Magmatic evolution and characteristics of magmatic fluid in the Qiagong porphyry system

Collision between the Indian and Asian plates has produced several metallogenic belts in the Tibet, Yunnan, Sichuan, and Qinghai provinces of China since ~65 Ma. Based on their contained resources, the Gangdese belt is the most important economically, and it is the host to the Qiagong iron skarn in the central part of the belt. The mineralising intrusions at Qiagong are monzonite granite porphyries, which were emplaced at 68.8±2.2 Ma based on U-Pb (zircon) age determinations. Qiagong therefore formed at the start of the main collision stage, and is much older than porphyry copper and Cu-Au-Mo skarns in the eastern part of the Gangdese belt. New geological mapping, petrological and geochemical studies of chemistry of Qiagong have identified features typical of porphyry copper system associated with the Fe skarn. Isotopic data are consisted with mixing of mantle and lower crustal material. Microthermometric PIXE and LA-ICP-MS analyses of fluid inclusions related to the Qiagong porphyry system have identified high temperature, high pressure and high salinity magmatic-hydrothermal fluids that have high Fe (up to 23%), Pb (up to 10%) and Zn (up to 1.4%) and low Cu (up to 464X10⁻⁶) Concentrations compared to other porphyry copper systems. The porphyry system is inferred to have formed at depths of 6~7 km based on pressure estimation results, and has therefore been deeply eroded.