Genesis and ore-forming processes of Dulong Sn polymetallic skarn deposit: Constraints from u-pb dating and trace elemental analyses of zircon, garnet and cassiterite

Abstract

The Dulong Sn polymetallic skarn deposit, situated at the southern extremity of Gejiu-Malipo Sn-W metallogenic belt, is a representative skarn Sn polymetallic deposit in SW China. However, due to various types of magmatism and metal mineralization in Dulong deposit, the genetic link between different magmatism (TMMG and GP), skarnization and Sn mineralization (skarn-type and vein-type) has not been established. In this study, Zircons from TMMG and GP, hydrothermal garnet, skarn-type and vein-type cassiterite in Dulong deposit were picked for in-situ U-Pb isotopic and elemental analyses, to constrain the ore genesis and ore-forming processes. As results, the TMMG in the Dulong Sn polymetallic deposit yields a U-Pb age of 88.56 ± 0.33 Ma, and the GP yields a younger age of 82.47 ± 0.61 Ma. The hydrothermal garnet yields a U-Pb age of 88.2 ± 2.9 Ma. Whereas the skarn-type cassiterite coexisting with magnetite and the skarn-type cassiterite coexisting with sulfide yielded a U-Pb age of 87.50 ± 1.00 Ma and 87.00 ± 1.40 Ma, respectively. Meanwhile, the vein-type cassiterite yielded a U-Pb age of 81.50 ± 2.50 Ma. The above dating results probably suggest that two-mica monzonitic granite is responsible for skarnization and skarn-type Sn mineralization, while granitic porphyry is responsible for vein-type Sn mineralization. Zircons from two-mica monozonitic granite and granitic porphyry in Dulong Sn polymetallic deposit is characterized by low ΔFMQ values and Eu/Eu*, Ce⁴⁺/Ce³⁺, Ce/Nd)/Y ratios, indicating that the causative magma resources have low oxygen fugacity and low initial water content, and the Sn mineralization potenital of TMMG is significantly greater than GP. The elemental changes from core to rim in hydrothermal garnet grains reveal that early ore-forming fluids originating from magma were more acidic and reducing. During the evolution processes, significant meteoric water continuously mixed into the ore-forming fluid, which was the potential mechanism controlling ore precipitation in Dulong deposit.