The post-collisional Cihai iron skarn deposit, eastern Tianshan, Xinjiang, China

•Plateau 40Ar-39Ar age (~ 282 Ma) of amphibole separated from massive ore represents the ore-forming time of the Cihai deposit.•The Cihai iron deposit constitutes an important part of Cu-Ni-Au polymetallic mineralization in a post-collisional tectonic environment.

The Cihai iron skarn deposit is located in the southern part of the eastern Tianshan, Xinjiang, northwestern China. The major iron orebodies are banded and nearly parallel to each other. The iron ores are hosted in an early diabase dike and in skarn. Post-ore diabase dikes cut the iron ores and their hosting diabase. Hydrothermal activity can be divided into four stages based on geological and petrographic observations: initial K-Na alteration (stage I), skarn-minor magnetite event (II), retrograde skarn-magnetite main ore event (III), and quartz-calcite-sulfide veining (IV). Zircon U-Pb dating yields ages of 286.5 ± 1.8 Ma for early diabase and 275.8 ± 2.2 Ma for post-ore diabase dikes. Amphibole separated from massive magnetite ore gives a 40Ar-39Ar plateau age of 281.9 ± 2.2 Ma and is the time of ore formation. Formation of the Cihai iron deposit is closely related to post-collisional magmatism and associated Cu-Ni-Au polymetallic mineralization in the eastern Tianshan.