Thats not that rare. I can find gold, iron, coal, and redstone near each other in the mineshaft I'm in.
Hmmm... One time I found a vein of coal almost 80 blocks strong, with 12 blocks of iron ore inside. I also found 5 gold ore, and two diamonds in a spot directly adjacent XD The nearest redstone was about 15 blocks away. Ore hotspots do happen like this, that's why you should always mine the adjacent stone whenever you find a mineral vein.