Newswire Reuters did not identify the buyer and Xstrata has declined to comment.
News of the alleged offer saw Xstrata's share climb as much as 3.4% in London on Thursday.
It comes on the back of Xstrata's rejection of Vale's informal offer of around £40 per share, with news out yesterday that the Anglo-Swiss miner and Glencore were chasing at least £50 per share, valuing the bid at around $US95 billion ($A106 billion).
Brazil company Vale confirmed last month that it was in discussions with Xstrata over possible acquisition opportunities.