Magmatic Deposits

As a body of molten rock cools, minerals begin to crystallize, sinking to the base of the magma chamber or accreting to its sides. These are usually the ordinary rock-forming minerals, but sometimes they are useful minerals like base metal sulphides or the oxides of iron, titanium or chromium. Ultramafic or mafic igneous rocks can have concentrations of these minerals that are large enough to be minable.

Much of the world’s nickel and a large proportion of its copper and cobalt come from sulphide deposits of this kind in mafic igneous rocks, like the nickel-copper deposits of Sudbury, Ontario.

Platinum, palladium and several other precious metals are often recovered as a byproduct in nickel-sulphide deposits. These sulphides can be massive or disseminated though silicate rock. Pyrrhotite and chalcopyrite, plus one or more nickel sulphides, are common minerals. The mineralized bodies usually have tabular or a lens-like shape.

Titanium and chromium oxides can also form magmatic deposits, either massive or disseminated. The mineralized bodies can even be layered in appearance like sedimentary beds, although their origin is clearly not sedimentary.