Shares in Sibanye-Stillwater slumped more than 11% on Tuesday as its CEO confirmed the South African company is pressing ahead with a bid for Zambia's Mopani Copper Mines after a 37% slump in first-half profit.

The Johannesburg-based precious metals producer is on the investor shortlist of potential buyers for the state-owned copper mines. The Zambian government had expected to have chosen the winning bidder by the end of July.

Sibanye may bring in a partner at Mopani and the outcome of the bidding process would be known in about three weeks, CEO Neal Froneman said on a conference call.

The Mopani assets require a significant amount of investment but that's spread over a number of years, Froneman said.

"We prefer working with partners and prefer earning-in so don't factor into your thinking a large capital outlay for Mopani," he said.

The South African platinum mining giant is seeking to expand in metals that are key to the green revolution. It has already bought lithium and nickel assets in Europe and the U.S. while its domestic operations continue to be hit by rolling electricity blackouts and rising crime.

The process to select a new investor is competitive and Sibanye and one another company are the only bidders remaining in the process to acquire Mopani, Froneman said.

Sibanye's Johannesburg-listed shares had slumped by more than 11% by 1351 GMT.

The miner's push to acquire the copper assets in Zambia came despite its profit in the six months to June 30 declining 37% to R7.8-billion amid operational challenges at some gold and platinum mines and lower metal prices, it said on Tuesday.

Platinum and palladium output at its Stillwater operations in the US that were previously hit by flooding dropped by 11%.