ABSTRACT
Epigenetic gold deposits occur in diverse geological settings in the Lapland greenstone belt, which forms part of a NW-SE trending Palaeoproterozoic terrain in the northern Fennoscandian Shield. The Svecokarelian orogeny resulted in compressive deformation and greenschist facies metamorphism throughout the belt at ca. 1.9 Ga.
Sulfide and country rock lead isotope compositions of ten gold deposits have been determined in order to evaluate the lead isotope characteristics and the ages and the sources of gold mineralization. For comparison, sulfides from stratabound base metal deposits and unmineralized volcanics of the Upper Lapponian Group in central Lapland have also been analysed. In addition, several sulfur isotope analyses, suggesting an ultimately magmatic source for the sulfur in the sulfides, were performed.
On the basis of their sulfide lead isotope characteristics the gold occurrences can be divided into relatively nonradiogenic deposits and economically more important, radiogenic deposits. In the nonradiogenic group, the Soretiavuoma, Kiistala, and Kuotko deposits (Soretiavuoma group) show relatively lead rich sulfides with homogeneous lead compositions, resulting from orogenic mixing of mantle and upper crustal material. In the Kuotko area, another gold mineralizing phase involving mantle derived lead is apparent. In the radiogenic group (Saattopora, Hangaslampi, Pahtavaara, Bidjovagge) the sulfide lead isotope compositions form linear trends and exhibit low Th/U and high U/Pb. The similar lead isotope characteristics suggest a common genesis for the gold mineralization in these deposits, and the occurrence of uranium rich inclusions in the sulfides is interpreted as recording crystallization from a relatively oxidized fluid.
Relative timing with respect to intrusive ages of the syn- and postorogenic granites (1900-1880 and 1780 Ma, respectively) permits recognition of the following hydrothermal stages, of which the earliest appear to have been significant for gold mineralization: 1) mineralization of the Soretiavuoma group, associated with the (early- to) synorogenic phase of magmatism, 2) the Saattopora gold mineralization, which took place during the synorogenic phase, 3) a separate gold mineralizing phase at Kuotko, 4) (syn- to) late orogenic metamorphic resetting of the U-Pb systems in the middle and lower unit volcanics of the Upper Lapponi Group, and a distinct late orogenic regional hydrothermal event apparent at Pahtavaara, Hangaslampi, and Bidjovagge, possibly implying related and roughly contemporaneous processes, 5) thucolite and monazite formation at Saattopora coeval with the postorogenic granites, 6) an anorogenic disturbance (ca. 170 Ma) of the U-Pb system in the Saattopora wall rocks, and 7) Phanerozoic separation of Svecokarelian lead with the formation of radiogenic sulfides in the Bidjovagge ore and locally in the Kittilä area.
irmeli.manttari@gsf.fiGeological Survey of Finland
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