JOHANNESBURG--Lonmin PLC (LMI.LN) warned Monday that it would miss full-year production targets after it was forced to close a shaft at its Marikana mine in an effort to save cash, making it more likely the miner will have to tap equity markets to avoid breaching its debt obligations.

As strikes at the mine run into a sixth week, the world's third largest platinum producer said it would put its K4 shaft, which hadn't yet ramped up to full production, on "care and maintenance" and said it would terminate a contract to staff the mine with Murray & Roberts Holding Ltd. that employs about 1,200 workers.

As a result of the move, Lonmin expects to sell between 685,000 and 700,000 troy ounces of platinum in 2012, down from the 750,000 ounces it forecast in July. The company also said that the cost of production per unit of platinum would increase by more than 8.5% due to the lower output.

Several analysts said the lower than expected output makes it almost certain that Lonmin will have to raise equity in order to shore up its balance sheet.

Lonmin has been battling to get workers to return to Marikana, its main source of production, since 3,000 rock drillers downed tools Aug. 10. Workers are demanding a salary increase to 12,500 rand ($1,520) a month, a near tripling of what many on the ground say they make now, amid widespread industrial unrest in the key South African mining sector.

Lonmin's Acting Chief Executive Officer Simon Scott, said at the weekend that the company can't afford to meet the demand, which would cost ZAR2.3 billion to implement.

"We have to acknowledge that we are running a platinum mine in the most difficult trading conditions our industry has faced," Mr. Scott said in an opinion piece published in a local newspaper Sunday. "The people who provide the capital we need to invest in our mines have been seeing negative returns."

The company already cautioned in July that, in light of weak platinum prices and lower demand, it would cut capital spending plans for the next few years, halving what it planned to spend in 2013 and 2014 to $250 million a year.

Deutsche Bank expects the company will need to raise $700 million via a rights issue, if production setbacks at Marikana persist. The final amount depends on whether platinum prices recover from current depressed levels and how quickly Lonmin is able to restore production when the strike does end, and on whether it intends to raise capital just to cover short-term debt obligations or longer term debt obligations as well.

Analysts at Investec said they are assuming the company will have to raise $500 million in 2013 at least to shore up its balance sheet.

"The company's reputation has suffered and may cloud investor sentiment for some time," the Investec mining team said.

But SBG Securities' analyst Justin Froneman said the timing of a rights issue isn't as pressing as some market participants may expect.

"They could possibly hold on for another six months or so at current rand PGM [platinum group metals] basket prices, assuming a very near term return to work by striking employees," he noted. He forecast the company may look at raising about $400 million to cover its short-term debt obligations and working capital needs.

Unions, Lonmin management and worker representatives resumed wage talks Monday. Tensions remain high at the mine after 45 people died in strike clashes, including 34 shot by police Aug. 16, and as police step up efforts to track down and arrest those they believe are leading the strike, which has also affected production at Anglo American Platinum Ltd. and Gold Fields Ltd.

Write to Devon Maylie at devon.maylie@dowjones.com

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