Intra-Arc Rifting and Massive Sulphide Mineralization in an Early Proterozoic Volcanic Arc, Skellefte District, Northern Sweden

Abstract

The Skellefte district in northern Sweden consists mainly of pyrite-rich massive sulphide deposits intercalated in an early Proterozoic volcano-sedimentary sequence. The volcanic activity was predominantly submarine, and gave rise to abundant felsic rocks with minor mafic volcanic rocks and rare andesitic volcanism. A few dykes and volcanic flows of high Mg-Cr basalts, probably komatiitic, are also found within the volcanic pile.The geochemistry of the volcanic rocks is of subalkaline character and displays slightly tholeiitic to calcalkaline differentiation trends. The Ti, Zr, Y, Cr and rare-earth contents of the basaltic and andesitic rocks are consistent with a subduction related volcanic arc setting.The bimodal nature of the volcanic suite, the presence of highly primitive basaltic rocks and the abundance of felsic pyroclastic material suggests that rifting occurred within the volcanic arc. Long periods of tensional stress predominated and favoured the extrusion of large volumes of felsic pyroclastics and the formation of tectono-volcanic depressions. The convergence in time and space of a thick volcanic sequence with high permeability, a high geothermal gradient related to the rising of basaltic magma, and synvolcanic faulting resulted in extensive hydrothermal activity. The hydrothermal activity caused regional alteration of the volcanic rocks and locally deposition of pyrite-rich massive sulphide deposits within local, tectonically controlled, depressions.

Source of metals and age constraints for epigenetic gold deposits in the Skellefte and Pohjanmaa districts, central part of the Fennoscandian Shield

“…Tahkonsaari, Muurame and Korpilahti are located in the Central Finnish Batholith to the south of this figure ments in the upper parts (Lundberg 1980). The volcanic rocks are bimodal in the eastern and western parts of the Skellefte district, while the central part shows a more continuous differentiation trend (Vivallo and Claesson 1987). The chemistry of the volcanic rocks is consistent with a volcanic arc regime (Claesson 1985;Vivallo 1987;Vivallo and Claesson 1987).…”

“…The volcanic rocks are bimodal in the eastern and western parts of the Skellefte district, while the central part shows a more continuous differentiation trend (Vivallo and Claesson 1987). The chemistry of the volcanic rocks is consistent with a volcanic arc regime (Claesson 1985;Vivallo 1987;Vivallo and Claesson 1987). A U-Pb age determination of a meta-rhyolite belonging to the Skellefte Group has yielded an age of 1882 ___ 8 Ma (Welin 1987).…”

Lithostratigraphic and structural reconstruction of the Zn-Pb-Cu-Ag-Au Lemarchant volcanogenic massive sulphide (VMS) deposit, Tally Pond group, central Newfoundland, Canada

“…7e). Furthermore, the rocks at the Lemarchant area do not exhibit a continuous 551 spectrum of magmatic products, features common in most "arc" environments (e.g., 552 Tatsumi and Eggins, 1995), but rather are bimodal to trimodal in nature, features 553 commonly observed in rifted arc environments (e.g., Vivallo andClaesson, 1987, Wright 554 et al, 1996), and consistent with regional tectonics models (e.g,. Rogers et al, 2006;555 Zagorevski et al, 2010;Piercey et al, 2014).…”

Synvolcanic mixing of ore lead and the development of lead isotopic provinces in the Skellefte district, Sweden

“…These rocks are tholeiitic in the east, transitional tholeiitic in the centre and calc-alkaline in the west (Vivallo and Claesson 1987). On the basis of their geochemistry and petrology they resemble rocks from Phanerozoic subduction-related volcanic arcs (Claesson 1985).…”

“…The acid and mafic rocks of the volcanic sequence are suggested not to be co-magmatic, instead representing a bimodal suite, with a dominant rhyolitic mode and a subordinate mode of mafic rocks ranging from andesites to basalts (cf. Vivallo and Claesson 1987 and references therein). The volcanic rocks can be subdivided stratigraphically, into a lower felsic unit and an upper one with both mafic and felsic components.…”