Genesis of the Gaosong Sn–Cu deposit, Gejiu district, SW China: Constraints from in situ LA-ICP-MS cassiterite U–Pb dating and trace element fingerprinting

The Gaosong Sn–Cu deposit in the Gejiu ore district, SW China, is a large cassiterite–sulfide deposit, hosting total ore reserves of 67.4 Mt @1.3% Sn and 19.0 Mt @1.1% Cu. Two types of mineralization developed in the deposit, i.e. cassiterite-sulfide ore within the proximal skarn in contact with the Gaofengshan biotite granite, and fault-controlled stratiform-like cassiterite–iron oxide ± sulfide ore in the distal carbonate country rock. Cassiterite grains are disseminated in both ores. In situ U–Pb dating of one cassiterite sample from the cassiterite-sulfide ore and two from the cassiterite–iron oxide ± sulfide ore yielded weighted mean 206Pb/238U ages of 84.3 ± 1.4 Ma, 85.1 ± 1.0 Ma and 83.5 ± 2.1 Ma, respectively. The three mineralization ages are consistent with the emplacement age of the Gaofengshan granite (85.8 ± 0.6 Ma) within error, suggesting a close temporal link between tin mineralization and granitic magmatism. Cassiterite grains can be divided into two types based on cathodoluminescence (CL) illustrations, namely euhedral Cst I and veinlet-like Cst II. In the cassiterite–sulfide ore, Cst I has clear oscillatory zonation and is dull luminescing. In the cassiterite–iron oxide ± sulfide ore, Cst I core is dull luminescing (blue in OM−CL) and surrounded by a bright luminescing Cst I rim (yellow in OM−CL). Cst I is replaced by Cst II veinlets with bright luminescing (yellow in OM−CL). The SEM−CL luminescing intensity of cassiterite is mainly controlled by Ti content. Elevated W and Fe but low Nb, Ta, Zr and Hf concentrations of cassiterites from the Gaosong deposit, are distinctly different from those of cassiterites in VMS/SEDEX tin deposits, but similar to those from granite-related tin deposits. The Late Cretaceous crystallization ages and trace element characteristics demonstrate that cassiterites from both ores were formed in a magmatic-hydrothermal system associated with the Gaofengshan granite, and preclude the SEDEX origin of the stratiform-like cassiterite–iron oxide ± sulfide ore.