Do mines have an area of effect?
If you put two mines next to each other will that reduce the amount of ore extracted, and will an iron mine affect a coal mine?
No, the capacity of a mine is based strictly on the space it occupies. All mines of a given type have identical capacity, and you can change the type of mine at will. Line them up!
They do have "unhappiness circles" however, that reduce the happiness of all you guys who spend time in the radius (which is 20, I think), and the unhappiness effect is cumulative. You can counteract this by building structures with happiness circles in close proximity; I use cemeteries and wells for this.
" ... and it's dark as a dungeon
"Way down in the mine."