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 also set up around the site, just 10 km (6 miles) from the mining town of 300,000 people, where tests 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 the southeastern province of Katanga 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 of the 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, who headed a team of experts from the Environment Ministry and Congo's Atomic Energy Agency who visited the site on Thursday, said he was due to report his findings to Prime Minister Antoine Gizenga on Friday.

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

"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."

Though the team of experts had confirmed some radioactive minerals had been dumped, the minister said he did not believe all 19 tonnes were in the Mura.

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

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.

According to a report by Likasi's mayor's office, some 17 tonnes of the minerals seized because of their extreme radioactivity were destined for Chinese firm Magma.

Inspectors confiscated smaller amounts from Congolese company Chemaf and a mineral broker based in Katanga's capital, Lubumbashi. Executives at Magma and Chemaf 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, Nikanor and Freeport-McMoRan Copper & Gold. (Editing by Daniel Flynn and Mary Gabriel)

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