Magmatic ore forming processes

The formation of magmatic ore deposits follows the principles of magmatic differentiation that you learned about in the video on rock types and the rock cycle.

During the ascend of a hot melt, certain minerals crystallise when pressure and temperature decreases, and specific mineral species become stable. These crystallised minerals settle at the bottom of the magma chamber leaving the rest of the molten magma (the residual magma) with a new chemical composition which continues to ascend in the crust. At first, so-called mafic minerals crystalise. The term mafic originates from the two words magnesium and ferric (or iron), and therefore it comes as no surprise that these elements are very abundant in the minerals found in ultramafic and mafic rocks. However, these are not the only elements that can crystallise early. Early in the magmatic differentiation process, ore minerals can accumulate as layers of crystallised material at the bottom of the magma chamber. An example of an ore mineral that can form this way is chromite, which is a source of chromium.

Figure 1: Magmatic ore formation

Additionally, when mantle rocks become partially melted, sulfide minerals dissolve into the melt. At a certain point, the sulfide content within the melt increases to a saturation limit where the dissolved sulfide may form sulfide droplets that are unable to mix with the rest of the silicate magma, like oil in water. These droplets especially attract elements like copper, nickel, platinum, palladium and gold. Just like magnesium, iron, and chromium, those metal-sulfide droplets crystalise early as ore minerals, and deposit near the bottom of the magma chamber. Due to progressive changes in the magma composition as minerals crystallise out of it, the amount of mafic mineral crystallisation gradually decreases while felsic mineral content increases. Felsic minerals have higher silica content and are less dense than their mafic counterparts. These differentiated layers are present over the entire width of an intrusion, which generally ranges between a few hundred meters to several kilometers. However, the ore layers can be relatively thin with a thickness ranging from centimeters to several meters.

Figure 2a: Olivine xenolith, an ultramafic rock that contains a lot of iron and magnesium

Figure 2b: Granite, a felsic rock with high silica contents

When the magmatic differentiation has reached the point where almost all minerals have crystallised, the remaining residue is a melt very rich in fluids that carry metals that could not enter the crystal lattices of minerals formed earlier in the process. The rocks that form in this environment are called granite-pegmatites. They are light-coloured and carry very low portions of dark and dense minerals. These pegmatites are the host of many of the most precious gemstones in the world, such as emeralds. Lithium, cesium, and tantalum may enter the crystal lattice of certain minerals in this fluid-rich magmatic environment, and it could be that the lithium contained in the Li-ion battery in your own mobile telephone, or your electric car might be derived from this increasingly important ore type.