Well, one plot point in the manga early on was about "the origin of magic". Getting close to it had a seemingly tangible effect in hades power and obtaining it would have had an even greater effect. If we consider the literal words then it is indeed the actual source of magic we are talking about. Lucy made the point that the origin of magic was love and makarov made the point that it was everchanging.

Now, with that in mind lets move on to curses. The curses used by demons are distinct from magic, they were not even affected by face which would have destroyed magic. So the question is, where do curses come from? If they are a thing in on in themselves then they would need to also have a different source from magic. The situation is even weirder if we consider zeref himself seems to be a mage that uses magic. Unless zeref is using curses.... Or perhaps zeref has access to curses and magic? Perhaps this will be touched upon at some point and it will give us insight in regards to the origin of magic....