Volcanism, mineralization and metamorphism at the Xitieshan Pb-Zn deposit, NW China: Insights from zircon geochronology and geochemistry

- The post-mineralization metamorphism occurred under a high-temperature condition.
- Two cycles of volcanism have been differentiated in the Xitieshan Pb-Zn deposit.
- The exhalative Pb-Zn mineralization may have occurred at 450-440 Ma.
- Zircons were affected by massive fluid during orogenic metamorphism at 440-430 Ma.

The footwall volcanic rocks of the Ordovician Tanjianshan Group in the world-class Xitieshan Pb-Zn deposit have experienced prolonged arc volcanism followed by strong metamorphism and deformation. This has resulted in a complex thermal history and led to ambiguity in interpretation of zircon geochronological results. An integrated study involving textural characterization, CL imaging, trace element analysis, Ti-in-zircon thermometry and LA-ICPMS U-Pb dating has provided tight constraints on the age and genesis of the zircon groups in the volcanic rocks. The temperature of metamorphism and deformation indicated by metacryst minerals and micro-structures in the volcanic rocks ranges from 550 to 650 °C, which partially overlaps with the lower temperature range of zircon crystallization (600-750 °C) calculated using the Ti-in-zircon thermometer. Cathodoluminescence images and trace element compositions confirm a magmatic origin for the zircons, which have also been variably altered by metamorphic fluids. Two ranges of U-Pb ages, 475-470 Ma and 460-450 Ma, have been obtained on typical magmatic zircons and are interpreted to represent pre-mineralization arc volcanism in the Xitieshan deposit. A younger age group of 440-430 Ma for the fluid-modified zircons is considered to record post-ore metamorphism during the North Qadaim Orogeny. Thus, we propose that the original exhalative ores at the Xitieshan Pb-Zn deposit formed at 450-440 Ma.

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