These days not even the middle to upper management (on foreign mine sites) are mostly Americans.
Freeport McMoRan......few Americans on site
Newmont......few Americans on site
Southern Copper.....few Americans on site
Alcoa.....almost no Americans on site
Those same companies do employ plenty of Americans in their US head offices and their US operations.....but you were posting about out on foreign sites.
You are correct that they employ the best people they can get......and these days, there are several nations with bigger and smarter mining industries than the US. There are not even many universities in the US that offer Mining Engineering degrees anymore.