Re-Os DEPOSITIONAL AGE CONSTRAINTS ON THE TALVIVAARA BLACK SHALE-HOSTED Ni-Zn-Cu-Co DEPOSIT

The depositional history of the Talvivaara Formation, eastern Finland is linked to an organic carbon burial event 2.09-1.98 billion years ago, known as the Shunga Event. These “Shunga” rocks record highly anoxic conditions that likely developed on a post-GOE Earth as cyanobacteria began to proliferate across shallow marine settings. These highly anoxic, and likely ferruginous and euxinic, conditions provided an environment well-suited for redox sensitive elements, such as Ni, Zn, Cu, and Co, to accumulate at the weight percent level, ultimately leading to the formation of one of the largest nickeliferous black shale deposits (Kuusilampi and Kolmisoppi ore bodies) currently in production.

Obtaining Re-Os depositional age information from the Talvivaara Formation, however, is hindered by the fact that this sedimentary unit has been overprinted by amphibolite facies metamorphism (ca. 1.89 Ga), causing sedimentary organic carbon to become graphitized, and syn-depositional pyrite to become locally sheared and desulfidized into pyrrhotite. Hitherto, it has remained unclear whether syn-depositional age information can be recovered from the Tavlivaara Formation or if the deposit will remain imprecisely constrained to 100 Myr window between ca. 2.0-1.9 Ga.

Here, we examine pyritiferous black shales from the Kolmisoppi ore body of the Talvivaara Formation to further elucidate the timing of sediment deposition and, by extension, ore formation. Drill core was obtained from less deformed parts of the Kolmisoppi ore zone. Subsampling was concentrated within the pyritiferous mudstone member and focused on regions containing minimal shearing and pyrrhotitization.

Re-Os dating results of pyrite and pyritiferous black shale from the Talvivaara Formation provide the first direct, high precision, age constraints on the Talvivaara black shale-hosted Ni-Zn-Cu-Co deposit and show that the Talvivaara Formation was deposited during the “Shunga Event”. These results are then discussed in the context of local and global paleogeography and critical mineral formation.