Re-Os Dating of Molybdenite from the Jiama Copper Polymetallic Deposit in Tibet and Its Metallogenic Significance

Molybdenite is very common in the Jiama copper polymetallic deposit and occurs in the different types of ores, especially in skarn-type and hornfel-type ores, minor in porphyry-type ores, and extremely less in mable and limestone ores. Molybdenite samples with different occurrences were collected from skarn, hornfel and porphyry and tested by Re-Os isotopic dating. Mode age of 27 samples ranges between 14.2 and 17.5 Ma and the isochron age is 15.22±0.59 Ma. The isochron ages of porphyry and hornfel are 14.78±0.33 Ma and 14.67±0.19 Ma, which are consistent to metallogenic period of other porphyry copper deposits in the Gangdese metallogenic belt. The 187Re content in molybdenite is correspondingly higher in the range of 38.75~387.4 μg/g. Molybdenite in the Jiama ore deposit was supposed to form in one period. It was in the Langhian Stage of Miocene with the feature of postcollision. The metallogenic period of the Jiama ore deposit is close to that of molybdenite, therefore the previous overview of effusive sedimentary metallogeny was not scientific.