Interpretation of a gravity profile across the Engebøfjellet rutile-bearing eclogite deposit

Abstract

As a part of a collaborative project with DuPont, NGU has made gravity measure- ments across the eastern part of the Engebøfjellet rutile-bearing eclogite deposit in Fjaler commune, Sogn and Fjordane county. The main objective of the investigations was to delineate the morphology and subsurface extension of the eclogite body through gravity modelling. Gravity measurements were made at 39 stations along one 775 m long profile. Two alternative models are proposed in the interpretation of the gravity data. The first model (model 1) has one eclogite body with a density of 3400 kg\/m3 and a dip 70-75\u00B0 towards north-north-west. The length along the dip is on the order of 350-400 m. The average width (horizontal cross section) of the body is c.90 m, and it seems to become thicker towards the surface. The length of the modelled eclogite in the strike direction is set to 1200 m (although the eclogite is longer and wider towards west, but this has little or no effect on the calculated model response). With the dimensions given above, the total volume of the modelled body is c.40.10\/6 m3 equivalent to a mass of c. 136 Mt. Model 2 was constructed to get a better match between the calculated model response and the observed Bouguer anomaly values for some stations. An additional body had to be added to the south and in the close vicinity of the main eclogite body to get a better match. However, it was found that the stations in question suffered from erroneous or insufficient terrain corrections due to severe topography, and that these stations should be taken into little or no account in the modelling. The introduction of the additional body in model 2 thus seemed unlikely.