The venture, known as PT ChengMach Nickel, will produce 40,000 tonnes per year of nickel matte on a metal content basis, Chengtun said in a filing. Chengtun unit Hongcheng International will hold 70% equity, with the remainder held by a Singapore-registered company called Extension Investment Pte.

Total investment is pegged at $350 million, including Extension's share.

The venture will be Chengtun's second nickel matte project in Indonesia and - like the first, with capacity of 34,000 tonnes per year - will be located in Weda Bay on the island of Halmahera.

Nickel matte, an intermediate product that can be further processed into battery-grade nickel sulphate, is growing in popularity among Chinese investors in Indonesia.

Top global nickel producer Tsingshan Holding Group in March took the market by surprise when it announced plans to produce the material at scale.

Extension Investment Pte's sole shareholder is a company called Indigo International Investment Ltd, the Chengtun filing said. Indigo was last month also named as the partner in another 40,000 tonnes per year nickel matte project being pursued by Weiming Group

(Reporting by Tom Daly; Editing by Mark Potter)