Metallogenic Mechanism Of The Laoliwan Ag-Pb-Zn Deposit In The Xiaoshan District,Southern Margin Of The North China Craton

The newly discovered Laoliwan large Ag-Pb-Zn deposit is located in the Xiaoshan district,southern margin of North China craton.The ore bodies are mainly hosted in the Laoliwan granite porphyry,partly hosted in the Xiong'er Group,and controlled by the regional faults.These features make Laoliwan being different from other types of Ag-Pb-Zn deposit in the area.The mineralization could be subdivided into two periods:the hydrothermal period and the supergene period.The hydrothermal period consists of four stages:the quartz-pyrite stage,the siderite-polymetallic sulfide stage,the quartz-polymetallic sulfide stage,and the quartz-barite-carbonate stage.LA-ICP-MS zircon U-Pb dating results of two samples from the Laoliwan granite porphyry are 129.0±1.7 Ma and 129.0±1.1 Ma,respectively,indicating the granite porphyry intruded during early Cretaceous,which is also the upper limit of the age of Ag-Pb-Zn mineralization.Based on the rock geochemical analysis,the granite porphyry is characterized by high silicon,high alkali,and weak peraluminous.The REE and trace elements characteristics display enrichment of large ion lithophile elements?LREE,Rb,Ba,Sr,Pb?,depletion of high-field elements?HREE,Nb,Ta,Ti,P?and moderately negative anomaly of Eu.These features are similar with other early Cretaceous granites in the area,indicating these magmatic activities are probably related to contemporaneous tectonic transition and lithosphere thinning in eastern China.In Laoliwan granite porphyry,research on the fluid inclusion of quartz phenocryst indicates that multi-phase fluid inclusion containing daughter minerals is the main fluid inclusion type,with a few liquid rich two-phase fluid inclusions.Homogenization temperatures are mostly more than 300?,maximum up to 550?.The sanilities of the fluid are more than 30 wt%NaCl equiv.and the densities are more than 1.0g/cm³.There is also a certain amount of CO₂ detected in the fluid inclusion.In Conclusion,Fluid in the Laoliwan granite porphyry shows the feature of imtermediate-high tempetures,imtermediate-high salinity and intermediate-high density,indicating the fluid is H₂O-NaCl-KCl±CO₂ system.The dominated fluid inclusions in metallogenic hydrothemal period are liquid rich,and the homogenization temperature decreases gradually from early stage to late stage,with no more than 300?and 13 wt%NaCl equiv.mostly.This data shows the ore-forming fluids is characterized by intermediate-low tempreatures,intermediate-low sanility and low density fluid features.There is also a little CO₂ detected,suggesting that the fluid is H₂O-NaCl±CO₂ system.The decrease of homogenization temperature may be due to the mixing of meteoric water in the later period.H-O-C isotopes also indicate that metallogenic fluids have magmatic-hydrothermal sources,but may be mix with meteoric water in late stage.?D_H2O2O of hydrothermal period stage II-III ore-related sericite is-48.1^-59.7‰with an average of-49.6‰;?¹⁸O_H2O2O is 5.1⁷.3‰with an average of 6.7‰.The?D_H2O2O of barite in stage III is-97.2^-103.7‰,and the?¹⁸O_H2O2O is 7.8⁸.6‰.The?D_H2O2O of barite in stage IV is-92.6^-96.2‰,and the?¹⁸O_H2O2O is-0.4⁰.2‰.In the?D_H2O2O versus?¹⁸O_H2O2O diagram,the H-O isotope points shows drifting from magmatic area to the meteoric line.The?¹³C_PDBDB and?¹⁸O_PDBDB of siderite in the stage II is-3.7^-4.9‰and-17.5^-18.8‰,respectively.The?¹³C_PDBDB and?¹⁸O_PDBDB of calcite in the IV stage is-0.2^-5.6‰and-13.1^-22.5‰,respectively.After calculation,the?¹³C_fluidluid and?¹⁸O_H2O2O of siderite is-5.2^-6.6‰and4.1⁵.3‰,respectively,and the?¹³C_fluidluid and?¹⁸O_H2O2O of calcite is-1.6^-7.0‰and-5.7⁴.0‰,which means the ore-forimg fluid is mainly magmatic-hydrothermal source,and in the late stage the metoric water mixed up.In the Laoliwan deposit,the?³⁴S of sulfide ranges from-0.1 to 5.3‰,the average is 2.3‰.The?³⁴S of barite is 10.0¹1.9‰,with an average of 11.3‰.Sulfur isotope composition points that sulfur is mainly derived from magmatic-hydrothermal fluids.From the II stage to IV the sulfur isotope increases relatively,indicating that there is probably a little sulfur derived from the Xiong'er Group.Lead isotope compositions of galena about ²⁰⁶Pb/²⁰⁴Pb?²⁰⁷Pb/²⁰⁴Pb?²⁰⁸Pb/²⁰⁴Pb is 17.163¹7.400,15.382¹5.491,36.840³7.801,respectively,of which the distribution is centralized.In the lead isotope diagram,the distribution is on the upper part of Taihua group area,overlapping part of granite porphyry in the Early Cretaceous in Xiaoshan area.Sulfur-lead isotope shows that the metallogenic material is from magmatic-hydrothermal source,with the feature of deep magma source,which is not exactly the same from the source of granite porphyry of Early Cretaceous in Xiaoshan area.Based on mineralization characteristics,regional geochronological data and sources of metallogenic fluid and material.Although the Laoliwan deposit and granite porphyry have closed space relationship and the ore-forming fluid is closely related to the genesis of magmatic hydrothermal fluid,it is supposed that the Ag-Pb-Zn mineralization formed later than the granite porphyry and they have no direct genetic relationships.However,they may be the products of activities of the same deep magma chamber.Study of the Laoliwan Ag-Pb-Zn deposit suggests that the superposition of faults and granite rocks could be the prospective places for exploring Ag-Pb-Zn deposits in the Xiaoshan disctrit,which is confirmed by the recent prospecting work at Zhonghe in north of Laoliwan.Compared with Xiaoqinling and Xiong'ershan area,the exhumation of the Xiaoshan area is quite shallower.It is potential to find porphyry molybdenum deposit under the Ag-Pb-Zn ore bodies of Laoliwan.