Zircon fertility indicators compromised by mineral inclusion contamination: A case study from the Taoxikeng W deposit, South China

Abstract

Zircon trace element compositions have been widely used as a tool to assess the ore-forming potential of magmatic-hydrothermal systems. However, many zircons contain accessory mineral inclusions which, if accidentally analyzed, could compromise reported zircon trace element signatures, especially when the mineral inclusions are sub-micrometer and thus hard to monitor using conventional microscopes. However, how mineral inclusion contamination influences the robustness of zircon fertility indicators (e.g., Ce and Eu anomalies) is still not well illustrated. In this study, we report U-Pb ages and trace element compositions of zircons from ore-forming muscovite granite plutons of the Taoxikeng W deposit, South China. The studied zircons are characterized by abundant small apatite inclusions. Although these had little influence on zircon Eu/Eu* (and thus probably the interpreted level of magmatic fractionation and melt water concentration), they could have resulted in a misestimation of the magmatic oxidation state either via the zircon Ce/Ce* or Loucks et al. (2020) methods. This is especially important for granite-related W-Sn systems since their magmatic zircons generally contain abundant mineral inclusions, which may be accidentally sampled during zircon trace element analysis. In practice, we can use La as a monitor of REE-enriched mineral (i.e., apatite, titanite) inclusions, although the criterion for identifying pure zircon trace element data is still controversial and awaits further work.