In the Holy Roman Empire, from around the mid-12th century, miners were not serfs. Although the work was extremely dangerous, that was one of the benefits. Miners dissatisfied with the lord they were working under could--and sometimes did--decamp for another mine. This was practical at heart, of course: mines did get stripped of reachable resources eventually.

They also tended to live separately from other peasants, right by the mine rather than in farming villages/clusters or clustered around a previously-built castle. (Some lords built fortresses near mines, on the other hand).

I don't recommend the book as much for its actual purpose, but, Lyndal Roper's new biography of Martin Luther talks about medieval German mining in one of the opening chapters. If you're interested, you might find that a worthwhile and affordable discussion. (...She then goes on to attribute the entire Reformation to the fact that Luther's dad was a miner instead of a farmer. I thought we had passed the whole "Luther's dad something Freud something" scholarship in the 1960s, but evidently not.)

Just to follow up on u/sunagainstgold's post with a slightly different perspective: it is certainly true that "free miners" were an important feature of the medieval landscape, especially when the mines concerned produced precious metals – gold and silver. But this was not invariably the case. In Scotland, a distinct class of "collier serfs" evolved who were tied to their masters' mines, and the institution of serfdom in the mines there lasted until 1775.

What makes the institution of the Scottish collier serf so fascinating is that it was not a survival or mutation of medieval agrarian serfdom, but a new type of legal status that was a product of the Act of Union between England and Scotland in 1707, and a deliberate legal response to the difficulties coal-masters were having in retaining workers in the coalfields.

Source

Baron F. Duckham, "Serfdom in Eighteenth Century Scotland," History 54 (1969)

Reading through Christopher Dyer's Making A Living In The Middle Ages right now and the situation, at least for England, seems to be a big heaping dose of 'it depends'.

For things like the tin mines in Cornwall and Devon, which were huge concerns, one was more likely to find full-time wage earners rather than serfs. For less intense mining in things like iron, coal, or lead, one was likely to find serfs handling the workings part-time.

In addition to the traditional view of serfs (working the lord's lands in exchange for a plot to farm for their own profit), there were additional bonded peasants, known as cottars or cottagers, who held little or no land from their lord and provided labor services in exchange for a house and village plot. These individuals would have had to support themselves with some manner of wage labor, such as mining.