SYDNEY—South32 Ltd., the new global miner spun off by BHP Billiton Ltd., on Wednesday flagged write-downs mostly against its manganese operations that could be worth around 1.9 billion U.S. dollars.

The company, carved out of BHP to house assets including manganese mines and aluminum smelters, said it had been notified by its former parent that pretax impairment charges totaling that amount had been marked against the South32 assets in May, when the new miner was spun off. South32 said it was conducting its own review of the value of its assets, and would "keep the market informed should there be a material difference to those figures provided by BHP Billiton."

The demerger of South32, which is due to release its maiden financial result next month, was one of the largest corporate breakups in mining history.

The miner listed on the Australian stock market two months ago with a market value of 11.3 billion Australian dollars (US$8.4 billion). But tumbling commodity prices-and, more broadly, an aversion for risk among investors due to fears over the outlook for China and Greece, have since dragged that lower to A$9.7 billion.

Prices for many commodities, including some industrial metals and coal, have plunged to multiyear lows as a steep selloff in China's stock market magnified investor concerns about weaker demand from the world's largest consumer of many raw materials. Their value has also been knocked by expectations the Federal Reserve will raise borrowing costs in coming months, a move that investors expect to further boost the dollar and pressure the prices of commodities, which generally are priced in the U.S. currency.

Some analysts worry the prospects for South32's industrial commodities have deteriorated as China's economy slows.

South32 said BHP's assessment of impairments in May had included a US$445 million charge against its Australia manganese business, an US$882 million charge against its South Africa manganese operations and a US$539 million charge against the Wolvekrans Middelburg complex, in its South Africa energy coal unit.

Still, South32 highlighted the value of the manganese assets had been lifted by US$2.1 billion earlier in the year.

Its Australian shares recently traded down 0.8% at A$1.81.

South32 was spun off with a balance sheet that included little debt, which executives say will help the miner weather weak commodity markets. On Wednesday, Chief Executive Graham Kerr restated South32's commitment to investor returns.

"We have the right balance sheet for the times and intend to distribute a minimum 40% of underlying earnings to our shareholders from next year," Mr. Kerr said.

In an operational report for the three months through June, the miner recorded a 1% on-quarter increase in alumina output, to 1.3 million metric tons, and a 23% jump in metallurgical coal production to 2.0 million tons. Manganese alloy output was down 26%, however, at 81,000 tons, while thermal coal production fell 5% to 8.9 million tons.

Write to Rhiannon Hoyle at rhiannon.hoyle@wsj.com

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