Geology Forum for Geologists

Magmatic Sulfides and Cumulates

Mafic and ultramafic magmas, like all common magmas, contain the major elements oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium.

But they typically also contain other elements including sulfur, nickel, and less common metals such as platinum, palladium, and chromium.

As these magmas cool and crystallize, the first minerals to form are plagioclase, pyroxene, and olivine – all made of major elements.

Consequently, the concentrations of sulfur and other minor elements increase in remaining melt.

Eventually, sulfur concentration becomes great enough so that sulfide minerals begin to crystallize.

The sulfide minerals, typically containing iron and nickel, may also contain relatively high concentrations of platinum, palladium, and other minor metals.

Sulfides have greater densities than silicate minerals and the mafic or ultramafic melts.

So, the denser sulfide minerals will, over time, begin to sink. Eventually, after more cooling and crystallization, significant deposits of sulfide minerals may accumulate on the bottom of a magma chamber.

The deposits, which may form centimeters-, or meters-thick layer called a cumulate, are often entirely, or nearly entirely, composed of sulfide minerals.

This process produces magmatic sulfide deposits, which are the most important sources of platinum, palladium, chromium, and several other metals.

Cumulate sulfide minerals include pentlandite (Fe,Ni)9S8, chalcopyrite (CuFeS2), pyrrhotite (Fe1-xS), and pyrite (FeS2)

Cumulate sulfide deposits account for almost 60% of the world’s nickel production and more than 95% of platinum and palladium production.

These deposits are associated with mafic and ultramafic magmas but not, generally, with felsic magmas, because felsic magmas are so viscous that they cool and crystallize before dense minerals can settle.

Sulfides are not the only kind of mineral that can become concentrated in a cumulate deposit.

Oxides – including magnetite (Fe3O4), ilmenite (FeTiO3), and chromite (FeCr2O4) – may settle and collect at the bottom of a magma chamber, too.

These chromite cumulates produce not only significant amounts of chrome, but also very large amounts of platinum, palladium, and related elements.