| In Mesozoic,a large-scale gold metallogenic event occurred in the Jiaoxibei area.However,the genesis of gold deposits in this area remain controversial cause of the different understandings of the sources of ore-forming fluids and materials.In this paper,one typical gold deposit is selected from each of the three main metallogenic belts in the Jiaoxibei area as the research object,the systematic study of lithogeochemistry,mineralogy and isotope geochemistry is carried out,aiming to constrain the sources of ore-forming fluids and materials.Finally,a metallogenic model of the gold deposits in the Jiaoxibei area has been built and the main achievements are as shown below:1.The characteristics of major elements show that the host rocks of typical gold deposits are mainly calc-alkaline peraluminous grantiods.During the intense hydrothermal alteration process,K2O,Fe2O3,Au,Ag,Cu,Pb,Zn and S were brought in,while Si O2,Na2O,Ba and Sr were out.The REE distribution patterns of ores are rightward.Compared with metamorphic rocks of the Jiaodong Groups and Mesozoic grantiods,ores and basic dikes are more enriched in HREE,indicating that they are homologous.2.The gold minerals are mainly electrum,followed by natural gold and goldargentid.Visible gold mainly occurs as fissure gold,intercrystalline gold and wrapped gold in pyrite,quartz and other sulfides.Consequently,the gold particles have no fixed shape,and the particle sizes change greatly,mainly concentrated in 2～25μm,with exceptions>100μm.Scanning Electron Microscope with Energy Dispersive X-ray Spectrometry（SEM-EDS）analysis shows that all gold minerals have no core-rime structure,and some gold particles contain silicate mineral inclusions.Paragenetic minerals include pyrite,galena,sphalerite and chalcopyrite,as well as Bi and/or Te-bearing minerals such as tetradymite.The Au+Ag content of gold mineral is generally more than 98 wt.%.Furthermore,gold minerals generally contain a certain amount of Bi and Te.Except for the obvious negative correlation between Au and Ag,other elements show no correlation with Au.The results of correspondence analysis show that the gold minerals are characterized by Au-Bi and Ag-Te combination,which are comparable to those of mantle-derived-magmatic-hydrothermal gold deposits.This suggests the contribution of mantle derived components to the ore-forming materials of gold deposits in the Jiaoxibei area.3.The pyrite of gold deposits in the Jiaoxibei area has similar characteristics.According to structure,morphology and paragenetic mineral assemblage,it can be divided into three types,namely Py1,Py2 and Py3.Py1,formed in the early stage,is mainly euhedral crystal,almost contains no voids and mineral inclusions,enriched in Co and Ni,while depleted in Cu,Pb,Ag and Au,indicating a relatively stable formation environment;Subhedral to anhedral Py2 and Py3,which represent the main gold mineralization stage,are rich in nano-inclusions,and enriched in As,Cu,Pb,Ag and Au while depleted Co and Ni.They were formed via rapid crystallization,indicating that intense fluid boiling occurred in the main mineralization stage.Co,Ni,As and invisible Au in pyrite mainly exist in the form of solid solution,while Pb,Bi,Ag,Cu,Zn and other elements mainly exist in the form of submicron mineral inclusions.Pb,Bi and Ag in pyrite usually show a positive correlation,whereas other elements have no unified correlation in different types of pyrite.The content of invisible gold in all pyrites is extremely low,indicating that no significant component of the gold ores can be formed by mobilization of gold out of previous pyrite.The content and ratio characteristics of Co and Ni in Py2 and Py3 are similar to those of sedimentary pyrite,suggesting that the source of ore-forming metals is metasomatized by sediments.4.The geochemical charateristics of rocks,minerals,and multiple isotopes consistently show that the most possible source of ore-forming fluids and materials of gold deposits in the Jiaoxibei area is the lithospheric mantle which has been metasomatized by fluids released by the subducted paleo-Pacific plate and its overlying marine sediments.5.Despite the difference in the tectonic setting,the genetic model of gold deposits in the Jiaoxibei area is similar to the subduction-related model of orogenic gold deposit.Furthermore,the isotopic composition,wall rock alteration,mineralization types,fluid sources,temperature and pressure conditions and metallogenic depths of gold deposits in this area are highly comparable to those of typical orogenic gold deposit.Therefore,this study suggests that the gold deposits in the Jiaoxibei area belong to orogenic gold deposits. |
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