Abstract
About 20 mafic-ultramafic layered intrusions in the northern Fennoscandian shield were emplaced during a widespread magmatic event at 2.5–2.4 Ga. The intrusions host orthomagmatic Ni-Cu-PGE and Cr-V-Ti-Fe deposits. We update the magmatic stratigraphy of the 2.44-Ga Näränkävaara mafic-ultramafic body, northeastern Finland, on the basis of new drill core and outcrop observations. The Näränkävaara body consists of an extensive basal dunite (1700 m thick), and a layered series comprising a peridotitic–pyroxenitic ultramafic zone (600 m thick) and a gabbronoritic–dioritic mafic zone (700 m thick). Two reversals are found in the layered series. The composition of the layered series parental magma was approximated using a previously unidentified marginal series gabbronorite. The parental magma was siliceous high-Mg basalt with high MgO, Ni, and Cr, but also high SiO2 and Zr, which suggests primary magma contamination by felsic crust. Cu/Pd ratio below that of primitive mantle implies PGE-fertility. The structural position of the marginal series indicates that the thick basal dunite represents an older wallrock for the layered intrusion. A subeconomic reef-type PGE-enriched zone is found in the border zone between the ultramafic and mafic zones and has an average thickness of 25 m with 150–250 ppb of Pt + Pd + Au. Offset-type metal distribution and high sulfide tenor (50–300 ppm Pd) and R-factor (105) suggest reef formation by sulfide saturation induced by fractional crystallization. The reef-forming process was probably interrupted by influx of magma related to the first reversal. Metal ratios suggest that this replenishing magma was PGE-depleted before emplacement.
Highlights
The Paleoproterozoic mafic-ultramafic Näränkävaara layered intrusion, northern Finland, is the easternmost member of the Editorial handling: F
We describe a new uneconomic sulfide-associated stratiform PGE-enriched zone, located in the contact between the ultramafic and mafic zones of the Näränkävaara layered series, and present a model that explains the low grade of the PGE mineralization
Conclusions factor PGE reef forming event, similar to those interpreted for other sulfide associated PGE reefs in 2.45 Ga Finnish layered intrusions. – Low whole-rock PGE grades of the Salmilammit reef (SL reef) seem to be caused by low rate of sulfide accumulation relative to simultaneous silicate accumulation, leading to dilution of high-grade sulfides over an average thickness of 25 m
Summary
The Paleoproterozoic mafic-ultramafic Näränkävaara layered intrusion, northern Finland, is the easternmost member of the Editorial handling: F. The intrusions were emplaced during widespread mantle-driven igneous activity related to extension and rifting of the Archean nucleus of the Fennoscandian shield (Amelin & Semenov 1996; Tiira et al 2014; Huhma et al 2018). Approximately 20 early Paleoproterozoic layered intrusions with associated dyke swarms have been found in the NW region of the Fennoscandian shield (Finland, Russia, and Sweden), all with ages between 2.53–2.39 Ga (Alapieti et al 1990; Amelin et al 1995; Vuollo & Huhma 2005; Bayanova et al 2009). Mafic-ultramafic layered intrusions elsewhere with similar structure and composition as
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