When there is only one buyer of labor in a community, we talk of a?

Monopsony is the term you are looking for. I know of one such place: Trail British Columbia. If the smelter ever shut down, Trail would become a ghost town (or very nearly a ghost town) very quickly. Most small mining communities are Monopsonies. If the mine closes, then the people living there have to be resourceful enough to create another industry.