Reflection seismic investigations in the western part of the paleoproterozoic VHMS-bearing Skellefte district, northern Sweden

The Skellefte district forms part of the Svecofennian ca. 1.90 to 1.80 Ga, snpracrustal sequence and associated intrusive rocks in the northern part of Sweden. The western part of the Skellefte district, which is the most important metallogenic province in northern Sweden today, hosts major volcanic-hosted massive Sulfide (VHMS) deposits (e.g., the 23 million metric tons (Mt) Kristineberg Cu-Zn-Pb-Ag-Au deposit). In order to obtain a better understanding of the VHMS ore potential at depth, new seismic reflection data were acquired along two parallel and 25-km-long profiles in the Kristineberg area in 2003. The data were collected with the purpose of obtaining high-resolution images of the top 10 kin of the crust and are presented here for the first time. Although the structural setting is very complex, the stacked sections reveal numerous reflections that can be correlated with surface geology. Visible on both profiles is a pronounced north-dipping hand of reflections marking a boundary between relatively transparent crust above and significantly more reflective crust beneath it. We interpret this reflective crust to represent a structural basement to the ore-bearing Skellefte Group, possibly constituting Bothnian basin metasedimentary rocks bordering the Skellefte district to the south. This new interpretation is important for the understanding of the tectonic evolution of the Skellefte district and for defining exploration strategies in the area. The seismic results suggest that the Kristineberg and Ravliden deposits occur on the northern limb of a kilometer-scale local second-order syncline within the hinge zone of a major antiform. Results from a profile located approximately 8 kin to the west of the Kristineberg mine indicate that the Revsund granitoid has a thickness of about 3 to 3.5 km. Ultramafic rocks are also imaged clearly Diffraction patterns and bright-spot reflectivity is interpreted as originating from either mafic to ultramafic intrusions ora mineralization zone at 3- to 5-km depth. These results help to identify new prospective areas and mineral potential, both downplunge from known ores and on the same stratigraphic horizon on the southern limb of the ore-bearing syncline. The seismic reflection profiling has been effective in imaging the major structures around the Kristineberg orebody, demonstrating that this technique can be used for delineating complex structures significant for mineral exploration.