Primary metal enrichment and metamorphism at the Talvivaara Ni-Zn-Cu-Co deposit, Finland

The 2 billion tonne, black shale hosted Talvivaara Ni-Zn-Cu-Co deposit appears unique with regards to its metal endowment and formation. Denoting the Talvivaara-type deposits, there are no known analogues of this metal-rich black schist which appear to match its significant size and metal enrichment. We present whole rock, in-situ trace element data and δ³⁴S isotope analysis of sulfides from throughout the stratigraphy of the deposit. We propose an alternate model from those previously published, suggesting that the metal enrichment can be linked to redox chemistry and variations in the atmosphere-seawater cycle. Our data shows that the early, syn-sedimentary/diagenetic pyrite is metal-rich and can result from simple weathering of metal-rich crust and deposition in an anoxic basin were bacteria and sulfide formation were prevalent. Metamorphism subsequently caused the recrystallization of some pyrites and liberation of trace elements from early sulfides and organic matter to form distinct sulfide phases. We argue that the typical processes by which black shales become enriched in metals during Phanerozoic time was similar to that which deposited the Talvivaara shales, however, Talvivaara developed on a more profound scale during a period of perturbation in many geochemical cycles.