Does anyone knows of rules for mining as a permanent activity for the profit of player characters? One of my PCs will probably get some land in the near future, including a silver operation, and I'd like it to be more than "ok, its production pays for the maintenance costs of your keep"; it should feel like a real treasure. I'm looking for written rules, but ideas of how to handle it would be great as well.
Sounds like a pretty cool idea. A bit more information would be useful regarding "my PC's will probably get some land in the near future, including a silver operation", such as how much land? Is it settled, like they were awarded a village and a title like Squire or Baronet and tasked with running it for a lord? Or are they starting from scratch, having to clear the surrounding area and attract settlers to really make use of things? Which could be fun.
Should be careful regarding such an asset, as even moderately sized precious metal mines are VERY lucrative and in reality once they are up and running well would basically provide PC's with enough money that most DM's would consider it game breaking. Especially as in some campaign you can turn cash into magic items pretty readily. How to deal with millionaire PC's? But done right that amount of prosperity can be played into the campaign for any number of issues, it's true what they say, mo money mo problems, you just don't have to worry about the price of gas or the basics anymore. Rival mining operations may want to take over or sabotage your efforts, and local lords will want to steal it or help you to ensure a strong tax base. But I find most PC's are into fighting and adventure, and actually running a business operation is well, work, and boring work at that if you're used to taking out orc strongholds for funsies. Maybe hire NPC's to manage it, provide the expertise, or take on partners to run it for you and just cut you a fat sack of profits every month.
The Basic DND "Companion Set" is one of my all time favorites, and actually had a simple rule system for PC's who started owning land. Population, resources, taxes, keeping your folks happy, random events like crops failing, stuff like that. If you can find an old copy of it, I think it might provide you some good inspirations on this. But trust me, unless your PC's are a little weirder than average, they are going to want to get back to honest adventuring over commerce pretty soon so I wouldn't get too bogged down in things. But it sure is fun to map stuff like this out. Good luck!
P.S. - If you haven't watched that HBO series "Deadwood", that is a pretty neat show about a mining town/operation and all the odd stuff that goes on in the background as those things spring up out of nowhere.