Copper is everywhere. A random shovel of granite, basalt or soil contains some. So does seawater. What is rare is copper that is concentrated enough to be worth extracting. Geologists call that rock ore: rock from which one or more metals or minerals can be mined, processed and sold at a profit.

That last phrase matters. Whether a rock counts as ore depends on the metal price, the cost of energy and labour, the mining and processing technology available, how far the deposit is from a road, railway or port, and whether communities and regulators will allow a mine. The same rock can be ore one decade and waste the next. In the early 1900s, two new methods together turned huge, low-grade copper bodies in the Americas into ore: digging whole hillsides in bulk from open pits with steam shovels, and a separation technique called froth flotation. That is why the field is called economic geology.

Three words come up constantly:

Each element has an average abundance in the continental crust. Divide a typical ore grade by that average and you get the concentration factor: how many times richer than ordinary rock an ore has to be. It tells you how special the geological process must have been.

All figures are rounded. Published crustal averages differ between sources, by up to a factor of two or three for copper and gold. We use older textbook values; newer estimates would make the copper and gold factors two to three times larger. One unit to know: g/t (grams per tonne) is the same as ppm, and is how gold, silver and platinum grades are quoted. Notice the pattern. Aluminium and iron are so common that a modest natural upgrade is enough. Gold and zinc need the most concentration of the metals shown, and much more of the planet's gold is still locked in ordinary rock than will ever be mined.

Two numbers describe any deposit. Grade is the concentration of the valuable metal: % for base metals, g/t for precious metals. Tonnage is how much ore there is, usually in millions of tonnes (Mt). Multiply them and you get the contained metal, the total amount of metal in the ground.

Where you draw the edge of the ore body is a choice. Ore grade usually fades gradually into waste rock. The cut-off grade is the lowest grade that is worth sending to the mill rather than the waste dump. Raise the cut-off and you get fewer tonnes of richer ore. Lower it and you get more tonnes of poorer ore. Mines recalculate it as prices and costs change. The break-even cut-off is simply the grade at which the copper in a tonne of rock just pays for mining and processing that tonne.

Plotting deposits by grade and tonnage on logarithmic axes reveals a trade-off. Some deposit types are small but rich, others vast but lean. A huge low-grade body can still hold more metal than a rich vein, and can be mined cheaply in bulk from an open pit. Over the past century the average grade of copper mined worldwide has fallen steadily, as the rich, easy deposits were used up and mining moved to bigger, leaner ones.

How does nature achieve a 100× or 1,000× concentration? Every ore deposit, whatever its type, needs three things. This framework will organise the next three modules, so it is worth learning now.

A fourth ingredient is often added: preservation. Many deposits form close to the surface and are later eroded away. The deposits we find are the ones that were buried, tilted or protected long enough to survive. We return to this in Sedimentary & surface deposits.

Geologists classify deposits by the process that concentrated the metal, which really means by their transport agent and trap. Three broad families cover almost everything mined today.

A critical mineral is one that an economy depends on and whose supply is at risk, for example because production is concentrated in a few countries. Lists differ by country, but the energy transition has put the same handful at the top of most of them. Electric vehicles, batteries, wind turbines and power grids all need far more metal than the fossil-fuel systems they replace. The International Energy Agency has estimated that a typical electric car needs roughly six times the mineral inputs of a conventional car, mostly in its battery and motor.

Supply is lopsided. A single country often dominates mining or, even more often, refining of a given mineral. China refines most of the world's rare earths and graphite, and the Democratic Republic of the Congo mines most of its cobalt.

India published its first list of 30 critical minerals in 2023, including lithium, cobalt, nickel, graphite, copper, the rare earths and the platinum-group elements. The mining law was amended the same year so that the central government could auction exploration and mining rights for these minerals, and auctions of critical-mineral blocks have been running since late 2023. A state-backed company, KABIL, has also been securing lithium prospects overseas. All of this is creating demand for people who understand where these minerals come from, which is the subject of the next three modules, and how to find them, which is covered in Prospecting & the exploration pipeline.