Platinum (Pt) is a dense, silvery-white metal and one of Earth’s rarest elements. It belongs to the Platinum Group Elements (PGEs)—platinum, palladium, rhodium, ruthenium, iridium, and osmium—which share similar chemical and physical properties. Platinum is characterized by its chemical inertness, resistance to corrosion, and high melting point. Found in ultra-low concentrations, typically around 0.0005 parts per million in the Earth’s crust, forming economically viable deposits requires specific and rare geological concentration mechanisms. The journey of platinum involves processes spanning the formation of the solar system and billions of years of geological activity.
The Extraterrestrial Source
The formation of platinum atoms began far from Earth, a process that requires immense stellar energy. Platinum, along with elements heavier than iron, is primarily synthesized during the explosive deaths of massive stars in events known as supernovae or through the collision of neutron stars. These catastrophic cosmic processes generate the necessary energy to fuse lighter nuclei into the dense, heavy elements that eventually become part of interstellar dust clouds.
As the early Earth formed and began to differentiate, platinum’s strong affinity for iron—a property known as siderophile (iron-loving)—caused most of the planet’s initial supply to sink. During the planet’s molten phase, the vast majority of platinum was scavenged by molten iron and partitioned into the Earth’s core. This process left the silicate mantle and crust virtually stripped of highly siderophile elements (HSEs).
The platinum now accessible in the crust and mantle is thought to have been delivered much later, after the core had stabilized. This concept is known as the “Late Veneer” hypothesis, which posits that a barrage of meteorite and comet impacts bombarded the Earth approximately 4.5 to 3.8 billion years ago. These late-arriving space objects deposited HSEs, including platinum, onto the already-formed mantle and crust, providing the initial source for all of today’s deposits.
Deep Earth Magmatic Segregation
The primary platinum deposits are formed through magmatic segregation within large, layered igneous intrusions. Structures like the Bushveld Complex in South Africa or the Stillwater Complex in Montana formed when immense volumes of ultramafic magma intruded into the crust and cooled slowly over millions of years. As the magma chamber cooled, magmatic differentiation occurred: minerals crystallized sequentially based on their melting points, causing the remaining liquid to change composition.
PGEs remain dissolved in the silicate magma until sulfur saturation is reached, meaning the sulfur concentration can no longer be fully dissolved. This often happens when the magma is contaminated by sulfur-rich rock or through changes in pressure and temperature. Once saturation is achieved, an immiscible sulfide liquid separates from the silicate magma, forming tiny, dense droplets of molten metal sulfide.
Platinum atoms have a strong chemical preference to bond with sulfur and iron, causing them to efficiently partition into these sulfide droplets. This process acts as a powerful collector, concentrating platinum from low background levels into a highly enriched sulfide melt. Because the sulfide droplets are significantly denser than the surrounding magma, they sink through the chamber. They accumulate in thin, distinct layers, or “reefs,” which become the world’s richest primary platinum ores. These high-grade layers can yield ore grades of 6 to 10 grams of PGEs per ton of rock in complexes like the Bushveld.
Weathering and Secondary Deposits
Two main post-magmatic mechanisms concentrate platinum into mineable secondary deposits: hydrothermal action and mechanical erosion.
Hydrothermal Action
This process involves the movement of hydrothermal fluids (superheated water) through the solid rock. These fluids circulate through fractures, often at temperatures exceeding 300°C. As the fluids move, they leach trace amounts of platinum and other PGEs from surrounding sulfide minerals. The platinum is transported as dissolved complexes until a change in temperature, pressure, or chemical environment causes the metal to precipitate. This redeposition concentrates the platinum in veins, pockets, or disseminated bodies, creating localized, high-grade enrichments distinct from the original magmatic reef layers.
Mechanical Erosion
This mechanism involves the mechanical breakdown of the primary rock through weathering and erosion. When platinum-bearing rocks are exposed at the Earth’s surface, weathering frees the resistant platinum particles. Platinum is extremely dense and chemically unreactive, meaning it does not corrode or dissolve easily. Moving water, such as in rivers or streams, transports the freed platinum grains downstream alongside sand and gravel. Due to its high density, platinum particles quickly drop out of the water current when the velocity decreases. This hydraulic sorting creates placer deposits, where the platinum is mechanically concentrated in stream beds and floodplains, providing a historically significant source of the metal, such as those found in the Ural Mountains.