The Palaeoproterozoic Skellefte ore district, northern Sweden: Results from an integrated 3D geophysical and geological modeling

The Skellefte district is a major base-metal producing region located in northern Sweden. The region consists primarily of volcanic rocks, volcanoclastic and associated sediments along with various intrusive suites, all of Palaeoproterozoic age. Despite the great economic importance of the Skellefte district, the structural relationships between the ore bearing rocks of the Skellefte volcanics and the surrounding rocks have hitherto remained enigmatic. A major effort has been made to combine different available data to construct a pilot 3D geological model for the region. The derived 3D geological model has an internal geometrical consistency that is compatible with available geophysical data, such as deep seismic reflection, magnetic, gravity, geological and geochemical observations on a regional scale. The reflection seismic data have been essential for providing the framework for building the deep 3D geological model. 3D forward and inverse gravity modeling have been performed to validate and refine previous modeling with 2D techniques. One new and important feature of the 3D geological model is a structural basement to the Skellefte volcanics, which is believed to have formed in a fore-arc and subduction/collision zone. The new crustal models, however, show two alternatives: either a major crustal shear zone that brings the Skellefte volcanic rocks over a younger domain, or the presence of previously deformed, older 1.9 Ga crust. The latter suggests that crustal thickening predates the Skellefte volcanism and that the interpreted Bothnian Basin rocks constitute a Palaeoproterozoic structural basement to the Skellefte volcanics. The Skellefte volcanics would then originate from a supracrustal rift setting and not a juvenile arc. Interpretations of 3D gravity data in conjunction with seismic reflection data have led us to recognize that the post-orogenic Revsund granites could be divided into two groups based on their shape and thickness. All the Revsund intrusions in the south are represented by stoke/dome bodies with a maximum depth extent of about 3.5-4 km, while in the north they are represented by thin sub-horizontal sheet- like bodies with a depth extent of about 500-600 m. Generally, the margins of the Revsund granites dip steeply inwards suggesting that the current erosion level is near the middle of, or towards the base of the granites. The new deep crustal 3D geological model has shed new light on the tectonostratigraphy and tectonic evolution of the Skellefte ore district.