Geologian tutkimuskeskus, Tutkimusraportti 198 – Geological Survey of Finland, Report of Investigation 198, 2013 Current Research: GTK Mineral Potential Workshop, Kuopio, May 2012
AIRBORNE GEOPHYSICAL, PETROPHYSICAL AND GEOCHEMICAL CHARACTERISTICS OF PALAEOPROTEROZOIC BLACK SHALE UNITS IN FINLAND: APPLICATIONS FOR EXPLORATION AND ENVIRONMENTAL STUDIES by Eija Hyvönen1, Meri-Liisa Airo2, Hilkka Arkimaa2, Jouni Lerssi3, Kirsti Loukola-Ruskeeniemi2, Jouko Vanne3 and Satu Vuoriainen2 Geological Survey of Finland, P.O. Box 77, FI-96101 Rovaniemi, Finland Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland 3 Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland E-mail:
[email protected]
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Palaeoproterozoic black shales in Finland are easy to recognize on aerogeophysical maps. They have distinct geophysical properties, such as electrical conductivity and magnetization, enabling stratigraphically constrained units with high concentrations of graphite and sulphides to be followed for tens to hundreds of kilometers. Therefore, a combination of airborne magnetic and electromagnetic survey data can be used for the identification of black shale units and mapping their regional distribution (Arkimaa et al. 1999, 2000). Airborne geophysical, geochemical and petrophysical data have been collected for detailed interpretation in order to characterize different black shale units in Finland. Altogether 800 black shale samples from drill cores all over the country were selected (Loukola-Ruskeeniemi et al. 2011). All drill cores from which graphite or black shale had been reported, were also reinvestigated for the purpose of this comparative black shale study. The petrophysical properties of black shales depend on their mineral composition and the abundance of graphite and sulphides in particular, with considerable differences being observed from site to site (Airo & Loukola-Ruskeeniemi 2004). The mean density for “average” black shale is about 2800 kg/m3 where graphite reduces and sulphides increase the density. Mean susceptibilities are about 6000 x10-6 SI-units and depend directly on the abundance of ferrimagnetic pyrrhotite. Consequently, the intensity of remanent magnetization is about 3 A/m. Königsberger ratios (the ratio of remanent to induced magnetization) for ordinary black shales range between 10−100, which allows effective discrimination between pyrrhotite-bearing source rocks and magnetite-bearing rocks, in which Königsberger ratios are typically below 5. Black shales are highly conductive and resistivity values are mainly below 1 Ohm-m. Even though black shales are easy to identify in airborne geophysical data, their interpretation is challenging; geophysical properties vary within individual 59
Geologian tutkimuskeskus, Tutkimusraportti 198 – Geological Survey of Finland, Report of Investigation 198, 2013 Pentti Hölttä (ed.)
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-354 -576 -800 nT
Geophysical anomalies similar to those of known deposits Black shale units verified with chemical analysis and petrophysical studies 0
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Black shale units observed in outcrop or drill core
Fig. 1. Preliminary distribution of black shale units on a low-altitude airborne total intensity magnetic map. Red lines indicate the presence of black shale units, confirmed by chemical analysis and petrophysical studies. Blue lines indicate black shale units observed in outcrops or drill cores. Green lines represent potential black shale units, interpreted from low-altitude airborne data.
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Geologian tutkimuskeskus, Tutkimusraportti 198 – Geological Survey of Finland, Report of Investigation 198, 2013 Current Research: GTK Mineral Potential Workshop, Kuopio, May 2012
lithostratigraphic units and the measured responses are controlled by bedrock geology and structure, overburden and conductivity structures. Variation in the concentrations of C, S and Fe clearly affect the geophysical responses. Increased S abundances in geochemical data can be correlated with increased conductivity and consequently for C~10% the concentration of Fe decreases considerably. This affects the composition and abundance of the Fe sulphides– there is a linear correlation between magnetic susceptibility and the abundance of monoclinic pyrrhotite. Aeroradiometric data can be used to estimate the effects of alteration and weathering associated with mineralization. The map of Palaeoproterozoic, metamorphosed sedimentary rocks rich in organic C and S was compiled by correlating aeromagnetic and aeroelectromagnetic data (Fig. 1). The inferred black shale units were verified with chemical analyses and petrophysical studies of drill core samples. Statistical analysis and interpretation of airborne geophysical, geochemical and rock petrophysical data are used to characterize and classify black shales in Finland and the results are compiled in the black shale database which is designed to be part of the Bedrock of Finland − DigiKP –database. These results provide background constraints for both mineral exploration and environmental studies. Black shales can cause environmental problems (e.g. Loukola-Ruskeeniemi et al. 1998) and drilling a well into black shale bedrock for groundwater is not recommended. Metamorphosed black shales in Finland have long been known for their ore potential. The Talvivaara deposit contains more than 1550 Mt of Ni-Cu-Co-ZnMn-U ore and was deposited in a stratified marine basin 2.0−1.9 Ga ago (Loukola-Ruskeeniemi & Heino 1996, Kontinen, 2012). High concentrations of organic C and S similar to those in the Talvivaara deposit occur in many localities in eastern and northern Finland, but Ni, Cu, Co, Zn, and Mn concentrations were lower in the present sample set, represented by 800 samples from throughout Finland. Indications of high Pd abundances were noted from some prospects, whereas Urich black shale units were not encountered.
REFERENCES Airo, M.-L. & Loukola-Ruskeeniemi, K. 2004. Characterization of sulfide deposits by airborne magnetic and gamma-ray responses in eastern Finland. Ore Geology Reviews 24 (2004), 67–84. Arkimaa, H., Hyvönen, E., Lerssi, J., Loukola-Ruskeeniemi, K. & Vanne, J. 1999. Compilation of maps of black shales in Finland: applications for exploration and environmental studies. In: Autio, S. (ed.) Geological Survey of Finland, Current Research 1997−1998. Geological Survey of Finland, Special Paper 27, 111–114. Arkimaa, H., Hyvönen, E., Lerssi, J., Loukola-Ruskeeniemi, K. & Vanne, J. 2000. Suomen mustaliuskeet aeromagneettisella kartalla − Proterozoic black shale formations and aeromagnetic anomalies in Finland 1:1 000 000. Geological Survey of Finland. Kontinen, A. 2012. F029 Talvivaara Ni-Zn-Cu. In: Eilu, P. (ed.) 2012. Mineral deposits and metallogeny of Fennoscandia. Geological Survey of Finland, Special Paper 53, 276−280. Loukola-Ruskeeniemi, K. & Heino, T. 1996. Geochemistry and genesis of the black schist-hosted Ni-Cu-Zn Deposit at Talvivaara, Finland. Economic Geology, 91, 80−110. Loukola-Ruskeeniemi, K., Uutela, A., Tenhola, M. & Paukola, T. 1998. Environmental impact of metalliferous black schists at Talvivaara in Finland, with indication of lake acidification 9000 years ago. Journal of Geochemical Exploration 64, 395−407. Loukola-Ruskeeniemi, K., Hyvönen, E., Airo, M.-L., Arkimaa, H., Eskelinen, J., Lerssi, J., Vanne, J. & Vuoriainen, S. 2011. Onko Suomessa uusia Talvivaara-tyyppisiä malmeja? Geofysikaalisiin ja geokemiallisiin tutkimuksiin perustuva Suomen mustaliuskekartta. Abstract: Evaluation of the ore potential of black shale units in Finland – preliminary results of a geophysical and geochemical study. Geologi 63 (3), 68−79.
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