Study Of The Kangjiawan Lead-zinc Deposit Genesis And Relationship With Surrounding Magmatism In Changning City,Hunan Province

The Kangjiawan lead-zinc deposit is located in the north eastern part of Shuikoushan orefield,and is a large lead-zinc deposit.Based on field geological investigation,comprehensive analysis,including microscopic observation,LA-ICP-MS zircon U-Pb dating,whole-rock chemistry and Sr-Nd-Pb isotopic compostion,and in-situ zircon Hf isotopic geochemistry of the Shuikoushan granodiorite and Laomengshan rhyodacite,and microscopic observation,electron microprobe analysis,fluid inclusions,ore S-Pb isotope,ore-bearing quartz and light-colored sphalerite H-O isotopes,ore-bearing calcite C-O isotope as well as single mineral Rb-Sr dating of the Kangjiawan Lead-Zinc deposit have been performed.By referring to previous researches,the genesis of the Kangjiawan Pb-Zn deposit and relationship with surrounding magmatism are discussed in this thesis.LA-ICP-MS zircon U-Pb dating yields a weighted average age of 156.0±1.0 Ma for the intrusion of the Shuikoushan granodiorite.The granodiorite is characterized by enriched in potassium,light REE and large ion lithophile elements(LILEs,Rb,Th,U)and Pb,but depleted in heavy REE,high field strength elements(HFSEs,Nb,Ta,Ti)Sr,and Ba,which suggests shoshonitic and metaluminous to weakly peraluminous affinity.Samples displays relatively high(87Sr/86Sr)i ratios(0.710230 to 0.710624),low ?Nd(T)values(-9.90 to-9.27),old two-stage Nd model ages(TDM2=1.70 to 1.75 Ga),relatively homogenous Pb isotopic compositions[(206Pb/204Pb)i=18.400?18.468,(207Pb/204Pb)i=15.632?15.670,and(208Pb/204Pb)i?38.579?38.732],and negative in-situ zircon ?Hf(T)values(-10.80 to-8.71)as well as old two-stage Hf model ages(TDM2=1.75 to 1.88 Ga).LA-ICP-MS zircon U-Pb dating for the Laomengshan rhyodacite yields a weighted average age of 156.7±1.6 Ma.The rhyodacite is enriched in potassium,light REE and large ion lithophile elements(LILEs,Rb,Th,U)and Pb,while depleted in heavy REE,high field strength elements(HFSEs,Nb,Ta,Ti)Sr,and Ba.Samples are mainly shoshonitic,and metaluminous to weakly peraluminous.Samples display relatively high(87Sr/86Sr)i ratios(0.711648 to 0.711758),low ?Nd(T)values(-10.71 to-10.28),old one-stage Nd model ages(TDm?1.73 to 1.86 Ga),and relatively homogenous Pb isotopic compositions[(206Pb/204Pb)i=18.365?18.412,(207Pb/204Pb)i=15.663?15.680,and(208Pb/204Pb)i=38.625?38.666].The zircons present much negative in-situ zircon ?Nf(T)values(-16.22 to-9.86)and old two-stage Hf model ages(TDM2?1.82 to 2.22 Ga).Above petrochemical and isotopic characteristics indicate that both the Shuikoushan granodiorite and Laomengshan rhyodacite mainly originated from the Paleoproterozoic crust during the Late Jurassic,and maybe contaminated with a small amount of mantle material.The ?34S VCDT values of ores in the Kangjiawan lead-zinc mine,ranging from-2.71‰to 0.70‰,exhibit a magmatic sulfur source.The 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios of the ores are relatively homogenous and range from 18.427?18.573,15.671?15.696 and 38.762?38.964,respectively,which indicates a dominated crustal lead source with a little mixture of mantle substance.The ?D SMOW and ?18O H2O values of ore-bearing quartz range from-68.00‰ to-60.00‰ and 0.04‰ to 2.31‰,respectively,and the ?D SMOW and ?18O H2O values of light-colored sphalerite range from-108.00 ‰ to-101.00‰and-10.92‰ to-O.25‰,respectively.These hydrogen and oxygen isotopic compositions demonstrate that ore-forming fluid was mainly magmatic hydrothermal at the early stage and mixed with meteoric water in the late mineralization stage.The ?13C VPDB and ?18O SMOW value of ore-bearing calcites range from-3.70‰ to-1.80‰ and 12.20‰ to 16.00‰,respectively.These carbon and oxygen isotopic compositions of ore-bearing calcites are similar to those of the hosted limestone,inferring that the carbon in ore-forming fluids was mainly derived from limestone stratum.The Rb-Sr isochron ages of 154.6 ± 2.1 Ma in sphalerite single mineral(+quartz+calcite)and 151.7±2.5 Ma in light-colored sphalerite implies that the Kangjiawan lead-zinc deposit formed in Late Jurassic.According to microscopic observation inclusions of quartz,calcite,and light-colored sphalerite,liquid-rich two-phase aqueous inclusions are dominated with a small amount of vapor-rich two-phase inclusions in light-colored sphalerite and quartz were also found.The homogenization temperatures range from 95? to 389?,and salinities range from 0.2%to 21.5%NaCl eq.Before metallogenic period,the peak values of homogenization temperatures and salinities in fluid inclusions range from 310? to 330?,16%to 18%NaCl eq.,respectively.In metallogenic period,the homogenization temperatures peak values in fluid inclusions range from 300? to 320? and 120? to 140?,and salinities peak values range from 6%to 8%NaCl eq.and 2%to 4%NaCl eq.,respectively.After metallogenic period,the peak values of homogenization temperatures range from 100? to 120?,and salinities peak values range from 0%to 2%NaCl eq.Laser Raman analyses disclose that the conclusion fluids are mainly H2O-NaCl system,and then the ore-forming fluid depth is estimated as about 1.4 km.The research results indicate that the Kangjiawan lead-zinc deposit formed in a low temperature epithermal hydrothermal environment with fluid mixing of magmatic water and meteoric water predominated the ore-forming process.During Late Jurassic,the Xiangnan district(Shuikoushan orefield)experienced a transition of the Palaeo-Pacific slab from subduction to roll-back,which resulted in tectonic extension and thinning of the continental lithosphere of this region.The upwelling mantle materials induced melting of the South China Palaeoproterozoic metamorphic basement,and produced granitic magma by mixing with a fraction of mantle melts.The magma then invaded and then partly erupted to the surface along deep faults,and formed the Shuikoushan granodiorite and the Laomengshan rhyodacite.At the late period of the magma cooling,with the combination of structure decompression and atmospheric precipitation,Pb,Zn,Au,Ag,Cu precipitated from ore-forming fluids at suitable tectonic positions and the Kangjiawan lead-zinc mineral deposits are formed.