Under the deal, OceanaGold has agreed to pay 0.04006 of an OceanaGold share for each of the just over 80 per cent of Pacific Rim shares it doesn't already own.
It said the deal provides Pacific Rim shareholders with a 50 per cent premium over the company's 20-day volume weighted average trading price on the Toronto Stock Exchange and a 73 per cent premium to Monday's closing price of 3.5 cents per share.
On Tuesday, those shares were up two cents, or 57.14 per cent, to 5.5 cents in midday trading in heavy volume of 5.5 million shares.
OceanaGold stock moved down seven cents, of 4.64 per cent, to $1.44.
Pacific Rim is a mineral exploration company whose primary asset is the advanced-stage El Dorado gold deposit in El Salvador, where the company also owns several grassroots gold projects.
The company is in the final stages of arbitration with the International Centre for the Settlement of Investment Disputes over the "passive refusal" by El Salvador to issue a decision on Pacific Rim's application for environmental and mining permits for El Dorado.
OceanaGold has mines in New Zealand and in the Philippines.