Origin of Indosinian W–Sn Deposits in the Nanling Region, China: Case Study of the Nanzhuhe Sn Polymetallic Deposit in the Dupangling Area

Great breakthroughs have been made in the Indosinian W–Sn ore exploration in the Nanling region of China in recent years, but there is still relatively little research on its metallogenic mechanism. Nanzhuhe Sn polymetallic deposit is located on the southeast side of Dupangling complex in the western part of Nanling and is a typical altered fracture zone type of Sn polymetallic deposit. The ore body is controlled by the NE-trending fault and occurs in the Indosinian granite in the Indosinian contact zone and the secondary tension–torsion fault zone. In this article, zircon and cassiterite were selected for U–Pb dating and trace element analysis, and quartz was studied by microthermometry, Laser Raman compositional analysis and H and O isotopes. The results show that the zircon U–Pb age of the biotite granite is 224.2 ± 6.4 Ma (MSWD = 1.5), and the cassiterite U–Pb age is 217.9 ± 7.2 Ma (MSWD = 1.6). The cassiterite is enriched in high-field-strength elements such as Nb, Ta, Ti, Sc, V and Hf, which indicates that the deposit was formed by the highly evolved granitic magmatic activity in the late stage of the Indosinian. The fluid inclusions are mainly liquid-rich two-phase, and a little gas-rich two-phase. The gas component is mainly water vapour. The initial ore-forming fluid is NaCl–H₂O system with medium to high temperatures and medium to low salinity. Hydrogen and oxygen isotopes indicate that about 40% of the atmospheric precipitation is mixed along the extensive fault. The comprehensive study shows that the mixing of magmatic–hydrothermal fluid and meteoric water occurred in the Indosinian period of the Nanzhuhe Sn polymetallic deposit, which led to the continuous decrease of temperature and salinity of the ore-forming fluid, the increase of oxygen fugacity, the increase of pH value and the decrease of ligand activity, which destroyed the stability of tin complex and caused a large number of tin and other metal ions to break off from the complex carrier, and finally the precipitation of tin and other metals.