Density, magnetic susceptibility and remanent magnetization of more than 30,000 samples of crystalline rocks from the northern part of the Baltic Shield have been measured by the national geological surveys of the Nordic countries. These data were compiled into a database as part of a project involving Finland, Norway and Sweden-the Nordkalott Project. In addition, more than 20,000 in-situ susceptibility measurements are available for a large part of the Baltic Shield. From all these measurements, a number of general conclusions can be drawn regarding the distribution of magnetic minerals in the different lithologies: (1) The Precambrian metasupracrustal rocks show a rather typical bimodal distribution in susceptibility. The spatial distribution of these susceptibility modes gives rise to the banded patterns observed in high-resolution (low-altitude) aeromagnetic measurements. (2) The amount of remanence in the various rock types is generally low, as indicated by Q-values (Konigsberger ratios) of around 0.2 on average. Thus, magnetization is parallel to the Earth's magnetic field, simplifying the interpretation of the total field magnetic survey results. (3) Plutonic rocks as a whole have the same bimodal susceptibility distribution, but individual plutons have usually unimodal susceptibility distributions and thereby create characteristic local anomaly patterns. (4) Metamorphic processes significantly affect the distribution of magnetite. Well-documented examples include the amphibolite-granulite facies transition (Tectonophysics, Vol. 192), the serpentinization of ultramafic rocks (Pure Appl. Geophys., Vol. 114) and oxidation in fracture zones (Geoexploration, 15). The metamorphic development of magnetite appears to reflect in many cases the variation in primary composition. (5) The range in density variation and magnetic susceptibility in crystalline rocks is now well defined. In local structures it is, however, always necessary to determine the actual petrophysical contrasts in order to minimize the ambiguity in quantitative interpretation of potential field anomalies.
