Parental magma, magmatic stratigraphy, and reef-type PGE enrichment of the 2.44-Ga mafic-ultramafic Näränkävaara layered intrusion, Northern Finland

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 SiO 2 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 (10 5) 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.

Geodynamic Implications of Synchronous Norite and TTG Formation in the 3 Ga Maniitsoq Norite Belt, West Greenland

“…Although previous authors have highlighted the role of crustal contamination in the ∼2.7-2.0 Ga norite suite (Barnes, 1989;Bridgwater et al, 1995;Järvinen et al, 2019;Mansur and Barnes, 2020), this model differs substantially in the manner in which the mafic-ultramafic precursors of the norites are generated. Models for younger norites typically invoke melting of highly depleted lithospheric mantle (Hall and Hughes, 1987) or melting of more fertile mantle in a rising mantle plume (Wilson, 2012;Zirakparvar et al, 2019).…”

“…There has been considerable debate as to whether the ∼2.7-2.0 Ga intracratonic noritic intrusions reflect "boninitelike" processes of re-melting of previously depleted and metasomatised mantle (Hall and Hughes, 1987;Smithies et al, 2004;Srivastava, 2008), or alternatively "siliceous high magnesium basalt-like" (SHMB) processes of severe crustal contamination of an ultramafic melt (Barnes, 1989;Bridgwater et al, 1995;Järvinen et al, 2019;Pearce and Reagan, 2019;Mansur and Barnes, 2020). Understanding if one or both of these processes were dominant in producing the ∼2.7-2.0 Ga noritic suite is important for understanding the tectonic regime(s) that globally produced these norites.…”

The basal dunite of the Precambrian mafic-ultramafic Näränkävaara intrusion: Petrogenetic considerations and implications to exploration

“…1). The geology and mineral potential of the Näränkävaara intrusion is still relatively poorly known (Alapieti 1982;Järvinen et al 2020). The presence of a large mass of ultramafic cumulates below the layered series of the Näränkävaara intrusion ( Fig.…”

“…Barnes and Hill 1995). The recent discovery of a marginal series between the Näränkävaara layered series and the basal dunite indicates that the basal dunite must be at least marginally older (Järvinen et al 2020).…”

“…The Tornio-Näränkävaara belt is outlined with a red dashed line, and a white arrow points out Näränkävaara. The Koillismaa complex includes the tectonically dismembered western intrusions and Näränkävaara to the east, connected by a positive magnetic and gravity anomaly presumed to be a "hidden dyke" (thin black dashed line) (map modified after Alapieti 1982) data (Alapieti 1982;Iljina 2003;Vesanto 2003;Lahtinen 2005;Akkerman 2008;Telenvuo 2017;Järvinen et al 2020), we characterize the Näränkävaara basal dunite, model its petrogenesis, and estimate its Ni-(Cu-Co-PGE) potential.…”