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Study On The Regional Metallogenic Model Of Lode Gold Deposits In The North China Block

Posted on:2012-11-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:L WangFull Text:PDF
GTID:1100330335952044Subject:Mineralogy, petrology, ore deposits
Abstract/Summary:PDF Full Text Request
Located in the center of the North China ancient plate, North China block is one of the most important gold-producing area,both for our country and for the world.In this paper, on the basis of the regional metallogenic theory, geodynamic evolution of North China region, study of lode gold mineralization,and analysis of typical deposits and experimental test,the author summarized the characteristics of the region deposit, and mineralization conditions, times and space.Further more,regional metallogenic model is established, which is helpful to make further exploration wok.According to the study of geological setting and geodynamic evolution. It is suggested that the North China block had gone through three stages,which are:1.the formation of the Precambrian crystalline basement;2.the formation of sediments in the stable block from the period of Proterozoic to Paleozoic;and 3.activation of the block from the period of Mesozoic to Cenozoic.North China craton which used to be a typical block before Mesozoic changed into an tectonic active region after that,and turned into an orogenic belt during Yanshan period. During the early and middle Mesozoic,it is an compressive orogenic environment,and changed into an extended environment in the late Mesozoic.The author compared the geochemical characteristics between Precambrian basalt in North China block and the typical greenstone belt abroad. Precambrian basalt has a low content of MgO(<20%),which reflects the North China block has a small origin depth of the mantle at that time, and less heat, lower temperature,together with the relatively cold mantle,all of which caused a small percentage of mantle melting. This small-scale melting mechanism lead to great differences between the mafic volcanic rocks of the North China block in the early Precambrian and typical greenstone belt abroad. They are demonstrated as higher metamorphism,intense transformation, and lack of komatiites. It is suggested that Archean greenstone belt and related gold deposits are not exist in the North China block.The point that Archean gold deposits of North China block is mantle source is proposed in this paper. Because sulfide has the higher ability of gold concentration than the silicate, during the process of basalt lava contained saturated sulfur which derived from upper mantle move upward,the basalt lava could form immiscible sulfide phase,and case most of gold remaining in the upper mantle. Gold in the upper mantle has a high capacity of mobilization and migration,and can easily be carried upward to the crust to form gold deposits.There are many similar characteristics in mineralization between lode gold deposits in North China block and the gold deposit in Archean greenstone belt abroad, but they also demonstrated their Unique sides. The North China block has a later time of consolidation and stronger activity, and the lode gold deposits mainly formed during active period of the continental blocks.The tectonic setting can be summarized in the grounds of extrusion, collision period and extensional tectonics period.In other words,It transformed from compression to tensile period. In the mining area, lode gold deposits mainly distribute in the block edge,adjacent to the deep fault zone, orogenic belt, the plates collision zone and the terrane collage zone.Wall rocks of gold deposits are mainly mafic and granitic rocks which are medium-high grade metamorphic rocks. The distribution of gold deposits is controlled by ductile and ductile to brittle shear zones.Large gold deposits are generally located in the ductile-brittle superposition shear belt,such as Jiapigou, Jinchangyu, Ural mountains, Small Qinling and Jiaodong gold deposits. Most gold deposits in North China Block are closely related to the mediate to acid magmatic activities. Such as small Qinling, Shandong and Jiapigou and so on.In this paper, SHRIMP U-Pd dating of Monazite uesd in Jinling and Lingshangou gold deposits is first used in the domestic,and the age of ore forming is 122±6.3 Ma and 118±11Ma,determined the time of formation of Jiaodong gold deposits is in the early Cretaceous.It is an important advance in determining the age of epigenetic deposits and related hydrothermal alteration,and it is one of the direct dating methods of the metal deposit. Three ore forming periods are determined,which are gold forming period in 190-160Ma and 126~110Ma,and gold-silver forming period in 100-80Ma.The period of 126~110Ma is the main ore forming period.According to the nonlinear relationship between fluid pressure and depth in fault zone established by Sibson(1988), combined with the pressure determined by immiscible inclusion geobarometry of CO2-H2O fluid inclusions, refered to formula to calculate the depth of hydrothermal lode gold mineralization by Sun Fenyue(2000),the depth of gold deposits formed in the North China block is determined to be 6.45~9.8km. Combined with geochemistry of ore-forming fluid inclusion, fluid of lode gold deposits in the North China block is CO2-H2O-NaCl system,with the ore forming temperature of 234.6~386.7℃and pressure of 64.0~138.0Mpa.Salinity of fluid is 2.73%~11.46%NaCl, and the density of fluid is 0.64~1.0g/cm3. Hydrogen, oxygen and carbon isotope show that the ore forming fluid was mostly mantle derived fluids, mixed with magmatic fluid and meteoric water. Studies on sulfur, lead, helium and argon isotope show that ore foming minerals are mainly from the upper mantle. The phenomenon of widespread distribution of independent tellurium minerals and lamprophyres in gold deposits in the North China block,together with mineralogy and petrology evidence,help to understand the mantle derived ore forming fluids.The paper also pointed out that the high content of gold in lamprophyre is the important evidence of mantle derived gold.On the basis study of typical deposits on the North China block, combining with geological setting, deposit geological and geochemical characteristics, source of ore-forming fluids, mineralization, ore-forming conditions and characteristics of isotopic chronology, it can be concluded that lode gold deposits of different structures in North China formed in the same geodynamic setting,which is,a transition system from collision to extension condition in the late Mesozoic. At that time, crustal extended significantly, resulting in mantle material and energy upwelling, rapid thinning and delamination of the lithosphere, large-scale volcanic eruption, resulting in the mantle ore forming elements being draged out,and ore-bearing hydrothermal fluids discharged in the brittle fracture. In the paper, the regional metallogenic model of lode gold deposits in the North China block is established. Seven prospecting directions are proposed and a theoretical basis for further exploration in this area is provided.
Keywords/Search Tags:Monazite, SHRIMP U-Pb dating, fluid inclusion, metallogenic model, the North China block
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