Palaeoproterozoic mineralized volcanic arc systems and tectonic evolution of the Fennoscandian Shield: Skellefte District Sweden

摘要

The Skellefte 1.9 Ga volcanic arc in northern Sweden is one of the most mineralized (VMS, orogenic gold, mafic hosted Ni, porhyry style Cu-Au) Palaeoproterozoic arc systems in the world. The Skellefte District is interpreted to have accreted, or formed as a continental volcanic arc system, during accretionary processes related to the Svecokarelian Orogeny. Based on Sm-Nd isotope studies it has been concluded that the basement to the ore-bearing Skellefte Group cannot be much older than the volcanic arc and was thus probably juvenile Palaeoproterozoic crust. The basement is not known to outcrop and recently it was speculated, based on high resolution seismic work in the western part of the district, that the basement is dipping gently northwards beneath the ore-bearing Skellefte Group. It was further postulated from these studies that the basement could at least partly constitute the Bothnian supergroup, metasedimentary rocks that outcrop south of the Skellefte District. Part of this supergroup has been dated at 1.95 Ga. For economic reasons it is extremely important to understand the 3-dimensional extent of the Skellefte Group and this constitutes one direct aim of a future deep drilling proposal. The basic scientific aim for the drilling project is to better understand the accretionary processes that constitute the Svecokarelian Orogeny. One of the best places to study these processes is the Skellefte District where well preserved volcanic rocks form an arc system on the older Karelian Craton margin. A drilling programme in the Skellefte District will thus benefit the exploration and mining industry directly and at the same time address fundamental questions related to the tectonic processes that built the Fennoscandian Shield during one of the most intense orogenic periods in the evolution of the earth, between 1.95 and 1.80 Ga.