Geochronological and geochemical constraints on formation of the Tongling metal deposits, middle Yangtze metallogenic belt, east-central China

摘要

The Tongling district is one of the most important non-ferrous metal producers in China. The origin of Cu-Au deposits in the region is closely related to Late Mesozoic intermediate intrusions, which are mainly high-K calc-alkaline and shoshonitic series. Geochemical characteristics indicate that these granitic rocks are mixtures of more than two compositional end-members, i.e. mantle-derived melts and crust components incorporated through assimilation. Three important magmatic intrusions related to the Cu-Au deposits in the Tongling region - the Jiguanshi quartz monzodiorite, the Xishizishan quartz diorite and the Miaojia diorite porphyry - were selected for this study. Zircon U-Pb dating by LA ICP-MS yielded two groups of ages (approx 130-132 and 138-140 Ma) for these intrusions. Pyrite Re-Os age for the Xinqiao Cu-Fe-S deposit in the Tongling region is 126 + 11 Ma. Trace elements of zircon grains show that the earlier Cu-Au mineralization event was associated with adakitic rocks characterized by high positive Ce anomalies, produced at an elevated oxygen fugacity range. In contrast, later iron-sulphur mineralization was closely related to low positive Ce anomalies, reflecting low oxygen fugacities. Considering that Pacific lithospheric subduction was the dominant factor that controlled major tectonic evolution in eastern China during the Early Cretaceous, the geochemical characteristics of these coeval ore-forming intermediate intrusive rocks in the Tongling district were likely the result of Pacific plate underflow.