If genuine, I believe this is from the Tang Dynasty, possibly a Qian Yuan Zhong Bao, and dates to the 8th Century AD. If so, it'll be my oldest example of the genre.
It was mixed in with a bunch of world coins that also included some late Roman bronzes. Just under 25 mm in diameter.
Edit: I hasten to add that my "dark ages" reference should only be taken as applying to the state of civilization in Europe at the time. I recognize that in contemporary China, culture was in full bloom.