Zinc and cadmium isotopic constraints on metal sources of the Xitieshan Pb-Zn deposit, NW China

Abstract

Over the past two decades, Zn-Cd isotopes in sphalerite have been employed to study ore formation processes, identify ore metal sources, and investigate post-ore metamorphism and deformation in metallic ore deposits. In this study, we examine the Zn-Cd isotope compositions of the Xitieshan intrusion-related carbonate-replacement Pb-Zn deposit situated in Qinghai Province, at the northern edge of the Tibet Plateau in Northwestern China. This deposit primarily contains two types of ore bodies, one embedded in marble and the other in schist. The ore body within the marble remains unaltered, while the sulfides in the schist exhibit distinct signs of metamorphism and deformation. Zn-Cd isotopes were analyzed in both the undeformed and the deformed sphalerite. The δ66Zn values (relative to JMC Lyon) ranged from 0.15 ‰ to 0.39 ‰ for the undeformed sphalerite and from 0.03 ‰ to 0.19 ‰ for the altered sphalerite. Likewise, the δ114/110Cd values (relative to NIST 3108) ranged from −0.10 ‰ to + 0.02 ‰ and −0.06 ‰ to −0.01 ‰, respectively. The undeformed sphalerite (Sp1) displayed a significant positive correlation between δ66Zn values and altitude, suggesting Rayleigh fractionation as a dominant factor. Conversely, the deformed sphalerite (Sp2) exhibited a pronounced negative correlation, possibly attributed to the involvement of sulfide melts during syntectonic remobilization. Additionally, we observed a decoupling of Cd isotopes from Zn isotopes, indicating a potential divergence in their sources. While the Zn in the deposit primarily originated from magmatic hydrothermal fluids, a minor fraction of the Cd might be derived from the surrounding carbonate rocks. For ore deposits that have undergone metamorphism and deformation, Cd isotopes might experience minimal or negligible fractionation during these processes, potentially preserving valuable information about the source characteristics. In contrast, Zn isotopes are more likely to undergo fractionation during metamorphism and deformation, which should be used with caution for tracing the metal sources.