If there was a realistic enormous certified deposit of every single rare earth mineral in the most Goldilocks location and the deck was stacked for this deus ex machina miracle, it wouldn't matter.

In mining a deposit of minerals is a concentration of minerals. Think Minecraft, in a 9x9x9 cube the vein takes up an 8x8x9 space and runs into the next 9x9x9 cube. That's what mining companies look for.

Rare earths are called rare earths not because there are few deposits, it's because they're more uniformly distributed in low quantities in mineral deposits. Again think Minecraft, but for every 9x9x9 cube there is a single block of antimony.

Rare earths cannot create mining economies of scale. The reason China has the most rare earth production is because they "use every part of the Buffallo" more than other country's mining companies do. Rare earths are usually not refined from minerals that contain them in America because the labor to do so is too expensive. America's access to rare earths has always been an economic question of cheap labor, not a question of locating the resources.