In recent plate tectonics, as the crust gets sub ducted and melted, the heavier stuff like the gold, nickel, other heavy metals, and diamonds, settle out down into the liquid, and the lighter elements and compounds rise toward the top.

The acclimation of the lighter material under the North American continent has caused it to rise above sea level, there are similar stories in the turnover of the ancient crust and the regeneration of newer crust, that would leave areas of the world with out much in the way of concentrations of heavy minerals.

(The Falkin Islands are rich in gold deposits) although most are below sea level, another antarctic gold mine story......Britain fought a war over the retention of the islands on these grounds...