Randy Fabi and Jonathan Thatcher, Jakarta – Indonesia's ban on shipping unprocessed mineral ore should cut export revenue by no more than 10 percent next year, the country's investment chief said, as the controversial rule is relaxed to limit the impact on Southeast Asia's largest economy.

The estimate, which could mean a loss of at least $470 million based on data on the top three metal exports, is the first a senior government official has publicly given on the potential economic impact of revisions to the mineral export ban.

Policymakers, grappling with an economic slowdown and a widening current account deficit, have been under pressure to water down the ban that threatens to halt billions of dollars worth of metal exports once it comes into effect in mid-January.

Uncertainty over how the final law will be enforced by Indonesia, the world's top exporter of nickel ore and refined tin, has hurt the country's credibility with foreign investors and rattled metal markets.

"We only see a slight impact (on metal export revenue from the ban)," Mahendra Siregar, chairman of the Investment Coordinating Board, said in an interview. "In total, a 10 percent decline would be the maximum."

The total value of the top three metal ore exports – nickel, copper and bauxite – was nearly $4.7 billion last year, according to Bank Indonesia.

Siregar said the ban would mainly impact shipments of copper and gold. Indonesia's major copper producers include U.S-based Freeport McMoRan Copper & Gold Inc and Newmont Corp, while the gold industry has state-owned Aneka Tambang and Australia's Newcrest Mining.

Export exemptions

Analysts believe that Indonesia does not have the capacity to process the enormous amount of ore it produces, and so the ban would strangle exports of minerals such as nickel.

The mining law was first passed in 2009 in a government bid to seize more control over Indonesia's natural resources.

But the government is close to finalising agreements with at least 12 companies that would allow them to continue shipping unprocessed ore after January's ban, said Siregar.

The dozen firms, which he declined to disclose, have already completed domestic mineral processing facilities or were close to completion.

Government officials said they expected an announcement of the changes once agreements were reached with a few more companies.

"The government plans to revise some of the regulations, some of the rules that apply to (the 2009 mining law)," Susilo Siswoutomo, deputy energy and mineral resources minister, at an investment conference on Thursday. "We are very confident that by the end of the year we will be able to do that."

Indonesia has received 28 "very serious" proposals to build smelters, some of which could be operational as early as next year, Siswoutomo said. There were 97 other proposals that were only at the stage of feasibility studies and were being reviewed by the government.

[Additional reporting by Fergus Jensen; Editing by Ed Davies.]