Geochemical characteristics and oxidation states of the Xietongmen ore-bearing porphyries: Implication for the genetic types of the Xietongmen No. I and No. II deposits, southern Tibet

The Xietongmen large deposit is the only known subduction-related porphyry Cu deposit in the Gangdese Belt, Tibet, which is composed of the No. I and No. II deposits. Previous work suggests that the No. I deposit is reduced while the No. II deposit is oxidized. However, it is ambiguous that both oxidized and reduced porphyry Cu deposits occur in close proximity, in the same tectonic setting, and within a short time period. Detailed examination of the ages, whole-rock geochemistry, zircon and apatite compositions of the Xietongmen No. I and No. II ore-bearing porphyries are conducted to investigate their magma characteristics and the genetic types of the Xietongmen No. I and No. II deposits. Zircon U-Pb dating results show that the Xietongmen No. I and No. II ore-bearing porphyries formed at 166.3 +/- 2.6 Ma and 175.5 +/- 1.0 Ma, respectively. The Xietongmen No. I and No. II ore-bearing porphyries have similar mineral assemblage (containing amphibole phenocryst), high Sr/Y ratios and negligible negative Eu anomalies, low Ti-in-zircon temperatures (633-749 and 633-763 degrees C, respectively), which indicate that their magmas are water-rich. In addition, the Xietongmen No. I and No. II ore-bearing porphyries have: (a) high whole-rock V/Sc ratios (10-15 and 8-16, respectively); (b) high zircon Ce4+/Ce3+ and Eu-N/Eu-N* ratios, falling into the field of the ore-bearing intrusions in northern Chile; (c) low apatite Ce-N/Ce-N* ratios (0.97-1.02 and 0.99-1.02, respectively), which suggest that their magmas are oxidized. The estimated magmatic S contents, based on the magmatic apatite SO3 contents, yield 0.0113-0.0507 wt% and 0.0012-0.0118 wt%, respectively for the Xietongmen No. I and No. II ore-bearing porphyries, which suggest their magma are sulfur-rich. Hydrous, oxidized and sulfur-rich magmas are the parent magma of oxidized porphyry Cu deposits, together with the oxidized minerals of hematite and magnetite, therefore, both Xietongmen No. I and No. II deposit are oxidized porphyry Cu deposits, rather than that the Xietongmen No. I deposit is a reduced porphyry Cu deposit