In situ LA ICP MS U Pb geochronology and trace elements analysis of polygenetic titanite from the giant Beiya gold polymetallic deposit in Yunnan Province, Southwest China
Abstract The Beiya gold polymetallic deposit, located in the middle of the Jinshajiang Ailaoshan alkaline porphyry metallogenic belt, is one of the largest gold deposits in China. The mineralization mainly occurs in skarn along the intrusive contacts between the alkaline porphyries and Middle Triassic limestone. In this paper, we present U Pb age as well as major and trace element geochemistry of titanite from the Beiya deposit, and distinguish the titanite into a magmatic and a hydrothermal suite. Our study indicates that the titanite from the ore related porphyry and from the mineralized skarn is texturally and geochemically very different. The euhedral, envelope shaped titanite from the ore related porphyry has lower FeO, F, HFSEs, Nb/Ta and Lu/Hf, together with higher TiO2 and Th/U than the subhedral titanite from the mineralized skarn. The titanite from the porphyry also displays higher LREE/HREE and more subtle negative Eu anomaly than its mineralized skarn counterpart. This suggests a magmatic and a hydrothermal origin for, respectively, the titanite from the ore related porphyry and from the mineralized skarn. In situ magmatic titanite U Pb dating has yielded an Eocene age of 36.0 5.9 Ma, consistent with the porphyry zircon U Pb age (36.07 0.43 Ma) obtained in previous studies. Hydrothermal titanite has yielded a weighted average 206Pb/238U age of 33.1 1.0 Ma (MSWD 2.0) which represents the age of the retrograde skarn alteration and the maximum age for the gold mineralization. Together with the previous molybdenite Re Os age, we have further constrained the Beiya gold polymetallic metallogeny to 33.1 34.1 Ma. The mineralization age is slightly younger than the porphyry emplacement, indicating that the Beiya metallogeny was likely to be a post magmatic hydrothermal product of the Himalayan orogenic event. The REE characteristics of hydrothermal titanite also reveal that the ore forming fluids may have been derived from a highly oxidized magma.