(Adds health minister paragraphs 10-13, Chemaf paragraph 18)

By Joe Bavier

KINSHASA, Nov 9 (Reuters) - Congolese authorities arrested six people in connection with the dumping of tonnes of highly radioactive minerals into a river near the southeastern town of Likasi, the environment minister said on Friday.

A quarantine zone was set up around the site, just 10 km (6 miles) from the mining town of 300,000 people. Tests on the river banks on Thursday revealed radiation levels nearly 50 times the limit for mineral exports from Democratic Republic of Congo.

Congo launched an inquiry on Wednesday after officials in Katanga province said radioactive copper and cobalt ore appeared to have been dumped into the Mura river, a source of drinking water for Likasi.

Authorities in Likasi had seized nearly 19 tonnes of radioactive minerals due for export in October and ordered their disposal at a nearby abandoned uranium mine last week.

The load never made it to the mine and the government says at least some ore was dropped from a bridge into the river.

"The entire commission charged with disposing of these minerals is now under arrest. There are six people now in custody, and I expect a seventh to be arrested today," Environment Minister Didace Pembe told Reuters on Friday.

Pembe took a team of experts from the Environment Ministry and Congo's Atomic Energy Agency to the site on Thursday and said he would report his findings to the prime minister.

"Nineteen tonnes would be a small mountain. We did not see that. All of the minerals were not dumped. That is sure," Pembe said.

The extent of contamination would be made public over the weekend and national copper mining company Gecamines had been asked to begin a cleanup of the dumping site, he said.

"The damage is enormous," Pembe said. "We've asked the population not to use the water from the river for consumption by either people or animals."

HEALTH RISKS

Congo's Health Minister Victor Makwenge Kaput said it was still too early to say if residents' health had been affected, and much would depend upon levels of exposure to radioactivity.

"It is very difficult to establish the links between radioactivity and pathologies. I will take weeks, months, or maybe years before the effects are seen," he said.

Police in Katanga were trying to locate the truck used in the dumping on Friday and Pembe said efforts were under way to trace the missing minerals.

A report by the Likasi mayor's office said some 17 tonnes of the minerals confiscated were destined for Chinese firm Magma.

Inspectors seized smaller amounts from local company Chemaf and a mineral broker based in Katanga's capital, Lubumbashi.

A Chemaf representative told Reuters by phone on Friday that the seized minerals had never belonged to the company.

Executives at Magma could not be reached for comment.

Ore mined in Katanga, home to one of the world's richest belts of copper and cobalt, habitually contains trace amounts of uranium, which Congo is currently banned from exporting.

Congolese officials said the dumped materials were believed to come from the nearby Kolwezi area, home to projects by several foreign mining groups including Katanga Mining <KAT.TO>, Nikanor <NKR.L> and Freeport-McMoRan Copper & Gold <FCX.N>. (Editing by Daniel Flynn and Philippa Fletcher)

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