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Towards an understanding of mineral systems – Contributions from magnetotelluric data from the Fennoscandian Shield in northern Sweden
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0001-7568-3895
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0002-1629-2920
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0002-5600-5375
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0003-1627-7058
2021 (English)In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 808, article id 228816Article in journal (Refereed) Published
Abstract [en]

Magnetotelluric data from 104 new stations in northern Sweden together with previously available MaSca project data are analysed with respect to a hypothesised genetic relation between mineral occurrences and commonly reported deep crustal electrically conductive structures in Precambrian Shield areas. A total of 165 stations from the north-eastern part of the Norrbotten ore province are modelled from an area characterised as having an Achaean basement with later reworking during the Paleoproterozoic. The data cover an area of 200 × 250 km2 and provide a significant improvement of spatial resolution compared to previously available data. The derived 3D model reveals the presence of strong crustal conductors with conductance of more than 3000 S at depth of tens of kilometres within a generally resistive crust. A subdivision into a central more conductive domain bounded towards north and south by resistive domains is revealed. The boundaries between these three domains are oriented NW-SE. The southern boundary coincides roughly with the boundary between Archaean and Paleoproterozoic crust defined by εNd data. The northern boundary coincides with an interpreted 2.4–2.3 Ga coastline of a marine rift basin, which is spatially defined by the occurrence of the Kovo group rocks. The location of major conductors within the central domain correlates with the presence of mapped Paleoproterozoic metasupracrustal belts. A significant part of the middle crust conductors is elongated in directions that coincide with major deformation zones that have been mapped from airborne magnetic data and geological fieldwork. Some of these conductors have near-surface expression where they spatially correlate with the location of known mineralisation. In these cases, the conductive structures may mark the pathways for metal-bearing hydrothermal fluids. Interpretations involving enhanced quantities of magnetite or graphite in the middle crust are discussed but the magnetotelluric data alone are not conclusive with respect to cause of observed high conductivities.

Place, publisher, year, edition, pages
Elsevier, 2021. Vol. 808, article id 228816
Keywords [en]
Magnetotellurics, 3D Inversion, Electrical Conductivities, Mineralisation
National Category
Geophysics
Research subject
Ore Geology; Exploration Geophysics
Identifiers
URN: urn:nbn:se:ltu:diva-83304DOI: 10.1016/j.tecto.2021.228816ISI: 000674649000005Scopus ID: 2-s2.0-85103982326OAI: oai:DiVA.org:ltu-83304DiVA, id: diva2:1538204
Funder
Luleå University of Technology
Note

Validerad;2021;Nivå 2;2021-04-20 (alebob)

Available from: 2021-03-18 Created: 2021-03-18 Last updated: 2023-09-05Bibliographically approved
In thesis
1. Integrated analyses and interpretations of regional scale magnetotelluric and potential field data from the Precambrian crust of northern Sweden
Open this publication in new window or tab >>Integrated analyses and interpretations of regional scale magnetotelluric and potential field data from the Precambrian crust of northern Sweden
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Norrbotten region in northern Sweden, as a part of the Fennoscandian (Baltic) Shield, is one of the most active mining areas in Europe. It is acknowledged that geological structures such as faults, shear zones, and associated fracture systems play a key role in providing a physical pathway connecting metal sources and the sites of mineral precipitation. In particular, several magnetotelluric (MT) surveys around the world have revealed a spatial relationship and correlation between mineral occurrences and regional scale conductive structures. Previous lithospheric studies in the Fennoscandian Shield have indicated enhanced conductivities in the Norrbotten region, and they served as a basis for the proposal to test the hypothesis of this genetic connection. Therefore, the current project was initiated with the aim of obtaining more detailed information on crustal and upper mantle structures in the Paleoproterozoic Norrbotten ore province by utilising a combination of regional geophysical data including, MT and potential field data. Potential field data (airborne magnetic, and ground gravity) are extensively used as the first choice in geophysical/geological mapping due to good areal coverage, whereas MT surveys are typically designed to answer more specific geological and geophysical questions. In order to improve utilisation of the geophysical methods, quantitative analyses and integration approaches have been developed and applied to the geophysical data.

This thesis covers four topics: (i) a regional-scale study of Norrbotten County using 3D inversion of newly measured MT data to reveal crustal geoelectric structures, (ii) integration of the resistivity model with regional-scale magnetic susceptibility and density models using an unsupervised classification approach to extract and analyse the possible correlation between modelled petrophysical properties and provide a geophysical domain classification as input to geological interpretations, (iii) identification of a regional-scale tectonic feature in northern Sweden by processing and analysing the potential field data, (iv) the development and evaluation of the feasibility of a classification approach designed to identify patterns in potential field data that are indicative of the type of geological environment.

The conductivity model of inverted new MT data reveals the presence of strong crustal electrical conductors. The conductance of thousands of Siemens within a generally resistive crust is modelled. Some of the conductors in the model have near-surface expressions and are spatially correlated with the location of known mineralisation. A significant part of middle crust conductors is elongated in a direction that coincides with parts of major deformation zones. The derived conductivity model provides a new insight into the tectonic unit boundaries in the area.

The magnetic susceptibility and density models were subsequently integrated with the results obtained from the MT data. The integration approach is based on a joint analysis of the three modelled physical properties by using an unsupervised classification approach referred to as Self-Organising Maps (SOM). The depth variations of the properties were included in the classification. The obtained domain classification is discussed with respect to the previously mapped/interpreted geological units and compositions. Some discrepancies between existing geological maps and the domain classification are noted for some areas. The discrepancies may partly be related to the fact that the surface geological features are compared with geophysical models that also include information at depth.

A detailed analysis of high-resolution potential field data in northern Sweden led to the discovery of a regional-scale strike-slip fault, which we refer to as the Norrbotten Mega Fault (NMF). The presence of a fault cutting through the entire area in a roughly N5E direction with horizontal displacement of 51.2 km can be traced from Karesuando at the Swedish-Finnish border in the north to the Archaean-Proterozoic boundary, which is marked by the Luleå-Jokkmokk Zone roughly 250 km towards the south. Altogether, the length, apparent displacement, and straight character of the proposed fault suggest that it represents a late-orogenic, brittle fault. The initial identification of the location of the fault was primarily based on visual inspection of potential field data. The estimation of the displacement and validity of the proposed NMF is supported by analyses of several higher-order spatial derivatives of the potential field data. The analyses included application of a neural network SOM classification and a visualisation approach with subsequent matching of well-defined anomalies and anomaly patterns.

The application of SOM for pattern recognition and classification were developed further with respect to analysis of potential field data. This involved various experiments on data feature definitions and extraction. Data features that capture information about spatial variations, both depth-wise and horizontally, are used as input to the SOM for domain classification. The feasibility of the methodology and the limitations are discussed with respect to the results derived from a synthetic model. The proposed approach is furthermore applied to measured data from the entire Norrbotten area.

The models derived from the geophysical data, as well as the new integration and classification approaches presented in this study, may be applied as a guidance for future exploration and investigations in the area, such as for planning or expanding geophysical surveys in high prospective areas, mapping and interpretation of the geology, mapping the mineral prospectivity or other prediction tasks, or used as a guidance for follow-up on geological and geochemical work.

Place, publisher, year, edition, pages
Luleå University of Technology, 2022
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Geophysics Geophysical Engineering
Research subject
Exploration Geophysics
Identifiers
urn:nbn:se:ltu:diva-93143 (URN)978-91-8048-145-8 (ISBN)978-91-8048-146-5 (ISBN)
Public defence
2022-10-26, E632, Luleå tekniska universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2022-09-20 Created: 2022-09-19 Last updated: 2023-09-05Bibliographically approved

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Vadoodi, RoshanakRasmussen, Thorkild M.Smirnov, MaximBauer, Tobias

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