Geologic Characteristic And Genesis Mechanism Of The Qingshuitang Pb-Zn Deposit In Qidong County,Hunan Province

The Qingshuitang Pb-Zn deposit is a medium deposit,located in the middle part of eastern Yangtze Block,about 38.5km northwest from Qidong County,Hunan Province.Based on geological investigation and microscopic observation,we study the Zhoujialing granitic stock on element geochemistry,zicon U-Pb ages and in situ zircon Hf isotope composition.Furthermore,we study the fluid inclusions,from sphalerite and ore-bearing quartz,calcite and barite by microscopic observation,microthermometry and micro-laser Raman spectroscopy of sphalerite.H-O isotopic data are obtained from ore-bearing quartz and C-O isotopic data are obtained from calcite and limestone.In this paper,we focus on the petrological property,forming age and material sources of Zhoujialing granite,as well as ore-forming material source and the physical and chemical conditions of Qingshuitang Pb-Zn deposit.With reference to previous research results,we discuss the genesis mechanism of Qingshuitang lead-zinc deposit.The Zhoujialing granitic stock is characterized by high potassium,rich in light rare earth element?LREE?and large ion lithophile element?LILE,i.e.Rb,Th,U?,while depleted in heavy rare earth element?HREE?and high field-strength element?HFSE,i.e.Nb,Ta,Ti?,Ba,and Sr.LA-ICP-MS zircon U-Pb dating indicates that the granite formed during the late Indosinian,with zircon age of 203.0 ± 1.4Ma?MSWD = 0.49,2?,N=19?.The in situ zircon Hf isotopic data of Zhoujialing granite exhibited low ?Hf?t?values??Hf?t?=-10.69?6.31?and old two stage Hf model ages?TDMC= 1.59?1.85Ga?,indicating that diagenetic substances of the Zhoujialing granitic stock were mainly derived from Paleoproterozoic lower crust.The microscopic observation shows that fluid inclusions are erratically developed in different minerals.There are more fluid inclusions occurring in sphalerite and calcite than quartz and barite.It mainly consists of liquid-rich two-phase inclusions.Microthermometry shows that the homogenization temperatures of sphalerite,quartz,calcite and barite range from 109 to 188?,122 to 239 ?,144 to 326? and 149 to 221? respectively,with the average of 146?,175?,246? and 171? respectively.Their salinities range from 9.47 to 20.97%NaCl eq.,0.18to 16.62%NaCl eq.,0.18 to 7.31%NaCl eq.and 0.18 to 1.57%NaCl eq.respectively,with the average of 16.4%NaCl eq.,8.2%NaCl eq.,2.7%NaCl eq.and 0.79%NaCl eq.respectively,of which the homogenization temperatures and salinities of sphalerite are in a narrow range.Based on inclusion data,metallogenic pressure of sphalerite,calcite and quartz is estimated about 10.6 MPa,9.4MPa and 11.7MPa,indicating the mineralization lay at depths of approximately 1.0km,0.9km and 1.2km below the surface.Micro-laser Raman spectroscopy of sphalerite shows the gas phase of the fluid inclusions mainly consists of water vapor.The synthetical analytical results indicate that Qingshuitang deposit formed in a low temperature epithermal environment.The evolution of the ore-forming fluid is simple and the mixing action is dominant during the mineralization process.?³⁴S data of pyrite,galena and sphalerite range from-7.41 to 2.91 ‰,with an average value of-1.93‰,and?³⁴S data of barite range from 11.49 to 12.34 ‰,with an average value of 11.98‰,indicating a deep source for magma and maybe partly contaminated by the crustal substance.Pb isotopic data are obtained from pyrite,galena and sphalerite.The 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb ratios are within the range of 17.810 to 18.710?average 18.297?,15.497 to 15.726?average 15.618?,37.858 to 38.834?average 38.463?,indicating Pb was mainly derived from crust and maybe mixed some mantle source.?Dsmow of ore-bearing quartz ranges from-98.9 to-79.3‰,with an average value of-85.3 ‰;?18OH2O ranges from 12.30 to 13.48‰,with an average value of 12.95 ‰,indicating a deep source of ore-forming fluid.Values of ?13 CPDB for limestone in strata range from 0.9 to 2.6‰,with an average value of 1.7 ‰,?18 O smow values range from 21.86 to 23.39 ‰,with an average value of 22.52‰;?13 CPDB Of ore-bearing calcites values range from-5.3 to-4.6‰,with an average value of-5.0‰,6 8O SMow values range from 12.30 to 13.48‰,with an average value of 12.95‰.The C-O isotopic compositions of ore-bearing calcites are different from the limestone and the ore-forming fluid was mainly derived from magmatic hydrothermal water,influenced by the formation water and meteoric water.At the end of the Middle Triassic and Late Triassic,the thickening of Paleoproterozoic lower crust in primary-deformation stage caused the rise of the temperature,and the melting of lower crust.With the stress receding or extrusion eased,the granitic magma transported along the tectonic faults,then emplaced in relative open environment and formed the Zhoujialing granitic stock.The mineralized ore-forming fluids containing Pb,Zn,Cu,Au elements migrated into the near-surface,setting and enriching at proper tectonic position under the mixing effect of tectonic decompression and atmospheric precipitation and formed Qingshuitang Pb-Zn deposit.