The idea of working with a private equity company like Blackstone is that BHP would be able to take Alcoa's core mining and exploration assets and then carve off other, unwanted units for its partner.

FORBES: Magazine Article

The mining group has said that it wants to focus on core businesses such as iron ore and coal.

BBC: BHP Billiton sells Arizona copper mine to Capstone

To determine whether a deposit has enough of a mineral to merit mining, companies use air-based geological mapping and drill for core samples.

WSJ: Exhausting the Earth's Resources? Not So Fast