When rock weathers and erodes, its minerals end up as grains in rivers and on beaches. Moving water carries grains along, but how far depends on size and density. Most sand is quartz, at about 2.65 g/cm³. Gold is about 19 g/cm³, seven times denser. Wherever the current slows, the heavy grains drop out first and the light ones are carried on. Repeat that over thousands of years and you get a placer: a concentration of heavy, durable minerals in sediment. Panning for gold is the same process done by hand.

A placer mineral needs two properties: it must be dense, and it must survive weathering and abrasion. Gold, cassiterite (tin oxide), diamond, garnet, zircon, ilmenite and rutile (titanium oxides) and monazite (a rare-earth and thorium phosphate) all qualify.

Size matters as well as density. How fast a grain sinks depends on both, so a large, light grain can sink as fast as a small, heavy one. A quartz grain about half a millimetre across settles at roughly the same speed as a gold flake a tenth of a millimetre across, so the two end up side by side. Grains that settle at the same speed are called hydraulically equivalent. It is why fine "flour" gold can travel a long way down a river, while coarse nuggets stay close to their source.

Most of the world's iron ore comes from banded iron formations (BIFs): rocks of alternating thin layers of iron oxide and chert (fine-grained silica). The biggest formed between about 2.5 and 1.8 billion years ago, but many are older, some as old as 3.8 billion years. The chemistry is simple. Iron dissolves easily in water that has no oxygen, as ferrous iron (Fe²⁺). The early oceans had almost no oxygen, so they held a lot of dissolved iron. When photosynthesising microbes began releasing oxygen, it reacted with that iron to make ferric iron (Fe³⁺), which is insoluble. At first this happened only locally, in sunlit shallow water where the microbes lived, before oxygen built up in the air. The ocean, in effect, rusted, and the rust settled on the seafloor. Iron formations kept forming before and during the Great Oxidation Event covered in A brief history of Earth.

A typical BIF contains roughly 30% iron. The richest ores, at 60% iron or more, formed where later fluids or deep weathering dissolved the silica away and left the iron behind, the same residual logic as the next section. The Hamersley Basin in the Pilbara of Western Australia is the world's largest iron-ore province. India's iron ore comes from BIFs too: the Odisha–Jharkhand belt around Noamundi and Joda–Barbil, Bailadila in Chhattisgarh, Bellary–Hospet in Karnataka, and Goa. Most of India's iron formations are Archaean, older than 2.6 billion years, so they formed well before oxygen built up in the air.

In Weathering, erosion & soil you saw that chemical weathering dissolves the soluble parts of a rock and carries them away in water. In a hot, wet tropical climate, over millions of years, this goes to an extreme. Calcium, magnesium, sodium, potassium and eventually even silica are leached out. What is left is a residual deposit, enriched in whatever was least soluble. The thick, red, iron- and aluminium-rich weathering profile that results is a laterite, from the Latin for "brick", a word coined in Kerala in 1807 for the material cut into building blocks there. Below the laterite lies saprolite: rock so rotted that you can dig it with a spade, yet still showing its original structure.

On ultramafic rock such as peridotite, the same weathering makes a nickel laterite. Olivine dissolves, magnesium and silica leave, iron stays at the top, and nickel does something in between: it dissolves near the surface, moves down with the water and is captured again lower in the profile, where the chemistry changes.

When a copper deposit is uplifted and exposed, weathering attacks it from the top down. Rain and oxygen react with pyrite (iron sulphide) to make sulphuric acid. The acid dissolves copper from the upper part of the deposit and carries it downward. When the solution reaches the water table, the level below which the ground is saturated and oxygen is scarce, conditions flip and copper re-precipitates, often as chalcocite (Cu₂S) replacing the original sulphides. Copper from a thick leached zone is packed into a thinner layer below. This is supergene enrichment ("supergene" means formed by water from above).

If the land is uplifted or the climate dries, the water table drops. The old blanket is now above it, so it is leached in turn, and its copper moves down to a new blanket. Each drop adds the copper of the old blanket to the new one, which is why the richest blankets formed where the water table fell in stages.

When water evaporates, the salts dissolved in it stay behind, and the water left becomes saltier. In a basin with no outlet to the sea and a dry climate, this can go on until minerals crystallise out in a predictable order: first carbonates and gypsum, then halite (rock salt), and finally the most soluble potassium and magnesium salts. The resulting rocks are evaporites. Ancient evaporite beds are the world's source of potash (potassium salts for fertiliser), as in Saskatchewan in Canada.

Lithium is so soluble that it rarely crystallises at all. Instead it builds up in the leftover brine in the porous sediments beneath a salt flat (a salar). In the high Andes, in the "lithium triangle" of Chile, Argentina and Bolivia, volcanic rocks and hot springs supply lithium, and intense sunshine and wind evaporate the water. The best brines, such as those beneath the Salar de Atacama, hold more than a thousand milligrams of lithium per litre, thousands of times more than seawater. Producers pump the brine into shallow ponds and let the Sun concentrate it further over many months.

Sediment-hosted copper is a cousin of these systems. Oxidised, salty basin water can carry copper (and cobalt) a long way. When it soaks into a layer rich in organic matter or sulphide, such as a black shale, the chemistry switches from oxidising to reducing and copper sulphides precipitate along that layer for tens of kilometres. The Central African Copperbelt of Zambia and the Democratic Republic of the Congo is the outstanding example, and the reason the DRC mines most of the world's cobalt. The Kupferschiefer ("copper shale") of Poland and Germany is another.

Across these four modules a pattern has emerged. Every deposit, magmatic, hydrothermal or surface, is the end product of a mineral system: a source of metal, an energy source and fluid to move it, a pathway, a trap, and luck with preservation afterwards. This way of thinking, developed largely by Australian geoscientists from the 1990s onwards, has changed how exploration is done.

The point is that each ingredient leaves its own evidence at a different scale, and you can look for it before you have seen any ore at all.

Instead of searching for "something that looks like a known mine", explorers map the probability of each ingredient being present and look where they overlap. How that is done in practice, with mapping, geochemistry, remote sensing and drilling, is the subject of Prospecting & the exploration pipeline, and the physics of seeing underground is covered in Geophysical methods.