Chinalco-Alcoa stake is blocking stake -BlackRock

LONDON Feb 1 (Reuters) - BlackRock (BLK.N) fund manager Graham Birch said on Friday the purchase of a 12 percent stake in Rio Tinto (RIO.AX) (RIO.L) by Alcoa (AA.N) and Chinalco was a blocking stake against BHP Billiton's (BLT.L) BHP.AX> offer.

"It has all the appearance of a blocking stake ... It stops anyone going to 100 percent as you can't squeeze minorities out," Birch told Reuters.

"BHP is probably mightily pissed off. I don't know what they will do ... It's timed to be a bit of spitting in BHP's soup."

Birch also said he had sold part of his holding in Rio Tinto to Alcoa and Chinalco at 60 pounds a share as "it made sense for our funds to participate".

BlackRock is a major shareholder in both Rio and BHP Billiton, which has made a three-for-one share proposal for Rio. (Reporting by Laurence Fletcher; Editing by Quentin Bryar)