In the source–transport–trap framework from What makes an ore deposit, magmatic deposits are the simplest case. The source is the mantle or deep crust that melted. The transport is the magma itself. The trap is a physical or chemical separation that happens as the magma cools: something heavy sinks, something immiscible separates, or something unwanted is left behind.

A crystal is a regular three-dimensional framework with sites, or "slots", of particular sizes and electrical charges. In olivine, the slots are built for magnesium and iron ions, which carry a charge of 2+ and have a particular size. As a crystal grows from magma, it takes in only the ions that fit those slots well. Think of a car park built for compact cars: small cars park easily, buses are turned away.

That simple rule sorts elements into two groups:

Geologists measure this with a partition coefficient (written D): the concentration of an element in the crystal divided by its concentration in the melt. Above 1 means compatible, below 1 means incompatible. Follow the logic through a whole cooling magma and you get a powerful result. The more of the magma has crystallised, the more the incompatible elements are crammed into the shrinking volume of liquid that remains.

In a large chamber of mafic or ultramafic magma, the first minerals to crystallise (see Bowen's series in Minerals & lithology) are olivine, pyroxene and chromite, an iron–chromium oxide. These crystals are denser than the liquid around them. Chromite is about 4.5–4.8 g/cm³; a basaltic melt is closer to 2.7. So they sink, like sand settling in a glass of water, and pile up on the chamber floor. This process is called fractional crystallisation with crystal settling, and the resulting crystal piles are cumulates.

Repeat this over thousands of years, with fresh pulses of magma being injected from time to time, and you build a layered intrusion: a body of igneous rock stacked like a layer cake, with each layer dominated by one or two minerals. Some layers are almost pure chromite, called chromitite. Others, often only a metre or less thick, are enriched in the platinum-group elements (PGEs): platinum, palladium, rhodium and their relatives.

A mafic magma can only dissolve a small amount of sulphur, roughly a tenth of a percent. If it takes in more, for example by melting sulphur-rich sediments it passes through, or if cooling and crystallisation lower how much it can hold, the excess sulphur does not form crystals. It separates as tiny droplets of a different liquid: a molten iron sulphide. The two liquids don't mix, just like oil and vinegar in a salad dressing. Liquids that won't mix are called immiscible.

Here is the trick. Nickel, copper, cobalt and especially the PGEs are chalcophile ("copper-loving"): chemically, they far prefer to bond with sulphur than to stay in the silicate melt. PGEs can be tens of thousands of times more concentrated in the sulphide droplets than in the magma around them. So each droplet acts as a sponge. As it sinks through the magma it soaks up these metals, and the more magma it meets, the richer it gets. Because sulphide liquid is much denser than silicate melt, the droplets collect in hollows in the floor of the chamber or in the narrow conduits magma flows through, forming magmatic Ni–Cu–PGE sulphide deposits.

How rich a droplet gets depends on how much magma it meets. Geologists call this the R factor: the mass of magma that a given mass of sulphide liquid has exchanged metals with. A droplet that meets little magma can only collect a little metal, however much it "wants" it. The stronger an element's preference for sulphide, the more magma the droplet must meet before it is full.

Diamond is pure carbon arranged in a very dense crystal structure. It is only stable at enormous pressures, which under the old, cold cores of continents (cratons) means depths of more than about 150 km. At shallower depth, carbon's stable form is graphite. Diamonds near the surface survive only because converting back to graphite is extremely slow at low temperature.

Kimberlite is a rare, gas-rich ultramafic magma that forms deeper still, at 150–250 km or more. It rises explosively fast, probably in hours to days, ripping off pieces of the mantle on the way. Some of those pieces contain diamonds that formed long before, often billions of years earlier. Near the surface, the expanding gas shatters the rock and blasts out a carrot-shaped pipe. The diamonds are passengers, not products, of the kimberlite. Even a rich pipe contains under about one part per million of diamond by weight, and only a small fraction of known kimberlites carry enough to mine.

India's diamonds have a long history. For centuries the world's diamonds came from river gravels along the Krishna valley in the Golconda region. Today India's only large mechanised diamond mine is at Majhgawan, near Panna in Madhya Pradesh, run by the state miner NMDC. The Majhgawan pipe is usually classed as a lamproite, a close cousin of kimberlite that carries diamonds up in the same way.

Go back to the explorer at the top of this module, set D near zero and push crystallisation past 90%: the element is 10× enriched at 90% and 20× at 95%. When a granite magma is almost entirely crystallised, the last few percent of liquid is loaded with everything the crystals turned away: lithium, beryllium, tantalum, niobium, tin, caesium, rare earths. It is also rich in water, fluorine and boron. These make the melt runny and let atoms move fast, so the crystals that finally grow can be enormous, centimetres to metres across. The resulting very coarse-grained rock is a pegmatite, usually found as dikes and pods near the top of a granite or in the rock around it.

A carbonatite is something stranger: an igneous rock made mostly of carbonate minerals, like a molten limestone. Carbonatites form from very small degrees of melting deep in the mantle, which concentrates incompatible elements from the start, especially the rare earths and niobium. Only one volcano on Earth erupts carbonatite lava today, Ol Doinyo Lengai in Tanzania.