Fluid-zircon interaction during low-temperature hydrothermal processes: Implications for the genesis of the Banxi antimony deposit, South China

摘要

Fluid-zircon interaction in hydrothermal systems may provide key information on fluid characterization and ore precipitation. In this study, in situ analyses of Hf isotopes in zircon, combined with U-Pb ages and major and trace element data, provide new insights into fluid genesis at the Banxi antimony deposit. The zircons separated from typical quartz-stibnite ores and altered wall rocks have undergone fluid modification to different degrees, showing hydrothermal overprints in reflected/transmitted light, back scattered electron (BSE) and cathodoluminescence (CL) images and variable concentrations of Si, Zr, Hf, U, Th, Pb, Ti, Nb, Ta, P, F, Y and rare earth elements. They possess identical U-Pb ages to the Banxi Group and underlying Lengjiaxi Group clastic rocks but distinct Hf-176/ Hf-177, Lu-176/Hf-177 and Yb-176/Hf-177 ratios, indicating the possible changing of Lu-Hf systematics during the hydrothermal alteration. The evidence suggests a contribution of ore material from the Banxi Group and a deeper fluid source, derived from the Lengjiaxi Group and the crystalline basement beneath it. A fluid mixing model is proposed wherein the deep-sourced, high radiogenic Hf fluid mixed with circulating near-surface meteoric component-dominated fluid, triggering antimony precipitation. The U-Pb chronometer was not reset during this low-temperature mineralization event whereas the Lu-Hf systematics in the fluid-altered zircons show evidence of lattice breakdown of the old zircons during fluid mixing, coupled with external input of highly radiogenic Hf from the crystalline basement. This research highlights the potential to use zircon structure and geochemistry in tracing complex fluid evolutionary processes.