"This unsolicited development, of which we had no prior notice, reinforces our view of the long-term value of Rio Tinto," says Rio Tinto Chairman Paul Skinner.

Alcoa said it will contribute up to $1.2 billion to the roughly $14 billion total investment.

The surprise move may throw a roadblock in the attempts by Australia's BHP (BHP - Cramer's Take - Stockpickr) to follow through on its bid for Rio Tinto, made last November.

The mining patch has seen a flurry of merger activity over the past few years with Freeport-McMoRan Copper & Gold (FCX - Cramer's Take - Stockpickr) purchasing Phelps Dodge, Companhia Vale do Rio Doce (RIO - Cramer's Take - Stockpickr) gobbling up Inco and Rio Tinto buying Alcan.

Shares of BHP were rallying about 8%, while those of CVRD were ahead by 4.5%. Alcoa was adding 1%, and Aluminum Corp. of China was up 11%.