MOSCOW — A river in the far north of Siberia turned bright red this week, and in the absence of any official explanation, Russians have taken to calling the eerie flow the "blood river."

One hint at the possible cause is the path the river, the Daldykan, takes past the Norilsk Nickel mine and metallurgical plant, by many measures one of the world's most polluting enterprises. The plant belches so much acid-rain-producing sulphur dioxide — 2 million tons a year, more than all of France — that it is surrounded by a dead zone of tree trunks and mud about twice the size of Rhode Island.

The metal smelters in this Russian Klondike produce copious amounts of copper, one-fifth of the world's nickel, a key alloy of stainless steel, and half of the global supply of palladium, a precious metal nearly as valuable as platinum.

The ore also contains iron, but that red-hue element is far less valuable than the precious metals extracted along with it, and is generally discarded in slurry ponds.

Which is the most likely source of the discoloration in the "blood river," environmental groups and Russian environmental regulators say, with the red hue coming from iron oxide, or rust.

That conclusion was firmed up Wednesday when the Ministry of Natural Resources issued a statement saying that "preliminary information of a possible cause of the pollution is a rupture of a slurry pipe at Norilsk Nickel."

If so, while certainly shocking, the nearly iridescent water would be mostly harmless to people, said Vladimir Chuprov, a researcher with Greenpeace in Russia. High concentrations could kill fish, however.

The red colour could also be, well, something of a red flag, for the iron slurry seems likely to be accompanied by heavy metals created by the Norilsk smelters, which could damage the fragile Arctic environment, Chuprov said.

President Vladimir Putin "promised that industrial development in the Arctic would progress with the utmost care," Chuprov said in a telephone interview. "Unfortunately, these words are only a formality."

Managers at the factory told Russian state news agencies they had reduced output at one smelter as a precaution but had found no leaks. In fact, the company said, "as far as we know, the colour of the river today is no different from usual."