"Any insite as to the precentage the belt holds in North America?"

Sorry, I don't know.  If it's any help, I understand that the belt across northern Mexico and parts of New Mexico, Arizona, and Southern California is second in production of copper after the mega mines in southern Chile. 

I would also guess that such features are very highly associated with calderas, which we know to occur in clusters such as we have across the area described above, in Alaska, and Chile, as a few examples. 

VP