Videos published on Sunday by Russia's Novaya Gazeta newspaper showed foaming liquid being pumped into wild tundra, described by Norilsk Nickel as "purified water" from a reservoir at a minerals enrichment plant.
The fluid was tainted with heavy metals and acids, asserted the newspaper.
As investigators arrived, the pipes were "hastily" removed, said Novaya Gazeta, and a car that had delivered officials to the scene was partly squashed — accidentally, without injury — by a heavy earthmoving machine.
Up to that stage, 6,000 cubic meters of liquid had been dumped over "several hours," reported the Russian news agency Interfax.
Rules are 'violated'
In a statement later Norilsk Nickel cited a "flagrant violation of operating rules," adding that it had suspended employees responsible for the discharge.
An internal investigation was underway, said concern spokeswoman Tatiana Yegorova. Russia's Investigative Committee, which probes serious crimes, said it too had opened an inquiry.
Russia's natural resources agency said the fluid from its Talnakh enrichment plant, near the city of Norilsk, was discharged after heavy rains that had left its reservoir nearly full.
'Crime against Nature'
Local emergency services claimed the wastewater was not likely to have reached the nearby Kharayelakh river.
Vassili Ryabkin, a former employee of the local environmental protection authority, described the fluid's discharge as "a crime against Nature and our children."
Arctic diesel spill unprecedented
Last month, 21,000 tons of diesel spilled from a fuel storage facility of a Norilsk Nickel subsidiary, seeping into the soil and polluting the Ambarnaya River.
Environmentalists said it was the worst such accident in the Arctic, with clean-up efforts hampered by swampy ground and a lack of access roads.
At the time, Russian President Vladimir Putin ordered a state of emergency. Concern oligarch Vladimir Potanin promised to pay the costs of the clean-up.
Norilsk Nickel blamed permafrost melt due to climate warming for the fuel tank collapse.
WWF expert Alexei Knizhnikov said a missing containment structure required by law would have kept most of the diesel spilled on site