Rio’s offer for Riversdale is its first attempt at a sizeable acquisition since a disastrous purchase of Alcan in 2007. Rio overpaid greatly for the Canadian aluminium producer and the debts it ran up to finance the deal, just as the credit crisis hit, were nearly fatal for the company. It has taken Rio a long while to repair the damage to its balance-sheet to the extent that it can again try to grow by acquisition.

The deal also shows the allure of coal, despite concerns over carbon emissions and the environment. Coal prices are surging. China’s hunger for energy turned it into a net importer in 2009 of the thermal coal that fuels power stations, a trend that is set to continue. And steelmakers, fearful of the rising prices of raw materials, are hunting for captive sources of metallurgical (coking) coal as well as iron ore.

Booming commodity prices are a burden as well as an immense boon for big miners. Rio, BHP Billiton, Vale and others are planning on vast increases in capital spending in the coming years, and have the bumper profits to finance it, yet there is only so much fruitful investment to be made in their current mines. As for the idea of growing through mega-mergers, Rio’s Alcan blunder and BHP’s recent rebuff by Canada’s government, in an unexpected show of resource nationalism over a $40 billion offer for PotashCorp, illustrate the difficulties of such a strategy.

Not all miners reckon that attempting mammoth mergers is the best use of their bulging wallets. Yet the remaining option of pursuing smaller, bolt-on acquisitions comes with problems too. Firms of the size of Riversdale are small enough for any number of potential bidders to be able to contemplate buying them. Anglo American has eschewed mega mergers (though it has itself been a bid target) but it now wants to expand beyond politically troublesome South Africa, where many of its mines are located, and is not averse to buying smaller targets. Thus it is said to be contemplating a rival bid for Riversdale. Switzerland’s Xstrata and Brazil’s Vale are also said to be interested.

Other firms that do not count mining as their main business are also on the prowl. India’s Tata Steel is considering its options, though its heavy debts may deter it from bidding. ICVL, a consortium made up of several state-run Indian steel companies with the aim of scouring the globe for desirable coal mines, has also been mooted as a potential bidder for Riversdale. And China, in the shape of Wuhan Iron & Steel, may yet show its hand even though the Chinese state-run firm’s plans for a big investment in one of Riversdale’s mines recently fell through.

So Rio and its many potential rivals could become involved in bidding war for a medium-sized mine in a part of Africa that, though not as troubled as it once was, is still far from risk-free. They should get used to it. In the quest for growth, big miners will increasingly be obliged to do battle with foes old and new in parts of Africa and elsewhere that are as forbidding as the deepest mineshafts.