Well, if you're interested in written materials in particular, the Maya and Aztec codices were in abundance at the time of the Spanish arrival, as were the Incan quipu (which seem to have been an actual writing system and not just an accounting method). They didn't survive contact - the Spanish burned as many of them as they could get their hands on, and today we've only really got a handful of extant examples left. That's unfortunately the answer for what happened to a lot of New World culture (although the climate and terrain deserves its due especially in Central America, where there's oral history of large cities which we're only now finding by lidar because the jungle's so aggressive).
The Olmec are an interesting case here, too - beautiful, just stunning sculptural works, going back to ~2000-1500BC. There's really quite a bit of cultural artifacts found and to be found.
I'm not really trying to downplay or deny the significance of pre-contact American cultures and civilizations. Everything I said was in the context of:
> It depends on your definition of treasure, of course.
Wouldn't it still be a tiny fraction of what was/can be found in Europe no matter how you define it?
Which I think still holds true (again certain materials (especially in combination with certain climatic conditions) just survive for much longer than others).
The Olmec are an interesting case here, too - beautiful, just stunning sculptural works, going back to ~2000-1500BC. There's really quite a bit of cultural artifacts found and to be found.