Fluid Inclusion and Titanite U-Pb Age Constraints on the Yuanjiang Ruby Mineralization in the Ailao Shan-Red River Metamorphic Belt, Southwest China

The Yuanjiang marble-hosted ruby deposit lies in the central segment of the Ailao Shan metamorphic massif of the Ailao Shan-Red River metamorphic belt. The mineralizing fluid and age were characterized by detailed petrography, Raman spectroscopy, microthermometry, and in situ titanite laser ablation-inductively coupled plasma-mass spectrometry dating. Some fluid inclusions in the corundum show an interesting morphology with a diaspore crystal fully separating the whole inclusion into two smaller inclusions. This morphological feature can be explained by morphological ripening and subsequent reactions between the trapped H2O and the host corundum during the cooling of the inclusion. Fluid inclusions in the ruby belong to the system CO2–H2S–COS–S8–H2S2–CH4–AlO(OH) with various daughter minerals, including diaspore, gibbsite, and native sulfur (S8). The observed seven-component fluid inclusion composition can be explained by two steps: (1) original fluid inclusion capture during deposit formation with compositions including CO2, H2S, COS, CH4, S8, and H2S2, and (2) post-entrapment fluid inclusion modification, such as diaspore and gibbsite. The presence of hydrous minerals in fluid inclusions strongly supports the idea that water was once present in the initial fluids. In the Yuanjiang deposit, petrographic evidence shows that titanite formed simultaneously with ruby, and U-Pb dating of titanite allows us to conclude that the ruby mineralization formed at 23.4 ± 0.3 Ma, in other words during the Himalayan orogeny.