July 21 (Bloomberg) -- Zambia's main mineworkers union said it opposes Vale SA's plan to invest $400 million in the development of a copper mine in a venture with African Rainbow Minerals Ltd. at Konkola North.

The Mineworkers Union of Zambia is concerned about the possible treatment of workers by Vale, Rayford Mbulu, the union's president, said in interview from the northern town of Kitwe today. A Vale spokeswoman declined to comment and said the investment is yet to be approved by the company's board. She declined to be identified in line with company policy.

"We have the powers to block any investment from coming into the country because we are internationally affiliated and we have information that their record is bad," Mbulu said, without giving more detail.

The Konkola North mine, located north of the Konkola Deep mine operated by Vedanta Resources Plc, will start production in 2013. Initial output is estimated at 50,000 metric tons a year, which will later be expanded to 100,000 tons. Zambia is Africa's biggest copper producer.