Hydrothermal modification of zircon geochemistry and Lu–Hf isotopes from the Hongtoushan Cu–Zn deposit, China

Abstract

The volcanic zircons, inherited pre-magmatic core domains and hydrothermal zircons in felsic meta-volcanic rocks and Cu–Zn orebodies from the Neoarchean Hongtoushan deposit in China were distinguished according to their texture, 207Pb/206Pb ages, Th/U and oxygen isotopic ratios. Their respective element compositional and Lu–Hf isotopic features were studied using LA–(MC)–ICP–MS methods, showing that the compositions of the pre-magmatic core domains are intensely modified by hydrothermal processes, exhibiting no differences with hydrothermal zircons. All zircons show a linearly decreasing trend in the ZrO2 vs SiO2 plot, but similar REE patterns, with steep slopes, Ce and Eu anomalies and enrichments of HREE, indicating their common petrologic origin. Nevertheless, compared with volcanic zircons, pre-magmatic core domains underwent compositional modification in the hydrothermal accretion processes by P and Y substitution mechanisms. Therefore, these core domains are highly enriched in P2O5, Y2O3, REEtotal, and Hf2O as well as some incompatible elements, such as Rb, Sr, Nb, Ta and Pb, similar to the hydrothermal zircons. Moreover, the Lu/Hf ratios are elevated by almost an order of magnitude, which is associated with the addition of Lu rather than changes in Hf, leading to increased 176Lu/177Hf ratios and decoupling of Hf–O isotope systematics. Nevertheless, the hydrothermal influence on the 176Hf/177Hf ratios seems to be undetectable, rising by 0.022% when compared with the ratios of the volcanic zircons. All 176Hf/177Hf ratios plot between depleted mantle and modern known plumes suggest a mantle plume setting corresponding to the ∼2.6Ga crustal growth in the Qingyuan district.