Chromite Mineralization in the Sopcheozero Deposit (Monchegorsk Layered Intrusion, Fennoscandian Shield)

In 1990, the Sopcheozero Cr deposit was discovered in the Monchegorsk Paleoproterozoic layered mafic-ultramafic layered intrusion (Monchepluton). This stratiform early-magmatic deposit occurs in the middle part of the Dunite Block, which is a member of the Monchepluton layered series. The Cr2O3 average-weighted content in ordinary and rich ores of the deposit is 16.65 and 38.76 wt.%, respectively, at gradually changing concentrations within the rich, ordinary and poor ore types and ore body in general. The ores of the Sopcheozero deposit, having a ratio of Cr2O3/FeOtotal = 0.9–1.7, can serve as raw materials for the refractory and chemical industries. The ore Cr-spinel (magnochromite and magnoalumochromite) is associated with highly magnesian olivine (96–98 Fo) rich in Ni (0.4–1.1 wt.%). It confirms a low S content in the melt and complies with the low oxygen fugacity. The coexisting Cr-spinel-olivine pairs crystallized at temperatures from 1258 to 1163 °C, with accessory Cr-spinel crystallizing at relatively low, while ore Cr-spinel at higher temperatures. The host rock and ore distinguish with widespread plastic deformations of olivine at the postcrystallization phase under conditions of high temperature (above 400 °C) and pressure (5 kbar). At the post magmatic Svecofennian stage (1.84 Ga), the deposit, jointly with the Monchepluton, was subject to diverse tectonic deformations.

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

Several mafic-ultramafic layered intrusions were emplaced in the Fennoscandian Shield during wide-spread mantle-sourced magmatism at 2.5–2.4 Ga. The Näränkävaara intrusion (surface area 5 × 30 km2), northeastern Finland, contains a 1.5–2 km thick basal dunite (not dated), and a 1.5 km thick layered series (2436 ± 5 Ma). A newly discovered marginal series between the basal dunite and the layered series indicates that the basal dunite is older, and highlights the need for further study on their relationship. Along its southern basement contact, the basal dunite contains a 200–300 m thick zone of olivine ortho- and mesocumulates, but the bulk of it is composed of olivine adcumulates and lesser olivine-orthopyroxene heteradcumulates. Based on whole-rock geochemistry, the basal dunite is divided into a low-Fe zone (average FeOt 10.2 wt% and Ni 2250 ppm) and a high-Fe zone (average FeOt 12.5 wt% and Ni 1700 ppm). Both zones have high MgO (32–47 wt%) and varying Cr (830–5160 ppm) and Al2O3/TiO2 (16–26). Textural and geochemical layering is similar along the 30 km strike of the basal dunite. A LREE-enriched high-MgO basaltic parental magma composition (13–18 wt% MgO) is inferred for the basal dunite from olivine–melt mixing trends in orthocumulates. The dunite exhibits at least two geochemical reversals as well as abundant low-porosity adcumulates, poikilitic chromite, and bimodal olivine, suggesting formation in a high-volume open magmatic system. Significant similarity in major and trace element compositions with the Näränkävaara layered series and the Burakovsky intrusion and Vetreny belt extrusives in Russian Karelia suggests that the basal dunite belongs to the Fennoscandian 2.5–2.4 Ga mafic layered intrusions. An Archean komatiitic origin for the dunite body cannot be completely ruled out, however. Distinct Ni-depletion in olivine is found in the basal dunite from the low-Fe zone to the high-Fe zone (3200 versus 2200 ppm). This depletion does not correlate with Fo contents, which suggests that it is not related to olivine fractionation. The basal dunite may thus have potential for Ni-(Cu-Co-PGE) sulfide mineralization.

Central sagging of a giant mafic intrusion: the Ediacaran Sept Îles Layered Intrusion (Québec, Canada)

Anisotropy of magnetic susceptibility data validated by image analysis were acquired on the Sept Îles Layered Intrusion. Most data come from the mafic Layered Series of the intrusion, although the overlying anorthositic Upper Border Series and felsic Upper Series were also investigated. In the Layered Series, Type 1 magnetic fabric defined by gently to moderately dipping foliations and horizontal to moderately plunging lineations dominates over Type 2 fabric characterized by steeply dipping magnetic foliations and lineations of variable plunge, although steep on average. The inverse relationship holds true for the Upper Border Series and only Type 1 fabric occurs in the Upper Series. Several observations, mainly centripetal disposition of the igneous layering, and inclination of the mean Type 1 foliation and lineation towards the deepest point of the intrusion, near its centre, demonstrate post-cumulus, central sagging of the Layered Series. The Type 2 fabric recorded the effect of synmagmatic discontinuities, possibly shear zones, in the Layered Series, its origin being speculative in the Upper Border Series. The mean Type 1 lineations of the Layered Series and the Upper Series are discordant, suggesting that the latter was slightly affected by the sagging of the underlying cumulates, possibly owing to its rapid waning.Supplementary material: Analytical methods (Appendix 1) and equal-area projections of the anisotropy of magnetic susceptibility principal axes (Appendix 2) are available at https://doi.org/10.6084/m9.figshare.c.5057499.v1

Evolution and metallogenic settings of the Pados-Tundra chrome-bearing ultramafic complex, Kola Peninsula: isotope Sm-Nd and U-Pb evidence

<p>The Pados-Tundra massif is located in the western Kola Peninsula and included in the Notozero ultrabasic rock complex (Vinogradov, 1971). The intrusion occurs as a body of ca. 13 km<sup>2</sup> stretched out to the north-east. Enclosing rocks are Archaean granite- and granodiorite-gneisses. There are three major areas in the massif structure (Mamontov, Dokuchaeva, 2005): endocontact area, rhythmically layered series, and upper area. The endocontact area with thickness of 10-20 m occurs as schistose amphibole rocks formed during the metamorphism of main rocks. The rhythmically layered series occurs as a number of rocks from dunites to orthopyroxenites and composes most of the massif. There are 7 rhythms in total, each of which starts with dunites and ends with orthopyroxenites. Dykes of mezo- and leucocratic gabbro, diorites, and hornblendites are developed in the series rocks. The upper gabbronorite area can be partially observed in the north-eastern massif. Presumably, its major volume has been overlapped by enclosing rocks as a result of the overthrust. In the massif, there are 4 horizons of disseminated stratiform chromite ores, which are confined to dunites and serpentinites, as well as to a number of lens- and column-like bodies (podiform type) of chromite ores (Mamontov, Dokuchaeva, 2005; Barkov et al., 2017). Previous isotope-geochronological studies have determined the massif rock age of 2.15 Ga (Shapkin et al., 2008). However, further geological field observations and analysis of the obtained data assume that the intrusive is much older.</p><p>New Sm-Nd geochronological data indicate that the massif rocks and its rhythmically layered series are of Paleoproterozoic age, which is similar to the age of the Cu-Ni-Co-Cr-PGE ore-magmatic system of the Fennoscandian Shield (Amelin et al., 1995; Bayanova et al., 2014, 2017, 2019; Hanski et al., 2001; Huhma et al., 1990, 1996; Layered intrusions ...; 2004; Maier, Hanski, 2017; Mitrofanov et al., 2019; Peltonen, Brugmann, 2006; Puchtel et al., 2001; Serov, 2008; Serov et al., 2014; Sharkov, 2006; Sharkov, Smolkin, 1997). Complex Sm-Nd and U-Pb isotope-geochronological studies have allowed determining the major formation and alteration stages of the Pados-Tundra complex rocks:</p><p>–  formation of the rhythmically layered series rocks of the intrusive 2485±77 Ma, harzburgites of the layered series – 2475±38 Ma;</p><p>– metamorphism of the massif rocks at the turn of 1.95 - 1.9 Ga;</p><p>– postmetamorphic cooling of the complex rocks tо 650°-600°С at the turn of 1872±76 Ma (Sm-Nd for metamorphic minerals) and then to 450°-400°С (U-Pb for rutile, 1804±10 Ma).</p><p>Therefore, the study results expand geography the East-Scandinavian large Palaeoproterozoic igneous province and are prospective for further study of analogous ultramafite-mafite complexes.</p><p>All investigations and were supported by the RFBR 18-05-70082, 18-35-00246, Presidium RAS Program #48 and are in frame of the Theme of Scientific Research 0226-2019-0053.</p>

Metallogenic Setting and Evolution of the Pados-Tundra Cr-Bearing Ultramafic Complex, Kola Peninsula: Evidence from Sm–Nd and U–Pb Isotopes

The article presents new Sm–Nd and U–Pb geochronological data on rocks of the poorly studied Pados-Tundra Cr-bearing complex. It is part of the Notozero mafic–ultramafic complex (western Kola Peninsula) and occurs at the border of the Paleoproterozoic Lapland Granulite Belt and the Archean Belomorian composite terrain. The Pados-Tundra complex hosts two major zones, the Dunite and Orthopyroxenite Blocks. Dunites are associated with four levels of chromite mineralization. Isotope Sm–Nd studies of dunites, harzburgites, and orthopyroxenites from the central part of the complex have been carried out. The isochron Sm–Nd age on 11 whole-rock samples from a rhythmically layered series of the complex is 2485 ± 38 Ma; the mineral Sm–Nd isochron for harzburgites shows the age of 2475 ± 38 Ma. It corresponds with the time of large-scale rifting that originated in the Fennoscandian Shield. When the rhythmically layered series of the intrusion and its chromite mineralization were formed, hornblendite dykes intruded. The U–Pb and Sm–Nd research has estimated their age at ca. 2080 Ma, which is likely to correspond with the occurrence of the Lapland–Kola Ocean. According to isotope Sm–Nd dating on metamorphic minerals (rutile, amphibole), the age of postmetamorphic cooling of rocks in the complex to 650–600 °C is 1872 ± 76 Ma. The U–Pb age on rutile from a hornblendite dyke (1804 ± 10 Ma) indicates further cooling to 450–400 °C. The conducted research has determined the early Proterozoic age of rocks in the rhythmically layered series in the Pados-Tundra complex. It is close to the age of the Paleoproterozoic ore magmatic system in the Fennoscandian Shield that developed 2.53–2.40 Ga ago. Later episodes of alterations in rocks are directly related to main metamorphic episodes in the region at the turn of 1.9 Ga. Results of the current study expand the geography of the vast Paleoproterozoic East Scandinavian Large Igneous Province and can be applied for further studies of similar mafic–ultramafic complexes.

‘They who document/paint the History hold the Power’: Retelling, Reimagining and Recreating New Narratives of Black Heroism in Toussaint I (1987) and Toussaint II (2002)

Across her decades-apart series, Scenes from the life of Toussaint L’Ouverture I (1988) and Scenes from the life of Toussaint L’Ouverture II (2002) which are the subject of this chapter, Himid excavates and examines Louverture’s mythical status as the leader of the ‘only successful slave revolution’. ‘History is painted/documented to glorify the living’, she declares, only to conclude at the end of this same page: ‘They who document/paint the History hold the Power’. In this multi-part and multi-layered series, Himid assumes the role not only of the artist but also of the historian, political commentator and social radical in order to ‘paint the History’ and ‘hold the Power’ over retelling, recreating and reimagining the life and works of this African Caribbean freedom fighter. No lone legend, exceptional icon or ‘single figure’, however, Himid’s Louverture is newly rooted in missing genealogies of African diasporic collective and collaborative militancy. In the Black Lives Matter era, the Louverture we need now is not a legendary lone icon but a fallible human being whose activism is embedded within and not separated from his life as a husband, father and grassroots community campaigner.

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

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.

Oxygen isotope composition of zircons from the Talnakh economic intrusion of the Noril’sk province: first data

This study presents the first results of oxygen isotope analyses (18O) collected on zircons from the Talnakh economic intrusion within the Norilsk province. Zircons from gabbro-diorite, gabbroic rocks of the layered series and plagioclase-bearing wehrlite have similar mantle-like mean 18O values (5,39 0,49; 5,64 0,48 and of 5,28 0,34, respectively), which differ from 18O in zircons from sulfide-bearing melanocratic troctolite with a taxitic texture in the lower part of the intrusion (mean 18O = 6,50 0,98). These new oxygen isotope data support (i) the mantle-derived origin of the primary magma(s), parental to the Talnakh intrusion, and (ii) possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks.