I imagine the main resources that USA had are the huge plains to grow food in, in a relatively favorable climate. The industry comes later.

Maybe there was a lot of wealth in silver, I don't know about that. It seems that it had some land-based oil that they started extracting in the beginning of the 20th century, but I've no idea if they were as extensive as in the US (where there was Standard Oil, an immensely wealthy corporation).

Mind you, I'm not saying it was only the natural resources, there are other factors as well - different climate, geography, cultural background (English vs. Spanish/Portuguese), colonial history, ... I somewhere read that it's a curious phenomenon how so many countries in SA had so many dictatorships in sequence...

It's really hard to argue that S. America, Africa, Asia, etc., all don't have immense natural resources and favorable climates.

A small note. I read a Statfor report that North American Rivers were very important.