Post-deposition hydrothermal alteration and U-Pb isotopic resetting of wolframite in the Tiantangshan deposit, Nanling Range (South China): Implications for complex isotopic ages of ore deposits

Tiantangshan deposit in the southeastern Nanling Range is a newly discovered Rb-Sn-W polymetallic deposit. In this paper, we present in-situ U-Pb isotope and major and trace element analyses of wolframite from the Tian-tangshan deposit to examine the possible post-deposition hydrothermal alteration and U-Pb isotopic resetting of wolframite, with additional insight into possible genesis of variably young mineralization ages in the Nanling Range. Wolframite in the W ore body (wolframite-cassiterite quartz veins) of the Tiantangshan deposit comprises two stages, i.e., Wolf I and Wolf II. The primary Wolf I occurs as euhedral crystals with oscillatory growth bands, and is rich in Fe relative to Mn, with Mn/(Mn + Fe) ratios varying from 0.15 to 0.26. The Wolf I yielded U-Pb age of 133.7 +/- 1.8 Ma (2 center dot:5; MSWD = 0.6). The Wolf II crosscuts and occurs as rims of the Wolf I with irregular boundaries, which has higher Mn and lower Fe contents than the Wolf I, with Mn/(Mn + Fe) ratios varying from 0.43 to 0.88. It shows similar REE pattern and concentrations of multiple elements (e.g., Sc, Ti, V, Zr, Hf and In) with the Wolf I, and also correlates with the Wolf I for elemental ratios of Nb vs. Ta, Zr vs. Hf, Nb + Ta vs. WO3 and Sn vs. WO3. These features suggest that the Wolf II was formed by replacing the Wolf I. The Wolf II yielded U-Pb age of 95.4 +/- 1.3 Ma (2 center dot:5; MSWD = 0.8). Collectively, the U-Pb age of Wolf I (-134 Ma) represents the W mineralization age in the Tiantangshan deposit, whereas, the age of the Wolf II (-95 Ma) does not represent a second stage of W mineralization, but isotopic resetting during post-deposition hydrothermal alteration of the Wolf I. The fluids accounting for the Wolf II was likely sourced from granites of the Late Cretaceous episode (-96 Ma) outcropping in adjacent areas. Our results indicate that wolframite and its U-Pb isotope system is susceptible to post-deposition hydrothermal alteration. This study also implies that later magmatism and hy-drothermal fluids may result in isotopic resetting and the variably young ages in the Nanling Range. Therefore, interpretation of ages obtained from a hydrothermal ore deposit should be based on detailed macroscopic to microscopic observation and multiple isotopic ages of different minerals, and particularly, abnormal young ages need to be treated with caution.