Spark when key is turned to the off position?

I have a 73 dart with a 318 and edelbrock carb. I went out a couple days ago and started and it run for about 5 minutes. Today I decided to wash it but when I went to start it it wasn't having it. Immediately went to check for spark and noticed that nothing was happening while cracking but it would let off a little spark whenever I disengaged the key and moved it to the off position. Anyone have any idea whats going on?