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Abstract

Metamorphosed sedimentary rocks rich in organic C and S, known as 'black shales', are encountered in Paleoproterozoic supracrustal rock units all over Finland. They have long been known for their ore potential, both for black-shale-hosted deposits and as an exploration tool for sulfide deposits occurring next to the black shale units. Black shales are easy to recognize on airborne geophysical maps because of their stratigraphy-related, coupled magnetic and conductive patterns. Geophysical responses of black shales vary within geological units and the measured responses are controlled by bedrock geology and structure, overburden and conductivity structures. Black shales are attributed with typical banded but irregular magnetic anomalies related to ferromagnetic monoclinic pyrrhotite, often present in mineralized environments. The regional distribution of black shales in Finland was mapped by correlating airborne magnetic and electromagnetic (EM) data in the late nineties. To our knowledge, the 1 : 1 million map was the first published nation-wide black shale map in the world. High-resolution airborne geophysical data (magnetic, frequency-domain electromagnetic and radiometric data) cover the whole country. Of the multi-frequency EM surveys, the 3 kHz data cover whole Finland. The maximum depth penetration of the system varies from 70 to 100 m, so that these data indicate conductivity variation only in the shallow subsurface. During 2009-2013 we have updated the black shale database and reinvestigated all the deep drill cores from which graphite bearing rocks or black shale had been reported during exploration. Black shale units as inferred from airborne geophysical data were verified with drill-core and outcrop information. The combination of airborne magnetic, electromagnetic and radiometric survey data has been used for the identification and classification of black shale units and mapping their regional distribution in the bedrock. Geochemical and petrophysical data was analyzed to characterize different black shale units in Finland. Petrophysical and geochemical data were analyzed with a variety of methods, for example by using non-metric multidimensional scaling (NMDS). Its application in this work led to the recognition of five black shale classes with distinctive characteristics. Even though black shales are easy to identify in airborne geophysical data, interpretation of their ore potential is challenging: their geophysical properties vary from site to site and within individual lithostratigraphic units. The surveyed airborne geophysical responses are controlled by the bedrock geology and structure, the overburden and the conductivity structures and texture. The black shale database composed of airborne geophysical, geochemical and rock physical data sets facilitates data integration, statistical analysis and interpretation. These results integrated with magnetic structural analysis provide a good tool for investigation, characterization and classification of black shales. They can be used both in predicting mineralized areas in regional context and also be applied in detailed geological and geophysical studies.

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/content/papers/10.3997/2214-4609-pdb.383.AEM2013_DAY1_SESSION_3B_Lerssi
2013-10-10
2024-03-29
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http://instance.metastore.ingenta.com/content/papers/10.3997/2214-4609-pdb.383.AEM2013_DAY1_SESSION_3B_Lerssi
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