Ores from molten rock
In some large bodies of magma, or molten rock, the minerals that crystallize settle to accumulate on the chamber floors, which slowly build up until the magma is completely solidified. The minerals accumulate in layers, depending on when they formed in the magma. This is one natural sorting process that may concentrate valuable metal-rich minerals.
Chromite is usually one of the first minerals to crystallize in basaltic magma, and is thus commonly found as a layer near the bottom of magma chambers.