The mine of Mountain Hall is the reason for Mountain Hall being founded in the Misty Mountains, and it has been delved steadily ever since. Iron and other metals are mined and traded with both the Woodmen as well as with the Beornings in exchange for food and other goods.
Occasionally gold and even gemstones have been found in the mines, but this has never been the primary goal - this is more the desire of Dwarves than Men.