Growth history of garnet from the Dulong Sn-Zn-In polymetallic deposit: Geochemical and UPb age constraints and their metallogenic significance

Abstract

The Dulong skarn-type tin‑zinc‑indium (Sn-Zn-In) polymetallic deposit contains 5.5 million tonnes (Mt) Zn, 0.4 Mt. Sn, and 7 kt In. It is the third-largest cassiterite-sulfide deposit in China, and is located in the Laojunshan WSn polymetallic orefield on the southern margin of the Youjiang basin. While it is widely accepted that the SnZn polymetallic mineralization is closely linked to the Yanshanian granites, the precise timing of skarn formation and its relationship to the granite magmatism has remained unclear due to a lack of reliable geochronological data. This has also hindered a comprehensive understanding of the ore-forming processes at Dulong. Garnet is a widely distributed major skarn mineral at Dulong. Field and laboratory studies have revealed two distinct garnet types (Grt I and II): Grt I is located near the main ore-controlling fault (F_M), while Grt II is found near a shallow granite porphyry in eastern Dulong. Both types of garnet exhibit a core-mantle-rim structure, indicating that they were formed by multistage fluid metasomatism. In this study, in situ LA-ICP-MS UPb dating was carried out on both types of garnet. Additionally, major and trace element analyses of the garnet and its coexisting pyroxene were conducted to examine the formation and evolution of the skarn. The results show that Grt I has generally higher ΣREE, Y, HFSE, and U concentrations, suggesting that it was formed under low W/R ratios in a relatively reducing environment. The garnet contains a grossular core (Grt Ia), andradite mantle (Grt Ib), and a grossular-andradite solid solution rim (Grt Ic), reflecting an initial increase and then decrease in the W/R ratio of the magmatic-hydrothermal system. During this process, the fluid pH was neutral-acidic, and the oxygen fugacity (fO₂) first decreased and then increased. In contrast, Grt II has lower ΣREE, Y, HFSE, and U concentrations, indicating its formation under higher W/R ratios in a more oxidizing environment. This garnet has also a grossular core (Grt IIa), andradite mantle (Grt IIb), and a grossular-andradite solid solution rim (Grt IIc). This reflects a system where the W/R ratio first increased and then decreased. The fluid pH shifted from neutral-acidic to acidic, and the fO₂ increased gradually. LA-ICP-MS UPb dating yielded 93 ± 2.4 Ma to 90.9 ± 0.7 Ma for Grt I and 80.4 ± 6.6 Ma for Grt II. Comparing these results with published data on the Cretaceous regional magmatism and Sn-polymetallic mineralization, we conclude that the magmatic-hydrothermal activity that formed Grt I and Grt II was associated with the concealed phase-II and phase-III Laojunshan granite, respectively. This study highlights the opportunities offered by garnet UPb dating for elucidating the formation age and ore genesis of SnZn skarn systems.