One of the horses I work with had an issue like this a few years ago...he's always been prone to skin issues, and one summer he started losing all the hair around his face. Turned out to be some kind of fungus, and proper treatment cleared it up. I've also seen stress (both mental and physical) cause hair loss around the neck and chest, and a horse with a completely bald forehead - has been like that for years and has no explanation, as the owners don't seem concerned enough to look into it
A mineral deficiency certainly sounds like a possibility as well. Good luck!