Zinc and cadmium isotopes in pyrite constraint on metal sources in the Guoluolongwa orogenic gold deposit, East Kunlun metallogenic belt

Abstract

The Guoluolongwa orogenic gold deposit is the most typical and largest gold deposit in the Gouli goldfield of the East Kunlun Orogen, and its mineralization was closely related to mantle magmatic hydrothermal fluid. However, its genesis study has been trapped due to the controversy regarding the relationship between mineralization and metamorphic wall rocks. Here, we use Zn and Cd isotopes in pyrite to trace the source of Guoluolongwa orogenic gold deposit. In the beginning, we verify that Zn and Cd in pyrite are closely connected with mineralization based on elemental compositions and mineralogy, which suggests that Zn and Cd in pyrite occur mainly as sphalerite inclusion and were precipitated from ore-forming fluid during pyrite precipitation. Pyrites from the Guoluolongwa deposit show obviously heavier δ⁶⁶Zn and δ^114/110Cd values (δ⁶⁶Zn_{JMC-Lyon 3-0749L} = 0.17 ∼ 0.44 ‰; δ^114/110Cd_{NIST SRM 3108} = 0.02 ∼ 0.56 ‰) than those of mantle magmatic fluid (δ⁶⁶Zn = 0.19 ∼ 0.27 ‰; δ^114/110Cd = 0.01 ∼ 0.21 ‰) and deposits which were closely associated with magmatism (δ⁶⁶Zn = −0.43 ∼ 0.23 ‰; δ^114/110Cd = −0.25 ∼ 0.05 ‰), indicating that the deposit was not formed solely by mantle magmatic fluid. The process of pyrite precipitation, including vigorous boiling of hydrothermal fluid and pyrite recrystallization, have limited impact on the Zn and Cd isotopes in pyrite. Therefore, Zn and Cd isotopes in pyrite inherit the isotopic composition of ore-forming fluid and can be used to trace the metal source of Guoluolongwa deposit. The δ⁶⁶Zn and δ^114/110Cd values of pyrites fall within the range of metamorphic wall rock (δ⁶⁶Zn = 0.47 ∼ 0.51 ‰; δ^114/110Cd = 0.52 ∼ 0.57 ‰) and mantle magmatic fluid, and they display a close positive correlation (R² = 0.71), This indicates that the Guoluolongwa deposit was formed by the mixing of metamorphic wall rock and mantle magmatic fluid. Furthermore, the systematic decrease of δ⁶⁶Zn and δ^114/110Cd values of pyrites reflects that the contribution from metamorphic wall rock decreased after stage I and that magmatic fluid became the dominant metal source in stage II and III. We propose that the Guoluolongwa deposit originated from the mixing of metamorphic wall rock and magmatic fluid and suggest that the combination of Zn and Cd isotopes in pyrite can be used to trace the metal sources of orogenic gold deposits. It is a promising indicator for future applications due to the widespread of pyrites in hydrothermal deposits.