Geophysical and petrophysical characterisation of the Tjårrojåkka IOCG mineralisation, Sweden

Sammanfattning: A geophysical-petrophysical study has been performed in an area WSW of the city of Kiruna, northern Sweden. Particular attention has been dedicated to the Tjårrojåkka iron oxide copper gold (IOCG) mineralisations. The bedrock is characterised by intermediate to mafic meta-volcanites, metamorphosed intermediate to mafic dykes, gabbroic-dioritic and granitic intrusions of Svecofennian ages (ca. 1.96-1.75 Ga). The aim of the study is to put the deposits into a tectonic framework, to test existing hypotheses for their occurrences and to produce a database of physical properties of rocks, in an attempt to define a petrophysical signature of the mineralisations. Glacial deposits cover almost the entire area, which means that outcrops are sparsely occurring and that geophysical data are crucial for the geological studies. For the tectonic analysis also anisotropy of magnetic susceptibility (AMS) was studied. More than 150 oriented samples were collected in a number of outcrops along a profile intersecting the major structures in the Tjårrojåkka area. From the airborne magnetic data two major linear features are interpreted as deformation zones. The strike of the deformation zones is approximately NW- SE and E-W, respectively. The same trend has been defined from other geophysical data, as airborne VLF and ground gravity data. A third important structural trend striking SW-NE has also been identified. Very good agreement has been found between geophysical lineaments, AMS directions and geological field observations. Magnetic foliations determined by AMS measurements confirm the existence of three major trends in the study area: SW-NE, E-W, NW-SE. Ground magnetic data indicate that the major mineralised bodies have a ca. SW-NE strike direction, which is parallel to the strike of magnetic foliation determined in outcrops ca. 1 km northwest of the mineralisations. This, together with the epigenetic nature of the mineralisations and their spatial relationship with deformation zones, suggests a structural control of the mineralisations. A later tectonic episode resulted in an E-W trending deformation in the central part of the area. Other SW-NE compressive deformation is indicated by petrophysical, geophysical and geological analysis. It resulted in a probable thrust in proximity of a granitic intrusion. The presence of such a thrust is supported by modelling of gravity data. A good spatial correlation has been observed between Cu-mineralisations and high K/Th values from radiometric data, values that are expressions of potassic alteration. Thermomagnetic measurements indicate that iron oxides (Ti-magnetite) are widely common in the area, while iron sulphides are almost absent. Multi- domain magnetite has been identified as the most common Fe-oxide in different rock types. An unstable magnetic mineral has been detected in metamorphosed volcanites and it may have originated from oxidation of (Ti)- magnetite in submarine/subaerial environments or from oxidation related to hydrothermal activity and metamorphism.