One of the great onsen(s) in Gifu prefecture is called Aboh-Kan, and it's famous for having highly mineral rich hot spring water. In other words, a very high iron content, or hard water.
I know a lot of people who get a little carried away with hot springs, I'm one of them. What I mean by that is, some people bathe too much, every day onsen dipping is not healthy for the immune system. When on vacation I typically hit up two or three different hot springs in a one week period, not every single day for one week. Your immune system needs time to recuperate.
Certain hot springs, like alkaline rich baths are fantastic for the skin, but iron rich...? If you have rheumatic conditions then hot spring water high in iron may help alleviate pain, so as you can imagine lots of elderly go here. It's not as bad as you imagine, the longest most of them stay in the water per day is about 10 minutes! Women stay longer and tend to chat the experience away, no offense.
The Abohkan, is not conveniently located. Anywhere you travel in Gifu, whether it be in Takayama or Shirakawa-go, a car is essential in order to maximize time and efficiency. I stayed one night in this hotel and enjoyed it, especially the wooded baths.
If you like hot water....45degrees centigrade and up....then this is the place to go. There isn't much scenery, but the water is excellent, so I highly recommend here if you have general fatigue and muscle pain.
If staying overnight don't expect much in terms of ambiance and luxury. It's an old rustic place with not much visual appeal and a bit dilapidated.