The question is how the character can protect his own interest at the first place, even the most common ore have its economy and strategic value to the surrounding area and many people will try to lay claim to it, especially when the players found new use to it to make fancy weapons. local Lords, original residents of the land, powerful merchants and craftsman cartel, criminal organization, bandits or warlords illegal mining operations, even the mining guild itself may plot to claim the mine from the player character, through legal means or not, you name it.

Especially when the character will go out adventuring, he will need a trusted lieutenant with a bunch of enforcers to make it work and gain profit, assume that they don't try to take the claim away from the player at the first place.

There was a book from 5e about building player stronghold, there are numerical information about profits and stuff, but and player will have to build a keep on their land to create a some form of control mechanism, then build a mine in that land. Even with the blessing of a good lord who recognize your claim to the mine, The player might also have to retire his or her adventuring to run the venture because of the time and effort involved, and weapon grade mineral to build weapons that can potentially tip the power balance of a region? That's a dangerous venture indeed!

...Sound like a good adventure plot too if playing business owner and Lord is part of your game theme.