Kirsti Loukola-Ruskeeniemi, Helsinki University of Technology (Finland)
Lisa M. Pratt, Indiana University (United States)
Timo Heino, Geological Survey of Finland (Finland)
Jana Hladikova, Czech Geological Survey (Czech Republic)
Sulfur isotope compositions of black shale and sulfide ore were compared at the Palaeoproterozoic Talvivaara and Outokumpu deposits, eastern Finland. The Talvivaara Ni-Cu-Zn deposit is hosted by metamorphosed black shale (black schist) and contains more than 300 Mt of low-grade ore. Exploitation of the Talvivaara ore will start in the near future. The Outokumpu Cu-Co-Zn-Ni-Ag-Au deposits were mined for decades. The total production of three major deposits was 50 Mt ore averaging 2.8 wt. % Cu. The sulfide ores at the Outokumpu-type deposits are hosted mainly by quartz rock, but also by calc-silicate rock and black schist.
Sulfur isotope δ34S values (CDT) were determined for pyrite, pyrrhotite, sphalerite, chalcopyrite, and alabandite. Sulfur isotope values in the Outokumpu ore ranges from -19.2 to +5.8 per mil, with an average of -3.5 per mil (258 samples: Bulletin of the Geological Survey of Finland 267). A detailed study at an Outokumpu-type occurrence at Kylylahti revealed that S isotope values in S-rich fragments in deformed black schist are comparable to those of the pyrite-rich ore: from -5.4 to -10.5 per mil in the black schist and from -5 to -16 per mil in the ore (Econ. Geol. 94, 1007-1028).
Sulfur isotope values in the Talvivaara deposit in two drill cores (3344: 308 and 329) range from -14.3 to +20.8 per mil (150 determinations, Econ. Geol. 91, 80-110). A detailed S isotope study from a drill core sample from the northern Kolmisoppi orebody revealed that S isotope value of spheroidal pyrite (<0.01 mm) varied from -7.1 to -6.4, that of recrystallized coarse-grained pyrrhotite and pyrite from+0.2 to +0.4, and pyrite dissemination exhibits -2.9 per mil (Geological Survey of Finland, Special Paper 20, 31-46). Previously unpublished S isotope data from Talvivaara reveal also more negative and more positive S isotope values: the minimum value for fine-grained pyrite obtained was -22.53 per mil and the maximum value for pyrrhotite +38.50 per mil.
The S isotope values of the Outokumpu and Talvivaara ores suggest bacterial reduction of seawater sulfate, with addition of hydrothermal S. Sulfides from eleven core samples of Talvivaara and Outokumpu black schist were recently analyzed in detail in the University of Indiana. The samples represent different parts of the sedimentary-hydrothermal system in terms of the relative importance of original hydrothermal minerals, submarine alteration, and biological activity. The S isotope results support a genetic link between sulfide ore and associated black shale, not only at Talvivaara but also at the Outokumpu deposits.