U-Pb age and geochemical characteristics of zircon in monzogranite porphyry from Qiagong deposit, Tibet, and geological implication

Qiagong Fe (Cu)-PbZn (Ag) skarn is genetically related to a monzogranite porphyry, and it together with Chazangcuo, Sinongduo, Jiaduobule, Qingdu and Nazha, all of which are also rarely studied, comprises a skarn-belt in the middle Gangdese metallogentic belt. In this paper, minerageny, micro-situ trace element and U-Pb age of zircons from two types of porphyry in southern part of Qiagong skarn deposit are studied using cathodeluminescence and LA-ICP-MS. The age of quartz porphyry is 66.83 +/- 0.72Ma (MSWD = 2.4, n = 9), and monzogranite porphyry associated with mineralization is 67.42 +/- 0.80Ma (MSWD = 3.8, n = 15). In U/Yb verse Hf and U/Yb verse Y discriminant diagrams, all the samples plot in continental zircon zone. Mineral structure, Zr/Hf, Th/U, Nb/Ta, Nd/Yb rate and T-Tiz estimated by Ti concertration of zircon from monzogranite porphyry show that the magma was sourced from upper mantle upwelling basite mixing or assimilating crustal components. It emplaced with crystallization differentiation when wall rock decompressed. These results imply that Qiagong deposit formed at the onset of collision of India to Asia continent, and is related to magma derived from both mantle and continental crust. The mantle source of the monzogranite porphyry was via upwelling through the tearing window of the subducted Neo-Tethys slab. The tearing window was caused by the different subducted angle induced by the roll-back of slab. The formation of Qiagong deposit represents a type of metallogenesis still needs further understanding in collision zone and additional ore potential in the northern part of middle-Gangdese.