When Rio Tinto chief executive Jean-Sebastien Jacques answers the constant questions over whether he will make a copper acquisition, he typically points to the high prices paid in recent years for mines such as Chile's Zaldivar and Africa's Tenke as proof there are no bargains in the copper market.

But there's an exception to every rule, and right now the market is valuing a stake in one of the world's most prospective copper and gold deposits at about 12 per cent of book value.