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Geophysical studies of structural features of northern part of the Tampere Schist Belt, southern Finland
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ABSTRACTS 18TH MEETING
The geological interpretation implies that the catchment area is composed of sediments with low apparent resistivities - here clayey till and that the cover apparently is at least 10 m thick. Singular high-resistivity areas are recognized. These are interpreted as sand-deposits,
with possible hydraulic contact with the main aquifer. From the investigation it can be concluded that the aquifer is generally well-protected by a thick series of low-permeable sediments.
Geophysical studies of structural features of northern part of the Tampere Schist Belt, southern Finland T. Pernu, H. Juntti, T. Ker/inen, E. Heikkinen and J. Mursu Department of Geophysics, University of Oulu, SF-905 70 Oulu, Finland
Some ten kilometers wide and over 100 km long in E - W direction, the Tampere Schist Belt in southern Finland, is composed mainly of greywacke metasedimentary rocks, of granitoid intrusions and of the zone of metavolcanic rocks (MVZ). This zone, partly well-exposed, composed of mafic, intermediate to felsic rocks, is located in the northern part of the schist belt, in the contact of the granitoid complex of central Finland. In the MVZ there occur lenses of hydrothermally altered, chloritized, seritized and strongly quartzified rocks, in which thin disseminated pyrite-bearing interbeds containing locally weak Zn, Cu, Au and Ag mineralization lenses have been found. The broad structural features of the MVZ can be readily m a p p e d by the aeromagnetic data. Detailed geophysical ground surveys in the MVZ have been carried out between 1986 and 1988 in six areas, from 6 to I0 km 2. One of the aims of this study was to find correlations between the MVZ and the electrically conductive zone, which were found in the upper crust by magnetovariational (MV) and magnetotelluric ( M T ) measurements. The MVZ contains numerous, parallel, both non-magnetic felsic layers and thin, near-vertical, from weakly up to relatively strongly
magnetized mafic layers. By interpretations of the ground and aeromagnetic data the vertical extent of these units can be estimated, at most, up depths of a few kilometers. The densities in each rock type and units vary in relatively broad ranges. According to an interpretation of the gravimetric data obtained in four profiles, 4-6 km in length, traversing the MVZ perpendicularly, the vertical extents of different volcanic units can be estimated mostly up to a few hundred meters only. Electrical conductivity structures were studied in detail by combinations of VLF and DC resistivity and IP central gradient profiling and in profiles with reversed axial dipole soundings (VAD), from 0.5 km up to 3 km in length. All volcanic rocks are very resistive, 1 0 - 4 0 lc.Q-m. Seritized and chloritized units have lower resistivity, which varies mostly between 1-5 kog-m, but the pyrite-bearing zones usually show clearly lower resistivities than these. The vertical extent of these zones can be estimated by VAD soundings. Conductive layers or lenses are located mostly near the magnetized units. According to surveyed sub-areas and to numerous reconnaissance electrical and magnetic profiles, a lot of local conductive zones,
NORDIC ASSOCIATION FOR APPLIED GEOPHYSICS
which are mostly due to weak pyrite mineralizations, can be found in the MVZ. Some fractured rock units, in which the resistivity is relatively low, can be traced too. Their vertical extent is in some cases at least few kilometers.
81
MT soundings, which were carried out in two profiles across the Tampere Schist Belt, show a long, vertical conductive zone near the MVZ. The vertical extent of the conductive zone must be at least several kilometers.
SAMPO wide-band EM system Tarmo Jokinen and Heikki Soininen Geological Survey of Finland, SF-02150 Espoo, Finland
SAMPO (GEFINEX 400 S) wide-band EM system was designed and constructed in 19861988 by Outokumpu Electronics in co-operation with the Geological Survey of Finland. SAMPO is an electromagnetic method, in which the horizontal loop, usually 5-50 m in diameter, acts as a transmitter. The receiver measures three ortogonal magnetic compo-
nents at a wide frequency band (2-2000 Hz). The coil separation can be augmented to more than 1000 m. The method has a good depth penetration and spatial resolution. SAMPO method has been studied theoretically by numerical modelling. Some examples of metal ore and kaolin exploration and impact crater studies are presented.
Geophysical exploration for gold in a tropical weathered greenstone terrain in Ghana, West Africa J.P.H. Larsen and J.S. Petersen Department of Geology and Geophysics, University of Aarhus, 8200 Aarhus, Denmark
Direct observation of structural and geological features related to mineralization is difficult in the tropics due to dense rainforest, general lack of outcrops and deep chemical weathering. Indirect geophysical and geochemical methods play a major role in the exploration of such areas. The survey area is located along the southeastern margin of the major Tarkwa-Eonongo greenstone belt in Ghana. The most important
gold deposits are related to greenstones and diorite with disseminated arsenopyrite, crosscut by gold-bearing quartz veins. These lode deposits are found especially along the margins of the Proterozoic, Birimian greenstones belts and mark NE-SW trending shear zones intersected by NW-SE traverse faults. Economic deposits are up to 500 m long with a thickness of up to 25 m. The geophysical methods used at this pilot
ABSTRACTS 18TH MEETING
The geological interpretation implies that the catchment area is composed of sediments with low apparent resistivities - here clayey till and that the cover apparently is at least 10 m thick. Singular high-resistivity areas are recognized. These are interpreted as sand-deposits,
with possible hydraulic contact with the main aquifer. From the investigation it can be concluded that the aquifer is generally well-protected by a thick series of low-permeable sediments.
Geophysical studies of structural features of northern part of the Tampere Schist Belt, southern Finland T. Pernu, H. Juntti, T. Ker/inen, E. Heikkinen and J. Mursu Department of Geophysics, University of Oulu, SF-905 70 Oulu, Finland
Some ten kilometers wide and over 100 km long in E - W direction, the Tampere Schist Belt in southern Finland, is composed mainly of greywacke metasedimentary rocks, of granitoid intrusions and of the zone of metavolcanic rocks (MVZ). This zone, partly well-exposed, composed of mafic, intermediate to felsic rocks, is located in the northern part of the schist belt, in the contact of the granitoid complex of central Finland. In the MVZ there occur lenses of hydrothermally altered, chloritized, seritized and strongly quartzified rocks, in which thin disseminated pyrite-bearing interbeds containing locally weak Zn, Cu, Au and Ag mineralization lenses have been found. The broad structural features of the MVZ can be readily m a p p e d by the aeromagnetic data. Detailed geophysical ground surveys in the MVZ have been carried out between 1986 and 1988 in six areas, from 6 to I0 km 2. One of the aims of this study was to find correlations between the MVZ and the electrically conductive zone, which were found in the upper crust by magnetovariational (MV) and magnetotelluric ( M T ) measurements. The MVZ contains numerous, parallel, both non-magnetic felsic layers and thin, near-vertical, from weakly up to relatively strongly
magnetized mafic layers. By interpretations of the ground and aeromagnetic data the vertical extent of these units can be estimated, at most, up depths of a few kilometers. The densities in each rock type and units vary in relatively broad ranges. According to an interpretation of the gravimetric data obtained in four profiles, 4-6 km in length, traversing the MVZ perpendicularly, the vertical extents of different volcanic units can be estimated mostly up to a few hundred meters only. Electrical conductivity structures were studied in detail by combinations of VLF and DC resistivity and IP central gradient profiling and in profiles with reversed axial dipole soundings (VAD), from 0.5 km up to 3 km in length. All volcanic rocks are very resistive, 1 0 - 4 0 lc.Q-m. Seritized and chloritized units have lower resistivity, which varies mostly between 1-5 kog-m, but the pyrite-bearing zones usually show clearly lower resistivities than these. The vertical extent of these zones can be estimated by VAD soundings. Conductive layers or lenses are located mostly near the magnetized units. According to surveyed sub-areas and to numerous reconnaissance electrical and magnetic profiles, a lot of local conductive zones,
NORDIC ASSOCIATION FOR APPLIED GEOPHYSICS
which are mostly due to weak pyrite mineralizations, can be found in the MVZ. Some fractured rock units, in which the resistivity is relatively low, can be traced too. Their vertical extent is in some cases at least few kilometers.
81
MT soundings, which were carried out in two profiles across the Tampere Schist Belt, show a long, vertical conductive zone near the MVZ. The vertical extent of the conductive zone must be at least several kilometers.
SAMPO wide-band EM system Tarmo Jokinen and Heikki Soininen Geological Survey of Finland, SF-02150 Espoo, Finland
SAMPO (GEFINEX 400 S) wide-band EM system was designed and constructed in 19861988 by Outokumpu Electronics in co-operation with the Geological Survey of Finland. SAMPO is an electromagnetic method, in which the horizontal loop, usually 5-50 m in diameter, acts as a transmitter. The receiver measures three ortogonal magnetic compo-
nents at a wide frequency band (2-2000 Hz). The coil separation can be augmented to more than 1000 m. The method has a good depth penetration and spatial resolution. SAMPO method has been studied theoretically by numerical modelling. Some examples of metal ore and kaolin exploration and impact crater studies are presented.
Geophysical exploration for gold in a tropical weathered greenstone terrain in Ghana, West Africa J.P.H. Larsen and J.S. Petersen Department of Geology and Geophysics, University of Aarhus, 8200 Aarhus, Denmark
Direct observation of structural and geological features related to mineralization is difficult in the tropics due to dense rainforest, general lack of outcrops and deep chemical weathering. Indirect geophysical and geochemical methods play a major role in the exploration of such areas. The survey area is located along the southeastern margin of the major Tarkwa-Eonongo greenstone belt in Ghana. The most important
gold deposits are related to greenstones and diorite with disseminated arsenopyrite, crosscut by gold-bearing quartz veins. These lode deposits are found especially along the margins of the Proterozoic, Birimian greenstones belts and mark NE-SW trending shear zones intersected by NW-SE traverse faults. Economic deposits are up to 500 m long with a thickness of up to 25 m. The geophysical methods used at this pilot