Genetic Mineralogy And Ore-forming Processes In Dayingezhuang Gold Deposit,Jiaodong Peninsula

Jiaodong area,located in the southeastern of North China Craton,is the largest gold producing regeion of China.Its proven reserves of the gold resources account for about 25%of the whole territory within 0.2%of the area.A lot of gold deposits with rich reserves distribute in the region.Among them,the Dayingezhuang gold deposit,in the middle of Zhaoyuan-Pingdu fault,is a typical fracture zone altered rock type gold deposit in Jiaobei uplift area.With gold reserve above 120 t,it is an important gold depostis in the area.Dayingezhuang gold mine is divided into two ore body by Dayingezhuang falt,and ore body I is obviously effected by the late silver polymetallic sulfide mineralization.The study of genetic mineralogy of siderite,quartz and other mineral and the ore-forming processes in Dayingezhuang gold deposit has been carried,and the muti-mineralization process has been discussed in this paper.Siderite mineralogy shows that there are four kinds of siderite in Dayingezhuang gold deposit,including coarse grain siderite,fine grain siderite,fragmentized siderite and siderite inclusions in euhedral crystal quartz.The Electron Microprobe Analysis(EMPA)results show that the coarse grain siderite and fragmentized grain siderite have the same composition,showing the feature of high manganese and low magnesium.Combined with their relationship with other metal sulfide,we can infer that the fragmentized grain siderite might have formed from the coarse grain siderite under structural stress.The fine grain siderite has the feature of high magnesium and low manganese,and should have been formed later than the coarse grain siderite.The metasomatism origin siderite is made up of calcium-bearing fluid and pyrite veins of metasomatism,and it is different from the other kinds of siderite in genetic type.The carbonate fluid in Dayingezhuang area has highly consistency with other gold deposits in background,combining previous study of carbon-oxygen isotope of carbonate minerals in Jiaodong area,we speculated that carbonate fluid in Dayingezhuang gold deposit might have derived from the enriched mantle.There are a variety of different types of quartz formed during the ore-forming processes in Dayingezhuang gold deposit,such as the coarse residual dependent-structured crystal quartz and fine quartz decomposed by the K-feldspar in beresitization stage,anhedral crystal and euhedral crystal in quartz vein stage,and anhedral crystal and euhedral crystal in siderite stage.Euhedral quartz in siderite vein are mostly in regular hexagon,and some rhombus quartz grains can be observed.We consider that they are pseudomorphous ?-quartz.EMPA results show that there isn't titanium element contained in the euhedral crystal quartz,which means that the formation environment could be Ti-unsaturated.Characteristic of ?quartz shows that the quartz might have been formed in low pressure environment,and was affected by the regional structural forces.The evidence,including the formation of fragmentized grain siderite and ?-quartz,and the study of fluid inclusions by other researchers,leads to the role of rigional WE fault,Dayingezhuang Fault.Dayingezhuang fault formed in the mid to late stage of gold mineralization,and the fault cut the ore deposit into two ore bodys.The fault activities broke the siderites and induced the abrupt release of pressure which could lead to the formation of ?-quartz.With the reduction of the pressure,the fluid“boiling”occurred,and the change of a series of physical and chemical conditions made the silver-bearing polymetallic sulfide enriched and mineralization in the fissure of siderite,in the foot wall of the Dayingezhuang Fault.This process resulted in the characteristic of silver mineralization in ore body I.Dayingezhuang Fault may be the key tectonic factor of silver mineralization.Therefore,it implies that the ore-controlling structure of Jiaodong area is not only in NE-NNE direction,but also in WE direction.