Geophysical and petrophysical study of an iron oxide copper gold deposit in northern Sweden

Abstract

A geophysical–petrophysical study has been performed in an area WSW of the city of Kiruna, northern Sweden. The sub-regional tectonic setting is dominated by two important shear zones, which define the boundary of a granitic body. Many Cu–Fe-occurrences are located in proximity of faults related to these major deformation zones. Particular attention has been given to the Tjårrojåkka iron oxide copper gold (IOCG) deposit. Here the bedrock is characterised by intermediate to mafic meta-volcanics, metamorphosed intermediate to mafic dykes, and gabbroic–dioritic intrusions of Svecofennian ages (∼1.96–1.75 Ga). The major Cu- and Fe-occurrences are hosted by the meta-andesites. The aim of the study is to put the deposits into a tectonic framework and test existing hypotheses for their occurrences. Glacial deposits cover almost the entire area, leading to a scarcity of outcrops and inferring that geophysical data are fundamental for geological understanding. In addition to this, petrophysical analysis is vital for the interpretation of geophysical data (gravity, airborne magnetics and radiometrics, very low frequency) and for the definition of geophysical signatures of the deposits. The anisotropy of magnetic susceptibility (AMS) was also studied for the tectonic analysis. 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 these deformation zones is approximately NW–SE and E–W, respectively. The same trends have been defined from other geophysical data such as airborne VLF and ground gravity data. A third important structural trend striking SW–NE has been defined by K/Th data and ground magnetic data. Very good agreement has been found between geophysical lineaments and AMS directions. Magnetic foliations determined by AMS measurements confirm the existence of three major trends in the study area: SW–NE, E–W and NW–SE. The major Fe-orebody shows approximately a SW–NE strike direction as defined from ground magnetic data. This is parallel to the strike of magnetic foliation determined in outcrops ∼1 km NW of the deposit. The epigenetic nature of the Cu and Fe occurrences in Tjårrojåkka and their spatial relationship with deformation zones suggest a connection between the formation of the deposits and a tectonic event. A later tectonic episode resulted in E–W trending deformation in the central area, affecting the orebodies themselves. Other, probable, compressive deformations have been indicated from petrophysical and geophysical analyses. Thermomagnetic measurements indicate that Fe-oxides (Ti-magnetite) are common in the area, while Fe-sulphides are almost absent. Multi-domain magnetite has been identified as the most common Fe-oxide in different rock types, while an unstable magnetic mineral has been detected in metamorphosed volcanics. A good spatial correlation has been observed between Cu-deposits and high K/Th values from radiometric data, values that are expressions of potassic alteration.