ORE POTENTIAL OF THE ARAMINLAMPI MASSIF. THE QUESTION OF EXTENDING THE KOTALAHTI NICKEL BELT TO THE LAKE LADOGA AREA (PRILADOZHYE)

Abstract

The Araminlampi massif is part of the Kaalamo clinopyroxenite-gabbro-norite-diorite complex dated at 1888.3 ± 5.2 Ma. This age is older than the ore-bearing intrusive units in the Kotalahti belt (1875–1885 Ma). Geodynamic reconstructions show that the Kotalahti belt is not traced into the Kaalamo intrusion area in the Russian territory, but it seems to extend only to the south of the Meieri thrust zone. The predominant petrotypes of Kaalamo rocks are diorite, gabbrodiorite and the less common metaclinopyroxenite; the Kotalahti intrusive units are dominated by peridotite, harzburgite, wehrlite, lherzolite, websterite, norite and gabbronorite. The distribution spectra of rare-earth elements and the Ce/Yb and Th/Yb ratios of Araminlampi rocks are similar to those of the ore-free massif differentiates in the Kotalahti belt. Olivin in Kaalamo ultramafics contains more iron (58–70 % forsterite) than Kotalahti (63–85 % forsterite; 78–85 % in ore-bearing rocks). The major ore minerals in the Kotalahti deposits are pyrrhotite, pentlandite and chalcopyrite, and the major ore mineral in the Araminlampi massif is pyrrhotite. In this structurally differentiated massif (olivine clinopyroxenite, clinopyroxenite and gabbro) pyrrhotite dissemination with minor quantities of other sulphides (chalcopyrite, pyrite, pentlandite, etc.) forms a ~ 10 m thick ore horizon with 0.1–0.2 % Cu, 0.03–0.1 % Ni, ΣPGM, 0.2–0.5 g/t Au and 70–90 g/t Sc in the olivine pyroxenite-pyroxenite transition zone. The 20–30 cm thick quartz veins cross-cutting the metapyroxenites near the ore horizon display abundant silver-bismuth mineralization (Ag – up to 550 g/t, Au – 0.2 g/t and Bi – 0.1 %). Palladium bismuthotellurids that predominate in the PGM-mineral association of the ore horizon were crystallized at a temperature (< 500 °C) below a typical sulphide solidus, and part of merenskyite phases under subsolidus conditions (~ 700 °С). All the rocks of the massif are highly amphibolized and cut by pegmatite veins. Primary minerals, such as diopside, olivine, chrome-spinels and bytownite, occur only as relics. Olivine, together with serpentinization, is altered to amphibole-magnetite symplectites. The Araminlampi ore occurrence displays signs of both late- and post-magmatic genesis, and the formation of molybdenite, scheelite, sphalerite, uraninite and uranothorianite, together with palladium bismuthotellurids at the final mineral formation stage suggests the involvement of fluids associated with the granites and pegmatites that cross-cut the massif.