Almost every rock contains a little copper, zinc or gold, spread thin among its other atoms. An ore is rock where geology has concentrated a metal enough to mine at a profit. In Renewable and Nonrenewable Resources you met minerals as resources; here we ask how nature gathers them, using the processes of weathering, magma, hot water and flowing streams.
| Process | How it concentrates metal | Examples |
|---|---|---|
| Magmatic | dense crystals settle and pile up as a magma cools | chromite layers; nickel and platinum ores |
| Hydrothermal | hot, salty water dissolves metals and deposits them in cracks as it cools | gold–quartz veins; porphyry copper around granite |
| Weathering | rain carries away soluble elements and leaves insoluble ones behind | bauxite (aluminum) in tropical soils |
| Evaporation | water dries up and leaves dissolved salts | rock salt, potash, gypsum |
| Placer | running water sorts heavy grains from lighter sand | gold and tin in stream gravels |
Why it works
What makes a deposit worth mining is its concentration factor, the ore grade divided by the metal's average abundance in the crust. Average continental crust holds only about 60 parts per million (ppm) of copper; typical copper ore is around 0.5% (5,000 ppm) or more, roughly 100 times richer. Rare metals need far larger factors: gold averages a few parts per billion, so gold ore of a few grams per tonne is concentrated about a thousand times. Each process above is a natural sorting machine that achieves this.
Key idea
Ores are rare places where geology concentrated a metal far above its crustal average, by magma, hot water, weathering, evaporation or running water. Concentration factor = ore grade ÷ crustal abundance.
Worked examples
Worked example: How concentrated?
Average crust holds about 60 ppm of copper. A mine works ore that is 0.6% copper. What is the concentration factor?
- $0.6\% = 0.006 = 6,000\,\text{ppm}$ (1% is 10,000 ppm).
- $6,000 \div 60 = 100$.
Answer: About 100 times.
Worked example: Metal from ore
A mine processes 50,000 tonnes of 0.5% copper ore. How much copper does it yield, if all is recovered?
- $50,000 \times 0.005 = 250\,\text{t}$ of copper.
- The other 49,750 t become waste rock (tailings), which is why mines leave such large piles behind.
Answer: 250 tonnes.
Common mistake
It is natural to think a metal can be mined wherever it occurs. In ordinary rock it is far too dilute: getting copper out of average crust would cost far more energy and money than the copper is worth. Check the concentration factor: ore needs about 100 times the crustal average for copper, and much more for rare metals like gold.