Is there a blight growth? If so, the main blight boil you have to destroy is hidden a bit. It's on the inside of the cave and you have to shoot it through a small opening.
If it's closed for another reason, then don't worry about it. I don't have a save file that far back, so I can't test this out right now, but a lot of areas don't open up until you progress in the story some more. I'd say, don't worry about it and check it out again at some later point.
I know those treasure chests can kinda taunt you, but their contents are randomized, so you're not really missing out.