The Jinchangyu Au deposit is the largest quartz-vein gold deposit in eastern Hebei Province, China, in the northern margin of the North China Craton (NMNCC). Three mineralization stages are identi？ed based on crosscutting relationships and mineral assemblages, with Au occurring mainly in pyrites in quartz–sulphide veinlets and quartz–albite veins. Five low-level highly radiogenic pyrite samples in the quartz–sulphide veinlets yielded a Re–Os isochron age of 223.5 ± 6.0 Ma. Auriferous pyrite He–Ar isotopic analyses indicate that the ore-forming ？uid was a mixture of mantle-derived and crustal ？uids. 3He/4He ratios are in the range 1.72–3.92 Ra, corresponding to 22%–50% mantle He contribution. These high 3He/4He ratios are related to lithospheric thinning in the NMNCC during the Middle–Late Triassic, with initial thinning during that period being supported by the wide distribution of Indosinian Au deposits and the regional tectonic and magmatic evolution. The mi- neralization of the Jinchangyu deposit and of other similar Triassic deposits was a response to post-orogenic to intraplate lithospheric extension after collision between the NCC and the southern margin of the Siberian Plate.