According to CNBC, Northern Star Resources shares jumped more than 9% Monday after the Australian gold miner rejected a $27 billion takeover proposal from South Africa's Gold Fields, saying the offer undervalued the company. Gold Fields proposed acquiring 100% of Northern Star with 0.3125 Gold Fields shares and A$7.25 in cash for each Northern Star share, according to a Northern Star statement Monday. The proposal, received Sept. 14, initially valued Northern Star at A$38.7 billion ($27.15 billion), a 22% premium to its Sept. 11 close, but the implied value had fallen to A$36.1 billion based on Gold Fields' closing share price on Friday, Sept. 25. Northern Star said its board unanimously rejected the bid, calling it materially undervalued and highly opportunistic. Chairman Michael Chaney said Gold Fields was trying to buy one of the world's premier gold portfolios at a price that fell well short of its fundamental value and at a highly opportunistic time. The miner also said most of the consideration would be paid in Gold Fields shares and that the proposal was subject to several conditions. Northern Star told Gold Fields on Friday that its board did not consider it appropriate to engage further on the proposal.