You have mixed up "fixed" and "limited". Zero-sum game assumes amount of wealth is fixed. What you describe is world that has limit on total wealth that have not yet been reached. But definitely it is not fixed. By your own definition you can always dig up some new natural resources and this would increase total wealth (so no, not fixed).

It is possible to create wealth. If I can do something that you can't and you can do something that I can not, then if we exchange services we can create wealth from nothing. I have more than at the start, you have more than at the start and nobody else is affected.

The maximum possible wealth I think is also not limited. There is always potential to take something and create something with more worth. Good, pure example is art. Painter takes resources that are relatively cheap and plentiful (paints, canvas, his time) and can create something of huge value to some people. Intel might take a bunch of sand and other resources and create processors from it.

There is also "intellectual property" which is not tied to any natural resources. Anybody can create something new (derive new chemical formula or write a book) that would be an example of creating wealth from nothing.