Outokumpu Deep Drilling Project 2003–2010 Edited by Ilmo T. Kukkonen Geological Survey of Finland, Special Paper 51, 229–252, 2011
GRAPHITE- AND SULPHIDE-BEARING SCHISTS IN THE OUTOKUMPU R2500 DRILL CORE: COMPARISON WITH THE Ni-Cu-Co-Zn-Mn-RICH BLACK SCHISTS AT TALVIVAARA, FINLAND by Kirsti Loukola-Ruskeeniemi
Loukola-Ruskeeniemi, K. 2011. Graphite- and sulphide-bearing schists in the Outokumpu R2500 drill core: comparison with the Ni-Cu-Co-Zn-Mn-rich black schists at Talvivaara, Finland. Geological Survey of Finland, Special Paper 51, 229–252, 7 figures and 3 tables. The Outokumpu R2500 Deep Drill Core intersects three types of graphite- and sulphide-bearing rocks: graphite- and sulphide-bearing schist, graphite- and sulphide-bearing calc-silicate rock, and black schist. Black schist refers to metamorphosed black shale containing more than 1% of both organic C and S. Organic C is mainly graphitic at present. Sheared black schist formations containing from 5.4% to 10.1% organic C and from 3.9% to 11.5% S are met from the depth of 1496.35 m downwards in the core intercalated with calc-silicate rock and mica schist. The thickness of the black schist formations varies from 0.5 m to 13.0 m in the core. The chemical composition and textures of the black schists are similar to those of black schists associated with sulphide mines and prospects in the Outokumpu region. Furthermore, they have many features in common with black schists in the Kainuu area, north of Outokumpu. In Kainuu, black schists also host a large NiCu-Co-Zn-Mn deposit at Talvivaara. The precursors of the present black schist formations were deposited in anoxic seawater basins 1.9–2.0 Ga ago both in Kainuu and in Outokumpu. In the present study, the major similarities and differences between Outokumpu and Talvivaara black schists are described and applied to interpreting the genesis of the black schist formations encountered in the Outokumpu R2500 core. Black schists associated with sulphide prospects and mines in Outokumpu and Talvivaara exhibit comparable median values for several elements such as organic C, Al, B, Cr, Fe, K, La, Mg, Mo, P, Rb, Sc, Si, Th, U, V and Y. C and S isotope values are comparable, suggesting deposition in sedimentary basins with the same type of vigorous organic productivity. During the deposition of the precursor of the Outokumpu R2500 core black schists, the Ni concentration in the seawater of the sedimentary basin was not as high as in Talvivaara. Furthermore, basin margins with higher oxygen levels did not exist, since black schist samples with elevated Mn concentrations (Mn ≥ 0.8%) are not met in the R2500 core or in other prospects and mines in the Outokumpu region. At Outokumpu, Pb isotope results suggest a younger age for black schist deposition (1.92 Ga) than for the deposition of the primary Outokumpu-type Cu-CoZn ore (1.94 Ga). Mixing of the primary Cu-Co-Zn ore lead and black schist lead occurred during Ca-Si alteration around the present serpentinite bodies at 1.91 Ga. Graphite- and sulphide-bearing calc-silicate rocks, also met in the Outokumpu R2500 core, were formed during this stage, as well as Ni occurrences, which are mainly hosted by Si-rich rocks. In contrast, at Talvivaara, Ni precipitated in organic-
229
Geological Survey of Finland, Special Paper 51 Kirsti Loukola-Ruskeeniemi
rich mud from Ni-rich bottom waters in a stratified marine basin. An altered serpentinite body enveloped by black schist, Outokumpu type calc-silicate rocks and Cr-bearing Si-rich rocks occurs just 30 km south of Talvivaara, but it is suggested that Outokumpu type Cu-Co-Zn ore bodies do not exist at Talvivaara, and Talvivaara-type Ni-Mn rich black schist deposits are not likely to occur in the Outokumpu area. Keywords (GeoRef Thesaurus, AGI): black schists, cores, chemical composition, organic carbon, sulfur, manganese, nickel, metal ores, genesis, Proterozoic, Outokumpu, Talvivaara, Finland Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland E-mail:
[email protected]
230
Geological Survey of Finland, Special Paper 51 Graphite- and sulphide-bearing schists in the Outokumpu R2500 drill core: comparison with the Ni-Cu-Co-Zn-Mn-rich black schists at Talvivaara, Finland
INTRODUCTION The Geological Survey of Finland drilled a 2516-m-deep research borehole during 2004– 2005 at Outokumpu, eastern Finland (Kukkonen 2011). In this paper, the Outokumpu R2500 black schists are compared with black schists in the Outokumpu region and Talvivaara. Talvivaara is a black-schist-hosted Ni-Cu-Co-Zn-Mn deposit, located in the Kainuu region 150 km north of Outokumpu (Figure 1). Black schists were previously investigated from more than 100 drill cores in the KainuuOutokumpu area (Loukola-Ruskeeniemi 1999). In addition, black schists in other localities in the Palaeoproterozoic of Finland were compared to evaluate the ore potential, both for black schisthosted ores and in order to examine the relationship between massive sulphide ores and adjacent black schists (e.g., Ruskeeniemi et al. 1986, Loukola-Ruskeeniemi et al. 1993, 1997). Previous study outcomes have also included the map and data-
base of Palaeoproterozoic black schist formations in whole of Finland (Arkimaa et al. 2000). The distribution of black schist formations has been determined with the aid of nationwide airborne low-altitude geophysical data, and a preliminary characterization of the ore potential of a black schist formation is possible through the interpretation of airborne magnetic and gamma-ray responses (Airo & Loukola-Ruskeeniemi 2004, Airo et al. 2009). In the present study, the major similarities and differences between Outokumpu and Talvivaara rift-related black schists are listed based on new and previously published data. In particular, differences in Mn and Ni concentrations in seawater during deposition, post-depositional evolution, and links to the genesis of the Outokumpu-type Cu-Co-Zn ore are discussed and applied to interpreting the genesis of black schists in the Outokumpu R2500 core.
MATERIALS AND METHODS Altogether, 15 black schist samples were selected from the Outokumpu R2500 Deep Drill Core between the depths of 926.50 m and 2247.15 m (Table 1). The samples were split in two: one half of the same drill core section was studied for 34S and 13 C values (Taran et al., this volume) and the other part was studied for major and trace element geochemistry in the present study. The samples were analyzed for major and trace elements by X-ray fluorescence and inductively coupled plasmamass spectrometry methods in the laboratories of the company Labtium Oy, Espoo, Finland. Rock samples were crushed using a jaw crusher with Mn-steel jaws, and pulverized in a carbon steel bowl. For ICP-MS and ICP-AES, fluoric acid-perchloric acid (HF-HClO4) digestion and sodium perborate fusion were used. Gold and
platinum group element concentrations were determined for one sample by ICP-MS with nickel sulphide enrichment. Total C, carbonate C and S contents were determined with LECO analyzers. The concentration of organic C was calculated as the difference between the total C and carbonate C contents. Organic C is presently mainly graphitic. The quality of the analytical results was assessed in several ways. For example, the same elements were analyzed using several methods and the results were compared with each other. Results from the Outokumpu R2500 core samples were also compared with published data (Loukola-Ruskeeniemi & Heino 1996, LoukolaRuskeeniemi 1991, 1995a, 1999) and by re-assessing the geochemical datasets of earlier studies.
REGIONAL GEOLOGY The Outokumpu area is located in the Palaeoproterozoic North Karelia Schist Belt within the Fennoscandian Shield (Figure 1). The North Karelia Schist Belt rests on a late Archean gneissic-granitoid basement (e.g., Gaál & Gorbatschev 1987) and comprises mainly metasedimentary rocks which are subdivided into autochthonous 2.5 to
2.0 Ga shallow-water deposits, and into
