Genesis and metallogenic potential of the ca.90 Ma Jiangcisha magmatic rocks in the western Shiquanhe suture zone, Xizang, China

Abstract

Numerous mineral deposits related to the ca.90 Ma magmatic activity were found in the northern and central Lhasa terrane. Recently, many Late Cretaceous intermediate and felsic magmatic rocks were discovered in the Jiangcisha area in the western Shiquanhe-Nam Tso Mélange Zone, separating the Lhasa terrane into northern and central subterranes. In this study, zircon UPb dating, whole-rock major and trace element geochemistry, zircon Hf isotopes and trace element compositions are used to determine the petrogenesis and mineralization potential of the Jiangcisha intermediate and felsic magmatic rocks. The zircon UPb ages of the diorite porphyry, dacite porphyry, and diorite sample are 92.2 ± 1.6 Ma, 90.0 ± 1.3 Ma, and 85.8 ± 1.7 Ma, respectively. The diorite porphyry and diorite samples have A/CNK values ranging from 0.65 to 0.91 and show elevated Mg# (44.78–53.11), Cr (108–338 ppm) and Ni (53.2–119 ppm) concentrations. These rocks also have high Sr/Y values (30.37–58.03) and low Rb/Sr ratios (0.04–0.33), with weak negative Eu anomalies (δEu = 0.75–0.95). Based on these values, these samples are high-Mg# calc-alkaline and metaluminous adakitic rocks. The dacite porphyry has a high SiO2 content (73.54–74.07 wt%) and significant negative Eu (δEu = 0.69–0.73) and Sr anomalies. Their Sr contents are 10 times lower than those of the diorite porphyry and diorite samples. The zircon εHf(t) values of these three magmatic rocks are high (7.61–12.69), and the Hf isotope two-stage model ages are young (0.41–0.78 Ga). The geochemical characteristics mentioned above indicate that the diorite porphyry and diorite were derived from the partial melting of the delaminated juvenile lower crust. However, the dacite porphyry is the product of plagioclase and K-feldspar fractional crystallization from the diorite porphyry. The zircons in the diorite porphyry and diorite samples have low Ce/Nd (3.66–29.04), logfO2 (−16.022-–12.976), and ΔFMQ (0.08–1.99) values and moderate Dy/Yb (0.22–0.37), 10,000 × δEu/Y (1.48–9.66), and (Ce/Nd)/Y (0.0025–0.0224) ratios. Compared to typical large porphyry-skarn CuAu deposits with dioritic rocks developed in the Tibetan Plateau, the relatively low oxygen fugacity and magma water content of the Jiangcisha dioritic rocks indicate their limited potential to form large porphyry and skarn CuAu deposits.