Palaeoproterozoic mineralized volcanic arc systems: the Skellefte District

 The Skellefte volcanic arc in northern Sweden is one of the most mineralized Palaeoproterozoic arc systems in the world. The main ore type is volcanogenic massive sulphide deposits (VMS).These are hosted by the mainly felsic Skellefte Group volcanic rocks which formed in a submarine environment 1.9 Ga ago. The bedrock of the Skellefte district is interpreted to have accreted to the Karelian Craton margin or formed as a continental volcanic arc system during the Palaeoproterozoic Svecokarelian Orogeny which belongs to one of the most intense orogenic periods of the Earth's evolution, between 1.95 and 1.80 Ga. This is also the main basic scientific goal of the drilling project, to try to increase our understanding about the tectonic processes which were acting on the Palaeoproterozoic Earth. For this the Skellefte district with its well preserved volcanic rocks and plenty of existing data is the ideal location. An undeniable advantage of this project is its direct relevance to economic geology. For the exploration and mining industry it is important to understand the 3-dimensional extent of the Skellefte group with its high grade ore deposits.

In recent years a pilot project with high resolution reflection seismic acquisitions and interpretations has been carried out in the western part of the Skellefte district. Based on the very promising results these studies were extended in a new VINNOVA-industry funded project. This on-going work integrates the results from old and new seismic studies with already existing knowledge, including potential field, bedrock geology and tectonics, and the information available from exploration drilling and underground exposures in mines. The highly anticipated outcome of the project at the end of 2012 are robust 4D-models of the top 3–5 km of the crust in the western and central parts of the Skellefte district. Thus, the results of the modelling project are ideal for planning a drillhole through the upper kilometres of the Skellefte District. The borehole, on the other hand, will then provide the unique possibility to constrain the established models.

In summary, a deep drillhole in the Skellefte district will directly support the following scientific studies: - volcanic facies architecture towards depth, including correlation with surface outcrops - structural relationship between the Skellefte Group and its basement, requiring a careful positioning of the drillhole based on the results of the on-going 4D-modelling of the Skellefte District - geochronology and petrogenesis of the basement/cover relationship - regional tectonic evolution - direct deep exploration and deep ore production – ore horizons towards depth

Principal investigators: Pär Weihed, Luleå University of Technology.