Geochronology and Geochemistry of the Tinggong Porphyry Copper Ore Deposit, Tibet

We have determined the ages of the ore-bearing Tinggong porphyries and the Eocene granites using the LA-ICPMS zircon U-Pb method. Zircons from one adamellite porphyry and two diorite porphyries yield ages of 15.54 +/- 0.28 Ma, 15.02 +/- 0.25 Ma and 14.74 +/- 0.22 Ma, respectively. The ages of two granites are 50.48 +/- 0.71 Ma and 50.16 +/- 0.48 Ma. Light Rare Earth Elements (LREE) are enriched in the ore-bearing adamellite porphyries, which are high-K calc-alkaline and metaluminous, while Heavy Rare Earth Elements (HREE) and Y are strongly depleted, indicating an adakitic affinity. The Large Ion Lithophile Elements (LILE) of the adamellite porphyries are highly enriched, whereas some High Field Strength Elements (HFSE) are depleted. The diorite porphyry in this study is chemically similar to the adamellite porphyries, except that the Mg-# of the diorite porphyry is a little higher, demonstrating more mantle contamination. Four samples from different rocks are selected for in situ zircon Hf isotopic analyses. The samples show positive epsilon(Hf)(t) values and young Hf model ages, indicating their derivation from juvenile crust. However, the adamellite porphyry and diorite porphyry formed in the Miocene exhibit more heterogeneous Hf isotopic ratios, with lower epsilon(Hf)(t) values than the granites formed in the Eocene, suggesting the involvement of old Indian continent crust in their petrogenesis. The geochronology and geochemistry of the adamellite porphyries and the diorite porphyries indicate that they formed from the same source region in a post-collisional environment, but contaminated by crust and mantle materials in different ratios. The metallic minerals formed mainly during the older adamellite porphyry stage, but they were recycled and reactivated by the diorite porphyry intrusion.