Garnet U-Pb geochronology and geochemistry of the Pusangguo skarn deposit in Tibet: Insights into the genesis and fluid evolution during prograde skarn formation

• Two different types of garnets (Grt-1 and Grt-2) at Pusangguo were formed in the same magmatic-hydrothermal event at ∼14 Ma. • Changes in external factors ( f O 2 , pH and W/R ratios) cause compositional variation from Grt-1 to Grt-2 at Pusangguo deposit. • Grt-1 and Grt-2 were formed in a hydrothermal environment under different skarn fluid conditions. The Pusangguo skarn deposit is located in the western part of the Gangdese metallogenic belt. Over the past few decades, the geology, petrogenesis, fluid inclusion and skarn mineralogy of this deposit have been studied in detail. However, there is still a lack of direct timing of the formation of the skarn at the Pusangguo deposit, which is critical to insight into the mineralization process. Two types of garnets were identified at the Pusangguo in this study, i.e., dark brown and coarse-grained garnet (Grt-1), and green to brown fine-grained garnet (Grt-2). At Pusangguo, the garnets mainly belong to andradite-grossular solid solution series. Grt-1 displays a more restricted composition range of Adr 87.85–98.55 Grs 0–8.96 , while the Grt-2 contains a broader range (Adr 54.09–87.87 Grs 11.53–44.73 ). LA-ICP-MS U-Pb dating of Grt-1 and Grt-2 yielded lower intercept 206 Pb/ 238 U ages of 14.7 ± 2.2 Ma (n = 30, MSWD = 2.1) and 13.9 ± 1.3 Ma (n = 30, MSWD = 0.6), respectively, coeval with the ore formation and ore-related granodiorite. Grt-1 shows a slight enrichment in LREE and a depletion in HREE without apparent Eu anomalies, while Grt-2 exhibits an enrichment in LREE and a depletion in HREE with clear positive Eu anomalies. Correlations between ΣREE and Al, Fe 3+ , Fe 2+ , Mn 2+ , and Ca 2+ together suggest that REE 3+ incorporation into the Pusangguo garnet is most likely controlled by a coupled substitution mechanism related to Ca-site vacancies, and may also be influenced by external factors such as pH and oxygen fugacity ( f O 2 ). The positive correlation between ΣREE and Y implies that Y 3+ and REE 3+ have similar geochemical behavior. Grt-1 has higher ΣREE, HFSE, U and Y contents than Grt-2. We propose that Grt-1 is probably formed by self-organizing at a relatively low growth rate, resulting in fluid Al/Fe fluctuations associated with changing external factors such as fluid mixing. The distinct geochemical properties of Grt-1 and Grt-2 indicate that Grt-1 likely originated in a nearly neutral to slightly acidic and oxidized environment with a lower W/R ratio, while Grt-2 could have formed in a more acidic and relatively reducing condition with a higher W/R ratio. Changes in redox, pH and W/R ratios led to a shift in composition from Grt-1 to Grt-2 as the hydrothermal fluid ascended. As the fluid underwent crystallization, there was a decrease in f O 2 , resulting in a transition from a weakly acidic to a strongly acidic state. This change in fluid chemistry ultimately led to the precipitation of metal sulfides.