Rock only becomes ore when mining it pays
Ore is not a type of rock but an economic verdict. A deposit counts as ore only when the metals or minerals it holds are worth more than the cost of digging them out and refining them. Change the price of copper or the efficiency of a smelter, and the same hillside can switch from rubble to riches.
The word comes from Anglo-Saxon for a lump of metal. The valuable minerals are usually oxides, sulfides or silicates, or native metals such as copper and gold, and they are almost always mixed with worthless rock called gangue. Mineral processing first liberates the good material and then concentrates it, using methods like froth flotation, gravity separation and magnetic or electric sorting. What remains is tailings: useless, potentially harmful waste produced in huge volumes, especially from low-grade deposits.
Deposit names can be wonderfully arbitrary. Some honour a place or discoverer, as with the Kambalda nickel shoots named after drillers; others borrow a god, a hometown or a company code, like MKD-5, the in-house label for the Mount Keith nickel deposit.
Geology decides where ore collects. Pegmatites, crystallising slowly at great depth, grow huge crystals and supply quartz, feldspar and lithium minerals. Carbonatites, over half carbonate minerals, hold more rare earth elements than any other igneous rock. Kimberlites, rising from around 150 kilometres down in the mantle, are a primary source of diamonds, and layered intrusions such as South Africa's Bushveld Complex yield chromite, the only ore of chromium. Canada's Sudbury basin is the only known nickel-copper source formed by an ancient impact.
Water does much of the work too. Porphyry copper deposits, formed above subducting plates and averaging only 0.8 percent copper by weight, are the leading source of the metal, while sediment-hosted copper supplies about 20 percent of world output. Orogenic deposits, created as mountain building drives gold-bearing fluids into cracks, account for about 75 percent of gold production. Laterites, weathered from rock near the equator in as little as a million years, provide iron, manganese and aluminium.
Source: Ore
