Reef-type PGE-enrichment in the 2.44 Ga mafic-ultramafic Narankavaara layered intrusion, Northern Finland

Several mafic-ultramafic layered intrusions were emplaced in the Fennoscandian shield during a widespread magmatic event at 2.5-2.4 Ga. The intrusions host several orthomagmatic Ni-Cu-PGE and Cr-V-Ti-Fe deposits. We update the magmatic stratigraphy of the 2.44 Ga Narankavaara mafic-ultramafic body based on new drilling by Geological Survey of Finland and Outokumpu Oy. The body consists of a basal dunite (1700 m), and a layered series comprising an ultramafic (600 m) and a mafic zone (700 m). Two reversals are found in the layered series, one in the ultramafic and one in the mafic zone. The composition of the layered series parental magma was approximated using a previously unidentified marginal series gabbronorite. The Cu/Pd ratio below primitive mantle suggests that the parental magma was PGE-fertile. Location of the marginal series indicates that the basal dunite represents an older wall-rock for the layered intrusion. A 25 m thick reef-type PGE-enriched zone with 150-250 ppb Pt+Pd+Au is found in the border between the ultramafic and mafic zones. The PGE reef-forming process may have been interrupted by influx of magma related to the first reversal. PGE-depleted metal ratios suggest that this replenishing magma had already undergone a sulfide saturation event.