How does chromite become concentrated at the bottom of a magma chamber?

The concentration of chromite in a magma chamber is a process of magmatic segregation, where minerals become locally concentrated during the cooling and crystallization of mafic or ultramafic magma.1

Historically, geologists believed that because chromite has a significantly higher density than the surrounding liquid magma, it simply sank to the bottom of the chamber as it crystallized.12

However, modern geological consensus suggests that other mechanisms are often more significant in forming these magmatic cumulates:

• In-place crystallization: Monomineralic layers of chromite may form directly on the floor of the magma chamber.1
• Density currents: Currents within the magma chamber can transport mineral grains from the walls and roof, depositing them on the chamber floor.1

These processes occur most effectively in low-silica, low-viscosity magmas, which allow mineral grains to move and segregate before the entire body of magma has finished crystallizing.1