Early Proterozoic bimodal volcanism, hydrothermal activity, and massive sulfide deposition in the Boliden-Langdal area, Skellefte District, Sweden

Stockholm Univ., Dep. Geol., Stockholm, Sweden

The Boliden-Laangdal area in the eastern part of the Skellefte district consists of an early Proterozoic volcanic-sedimentary sequence which includes massive sulfide deposits. Both the ores and their country rocks have been strongly folded and metamorphosed in the lower amphibolite facies and intruded by synorogenic (1,890 m.y.) and late orogenic (1,750 m.y.) granitoids.The calc-alkaline/tholeiitic volcanic suite displays petrological and geochemical features similar to Phanerozoic subduction-related volcanic arcs. Its bimodal character suggests that extensional forces dominated in the arc during long periods, probably producing incipient rifting. In connection with this tensional stress large volumes of felsic pyroclastic and subordinate lava flows were extruded and large-scale tectono-volcanic depressions formed (e.g., calderas). This event, with predominantly felsic volcanism, culminated with the extrusion of genetically unrelated mafic and ultramafic rocks. Most of this activity occurred as submarine volcanism.The convergence in space and time of a thick volcanic sequence with high permeability, synvolcanic faulting, and a high geothermal gradient produced by the magmatic activity favored the starting of a convective hydrothermal system where the fluid was modified seawater. The hydrothermal activity produced extensive alteration in the volcanic rocks under low water/rock mass ratio conditions, changing their original chemical compositions and producing ore-bearing hydrothermal solutions. The geochemical data suggest that most of the Cu and Zn were leached from the volcanic rocks whereas the provenance of the Pb is more uncertain because it is normally enriched in these rocks. When the ore-bearing hydrothermal solutions reached the sea floor, they caused the deposition of massive sulfide deposits within local tectono-volcanic depressions.

This record provided courtesy of AGI/GeoRef.

This article has been cited by other articles:

				T. Wagner and E. Jonsson MINERALOGY OF SULFOSALT-RICH VEIN-TYPE ORES, BOLIDEN MASSIVE SULFIDE DEPOSIT, SKELLEFTE DISTRICT, NORTHERN SWEDEN Can Mineral, June 1, 2001; 39(3): 855 - 872. [Abstract] [Full Text] [PDF]