Resources

Resources generally represent non-renewable sources of workable minerals. Every tile with a resource has a number called accessibility associated with it. It's used to determine how difficult it is to extract resources from the tile. At the end of each year, a tile has a chance to deplete some of it's veins, decreasing it's accessibility. Chances for that are higher, if the tile is actively mined, scaling with amount of extracted resources. Once accessibility is low enough, mining on the tile may stop being economically viable. Use technology and magic to keep up with the rate of extraction if your veins are poor.