You will find dwarf mine-holds scattered around The Empire, and their leaders are generally recognized as Imperial Lords with few responsibilities to the local rulers or the empire. In truth, most of those mine-holds control a route into dwarf controlled sections of the darklands , rather than a single mine entity. The ores and metals they export, are often mined deep underground or, perhaps, some distance from the mine hold, by independent clans of dwarves, then transported along underground roads to the sales outlet, the mine-hold. Because of this, a single mine-hold might produce a number of different metals and minerals.
A minehold has a mine to work, but it also functions between the underground and surface societies. The Sovereign Lord of the minehold is effectively king of the dwarven community at that minehold Leaders of other clans see themselves as equal to the Sovereign Lord and have ultimate control over their own holdings. The underground communities vary from small groups working a small and specific vein of ore, to larger communities who have found a motherlode.
While there are Dwarves born in the minehold, there are may also be dwarves, and other races from other underground holdings living there or visiting - they recognize the sovereignty of the Imperial Lord, and follow his directions when they are in his domain.
Trade routes through the darklands are dangerous, sparsely populated and not well travelled, they wind and twist along (mainly) natural caverns, that can be home to all sorts of hazards. Other humanoid races send hunting parties out to monitor the routes, so merchants either have to be strong or stealthy to travel them - so many of the ‘merchant caravans’ are small and only carry a few trade goods with them. Whatever they trade, has to be valuable, or the risk is too high. A single bottle of whiskey can be worth many hundreds of gold pieces when you are two miles underground!
This may be incomplete.