Metallogeny of indium in the Svecofennian Domain
|Location||International Geological Congress,oslo 2008|
|Author||Sundblad, Krister۱; Ahl, Martin۲|
|Holding Date||11 October 2008|
The Svecofennian Domain is the most important ore-bearing segment in the Fennoscandian Shield, where a number of metals, including Fe, Mn, Cu, Zn, Pb, Ag, Au, W, Mo and Sn, have been produced from a number of ore types in numerous mines in Sweden, Finland and Russia. In the present investigation, 30 sulphide deposits in the Svecofennian Domain were examined with respect to their In potential.
Ores genetically related to Early Svecofennian (1.9 Ga) geological processes (including Falun, Silvberg, Sala, Regna, Utö, Bersbo and Pyhäsalmi) have (with one exception) In contents <15 ppm; one Zn-rich sample from Pyhäsalmi yielded 39 ppm In. Ores hosted by post-Svecofennian 1.80 Ga granitoids of the Transscandinavian Igneous Belt have also In contents <15 ppm.
Significantly higher In contens (40-600 ppm) were obtained for ores genetically related to 1.85-1.54 Ga post-Svecofennian and anorogenic granites. Six targets with >100 ppm In were identified in ores related to the 1.64 Ga Wiborg and 1.54 Ga Salmi rapakivi granites, while lower contents were recorded for ores in the 1.67 Ga Dala igneous complex and the 1.85 Ga Kamptjärn granite. 40-100 ppm In were recorded in supracrustal-hosted ores in westernmost Bergslagen, an area which comprises the only sites (Långban and Gåsborn) where roquesite so far has been recognized in the Svecofennian Domain. Three main types of In-bearing ores can be distinguished in the Svecofennian Domain:
1. Greisen veins (e.g. Jungfrubergen and Pahasaari, Wiborg batholith; Norra Hålen, Dala igneous complex and Gillerdrågen, Kamptjärn granite).
2. Massive Mn-bearing magnetite-sphalerite ore (Getmossmalmen, near the border between rapakivi granites and ignimbrites within the Wiborg batholith).
3. Exoskarn deposits (e.g. Hopunvaara in the Pitkäranta district, on the western margin of the Salmi batholith).
The In-bearing supracrustal-hosted ores in westernmost Bergslagen are controversial in this context. They include the Getön, Gruvåsen and Björkskogsnäs exoskarn deposits, Ag-rich sulphide veins and Zn-Pb-Mn-rich massive ores at Hällefors as well as parts of the Långban and Gåsborn ores. Although many authors have proposed volcanogenic models for these ores, their similarities with respect to Pb isotopic, paragenetic (e.g. presence of cubanite) and geochemical (particularly In, Sn, As, Ag, Fe and Mn) patterns, compared with the rapakivi-related ores, suggest that the In-bearing polymetallic ores in westernmost Bergslagen belong to an anorogenic epithermal system. A set of large-scale NS-trending faults, connecting westernmost Bergslagen and the 1.85 Ga Kamptjärn granite, support such a link between the polymetallic ores in westernmost Bergslagen and post-Svecofennian/anorogenic granites. The understanding of the complex Mn-bearing iron and base metal ores in western Bergslagen may thus benefit from studies of the In-bearing polymetallic greisen, skarn and magnetite ores related to the anorogenic Kamptjärn, Dala, Wiborg and Salmi granites.