Ore-Forming Fluid Controlling Mineralization in Qulong Super-Large Porphyry Copper Deposit, Tibet

The Qulong porphyry copper deposit is the largest recently found in the Gangdise metallogenic belt. It was formed in the Miocene. The original fluid inclusions can be divided into five types according to the compositions of liquid, gas and single minerals The temperature of the main ore-forming phase ranges from 240 to 650 ℃ and the salinity is about 0.18wt.%-52.04wt.%. The fluids can be classified into two types including high salinity and high density, low salinity and low density. Daughter mineral inclusions, liquid and gas inclusions are coexistent and their homogeneous temperature is approximate, however salinity is quite different, showing that the fluid underwent the process of boiling. A study of the H and O isotopes and trace and rare elements of a single mineral indicates that the mineralization substance comes from the system of porphyry magma and fluid belonging to the NaCl (F) -KCl (F) -C₂H₆-HCO₃-CaSO₄ type fluid, which comes from magma and natural hot-brine. The fluid is characteristically reducing and acid, with high temperature, high salinity, high mineralization degree, and high oxygen fugacity.The fluid has high concentrations of Na⁺, K⁺, Cl^-, SO₄^2-, CO₂ etc., with high F^-(F^-/Cl^- > 1, average 5.66), which was good for the transfer of Fe²⁺ and Cu²⁺, finally concentrated and formed the deposit, and is the necessary condition for the Qulong super-large porphyry copper deposit to form. Decompression and the mixing of different quality fluids is the main mechanism for the depositing of metalions. The mineralizated depth is also discussed and could be regarded as the prospecting base of mineral deposit.