GEOCHEMISTRY AND PALEOSEDIMENTOLOGY OF IRON SULFIDE-RICH CARBONACEOUS MUDSTONES IN DRILL CORE DDKS-010: A TIDALLY RESEDIMENTED ESTUARINE MUDFLAT ORIGIN FOR SURFICIALLY NI-ZN-CU-CO –ENRICHED PALEOPROTEROZOIC TALVIVAARA FORMATION (2.1–2.0 GA), FINLAND

The metasedimentary Talvivaara fm (2.1–2.0 Ga) in Sotkamo, Finland, hosts one of the largest nickel deposits in Europe. We report here the preliminary results of our ongoing studies, which contradict the current genetic model relating the elevated Ni-Zn-Cu-Co –content of the Talvivaara "black shales" to hydrothermal fluids. Drill core DDKS-010 (476.40 m) from the Kolmisoppi orebody was analyzed as detailed M.Sc. project, also additional high-resolution isotopic and trace element data was obtained. The section from 476.40 m to 390 m forms stratigraphically nearly continuous well preserved succession, allowing an integrated study that has not previously been viable.

The exact depositional age is not tightly constrained, but the TV fm is correlated with the voluminous accumulation of C_org-rich sediments (the "Shunga event") immediately following the Lomagundi–Jatuli -isotopic excursion. Also, the regional post-Jatulian basin-margin iron formations of the Kainuu Belt display in several localities laminated sulfide-facies units that likely correspond closely in time with the C-S-Fe –rich TV fm.

We document rhythmic, mainly tidally pulsed to massive fluid-mud deposition combined with river-plume sedimentation for the majority of the metalliferous organic-rich mudstones–siltstones of the TV fm. Thicker, graded mass-flow interbeds of fine to coarse sands are common, also material from nearby siliciclastic intertidal flat is positively identified. The highest "primary" Ni-Zn-Cu –concentrations (Ni up to 5226–5740 ppm) occur in most undiluted graded-laminated fine clay- and organic-rich pyrrhotitic mudstones. A classification scheme was established for the various sulfidic mudstone and claystone lithofacies.

Our results indicate a mud-dominated, near-estuarine subtidal environment in a narrow unrestricted craton-margin seaway. All element behavior and ratios feature paleomarine hydrogenous trends, and the organic productivity is attributed to riverine P and nutrient loading. High Ni-Zn-Cu levels correlate with interpreted Fe-Ti –rich smectitic protolith clay component, likely sourced from weathered continental basalts and hydrodynamically sorted to proximal offshore zone with deeper ferruginous watermass, euxinic chemocline and shore-sourced replenishment of oxic, sulfatic water.