Xanthates, Sodium Isobutyl Xanthate (SIBX) to be exact. Originally they planned on dumping 250 million tonnes of the stuff, but after revising their processes they are now planning a "significant reduction" of that, but not specifying how much exactly. Based on that revision they were granted an environmental license in 2020.

The exact impact on marine life is speculative, but I wouldn't eat anything from that fjord or go swimming there for the next 50 years. It's pretty toxic stuff.

It breaks down in carbon disulfide, carbonyl sulfide, hydrogen sulfide and hydrogen peroxide.

Edit: Here are some sources: https://web.archive.org/web/20221230071002id_/https://www.scirp.org/pdf/gep_2022122715084300.pdf https://open.uct.ac.za/items/f198e26b-097d-47db-b064-12cd7f306de2 https://link.springer.com/article/10.1007/s11356-016-7905-5 https://www.waterquality.gov.au/anz-guidelines/guideline-values/default/water-quality-toxicants/toxicants/xanthates-2000

And info about the mine itself by Nordic Mining: https://www.nordicmining.com/operations/engebo/