Granites associated with tin mineralization in the Qaidam Mountain, South Qilian Belt: Petrogenesis, metallogenic setting and exploration implications

The newly discovered Erlikenda greisen-type tin deposit indicates that the Qaidam Mountain in the western segment of the South Qilian tectonic belt has a possible potential for forming large-scale tin deposits. However, the formation time and petrogenesis of granites associated with the newly discovered tin deposit in the Qaidam Mountain, and related metallogenic geological setting and prospecting potential in this area remain unclear. This study reports geological and geochemical characteristics and formation time of the Erlikenda granites. The Erlikenda granites contain no dark ferromagnesian micro-inclusions and xenoliths. They contain biotites (less than 5vol%) with a very small amount of muscovite and tourmaline. LA-ICP-MS zircon, monazite and cassiterite U-Pb isotopic dating indicates that the Erlikenda granites and tin mineralization occurred in 435~434Ma, and 435~432Ma, respectively, indicating that the magmatism and tin mineralization are coeval (the Early Silurian). The Erlikenda granites have high contents of SiO2 and Na2O+K2O, low contents of CaO, MgO, and total Fe2O3, and are weakly peraluminous. They have a typical "seagull-type" chondrite-normalized distribution patterns, with high Rb/Sr (11.4~133) and Rb/Ba (1.77~115) and low Zr/Hf (19~30) and Nb/Ta (4.8~9.2) ratios. The zircon εHf(t) values of these granites range from -10.49 to -5.18 with two-stage model ages of 2.08~1.74Ga. Furthermore, these rocks have whole-rock εNd(t) values ranging from -9.15 to -8.34 with the Nd isotopic model ages (tDM2) of 1.93~1.86Ga. The Erlikenda granites have highly fractionated granite characteristics, which mainly originated from the Paleoproterozoic crust. They may be formed in a syn-collisional tectonic environment, which was possibly related to the underplating of mantle-derived magmas as a result of the breakoff or rollback of the subducted plate. The Erlikenda granitic magmatism possibly caused the greisen-type tin mineralization in this area. These highly fractionated Early Silurian granites with zircon Hf isotopic compositions similar to those of the Erlikenda granites which widely occur in the Qaidam Mountain. In the recent mineral investigation, evaluation and exploration work, a large number of tin deposits or mineralized spots associated with Early Silurian granites have been discovered in the Qaidam Mountain, indicating that this area has an important potential for forming large-scale tin deposits, i.e., the Qaidam Mountain tin ore concentration area will be developed soon.