Copper - keeping up with demand China's fascination with resource-rich Africa's buried treasure knows no bounds. And if the metal is red, so much the better: China is the world's biggest consumer of copper. Last week Discovery Metals, whose main asset is a copper mine in Botswana, brushed off a A$830m (US$850m) offer from private equity group Cathay Fortune. The Australian exploration company wisely left the door open for a higher offer. It must recall the generosity of Minmetals Resources, which paid C$1.3bn in February for Toronto-listed Anvil Mining, a miner in Democratic Republic of Congo. Copper has recovered from its midyear funk to trade around $8,100 a tonne. One reason for remarkable resilience when demand is subdued is that liquid stocks of the metal hit a historic low (put at 2.6 weeks of consumption, by IntierraRMG) in the third quarter. But copper's price is not just a bet on Chinese demand - or monetary easing programmes. Supply - constrained variously by bad weather, earthquakes, permits, and declining grades from old mines - also plays a part. The spat between Anglo American and Codelco did not affect supply, but labour disruption in Chile did. The world's top 10 mines produced 600,000t less copper last year than in 2010, Rio Tinto notes. It expects industry-wide mine closures to cut copper output by 1.8mt in the next five years, and declining grades by a further 1.6mt over the same period. In other words 3.4mt of new production, equivalent to about a fifth of this year's output, will be required to stand still. Demand in the longer term will be supported by growing electrification in emerging markets in Asia, and motor industry growth in China and India. The outlook emboldened some miners (Anglo, Rio and Xstrata) to go ahead with development projects; others (BHP Billiton) slowed them, amid fears of a looming glut. Either way, investors should prefer names with access to high-grade ore, and lower cash costs. Wood Mackenzie data put Rio, Freeport-McMoRan and BHP Billiton at the low end of the cost curve. New projects for Xstrata could reshuffle the pack. As ever, the real winners will be those suppliers that best gauge demand.