Polymetallic metallogeny in the Wiborg batholith, Fennoscandian shield
|Location||International Geological Congress,oslo 2008|
|Author||Sundblad, Krister۱; Cook, Nigel۲; Nygهrd, Rune۳; Eklund, Olav۱; Valkama, Mira۱; Penttinen, Karoliina۱; Nygهrd, Niklas۴; Rimaila, Kristoffer۴; Paadar, Johanna۱; Lammi, Marjaana۱; Seifert, Thomas۵; Karell, Fredrik۴; Huhma, Hannu۶; Airo, Meri-Lisa۶|
|Holding Date||11 October 2008|
The 1.64-1.63 Ga Wiborg rapakivi batholith consists of a variety of A-type granites in the southeastern part of the Fennoscandian Shield. Wiborgites and pyterlites dominate in the entire batholith but the presence of anorthosites in the east, porphyric and recently found volcanic varieties in the west, suggest that the western part represents a higher erosion level. Support for this is provided by the presence of frequent sulphide mineralization, closely associated with ellipsoid structures along the western margin of the Wiborg batholith. This metallogenetic zone extends from the Gulf of Finland and 70 km towards northeast, compring three ore types, considered as end-members of one complex hydrothermal system:
Zn-Cu-Pb-Ag-In-bearing greisen veins is the most extensively distributed ore type. The highest metal contents are at Jungfrubergen (14-28 % Zn, 500-590 ppm In, 300-900 ppm Ag and 0.09 % Mn) and Pahasaari (3-13 % Zn, 40-280 ppm In, < 10 ppm Ag and 0.9 % Mn). The low Mn-grades in Jungfrubergen correlate with an absence of magnetite while the higher Mn-grades in Pahasaari match with the presence of magnetite in the greisen veins and adjacent massive magnetite-sphalerite lenses (see below). The Jungfrubergen vein contains dominant sphalerite and minor chalcopyrite, pyrite, galena, arsenopyrite and cassiterite. No discrete minerals of Ag or In were identified. Coarse-grained sphalerite is homogeneous with no identifiable zoning; 10-100 µm inclusions of cassiterite are conspicuous. The sphalerite contains ca. 1 wt. % Fe and on average 1500 ppm In; Ag, Cd, Sb, Mn, Cu and Se are detected at trace concentrations. Sphalerite, gahnite, herzenbergite, teallite and Mn-rich garnet have been reported from Pahasaari.
In-bearing magnetite-sphalerite ore occurs as dm-sized lenses at Pahasaari in pyterlites and along a km-long structure at Getmossmalmen, where several m-thick massive magnetite-sphalerite lenses occur between hematite-altered rapakivi granites and ignimbrites. Neither the ore type nor the ignimbrites were known in the Wiborg batholith before. Metal contents in the Getmossmalmen magnetite-sphalerite lenses are 3.7-17.3 % Zn, 20-43 ppm In, 50-200 ppm Sn, 0.2-7.5 % Mn, <2 ppm Ag, <200 ppm Pb and <100 ppm Cu. A positive correlation between Sn and Mn is noted. The Getmossmalmen ore contains dominant sphalerite and magnetite (the latter formed from hematite) in a matrix composed of calcite, fluorite, Mn-rich garnet and quartz. Sphalerite displays no identifiable zoning and all In is probably present as solid solution in sphalerite.
Sphalerite and galena, associated with epidote, quartz and K-feldspar, occur at Björnträsket in the inner parts of the ellipsiod structures, probably representing higher depositional temperatures.
The western part of the Wiborg batholith thus represents a new metallogenetic belt, which is the first of its kind in Finland and characterized by anomalous (to ore grade) contents of In, Zn, Cu, Pb, Ag, As, Sn, Fe, Mn and Be.