(Sharecast News) - BHP Billiton has agreed to buy Guyana Goldfields' 6.1% interest in SolGold, which is the majority owner and operator of the Cascabel porphyry copper-gold project in Ecuador, for 26.592p per share.The price represents a 20% premium to the 20-day volume-weighted average LSE price of 22.16p per share.Under the terms of the agreement, BHP will buy 103.1 million shares in SolGold, which is listed on the LSE and the Toronto Stock Exchange.BHP's chief executive officer Andrew Mackenzie said the investment will give the company exposure to a high quality copper exploration project in Ecuador, which is a highly prospective location for BHP."Consistent with our positive long-term outlook, copper is a key exploration focus for BHP as we seek to replenish our resource base and grow this important business," he said.