Geochronologies of the Huaixi copper-gold deposit and the Caomen alkaline granite, SE China: implications for tectonic evolution

Abstract

The Huaixi copper-gold polymetallic deposit of SE Zhejiang Province, China, is a typical hydrothermal-vein ore body. The Caomen K-feldspar granite porphyry, the dominant intrusion in the mining district, has been dated by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses of zircon, which yielded a weighted mean 206Pb/238U age of 101.6 ± 0.9 Ma (MSDW = 1.3). Rb-Sr dating of fluid inclusions in auriferous quartz from ore bodies yielded an isochron age of 101 ± 2 Ma. These results indicate that intrusion of the Caomen pluton and Cu-Au mineralization was contemporaneous and corresponds to the third episode of Mesozoic magmatism and metallogenesis in South China. Calculated δ18O values of fluid inclusions from ore-bearing quartz range from −0.89 to −1.98‰ and δD from −42.60 to −60.20‰, suggesting that the ore-forming fluids are derived from a mixed source of magmatic and meteoric waters. δ18S values of 8 pyrites range from −2.14 to +4.14‰ with a mean of +1.67‰, similar to magmatic sulphur. These isotope data support a genetic relationship between the Huaixi copper-gold deposit and the Caomen alkaline granite and probably indicate a common deep source. Petrography and chemical compositions show that the Caomen alkaline granite crystallized from shoshonitic magmas characterized by high SiO2 (75.64–78.00%) and alkali (K2O + Na2O = 7.96–8.82%) but low FeOT (1.34–3.31%), P2O5 (0.05–0.13%) and TiO2 (0.12–0.18%). The granitic rocks are enriched in Ga, Rb, Th, U, and Pb but depleted in Ba, Nb, Sr, P, and Ti. REEs are characterized by marked negative Eu anomalies (Eu/Eu* = 0.06–0.13) and exhibit right-dipping ‘V’ patterns with LREE enrichment. These are similar to the Late Cretaceous alkaline granites in the coastal areas of Zhejiang and Fujian provinces, implying that the Caomen granite formed in a post-collisional extensional tectonic setting. Combined with previous studies, we interpret the Huaixi copper-gold deposit and the associated Caomen alkaline granite as related to back-arc extension due to high-angle subduction of the palaeo-Pacific plate, caused by northward movement of the Indian plate.