have little islands branch off from your island that have more metal mines. maybe make it to where your geologist needs a special skill to reach that island mine. and you have to clear off each island mine you explorer finds with your general. and each little island is just a mine and a store house already on the island that the bandits have left behind. so all that can be on the island is 1 store house and 1 mine. you just have to repair the storehouse thats already there.
i also suggestion that you gain exp whenever you mined a whole mine, and your explorer finds somthing. the rarer the more xp you get. for those who dont like to do adventues as much. maybe some exp when you upgrade a building