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Fluid inclusion and H-O isotope study of gold mineralization in the Penglai gold field,Eastern Shandong

HOU MingLan JIANG ShaoYong SHEN Kun LIAN GuoJlan LIU QiChen XIAO FengLi State Key Laboratory for Mineral Deposits Research,Department of Earth Sciences,Nanjing University,Nanjing 210093,ChinaShandong Burean of China Metallurgical Geology Bureau,Jinan 250014,ChinaShandong lnstitute and Laboratory of Geological Sciences,Jinan 250013,ChinaShandong Gold Group CO.,Lid,Jinan 250014,China  
Gold deposits in the Penglai gold field,Shandong peninsula are mainly auriferous quartz-veins with subordinate disseminations in altered granites(e.g.pyrite-sericite-quartz rock).The mineralization conditions are similar to those of the famous Linglong gold deposits.Gold occurs mainly in pyrite and in polymetallic sulphide quartz veins.Fluid inclusion study indicates that there are two major types of fluid inclusions in the gold deposits:CO_2-H_2O inclusions and NaCl-H_2O inclusions of medium-low salinity.CO_2 is the dominant component of the vapour phase of CO_2-H_2O inclusions which sometimes contains minor CH_4,H_2S and CO.Abundant CO_2-H_2O inclusions are present in pyrite-sericite-quartz rock whereas CO_2-rieh and H_2O-rich CO_2-H_2O inclusions with variable phase ratios coexist with each other in pyrite-quartz veins and polymetallic sulphide-quartz veins.Microthermometric results have shown that the homogenization temperatures of CO_2-H_2O inclusions in pyrite-sericite-quartz rock range between 230℃~300℃ whereas those for fluid inclusions in pyrite-quartz veins and polymetallic sulphide-quartz veins vary from 220 to 390℃.As the CO_2- H_2O inclusions with variable phase ratios were trapped from the immiscible CO_2-H_2O fluids the lower temperature range of ca.220~ 250℃ is considered to be their formation temperature.The fluids in the early mineralization stage were volatile-rich and medium-low salinity(4.15%~5.23% NaCl eqv)ones with densities from 0.92g/cm~3 to 0.985g/cm~3.During the main mineralization stage the fluids evolved to higher-temperature CO_2-H_2O fluids containing a NaCl-dominated aqueous liquid(1.02%~15.5% NaCl eqv)and a CO_2 vapour with minor CH_4,H_2S,CO and even organic substance;its density vary from 0.32 to 0.99g/cm~3.In the post mineralization stage the temperature,salinity and CO_2 content of the fluids decreased to the minimum.The δ~(18)O values of quartz in the deposits vary between 13.8‰ and 18.3‰ while the δ~(18)O values of the mineralizing fluids between 4.9‰ and 10.9‰.The δD values of fluid inclusions show a narrow range between-78‰ and-101‰ with a mode of-78‰~-88‰,suggesting that the mineralizing fluids originated mainly from magmatic water and accompanied with participation of meteoric water.The mineralizing fluids of the main mineralization stage exhibit feature of multiple sources.Temperature decrease and fluid immiscibility are the important mechanisms for deposition and enrichment of gold and other mineralizing elements.
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