Not an expert on iguana behavior but mouth opening without any hissing or turning towards you (all the better to bite etc.) may be contentment or she may just be warm and thermoregulating by 'gaping' which is opening the mouth to allow heat to escape, much like a dog panting.
It could also be she doesn't particularly like the place you are petting to be touched, maybe try moving elsewhere to pet. I think they like scratching around their dewlap, but I am not sure here. Unfortunately, as I already mentioned, not the expert on where iguanas like to be touched or their more subtle body language. I do know a few regular chatters that come to the kingsnake reptile chats friday, sunday (9pm-11pm est) and tuesday (8pm-9pm est) who have owned green iguanas for years so if I see them tonight or after the long weekend, I will see if they have other ideals about what your iguana is trying to tell you.
You are ofcourse, most welcome to join us for the chat. The more Canadians there the better. 