Isotopic Geochemistry And Ore-Forming Fluids Evolution Of The Sadaigoumen Mo Deposit, Hebei Province, China

Located in Fengning County,Hebei Province,the Sadaigoumen molybdenum deposit is the largest molybdenum deposit in this region so far.The Sadaigoumen molybdenum deposit is a typical porphyry molybdenum deposit in the Yan-Liao Mo metallogenic belt on the northern margin of North China Block.The ore bodies are mainly distributed in the monzogranite,and the molybdenum mineralization is closely related to micro-oblique petrochemistry,silicification,and muscovite.The Mo mineralization can be divided into three periods,which are pre-ore period?quartz vein stage without mineralization?,the syn-ore period?including?1?quartz+magnetite+molybdenite+pyrite+K-feldspar stage,?2?quartz+molybdenite stage,?3?quartz+mica+molybdenite+pyrite stage,and?4?quartz+molybdenite+pyrite stage?,and the post-ore period?late hydrothermal stage without mineralization?.The total characteristics of ore bearing joint veins in the mineralization show that the dominant positions are NWW and SWW directions.In addition,a statistical analysis suggests that the mineralization is closely related to the strength of the fractures,as the thickness of ore veins obeys the law of power distribution.From early to late ore-forming stages,the fractal dimension D value of vein thickness?0.28¹.047?shows an increasing trend.It indicates that the connectivity of the vein system gradually becomes worse and worse,and the mineralization becomes weaker and weaker,which is consistent with our observation in the ore district.There are two types of fluid inclusions in this deposit,which consist of two-phase vapor-liquid inclusions and CO₂-bearing three-phase inclusions.The quartz from the pre-ore and post-ore periods are predominantly characterized by the two-phase vapor-liquid inclusions,which have the homogenization temperatures ranging from 248? to 296?,130? to 197?,and the salinities varying from 6.0%to 10.2%NaCleqv,and 0.2%to 5.9%NaCleqv for the pre-ore and post-ore periods,respectively.During the syn-ore period,the two types of fluid inclusions are both developed in quartz,with the homogenization temperatures and salinities ranging from 200? to 260?,and from 7.0%to 17.5%NaCleqv.Raman spectrum and inclusion groups components studies indicate that ore fluids are mainly composed of CO₂,H₂O,N₂,Na⁺,Ca²⁺,NO₃^-and Cl^-.Therefore,the ore fluid in the Sadaigoumen Mo deposit is a CO₂-H₂O-NaCl system dominated in a medium-low temperature and medium-low salinity.The ?D and ?¹⁸O_H2O values for quartz from Mo ores vary from-88‰to-102‰,and-1.90‰to 3.12‰,thedD values for muscovitere from ores vary from-92‰to-71‰,spectively.Therefore,the ore fluid is dominated by the magmatic water.The sulfur isotope composition of pyrite indicates that the source of ore-forming material may be magmatic hydrothermal fluids.Based on characteristics evolution associated with the ore-forming fluids of the Sadaigoumen Mo deposit,we believe that molybdenum may migrate in the form of KHMoO₄.The involvement of tectonic movement and deep-seated magmatic hydrothermal fluids have changed the physical and chemical conditions such as temperature and pressure in fluid systems,and lead to the precipitation of molybdenum bearing metallic minerals and eventually form the Satai gully porphyry molybdenum deposit.