Chronology and Crust‐Mantle Mixing of Ore‐forming Porphyry of the Bangongco: Evidence from Zircon U‐Pb Age and Hf Isotopes of the Naruo Porphyry Copper‐Gold Deposit

AbstractThe Naruo porphyry copper‐gold deposit (hereinafter referred to as the Naruo deposit) in Tibet is a potentially ultra‐large, typical gold‐rich porphyry copper deposit, which was recently discovered in the Bangongco‐Nujiang metallogenic belt. This study analyzed U‐Pb chronology and Hf isotopes of the ore‐bearing granodiorite porphyry in the Naruo deposit using the LA‐ICPMS dating technique. The results show that the weighted average age is 124.03±0.94Ma (MSWD=1.7, n=20), and 206Pb/238U isochron age is 126.2±2.7 Ma (MSWD=1.02, n=20), both of which are within the error. The weighted average age represents the crystallization age of the granodiorite porphyry, which indicates that the ore‐bearing porphyry in the Naruo deposit area was formed in the Early Cretaceous and further implies that the Neo‐tethys Ocean had not been closed before 124 Ma under a typical island‐arc subduction environment. The εHf(t) of zircons from the granodiorite porphyry varies from 2.14 to 9.07, with an average of 5.18, and all zircons have εHf(t) values greater than 0; 176Hf/177Hf ratio is relatively high (0.282725–0.282986). Combined with the zircon age—Hf isotope correlation diagram, the aforementioned data indicate that the source reservoir might be a region that is mixed with depleted mantle and ancient crust, which possibly contains more materials sourced from depleted mantle. Rock‐forming ages and ore‐forming ages of the Duolong ore concentrate area are 120–124 Ma and 118–119 Ma, respectively, which indicate 124–118 Ma represents the main rock‐forming and ore‐forming stage within the area. The Naruo deposit is located in the north of the Bangongco‐Nujiang suture, and it yielded a zircon LA‐ICPMS age of 124.03 Ma. This indicates the Bangongco‐Nujiang oceanic basin subducted towards the north at about 124 Ma, and the Neo‐tethys Ocean had not been closed before the middle Early Cretaceous. It is possible that the crust‐mantle mixing formed the series of large and giant porphyry copper‐gold deposits in the Bangongco.