The Silangan copper and gold mine could be Philex's biggest source of revenue after its 61-year-old Padcal mine in the north is expected to close in 2022.

Silangan was originally slated to begin production by 2018, but that has been set back by a ban on new open-pit mining introduced in 2017 as the government in one of the world's top copper, gold and nickel producers tries to step up environmental protection.

Open-pit extraction has been used by many miners in the Philippines, but is blamed for massive environmental destruction in some areas.

Philex said in a statement on Friday that it was now looking at an infrastructure design that would allow it to use underground mining to extract ores.

"We are currently working on securing all requisite permits and approvals to operate Silangan," said Chief Executive Officer Eulalio Austin.

"We look forward to realising the massive potential of a project of this magnitude."

Philippine President Rodrigo Duterte has warned he might declare a total ban on open-pit mining as he ordered mining companies to reforest denuded sites. The existing ban also covers the $5.9 billion Tampakan copper-gold project in southern Mindanao island.

Silangan consists of three deposit areas - Boyongan, Bayugo and Kalayaan - with the latter a joint venture with Manila Mining Corp. Boyongan is expected to be the first to operate, by 2022.

Silangan's development was previously estimated to cost $1.2 billion, based on open-pit extraction. Philex did not give any new estimate for the cost of the project.

Padcal accounted for 9 percent of gold output in the Philippines last year at 29,782 kilograms, and 20 percent of its copper concentrate production at 282,391 tonnes.

(Reporting by Enrico dela Cruz; Editing by Joseph Radford)