Svecofennian volcanism in northernmost Sweden

Abstract: Volcanism and volcanic processes are important factors for ore formation. The environment of the volcanism plays an important role for which type of ore that will be formed. For instance, massive sulphide deposits are now being formed in mid-ocean ridges and submarine back-arc environments, whereas epithermal gold deposits are being formed in volcanic fumarole fields. The ultimate mechanism of formation for both these ore types is the thermal activity of the volcanism combined with circulation of water, in the first case seawater circulation and in the second case groundwater circulation. In areas with older bedrock, the tectonic environment that resulted in the formation of volcanism and ores can often be determined by the geochemical character of the volcanic rocks in combination with the distribution of associated rock types. During the c. 1.9 Ga old Svecofennian orogeny large amount of volcanic rocks were formed in Bergslagen, the Skellefte District and Norrbotten. Both the iron ores and the sulphide ores in the Precambrian of Sweden mainly occur in the volcanic areas. The genesis of the ores were closely associated to volcanic processes. Geochemical studies of the volcanic rocks and alterations related to the ores have been. performed in Bergslagen and the Skellefte District, but in Norrbotten no major study has been carried out. Only minor studies in close connection to the ores in northern Norrbotten have previously been done. The purpose of this thesis is to make an overall characterization and classification of the volcanic rocks north of the Skellefte District. This improves the understanding of the tectonic setting prevailing when the volcanic rocks were formed, and provides a better basis for the exploration of ores in the area. The volcanic rocks north of the Skellefte District were previously considered to have been formed by processes related to a subduction zone directed to the north below the Skellefte District. The results of this thesis show that the volcanic rocks can be divided into several types with a geographical distribution. Calcalkaline volcanic rocks are found in the Luleå area, in the Arvidsjaur area and in parts of eastern and northern Norrbotten, while alkali-calcic and alkaline volcanic rocks are found in the westernmost parts of Norrbotten and in the Kiruna area. Some of the volcanic rocks have been formed in compressional environment similar to that found in the Andes, whereas others were formed in extensional environments. Stratigraphical evidence and isotopic age determinations indicate that the Svecofennian volcanic rocks in the areas north of the Skellefte District were emplaced during several periods. The results show that a more complex model is necessary to explain the formation of the volcanic rocks. Probably more than one subduction zone have been active, and extension has occurred in at least two events.