Orthomagmatic deposits

During magma crystallization, fractional crystallization and liquid segregation often lead to an enrichment of certain ore minerals. In fractional crystallization, cumulates accumulate in specific parts of the magma chamber, resulting in the layered mineralization of rock and ore. If the cumulate accumulates at the bottom of the melt, this is called gravitational settling; lighter (less dense) crystals can also float to the surface. Processes such as convection, fluid circulation, nucleation, and density currents also play a role in fractional crystallization. The ore body may represent an early crystallizate of the magma, but this is not always the case. Fractional ore melts, which are relatively mobile and low-viscosity due to a high content of volatile components such as _CO₂ or _H₂O , can be "pressed" into the surrounding rock before crystallization. This process is called filter pressing.

This filter pressure can also lead to the liquid separation of a parent magma into partial melts . However, other processes can also cause a homogeneous melt to separate into two immiscible partial melts as it cools. Melts with limited miscibility include silicate and sulfide melts, as well as oxide, carbonate, or phosphate melts. If, for example, two melts with different chemical compositions mix, certain ions or elements may preferentially remain in one of the melts, leading to separation. This is known as liquid-liquid separation .

Cover image: Stratiform chromite-PGE deposit in anorthosites from Bushveld/South Africa ( kevinzim / Kevin Walsh , Chromitite Bushveld South Africa , CC BY 2.0 )