Krister Sundblad

Geochemistry and age of two orthogneisses in the Proterozoic Mj⊘sa-Vänern ore district, southwestern Scandinavia

Abstract Two orthogneisses in the southwestern part of the Fennoscandian Shield have been investigated with respect to geochemistry, age and lead isotopic composition in feldspar. Both orthogneisses host gold-bearing quartz veins in the Mjøsa-Vänern ore district. The alkali-calcic Brustad augen gneiss of granitic composition is situated within the Mylonite Zone (MZ), close to Mjøsa in southeastern Norway. The calc-alkaline Harnäs gneiss of granodioritic composition is situated 50 km west of the MZ, northwest of Vänern in southwestern Sweden. U-Pb zircon dating yielded protolith ages of 1674±10 Ma for the Brustad augen gneiss and 1595+24 −17 Ma for the Harnäs gneiss, indicating affinity to the Transscandinavian Igneous Belt (TIB) and the Gothian Åmål granitoid suite, respectively; the extension of TIB units to the northwestern margin of the MZ is thus confirmed. Both rocks were deformed during Sveconorwegian orogenesis (c. 1.15–0.9 Ga). Their feldspar lead isotopic compositions are significantly more radiogenic than expected for initial lead and no vestiges of more primary lead could be detected in pre-deformational grains. Resetting of feldspar lead is suggested to have taken place at around 1000 Ma, in conjunction with Sveconorwegian ductile deformation. A revision of the domain boundaries within the Fennoscandian Shield is proposed, such that the Torsby and Borås Segments are conceptually excluded from the Southwest Scandinavian Domain. Thereby, the eastern boundary of the Southwest Scandinavian Domain is outlined by the MZ. It is further proposed that the Southwest Scandinavian Domain be renamed as the Gothian Domain.

Syngenetic Pb‐Zn‐Cu sulphide mineralization in 1.8 Ga subaerial volcanic rocks, southeastern Sweden

Abstract A Pb‐Zn‐Cu‐(As‐W)‐bearing sulphide mineralization, hosted by the 1800 Ma Smaland volcanics, has been investigated at Alatorp, 40 km ENE of Vaxjo in southeastern Sweden. The Alatorp deposit was subject to small‐scale mining for silver in the 18th century, but due to very low silver contents it has never been economically viable. Galena, sphalerite, and chalcopyrite occur in a dense network of quartz‐carbonate veinlets, hosted by intensely hydrothermally altered volcanic rocks, which are strongly depleted in sodium, calcium, and strontium, and strongly enriched in lead, zinc, copper, tin, and tungsten when compared to unmineralized and unaltered equivalents. The lead isotopic ratios of galena suggest that the hydrothermal process leached metals from the volcanic rocks immediately after their deposition. The Alatorp Pb‐Zn‐Cu mineralization is thus a typical example of a synvolcanic ore‐forming system, which may be compared to the much larger porphyry‐type systems in Cretaceous and Tertiary subaerial...

A Group of Papers Devoted to the Metallogeny of Gold in the Fennoscandian Shield

The Fennoscandian and Ukrainian shield areas contain some of the largest and richest gold deposits in Europe. The earliest records of gold mining in northern Europe go back more than 200 years, but it was not until the last two decades that gold has become the major target in the Fennoscandian shield for most exploration and mining companies. The extent of …