Magmatic ore deposits, also known as orthomagmatic ore deposits, are deposits within igneous rocks or along their contacts in which ore minerals crystallised from a melt or were transported in a melt. Ore deposits that form in and around igneous rock units as a result of mineral precipitation from aqueous solutions or hydrothermal fluids are hydrothermal ore deposits. Magmatic-hydrothermal ore deposits are hydrothermal ore deposits in which the aqueous solutions were derived from magma. These ore deposits are described further in Chapter 3.

Petrological and geochemical background to magmatic ore formation

Processes of magma development

Magmas evolve in magmatic systems. A magmatic system is the site of melting, the pathway of magma migration and the site of crystallisation within a specific geological and tectonic environment. A magmatic system will extend from the mantle to the upper crust in most cases (Figure 2.1). The composition of an igneous rock is the result of the complex set of petrological processes in the development of the magmatic system from the site of melt formation in the mantle, during melt transport through the upper mantle and crust, to the site of final crystallisation. The processes that control magma compositions and compositions of igneous rock that crystallise from the magmas include: partial melting at the source and interaction between the melt and unmelted, restite minerals; interaction with wall-rocks the magma comes in contact with; assimilation of partial melts of wall-rocks; mixing of magmas from different sources; separation of a magma into two or more immiscible melts; fractional crystallisation of minerals; and mineral accumulation.