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Rock magnetic measurements were carried out on drill core material and hand specimens from the Blotberget apatite--iron oxide deposit in the Bergslagen ore province, south-central Sweden, to characterise their magnetic properties. Measurements included several kinds of magnetic susceptibility and hysteresis parameters. Petrographic and scanning electron microscopy were used to independently identify and quantify the amount and type of magnetite and hematite. Two hematite-rich samples were studied with laser ablation inductively coupled plasma mass spectrometry to quantify the trace element chemistry in hematite and investigate the potential influence of trace elements on magnetic properties. Three aspects of this study are noteworthy. (1) Hematite-rich samples display strong anisotropy of magnetic susceptibility, which is likely to affect the appearance and modelling of magnetic anomalies. (2) The magnitude-drop in susceptibility across Curie and Neel temperature transitions shows significant correlation with the respective weight percentage of magnetite and hematite. Temperature-dependent magnetic susceptibility measurements can therefore be used to infer the amounts of both magnetite and hematite. (3) Observations of a strongly depressed Morin transition at approximately -60 to -70 [degrees]C (200 to 210 K) are made during low-temperature susceptibility measurements. This anomalous Morin transition is most likely related to trace amounts of V and Ti that substitute for Fe in the hematite. When taken together, these magnetic observations improve the understanding of the magnetic anomaly signature of the Blotberget apatite--iron oxide deposits and may potentially be utilised in a broader context when assessing similar (Paleoproterozoic) apatite--iron oxide systems. Key words: apatite--iron oxide, magnetic properties, magnetite, hematite, Kiruna-type deposit. Des mesures du magnetisme des roches ont ete realisees dans du materiau de carottes et des echantillons de terrain du gisement d'apatite--oxyde de fer de Blotberget dans la province miniere de Bergslagen du centre-sud de la Suede, afin de caracteriser leurs proprietes magnetiques. Des mesures de plusieurs parametres associes a la susceptibilite et aux hystereses magnetiques ont ete faites. La microscopie optique et la microscopie electronique a balayage ont ete utilisees pour identifier et quantifier de maniere independante les proportions et types de magnetite et d'hematite. Deux echantillons riches en hematite ont ete etudies par spectrometrie de masse a source a plasma inductif jumelee a l'ablation au laser afin de quantifier la composition des elements en traces dans l'hematite et d'examiner la possible influence de ces elements sur les proprietes magnetiques. Trois aspects de l'etude meritent d'etre soulignes, a savoir. (1) Les echantillons riches en hematite presentent une forte anisotropie de la susceptibilite magnetique, qui a probablement une incidence sur l'apparence et la modelisation des anomalies magnetiques. (2) La baisse de susceptibilite associee aux transitions thermiques de Curie et de Neel est significativement correlee aux pourcentages en poids respectifs de la magnetite et de l'hematite, de sorte que les mesures de la susceptibilite magnetique dependante de la temperature peuvent etre utilisees pour inferer les proportions de magnetite et d'hematite. (3) Une transition de Morin fortement reduite est observee autour de -6 a -70 [degrees]C (200 a 210 K) durant les mesures de susceptibilite a basse temperature. Cette transition de Morin anormale est vraisemblablement reliee a des traces de V et de Ti qui se substituent au Fe dans l'hematite. Collectivement, ces observations magnetiques ameliorent la comprehension de la signature des anomalies magnetiques des gisements d'apatite--oxyde de fer de Blotberget et pourraient eventuellement etre utilisees plus largement pour l'evaluation de systemes a apatite--oxyde de fer (paleoproterozoiques) semblables. [Traduit par la Redaction] Mots-cles: apatite--oxyde de fer, proprietes magnetiques, magnetite, hematite, gisement de type Kiruna., Introduction Characterisation of magnetic properties of iron oxide deposits is important to improve interpretation of magnetic anomalies for exploration purposes and modeling of anomalies (i.e., Clark 1997, 1999). Laboratory rock [...]

The rock fabric can be expressed by both shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) of rock-forming elements (minerals, grain aggregates, pores, inclusions). The study of fabric properties (orientation, planar vs. linear fabric, anisotropy) is key to unravel the geological processes that forged it. State-of-the-art methods either assess the CPO by electron backscatter diffraction (EBSD), or a combination of CPO and SPO by anisotropy of magnetic susceptibility (AMS), whereas the documentation of solely SPO requires serial sectioning of rock samples. With advancements in data acquisition and treatment, X-ray computed microtomography (µXCT) represents a powerful alternative to analyse SPO in rocks.Here, we propose and showcase the use of µXCT for rock fabric analysis. After reconstruction and segmentation of µXCT images (using Blob3D), we developed an approach based on the analysis of the fabric tensor, a 2nd rank symmetric tensor constructed using the orientation tensor and the length of the three characteristic axes of each grain (simplified to best fit ellipsoids). The analysis of the fabric tensor permits to calculate mean principal directions and associated confidence ellipses, but also to quantify the degree of anisotropy (P') and the "shape" (T) of the fabric ellipsoid by eigenvalue and eigenvector analysis.We implemented this method in an open-source MATLAB package and designed the µTomoFab analysis tool. The code integrates a graphical user interface that allows the visualization of the full set of ellipsoid orientation, shape and size. Density plots and contouring can be utilised to graphically identify fabrics and sub-fabrics, and a full set of fabric parameters can be calculated based on the analysis of the fabric tensor (P', T) as well as the analysis of each principal direction orientation tensor (PGR values, K-index, LS-index).We demonstrate the strength of µTomoFab analysis tool with synthetic dataset and a field- and laboratory-based investigation of the mechanism of emplacement of the Ivrea Mafic Complex within the lower crustal section of the Ivrea-Verbano Zone (N Italy). The new dataset provides crucial insights into the micro- and macro-scale evolution of the Mafic Complex during its emplacement, construction and deformation history: (1) at micro-scale, we have discriminated the magmatic fabric from the fabric recording sub-solidus static recrystallization ("metamorphic overprint"); (2) at macro-scale, we have revealed the internal architecture of the pluton made by magma sills.In the light of these developments, we stress that µXCT represents a pertinent tool for rock fabric analysis to characterize the SPO of rock components. This approach can be performed parallel to other rock fabric quantification methods (e.g. AMS, EBSD) and applied to various rock types. [ABSTRACT FROM AUTHOR]