Geochemistry And Ore Genesis Of Cu And Mo Mineralization In Yongping Ore Field,Jiangxi Province

Yongping ore field is located in the northeastern section of Qin-Hang metallogenic belt.It includes a large sized Yongping copper deposit and a medium-sized Shizitou molybdenum deposit.Previous researches on copper ore genesis in Yongping copper deposit indicate the stratiform-like copper ore bodies is either of Hercynian Sedex type or of the Yanshanian magmatic hydrothermal origin.In South China,a certain amount of stratiform copper polymetallic deposits are sharing the same controvisal issues on ore genesis,such as Jiangde,Dongxiang,Chengmenshan,Wushan,Tongling and Mashan,etc.Recently,a porphyry molybdenum ore bodies were found around Shizitou granite in southeast of Yongping ore field through general investigation.Therefore,we can make it clear how stratiform ore bodies got formed and what role the Yanshanian intrusions played in South China by systematic study on Yongping ore genesis.Yongping copper ore bodies are hosted in late Carboniferous Outangdi formation of marine carbonate rocks and terrigenous clastic rocks with siliceous rock formation.The Houjia inverted anticline and a series of NNE-trending compresso-shear fault belts are the main rock-and ore-controlling structures in the Yongping copper orefield.Moreover,main intrusive rocks in area are porphyritic biotite granite and quartz porphyry in stocks and dikes.In Yongping deposit,stratiform like copper ore bodies are hosted in Outangdi Formation and close to Huoshaogang granite.Stratiform ores and skarn ores are main ore types.Below the stratiform orebodies,a stockwork of quartz-pyrite veins are developed.In Shizitou,molybdenum ore bodies are located in interior and contact zone of porphyritic biotite granite intrusion,occurring in veinlets or thin film shape.The ore minerals are pyrite,chalcopyrite,molybdenite,galena,sphalerite,wolframite,etc.The gangue minerals are quartz,sericite,garnet,diopside,scapolite,wollastonite,tremolite,epidote,calcite and chlorite.In this study,we present systematic LA-ICP-MS zircon U-Pb dating,whole-rock major and trace elements,REE,Sr-Nd-Hf isotopic data.Meanwhile the fluid inclusions,H-O-S-Pb-He isotopes,pyrite Pb-Pb dating,quartz inclusions Rb-Sr dating and molybdenite Re-Os dating were conducted on ore forming fluid and metallogenic chronology.Shizitou and Huoshaogang granite dykes were formed during Yanshanian with the chronology data 160.1±1.OMa and 160.12±0.95Ma respectively.The quartz porphyry stocks intruded later(147.5±1.6Ma).Geochemically,these two intrusions are featured by their high K calc-alkaline nature.Trace elements have similar consistent evolutionary trend.It shows the relative enrichment of large ion lithophile elements(Rb,K)and a loss of high field strength elements(Nb,Ta,Ti).The LREE/HREE ratio of the porphyritic biotite granite is 10.0 to 20.1.The(La/Yb)N value varies from 12.4 to 40.6,not obvious HREE fractionation.The(Gd/Yb)N value is only 2.0 to 5.5 with low MgO content(0.4 to 1.88%)and high ratio of 207Pb/204Pb value(15.542-15.578).The LREE/HREE ratio of the quartz porphyry is 13.6 to 23.6.The(La/Yb)N value varies froml3.6-23.6,not obvious HREE fractionation,neither.The(Gd/Yb)N value is low at 1.8 to 2.9 with low MgO content(0.68 to 0.93%).Low MgO content and high(La/Yb)N value indicate the intermediate acidity magmatic rocks in Yongping deposit were formed by partial melting of the thickened lower crust.Sr-Nd-Hf isotopes results show that the initial 87Sr/86Sr ratios of porphyritic biotite granite in Huoshaogang and Shizitou of Yongping copper deposit vary from 0.708397 to 0.712051.The ?Nd(t)value range from-5.0 to-10.4,and the Nd model age is 1.47 to 1.96Ga.Quartz porphyry of the lower ?Hf(t)values represent the magma compositions were not affected by the influence of mantle derived coUponents in the melting of the crust formation,and the(TDM2)value indicating the source rocks may be Mesoproterozoic crust.The relatively high ?Hf(t)value of the porphyritic biotite granite may be contaminated by the mantle.The stratiform-like copper ore bodies,with minor quartz-polymetallic veins,were formed in the main mineralization stage.The post-ore stage is characterized by late quartz-calcite±pyrite veins.Fluid inclusions in quartz from the pre-ore barren quartz-pyrite veins,stratiform-like copper ore bodies,and post-ore quartz-calcite±pyrite veins were studied.Fluid inclusion petrography shows that two-phase liquid-rich(type ?),two-phase vapor-rich(type ?),and halite-bearing(type?)fluid inclusions.Type ? and type ? inclusions occur only in quartz from the stratiform-like copper ore bodies,whereas type I inclusions occur in all stages of hydrothermal quartz.Type ?inclusions in pre-ore quartz-pyrite veins have homogenization temperatures of 300 to 398? and salinities of 3.6 to 8.1 wt.%NaC1 equivalent.Type ? and coexisting type ? inclusionsin the stratiform-like copper ore bodies share similar homogenization temperatures of 233 to 324? and two contrasting salinity groupings of 0.4 to 3.2 wt.%and 28.2 to 39.0wt.%NaCl equivalent,respectively.The coexistence of type ? and type ? inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred.Type ?inclusions in the post-ore quartz-calcite veins have homogenization temperatures of 122 to 250?and salinities of 0.2 to 6.8 wt.%NaC1 equivalent.The hydrogen and oxygen isotopic compositions of hydrothermal quartz indicate that the mineralizing fluids gradually evolved from a magmatic to meteoric source.Fluid inclusion petrography shows that two-phase liquid-rich(type ?),two-phase vapor-rich(type ?),and halite-bearing(type ?)fluid inclusions are present in three stages of molybdenum orebodies forming process,including pre-ore,ore and post-ore stage.Type ? and type ?inclusions occur only in quartz from the quartz-molybdenite veinlets,whereas type ? inclusions occur in all stages.Type ? inclusions in the pre-ore stage have homogenization temperatures of 291 to 376? and salinities of 2.2 to 0.5 wt.%NaCl equivalent.Type ? and coexisting type ?inclusions in the ore stage share similar homogenization temperatures of 245 to 351? and two contrasting salinity groupings of 0.6 to 3.5 wt.%and 28.0 to 36.2 wt.%NaCl equivalent,respectively.The coexistence of type ? and type ? inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred.Type ?inclusions in the post-ore stage have homogenization temperatures of 209 to 288? and salinities of 0.5 to 5.0 wt.%NaCl equivalent.The hydrogen and oxygen isotopic compositions of hydrothermal quartz indicate that the mineralizing fluids gradually evolved from a magmatic sourceThe Pb-Pb dating of pyrite with all data from strat?form-like copper ore bodies and the Re-Os isotope dating of molybdenite suggest that mineralization took place in the Jurassic,consistent with the age of Huoshaogang and Shizitou porphyritic biotite granite.The Pb-Pb dating result of pyrite with stepwise leaching method of massive ore is 144±13Ma.And the Rb-Sr isotope dating of inclusions in quartz veins indicates the age of 147.9±5.2Ma,similar with the quartz porphyry stock.The similar age probably infers the result of early Pb-Pb isotope system reset for late thermal events.The S isotope composition of sulfide indicates that the S source is from magmatic sources.The Pb isotope data show that the lead is of Yanshanian magmatic hydrothermal genesis.And the He isotopic results of pyrite indicate that the source of copper ore is a mixture of the mantle and the crust.In summary,the skarn alteration,inferred boiling fluid inclusion assemblages,stable isotopic features of magmatic fluids,and Jurassic ages imply that the Yongping copper deposit is inclined to be a Jurassic skarn copper deposit.But we can not invalidate the contribution of Carboniferous submarine exhalative-sedimentation.Moreover,the Shizitou molybdenum ore bodies are of Endako porphyry type.