Characteristics and evolution of ore-forming fluids from the Sadaigoumen porphyry molybdenum deposit,Hebei.

The Sadaigoumen deposit lies in Hebei province and is the largest molybdenum deposit in this region thus far reported. Its major molybdenite orebodies are mainly hosted inside monzogranite. Molybdenite mineralization is closely associated with microclinization,silicification and muscovitization.Fluid inclusion studies suggest that three types of fluid inclusion are present in this deposit:two-phase vapor-liquid inclusions,CO_2-beating three-phase inclusions and pure CO2 phase inclusions.Fluid inclusions in quartz in the pre-ore stage and post-ore stage are mainly two-phase vapor-liquid ones with small amount of CO_2 phase.The homogenization temperatures and salinities of the former stage vary from 280 to 452℃and from 5.4 to 18.4 wt% NaCl equivalent respectively,and those of the latter stage vary from 153 to 279℃and from 3.9 to 9.7 wt% NaCl equivalent respectively.During the mineralization stage,all the three types of fluid inclusion were formed in ores.Microthermometric studies indicate that their homogenization temperatures and salinities range from 170 to 370℃and from 4.3 to 14.4 wt% NaCI equivalent respectively.Laser Raman spectrometric and hydrogen and oxygen isotope studies of the fluid inclusions in ores indicate that quartz in the Sadaigoumen deposit contains-82‰to-98‰δD and 0.1‰to 6.2‰δ~(18)O_(H_2O)and that the ore-forming fluids once belonged to the immiscible NaCl-H_2O-CO_2 system and were mainly derived from magmatic water,which were mixed with meteoric water in the post-ore stage. During the formation of the ore-forming fluids,three stages of fluid evolution occurred in the Sadaigoumen deposit :magma dehydrating and degassing in the pre-ore stage,fluid immiscibility in the mineralization stage and meteoric waters mixing in the post-ore stage, respectively.The decrease of temperatures and fluid immiscibility are the dominant factors for molybdenite deposition.