By Wendell Roelf

CAPE TOWN (Reuters) - Shares of ArcelorMittal South Africa dropped more than 6 percent on Wednesday after the government said it would tax carbon emissions from 2015, sparking concern heavy polluters could see their earnings hit.

As the continent's top greenhouse gas emitter, South Africa is looking to curb emissions, although it also plans some exemptions to protect industry, Finance Minister Pravin Gordhan said during his 2013 budget speech.

ArcelorMittal South Africa, a unit of the world's biggest steelmaker and one of the biggest polluters in the country, finished down 6.1 percent at 28.83 rand following the speech, becoming the biggest percentage decliner on Johannesburg's All-share index.

A carbon tax will add to the woes of the steelmaker, which has been struggling with lower domestic sales, high input costs and a fire at one of its plants.

"They have been spending heavily on curbing pollution in recent years, but there's probably still a way to go," said Stephen Meintjes, an analyst at brokerage Imara S.P. Reid in Johannesburg.

The tax, set at 120 rand ($14) per tonne of carbon dioxide equivalent, has been criticized by carbon-intensive companies such as ArcelorMittal and petrochemicals giant Sasol.

Shares of Sasol, the world's top producer of motor fuel from coal and one of the heavyweights on the Johannesburg Stock Exchange, fell nearly 2 percent.

Some of the exemptions proposed by the Treasury include a 60 percent tax-free threshold until 2020 on annual emissions for all industries, including electricity, petroleum, iron, steel and aluminum.

Companies will also be able to claim additional relief of up to 10 percent by investing in external green projects to reduce their carbon tax liabilities.

"It is a mixture of the ugly, the good and the uncertain," said Mike Rossouw, chairman of the Energy Intensive Users Group, an industry body. Continued...