TripAdvisor says
"DianCi ChunTian (The spring DianCi), anytime of the night to take a warm natural spring bath"

Can anyone help with more information. Is it mixed, can you book your own room, do you wear swimming costumes, where and how much?

You can get your own room with hotsprings at Dianchi Chuntian, but it's something like 600-800 yuan/night. There are also shared outdoor hot springs for around 85 yuan. The outdoor hot springs are mixed with no nudity, and there is a nice little swimming pool. There is also one pool that is filled with little fish that eat your dead skin off of you. Really. Address and phone number are listed on the Spas page in this site.