Geochemistry of similar to 2.45 Ga mafic dykes in northern Finland: constraints on the origin of PGE mineralization in coeval layered intrusions

The Karelian craton contains abundant similar to 2.45 Ga mafic dykes, spatially and temporally related to PGE mineralized ultramafic-mafic layered intrusions. The mafic dykes include siliceous high magnesium basalt (SHMB), gabbronorite (GBNO), Ti-poor tholeiite (TTH) and Fe-tholeiite (FTH). The SHMB and GBNO have Sr-87/Sr-86((i)) ratios (in situ laser ICP-MS analyses on plagioclase) ranging from 0.7028-0.7036, and initial epsilon Nd (whole rock) ranging from -1.0 to -2.5. These data could indicate moderate degrees (about 10%) of contamination with Archean basement or magma derivation from an enriched mantle source. The TTH show a lower radiogenic Sr isotope composition with an average Sr-87/Sr-86((i)) ratio 0.7022, and a higher initial epsilon Nd ranging from +0.3 to +1.7. These data are broadly consistent with magma derivation from a depleted asthenospheric mantle plume. Most dyke types are undepleted in PGE, with 10-20 ppb Pt and Pd, and mantle-like Cu/Pd ratios in the more primitive members, suggesting sulfide undersaturation both in the source and en route to the surface. Sulfide saturation was achieved in the more evolved magmas, and in many cases the sulfides were entrained by the magma, resulting in high Cu and PGE contents (up to 37 ppb Pd). The data indicate that all 4 magma types represented in the dykes are prospective for PGE mineralization.