Ulf Bergström

Marginal basin magmatism in an ancient volcanic arc: Petrology of the Palaeoproterozoic Malå‐group basalts, Skellefte District, northern Sweden

Abstract A new stratigraphic unit, informally called the Mala group, is tentatively suggested for the interval between the arc‐related volcanic Skellefte and Arvidsjaur Groups in the western part of the Skellefte District. The Mala group comprises turbiditic grey wackes and high‐Mg basalts. The basalts vary from sills and lavas in the lower levels to mainly volcaniclastites at the top. The basalts contain characteristic actinolite pseudomorphs after pyroxene. Geochemically the basalts have high contents of Mg, Cr, and Ni and relatively high LIL elements. Tectonic discrimination using trace element geochemistry suggests a volcanicarc setting. The geological setting is interpreted as a marginal basin formed at an advanced stage of the development of the Skellefte extensional continental margin arc. Bergstrom, U., 1997: Marginal basin magmatism in an ancient volcanic arc: Petrology of the Palaeoproterozoic Mala‐group basalts, Skellefte District, northern Sweden. GFF, Vol. 119 (Pt. 2, June), pp. 151–157. Stoc...

The Palaeoproterozoic Malmbäck Formation in S Sweden: age, composition and tectonic setting

The Palaeoproterozoic Malmbäck Formation in southern Sweden comprises mafic to felsic volcanic rocks and syn-eruptive volcaniclastic and volcanogenic sedimentary deposits. Field observations indicate terrestrial to fluvial deposits – predominantly with massive to highly vesicular lavas, but also with ignimbrites, ashes, redeposited volcanic rocks and conglomerates. Three groups of volcanic rocks can be distinguished geochemically: (1) primitive basalts to basaltic andesites, (2) dacites-andesites and (3) rhyolites. The groups constitute a calc-alkaline to shoshonitic series. Large ion lithophile elements and light rare earth elements enrichment, combined with depletion in heavy rare earth elements and high field strength elements, particularly in Nb (and Ta), are characteristic of typical arc rocks. Discriminant diagrams suggest emplacement along a one-time active continental margin, which is now marked by the Transscandinavian Igneous Belt (TIB): an array of ∼1.85–1.65 Ga granitic to gabbroic plutonic rocks and associated volcanic rocks more than 1000 km long. In the TIB, the Malmbäck Formation forms mega-xenoliths and other inclusions. U–Pb secondary ion mass spectrometry dating of zircon from a rhyolite of the Malmbäck Formation yielded a concordia age of 1796 ± 7 Ma, which is interpreted as the crystallisation age of the rhyolite and the age of deposition of the Malmbäck Formation, during a relatively early part of the formation period of the TIB. Altogether, the setting, composition and age of the Malmbäck Formation and the TIB confirm the existence of an Andean-type active continental margin in the present southern part of the TIB at c.1.8 Ga.

Fluid evolution in gold‐bearing quartz veins associated with feldspar porphyry dikes at Vinliden in the Skellefte District, northern Sweden

Abstract The Vinliden gold mineralization is located in the northwest‐em part of the Skellefte District in northern Sweden. The mineralization consists of a number of gold‐bearing quartz veinlets in a greywacke sequence. The quartz veins have been emplaced in small‐scale second order fault and shear zones within one of the major fault structures, which transects the folded supracrustal pile in the Skellefte District The Vinliden gold occurrence is spatially related to feldspar porphyry dikes which have intruded the greywackes. Alteration zones with albite or sericite surround the quartz veinlets. The mineralization also includes a sulfide assemblage dominated by arsenopyrite and pyrrhotite. Aqueous and carbonic (CO2‐CH4) fluid inclusions in the quartz veinlets indicate initial deposition of quartz and an early generation of arsenopyrite at a temperature exceeding 400 °C in association with a pressure drop from 5 to 1 kbar. Secondary fracturing opened passageways and allowed fluid pressure to adjust to hyd...