Gravity and Magnetic Survey, Modelling and Interpretation in the Blötberget Iron-Oxide Mining Area, Bergslagen, Sweden

Abstract: The Blötberget mining area, the focus of this MSc project, is located about 230 km northwest ofStockholm and 12 km southwest of the city of Ludvika (central Sweden). The mining area has beenknown since 1600 for its various types of mineralization particularly iron-oxide deposits (magnetite andhematite) with the mining commenced in 1944. Previous geoscientific research in the area providesdetailed information about lithological variations and structure of the bedrock near the surface.However, knowledge of the depth extent of the mineral deposits and their host rocks is limited. To shedlights on these issues and support deep mineral exploration potential in the study area, within the recentlylaunched StartGeoDelineation project, new ground gravity data, 180 data points on average 150 m apart,were collected during two field campaigns in 2015 and 2016. Aeromagnetic data were obtained fromthe Geological Survey of Sweden (SGU) to complement the ground gravity measurement interpretationsand modelling. After a careful inspection of the field gravity data, they were reduced to completeBouguer anomaly with a maximum error estimate of about 0.6 mGal due to uncertainty in theinstrumental drift, slab density, geodetic surveying, diurnal variations and terrain (or topography)correction. The Bouguer gravity data after separation of regional field (second order polynomial at theend was used) were used (~ 8 mGal range) for interpretation and 3D inverse modelling. Clear anomalouszones are noticeable in the gravity data particularly due to mineralization and a major boundaryseparating a gravity low from gravity high in the southern part of the study area likely representing afault boundary separating two different lithological units. In my study, both forward and inversemodelling using rudimentary objects/shapes and voxel-type (mesh) approach were carried out. Effect ofinitial and reference models were tested on both gravity and magnetic datasets. While the constrainedmodels have still significant ambiguity, they help to suggest structural control on the location ofmineralization and may allow estimating an excess tonnage due to the presence of mineralization in thestudy area. Due to access limitations (e.g., unable to measure on the water-filled pit) the gravity modelis sensitive to the measuring positions and constraints using known shape of mineralization was not atthe end successful to overcome this. Collecting more gravity data on the target area and repeated test of3D inversion by adjusting the inversion parameters might help to improve the final result.