And what will happen with existing mines?
Don't forget whole towns have been build in a certain place because of the location! (Which likely includes a mine!)
Effectively removing the mine, makes a location useless.
[edit:]
And what about copper-ore/mines?
Will they become more useful?
[edit2:]
Concerning existing mines in general, I have no clue of what you are going to do with them...
- Resetting all existing mines might be harsh, but everyone gets a fresh start, so that would be ok i guess, just inform people ahead of time so they don't lose all they have stored in their "basement"!
- Restricting any meaningful purity to mines opened after the patch, would be... unfortunate, pretty much as bad as removing old mines right away (which would not be necessary ofc in any case).