Geochemistry and genesis of the black shale-hosted Ni-Cu-Zn deposit at Talvivaara, Finland

Abstract

The Talvivaara deposit is hosted by metamorphosed black shales (black schists) and contains 300 million metric tons (Mt) of low-grade ore averaging 0.26 percent Ni, 0.14 percent Cu, and 0.53 percent Zn. As a result of tectonic deformation, the horizon containing > or = 0.8 percent Ni-Cu-Zn is up to 330 m thick. Talvivaara is situated in the Early Proterozoic Kainuu schist belt, which consists mainly of metasediments, with discontinuous serpentinite remnants 1.97 to 1.96 Ga in age. Serpentinite-associated Cu-Co-Zn-Au-Ni deposits and prospects of the Outokumpu type are present in the Kainuu-Outokumpu region; talc-carbonate rocks (altered serpentinites) are also found in the Talvivaara area.Pyrite and pyrrhotite are the dominant sulfide minerals in the Talvivaara deposit. Pyrite, pyrrhotite, chalcopyrite, sphalerite, alabandite, and pentlandite occur both as fine-grained disseminations ( 1% and S > 1%) were divided into three groups according to their Ni and Mn values: low Ni-Mn (Ni or = 0.1% and Mn or = 0.8%). Black calc-silicate rocks (Ca > or = 3.5%) are encountered as intercalations 10 cm to 3 m thick within the black schists. Even the so-called low Ni-Mn black schists at Talvivaara have anomalously high median C graphite , S, Ni, Cu, and Zn values compared with black schist intercalations within mica schists elsewhere in eastern Finland. The median C graphite values in the Talvivaara black schists are 7.1 to 7.7 percent for all three black schist types, and C graphite does not correlate significantly with any other element studied. Median S/C graphite ratios are 1.0 to 1.7 for the low Ni-Mn and Ni-rich black schists and 2.3 for the Mn-rich black schists. Black schists are also characterized by higher Al, Au, B, Ba, Fe, Hg, K, Li, Mo, Na, Pd, V, Zr, and rare earth element (REE) concentrations and by lower Ca, Mg, Ag, and F values compared with the intercalated black calc-silicate horizons.The degree of pyritization of black schists containing fine-grained sulfide disseminations is 0.8 to 1.0, suggesting an anoxic depositional environment. The precursors to the Talvivaara black schists were metalliferous organic-rich muds deposited in anoxic conditions. Geochemical evidence for hydrothermal influx includes elevated Ni, Cu, Zn, and Mn values relative to those of average Finnish and North American black shales. In addition, elevated S isotope delta 34 S values (median values of -5.2ppm for pyrrhotite, -4.3ppm for pyrite from the low Ni-Mn black schists, and -3ppm for both pyrrhotite and pyrite from the Ni-rich black schists) and positive Eu anomalies are regarded as further indications of hydrothermal activity. The Ni-rich black schists record Na depletion, which suggests hydrothermal alteration. Black calc-silicate rocks exhibit heavy S isotope values (-3.8-+20.8ppm for pyrrhotite and -7.1-+20.8ppm for pyrite) and are depleted in light REE, thus resembling Mn-rich seawater from recent hydrothermal sites. The Ni-rich and Mn-rich horizons with black calc-silicate rock intercalations probably result in part from precipitation of upwelling hydrothermal solutions through the Talvivaara sediments, analogously with the processes encountered, for example, in the recent Galapagos mounds hydrothermal field.The Talvivaara Ni-Cu-Zn deposit and the Outokumpu serpentinite-associated Cu-Co-Zn-Au-Ni deposits have features in common, such as the C concentration and texture of associated black schists and S isotope compositions. Cobalt and nickel concentrations of the Talvivaara Ni-rich black schists are similar to those of the Ni occurrences hosted by quartz rocks at Outokumpu. There are, however, distinct differences between the two deposits. In contrast to the black schists at Talvivaara, which have horizons with Mn values of 0.8 or more percent, those of the Outokumpu region do not contain elevated Mn concentrations. At Talvivaara, Ni predominates over Cu, and the deposit is hosted by graphite-rich rocks. At Outokumpu, Cu predominates over Ni and the deposit is mainly hosted by fine-grained quartz rocks. In the case of Talvivaara, the nearest serpentinite massif with Outokumpu-type fine-grained quartz rocks anomalous with respect to Cr and Ni is encountered 20 km to the south.The high C graphite concentrations, which are very characteristic of the rift-related black schists in the Kainuu-Outokumpu areas, point to vigorous organic productivity and extensive preservation of organic material in the sediment. Thick (>120 m) formations 2.1 to 1.9 Ga in age with a high median C graphite value are also encountered in several places elsewhere in the Fennoscandian Shield.