk-rock geochemical data,zircon U–Pb,Hf–O and whole-rock Sr–Nd isotopic data of the granitic rocks are analyzed and studied to constrain the timing of tin mineralization,decipher its genetic link with the granitic magmatism and discern the petrogenesis and tin mineralization potential of the granitoids.The Gejiu Sn?Cu district consists of Malage,Songshujiao,Gaosong,Laochang and Kafang deposits.Two major styles of tin mineralization developed in the district,including cassiterite-sulfide ore within the proximal skarn and stratiform-like cassiterite-iron oxide±sulfide ore in the distal Triassic carbonate country rock.In-situ cassiterite U–Pb dating of two ore types from the Gaosong deposit yields ages of 85.1–83.5 Ma,which agree well with the zircon U–Pb age of the Gaofengshan biotite granite.The Dachang Sn?Zn district is composed of Tongkeng-Changpo,Gaofeng,Dafulou,Huile and Kangma deposits.Economic tin orebodies are hosted by Devonian strata,as massive/stratiform and vein/stockwork mineralization.In-situ cassiterite U–Pb dating of five tin deposits yields U–Pb ages of 95.4–90.3 Ma,which agree well with the zircon U–Pb ages of the Longxianggai biotite granites.The Cretaceous U–Pb ages,combined with high Fe–W–Mn but low Nb–Ta concentrations of cassiterite,suggest that Sn-polymetallic deposits in the Gejiu and Dachang districts were formed in magmatic-hydrothermal systems associated with the Late Cretaceous granites.Late Cretaceous granitic plutons are well developed in the Gejiu area,i.e.the Longchahe and Shenxianshui plutons in the west,and the Baishachong,Masong?Beipaotai?and Laoka plutons in the east.Large-scale tin deposits are distributed in the east,whereas there are only sporadic medium-/small-scale tin deposits and tin occurrences in the west.As the largest granitic pluton in the district,however,the tin mineralization potential of the Longchahe coarse-grained porphyritic biotite granite remains obscure.Compared with the biotite granites in the east,the Longchahe porphyritic biotite granite exhibits low SiO₂ and Sn contents,low Rb/Sr but high K/Rb,Nb/Ta and Zr/Hf ratios.Magmatic zircon from the Longchahe pluton experienced ignorable or weak metamictization,characterized by low Hf and Sn concentrations.Apatite exhibits clear oscillatory zoning and has low concentrations of Mn and Sn.Meanwhile,they both have weak Eu anomalies in REE patterns.These features indicate that the Longchahe pluton is less evolved.Its relatively high oxygen fugacity reflected by zircon Ce⁴⁺/Ce³⁺?up to 411?and whole-rock Fe₂O₃/FeO?>0.5?ratios,shows affinities of magnetite-series granites.Limited fractionation,as well as elevated oxygen fugacity of the Longchahe pluton,hinders the Sn enrichment in granitic melt,suggesting limited ore-forming potential of tin.By contrast,the low oxygen fugacity and highly fractionated nature of the biotite granites in the east are similar to Sn-bearing granites in South China,indicating a great potential of tin mineralization.Late Cretaceous granitic rocks in the Dachang area comprise the Longxianggai biotite granite pluton and late granite porphyry dykes with abundant mafic microgranular enclaves?MMEs?,and the former is spatially and temporally associated with the tin mineralization.LA-ICP-MS zircon U–Pb dating suggests that the Longxianggai biotite granites were emplaced at⁹3 Ma,and the granite porphyry dykes and their MMEs were formed at⁸6 Ma.The biotite granites are strongly peraluminous,enriched in?Na₂O+K₂O?,P,Li,Rb,Cs,Sn,W and U,depleted in Ba,Sr,Eu and Ti,which belong to highly fractionated S-type granites.Whole-rock Zr saturation and Ti-in-zircon temperatures suggest that granitic melts have relatively high initial generation temperatures??780 ^oC?.Whole-rock?_Nd?t?,zircon?_Hf?t?and?¹⁸O values are-10.0–-7.8,-9.9–-3.9,6.2–8.1‰,respectively,with Mesoproterozoic two-stage model ages for Nd and Hf isotopes.Geochemical and isotopic data suggest the Longxianggai biotite granites were derived from partial melting of Proterozoic meta-sedimentary rocks,possibly with minor input of mantle manterial.The post-ore granite porphyry dykes are also highly fractionated,showing S-type affinities.Zircon Hf–O isotopes[?_Hf?t?=-9.0–-4.9 and?¹⁸O=6.5–8.2‰]and whole-rock geochemical data indicate a similar source with the biotite granites.The elevated?_Nd?t?values of-5.0–-4.3 for the granite porphyry dykes relative to the biotite granites reveal an increasing mantle input.The MMEs show bulk-rock?_Nd?t?,zircon?_Hf?t?and?¹⁸O values of-0.4–-0.3,1.5–4.9 and6.5–7.2‰,respectively,with Neoproterozoic model ages(T_DM)for Nd and Hf isotopes.The MMEs represent hybrids between felsic and mafic melts,in which the mafic magma member is likely derived from asthenospheric mantle.The magmatic sequence in the Dachang district,is indicative of an extensional tectonic setting during the Late Cretaceous.In this extensional tectonic context,we suggest that mafic magma,most likely derived from partial melting of upwelling heterogeneous asthenosphere,intrudes the crust,locally yielding necessary heat for partial melting of overlying mature meta-sedimentary rocks.Partial melting involves Sn-carrier phases,especially the decomposition of biotite,could lead to initial Sn enrichment in granitic melts,which is precondition to form the Longxianggai Sn-bearing biotite granites.The further mantle upwelling results in magma mixing,generating the post-ore granite porphyry dykes and their MMEs.Tin-polymetallic mineralization in the Gejiu and Dachang districts was formed in magmatic-hydrothermal systems associated with the Late Cretaceous biotite granitoids,precluding the syngenetic origin.The medium-fine grained biotite granites in eastern Gejiu,and the biotite granites and granite porphyry dykes in the Dachang area show potential to form hydrothermal Sn mineralization,which should be strengthened in further exploration.