BHP Billiton Ltd. (BHP.AU) said Tuesday it has boosted the total ore reserves at its Olympic Dam copper and uranium mine in South Australia state, although ore grades have fallen slightly.

The miner said proven and probable ore reserves at the mine stood at 589 million metric tons as at June 30 with a copper grade of 1.81% and a uranium grade of 0.59 kilograms per ton.

This compares with 473 million tons a year ago when the grades stood at 1.86% copper and 0.6 kilograms of uranium per ton.

The new reserve consists of 188 million tons of proven reserves and 401 million tons of probable reserves.

BHP plans a major expansion of the mine, tipped by analysts to cost up to US$15 billion, to boost annual output to 750,000 tons of copper, 19,000 tons of uranium and 800,000 ounces of gold.

Olympic Dam produced 194,000 tons of copper, 4,007 tons of uranium and 108,039 ounces of gold in the year ended June 30.

The reserve upgrade, which follows a resource upgrade at the mine announced last year, was contained in the miner's U.S. annual report, released Tuesday.

In the report, the miner reiterated previous statements that the global economy looks set to remain volatile for the immediate future.

"While the global economy is showing signs of stabilizing, the large developed economies are not expected to show real growth until at least the end of 2010," Chairman Don Argus said in the report.

Argus said there is evidence of increasing stability in the U.S, U.K., Europe and Australia and that China is showing early signs of improvement, providing strong support for short-term economic growth.

"Over the longer term, we believe that emerging economies such as China and India will contribute the majority of world economic growth as they continue to industrialize, which will see demand for commodities continue to grow," he said.

-By Alex Wilson, Dow Jones Newswires; 61-3-9292-2094; alex.wilson@dowjones.com