| Summary: | The 2 billion tonne, black shale hosted Talvivaara Ni-Zn-Cu-Co deposit appears unique withregards to its metal endowment and formation. Denoting the Talvivaara-type deposit(Loukola-Ruskeeniemi, 1991) there are no known analogues of this 2.0 Ga metal-rich blackschist in terms of its significant size and metal enrichment. We present whole rock, in-situ traceelement data and δ 34 S isotope analysis of sulfides from throughout the stratigraphy of thedeposit. We propose a model suggesting that the metal enrichment can be linked to redoxchemistry and variations in the atmosphere-ocean cycle. Ore at Talvivaara is mainly associated with fine-grained pyrite, pyrrhotite, with pentlandite, aswell as accessory sphalerite and chalcopyrite. The sequence was metamorphosed during theSvecofennian Orogeny (~1.9- 1.8 Ga). Using LA-ICP-MS we have been able to track elementbudgets within pyrites and pyrrhotites through the metamorphic sequence. Extremely elevatedvalues of nickel and other metals (Zn, Cu, Pb, Se) are associated with early stage, synsedimentary-diagenetic pyrite, and organic matter. δ 34 S isotopic values of the pyrite suggest it isderived via bacterial reduction of seawater sulfate and this signature is common for most of theore minerals. This implies large-scale metal enrichment during sedimentation. Metamorphismsubsequently caused the recrystallization of some pyrites and liberation of trace elements to formdistinct sulfide phases i.e sphalerite and chalcopyrite. We argue that the typical processes by which Phanerozoic black shales become enriched inmetals during Phanerozoic time were similar to that which deposited the Talvivaara shales,however, Talvivaara developed on a more intense scale, in a particularly favorable time period.Talvivaara shales were deposited around 2 Ga, a period marked by a large carbon isotopeexcursion, increased bioproductivity and deposition of vast quantities of organic-rich shale(Melezhik, et al., 2013). Conditions during this time would be conducive to the hyper-enrichedblack shale deposition that formed this extraordinary ore. The rise of oxygen (GOE) would haveled to enhanced oxidative weathering of the Fennoscandian Shield, flooding the ocean withdetritus, where trace elements acting as essential nutrients, drove high biological productivity.Decaying organisms, along with a strong redox gradient between the oxygenated surface andanoxic deep waters, helped to deliver a vast quantity of metals to the seafloor and facilitatedconditions for their accumulation.
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