Mineralogical Constraints On Hydrothermal Ore-forming Process Of The Songshugang W-Sn-Nb-Ta Deposit,Northeastern Jiangxi Province

The ore-forming process of tungsten-tin deposits is always the highly focused topic for geologists all over the world.The Songshugang W-Sn-Nb-Ta deposit in northeastern Jiangxi Province is an important Yanshanian rare-metal deposit in South China and is genetically associated with a highly fractionated granite.This deposit shows a distinct zonation of deep Nb-Ta mineralization and shallow W-Sn mineralization.The Nb-Ta ore bodies are mainly hosted by granite and the W-Snbearing quartz veins are distributed in the peripheral Sinian strata composed of lowdegree metasedimentary rocks.Detailed works have been carried out on the deep NbTa ore-forming process in previous studies and a two-stage model of Nb-Tamineralization was proposed.However,the shallow hydrothermal W-Sn ore-forming process is still unclear and need further researches.The compositional evolution of ore and gangue minerals records abundant ore-forming information.Detailed mineralogical studies can provide important constraints on hydrothermal ore-forming process.Based on detailed petrographic observation,combined with EMP and LA-ICP-MS analyses,the shallow hydrothermal ore-forming process of the Songshugang W-Sn-Nb-Ta deposit is studied by compositional evolution of mica,wolframite,and sulfide minerals.Based on detailed observations of hand specimens and thin sections,the quartz veins in the shallow W-Sn ore bodies of the Songshugang deposit can be devided into wolframite-quartz vein,cassiterite-quartz vein,sulfide-quartz vein,and barren quartz vein from early to late and from deep to shallow.The quartz veins at depth are dominated by wolframite-quartz veins,whereas,the quartz veins at shallow parts are more complex with different types.All these four types of quartz veins contain earlyformed zinnwaldite and late-formed aggregates of muscovite and iron oxides.Micas in the earlier and later veins are dominated by zinnwaldite and muscovite,respectively.Wolframite mainly occurs along the borders of quartz veins and sometimes in the center as euhedral to subhedral platy crystals without obvious generations and zonal texture.Sulfide minerals are distributed in the granite,quartz veins,and metasedimentary strata as sparsely disseminated texture and mainly include chalcopyrite,sphalerite,galena,and pyrite.According to EMP and LA-ICP-MS compositional analyses,compared with the early-formed zinnwaldite,the late-formed muscovite has distinctly lower contents of Ti,Na,Rb,Cs,W,Nb,Zn,Li2O and higher contents of Pb,Cu,B.From the earlier veins to the later veins,the micas show decreasing Ti,Na,W,Nb contents and increasing Pb,Zn,Cu,Li2O,B contents.Two different types of wolframite,which are considered as the products of hydrothermal evolution,have been distinguished in the wolframite-quartz veins at different depths.Compared with the early-formed wolframite,the late-formed wolframite has distinctly lower Nb,Ta,Zr,Hf,Ti,Sn,U,In,Sc contents and higher Mo contents and FeO/MnO ratios.The compositional evolution of mica and wolframite reveals that magmatic fluids dominated the early stage of hydrothermal ore-forming process,whereas more wall-rock contributions were involved in the late stage due to enhanced water-rock interaction.Besides,the variations of trace elements(Ag,Pd,Ni,W)in sulfides from quartz veins and wall rocks at different depths also indicate that the proximal earlier veins were mainly formed by crystallization of magmatic fluids,whereas more wall-rock contributions by water-rock interaction were involved in the formation of the distal later veins.Although both granite and wall rocks have provided ore-forming materials,the granite is the main source.The trace element compositions of sulfide minerals do not show systematic variations overall,however,several inferences could be obtained as follows.The oreforming materials are mainly derived from magma;the early wolframite-quartz veins mainly formed by crystallization of magmatic hydrothermal fluids;concentration of ore-forming materials content gradually decreases along with the evolution of oreforming fluids.