Age, Isotopic Features, and Formation Type of Rocks and Ores of the Allarechka Cu-Ni Sulfide Deposit, Fennoscandian Shield

The Neoarchean U-Th-Pb (SHRIMP-II) age of 2718 +/- 7 Ma was established for the large copper-nickel Allarechka deposit located in the Kola-Norwegian region of the Fennoscandian shield. The deposit is associated with metamorphosed and deeply eroded volcano-plutonic complexes, which are the age and formational analogues of the Neoarchean greenstone belts. The compositional similarity of hyperbasites of the Allarechka ore district and host metavolcanic rocks of the Annama Formation with komatiite-tholeiitic metavolcanic rocks of the similar age of the Ura Guba-Kolmozero-Voron'ya greenstone belt suggests that they are ascribed to the komatiite-tholeiite volcanoplutonic series, the parental high-Fe komatiite melts of which were derived by partial melting of depleted upper mantle epsilon(Nd)(t) = +7.0 +/- 0.4) at high PT parameters. The petrographic and geochemical composition of ore peridotites indicates that they are a cumulate formed by fractionation of Ol + Opx +/- Pl, Mag from the primary high-Fe komatiite melt in a shallow (P similar to 4 kbar) magma chamber at a liquidus temperature of at least 1200 degrees C. The absence of a significant crustal contamination of the hyperbasites by Mesoarchean TTG-complexes is proved by the trace and rare-earth element geochemistry, the Nd isotopic composition, as well as by the homogeneous morphology and isotopic composition of the accessory magmatic zircon. In contrast, Re-Os and sulfur isotopic systematics of sulfide ores indicate the contribution of not only mantle but likely crustal component in the evolution of sulfide melt. After separation from a primary mantle melt under subcrustal conditions, the silicate and sulfide melts evolved independently. Obtained data suggest that a local Ni, Cu, and PGE-rich mantle anomaly was formed through a plume-lithosphere interaction beneath the Kola-Norwegian Terrane (in its present-day outlines) of the Fennoscandian Shield at 2.75-1.9 Ga. This anomaly was a source of ore matter for three Early Precambrian copper-nickel ore epochs, in particular, for Neoarchean (similar to 2750 Ma) deposits of the Allarechka ore field, Paleoproterozoic deposits (similar to 2500 Ma) of the layered intrusions (e.g., Monchegorsk and Fedorova-Pana), and Pechenga group (similar to 1900 Ma).

Automated summary

The study establishes the Neoarchean age of the Allarechka deposit in the Kola-Norwegian region of the Fennoscandian shield, and suggests a compositional similarity with other metavolcanic rocks in the same age greenstone belt. The ore peridotites are believed to have formed from high-Fe komatiite melts in a magma chamber, while the sulfide ores indicate contributions from both mantle and crustal components.