| As one of the most significant and typical ore fields of the massive sulfide deposits metallogenic province in North Qilian Mountain,Baiyinchang copper polymetallic ore deposit,Gansu consists of 5 industrial deposits:Zheyaoshan Cu-Zn large deposit,Huoyanshan Cu-Zn medium deposit,Tongchanggou Zn-Pb-Cu small deposit,Xiaotieshan Pb-Zn-Cu large deposit and Sigequan Pb-Zn-Cu regular deposit,respectively.In this study,taking the Xiaotieshan ore deposits as the key investigating object,with the comprehensive researches of the fluid inclusion and isotope geochemical characteristics,it preliminary discussed the sources of ore-forming materials,ore-forming fluid sources and physical and chemical properties,and combined with regional tectonic evolution and mineralizing chronology information discussed ore deposit genesis of the Xiaotieshan lead-zinc polymetallic ore deposit,further constructed the possible geological prospecting model.Xiaotieshan ore deposits both occur in the keratophyric quartz tuff,controlled by the related volcanism.The upper plate of the ore body mainly tends towards NEE with bed and lens forms,whereas the footwall mostly presents in veins and mesh-veins structure.Eight types of ores could be identified,namely massive pyriteore,massive chalcopyrite-pyrite ore,massive sphalerite-galena-pyrite ore,massive chalcopyrite-sphalerite-galena-pyrite ore,disseminated chalcopyrite-pyrite ore,disseminated sphalerite-galena-pyrite,disseminated chalcopyrite-sphalerite-galena-pyrite ore,and quartz-vein copper ore.The near-ores country rock alterations are chiefly chloritization,sericitization,silicification and barite gasification.Combining sulfur,lead isotope result and mineral trace elements data,the mineragenesis of Baiyinchang Deposit was volcanic rock formation-spilite keratophyre sequence.The studies of helium,argon,hydrogen and oxygen isotopes together with inclusions indicated that ore-forming fluid was mixtureof volcanic hydrothermal fluid and seawater introduced by the associated volcanism.Through the fluid inclusion studies,the mobile ore-forming fluid in H2O-NaCl-CO2-CH4 system with medium-high temperature,low salinity,rich Cl-and SO42-and poor F-was featured in the relatively oxidizing and acidic environment,whereas the ore-forming fluid in H2O-NaCl-CO2-CH4 system during mineralization with medium-low temperature was under the moderately reducing and neutral environment.At the same time according to the characteristics of alteration mineral barite,chlorite was studied by means of electron probe,once again verified that metallogenic stage ore-forming fluid from the early stage of the metallogenic process,there existed a typical changes in pHand REDOX process.Dating work show that the age of the igneous rocks from Baiyingchang was 468-472Ma,the mineralization age of this kind of deposit is considered to be consistent with its age of rock forming,so the mineralization is formed in island arc setting in Ordovician.In the process of mineralization the contain quartz vein in the vein-net orebody footwall indicated the channel of hydrothermal rising.The vivid acidic volcanic rock in the deep segregated out hydrothermal fluid carry with metal.It flowed up through the felsic volcanic rock by crack or fault and formed deposit when it reach the bottom of the sea.Part of the hydrothermal moved along the high permeability of pyroclastic rock with lateral migration which formed in net vein ore belt.When thermal fluid continued upwelling,the discharge runed into the sea and met with cold water and hot water leaded to a large number of silica,barite and iron oxide rapidly accumulate forming of layered solid crust.It prevented the sea water from the rising hydrothermal,Magmatic hydrothermal continued moving through crack channels with large numbers of ore-forming materials,the met between the fluid and the convective circulated water prompted the formation of ore deposits. |
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