Mafic-ultramafic Layered Intrusions Research Articles

Formation of Intergrowths of Platinum-Group Minerals and Gold from Magmatogenic Ores in Relation to Phase Changes in Pt-Pd-Fe-Cu-Au System

The article discusses the features of the chemical composition and the formation of intergrowths of platinum-group minerals, gold, gold-bearing phases, and other ore minerals present in placers collected from the Anabar River in the northeast part of the Siberian platform. Based on an analysis of changes in the phase compositions of these intergrowths of noble metals with other ore minerals on (Pt, Pd)-Fe-Au and Pd-Cu-Au phase equilibrium diagrams, potential trends in the crystallization of natural polymineral alloys from multicomponent low-sulfide metallic liquids are discussed. The similarity of the microstructures of natural and metallurgical alloys indicates that the formation of natural multiphase Au-PGE intergrowths occurred in a similar manner to the crystallization of multicomponent synthetic alloys. The authors suggest that magmatic Au-PGE mineralization occurs during the crystallization of a noble-metal-containing, low-sulfide, Cr-rich oxide melt separated from silicate mafic–ultramafic magma. Magmatic gold–platinum deposits are commonly associated with sulfide or oxide disseminated-schlieren ores in layered mafic–ultramafic intrusions. However, due to the high solubility of gold and platinoids in sulfide minerals, PGMs in sulfide ores occur as isomorphic impurities or as microphases and dispersed inclusions that cannot form placers. Therefore, the authors suggest that magmatic Au-PGE mineralization occurs during the crystallization of an immiscible low-sulfide, high-Cr oxide liquid separated from silicate mafic–ultramafic magma. In the northeast part of the Siberian platform, potential sources for these placers are likely alkaline, high-Ti mafic–ultramafic intrusions, as confirmed by the presence of silicate inclusions in ferroan platinum similar in composition to melteigite.

Microstructural constraints for the formation of net-textured Fe-Ti oxide ores of the Baima layered intrusion in the Emeishan large igneous province, SW China

The mafic–ultramafic layered intrusions in the Emeishan large igneous province (LIP) host significant amounts of Fe, Ti and V resources. The Fe-Ti oxide ore layers hosted in these layered intrusions are variable in thickness and it is enigmatic how the large amounts of Fe-Ti oxides are accumulated to form the Fe-Ti oxide ore layers. The Fe-Ti oxide ore layers are mainly composed of net-textured ores, however, the processes to form the net-textured ores have not yet been well constrained due to lack of diagnostic microstructural evidence of Fe-Ti oxides in the ores. Here we use the electron back-scattered diffraction (EBSD) technique to characterize the microstructures of Fe-Ti oxides in the net-textured ores of the Baima layered intrusion in the Emeishan LIP, e.g., crystallographic orientation and neighbor-/random-pair misorientation, and to assess the formation model of net-textured ores. The net-textured ores are composed of silicate crystals (olivine, and minor plagioclase and clinopyroxene) isolated by interconnected matrix composed of Fe-Ti oxides (titanomagnetite and ilmenite) and minor sulfides. It is likely that the Fe-Ti oxides simultaneously crystallized from a common Fe–Ti–(P)-rich melt, but postdated olivine and other cumulus minerals. The compaction induced by the gravity of overlying cumulus minerals only caused weak intragrain deformation of Fe-Ti oxides so that the Fe-Ti oxides did not experience intense dynamic recrystallization. Instead, the relatively high temperature and water-rich condition facilitated the operation of static recrystallization, resulting in the straight or smoothly curved grain boundaries of Fe-Ti oxides. The neighbor-pair misorientation patterns of Fe-Ti oxides show the preference for low-angle misorientation, supporting the idea that adjacent crystallized Fe-Ti oxides were rotated to coincident orientation in a melt. This study provides new microstructural evidence for the presence of immiscible Fe-rich melt from which the Fe-Ti oxides crystallized to form the net-textured ores.

Within-plate magmatism in the southern Borborema Province (NE Brazil): Mantle plumes associated with the Nuna-Columbia and Rodinia breakup?

Formation and fragmentation of landmasses, such as the Nuna/Columbia and Rodinia supercontinents, are influenced by mantle plume activity during rifting and breakup. In the Borborema Province, NE Brazil, within-plate magmatism at ca. 1.6 and at ca. 0.8 Ga is evidenced by the formation of mafic-ultramafic volcanic and plutonic rocks, mafic-ultramafic layered intrusions, and A-type granite intrusions. The Paulistana and Santa Filomena complexes, located in the Riacho do Pontal Orogen, southern Borborema Province, correspond, respectively, to metaplutonic/volcanic-sedimentary and metavolcanic-sedimentary sequences. These sequences are composed of metamorphic rocks of the greenschist to amphibolite facies whose basement corresponds to the Paleoproterozoic rocks of the Itaizinho Complex, which is located in the southern portion of the São Pedro Terrane. The combined analysis of new field, petrographic and geochemical data obtained for the metavolcanic rocks of the São Pedro Terrane and Riacho do Pontal Orogen aided the determination of their tectonic setting. Whole-rock geochemical analyses revealed the predominance of andesites, basaltic andesites and tholeiitic alkali basalts generated in a within-plate environment (E-MORB) with shallow melting of the metavolcanic rocks of the Itaizinho Complex. In the Riacho do Pontal Orogen, basaltic to alkali-basaltic tholeiites are the dominant metavolcanic rocks of the Paulistana and Santa Filomena complexes. Tectonic setting discriminant diagrams indicate within-plate (E-MORB/OIB), deep and shallow melting environments, respectively. This study, combined with data available in the literature, suggests that the metavolcanic rocks of the Itaizinho Complex and metavolcanic rocks of the Paulistana/Santa Filomena complexes were formed in an extensional setting related to Calymmian (1631 Ma) and Tonian (882 Ma) mantle plumes, respectively, precursor to the oceanic crust between the São Pedro Terrane and the São Francisco Craton, represented by the metabasalts of the Monte Orebe complex, which underwent re-accretion and reworking during the Brasiliano-Pan-African Orogeny.

Geochemistry of 2.45 Ga mafic dykes in northern Finland: Constraints on the petrogenesis and PGE prospectivity of coeval layered intrusions

The Karelian craton contains abundant ~2.45 Ga mafic dykes that are compositionally, temporally and, in some cases, spatially related to PGE-mineralised mafic-ultramafic layered intrusions. The dykes can be sub-divided into four groups, namely siliceous high-magnesian basalts (SHMB), gabbronorites (GBNO), low-Ti tholeiites and Fe-rich tholeiites. In this study, we group the SHMB and GBNO dykes to one group as SHMB group, and the two tholeiitic group dykes as tholeiite group, based on their similar geochemical and mineralogical features. In the SHMB group dykes, plagioclase has initial 87Sr/86Sr ratios of 0.7028 to 0.7036 and initial bulk-rock εNd values vary from −2.5 to −1.0, indicating moderate degrees of contamination with Archaean basement. Tholeiitic dykes show a less-radiogenic Sr isotope composition with an average initial 87Sr/86Sr ratio of 0.7024 and higher initial εNd values ranging from +0.3 to +1.7. Thermodynamic and geochemical modelling suggests that the SHMB group dykes could have formed by crustal contamination of a komatiitic magma at deeper crust followed by fractional crystallisation at shallower depth, whereas the tholeiitic group mainly experienced fractional crystallisation with less crustal contamination. Alternatively, the SHMB dyke s may have been derived from a SCLM mantle whereas tholeiitic dykes from a plume mantle, or the two types of dykes derived from different part of a mantle plume with different melting degrees, though these latter two models are not favoured in this study. Based on trace element and isotope characteristics, the SHMB dykes are suitable candidates for the parental magmas to some of the Finnish PGE-mineralised intrusions (e.g., Penikat and Portimo), whereas the tholeiitic dykes may represent the parental magma of the Tsipringa layered intrusion in Russia. Both the SHMB and tholeiitic dyke types are fertile with regard to PGE, with up to 10–20 ppb Pt and Pd and mantle-like Cu/Pd ratios in their least evolved members, suggesting that the magmas remained sulphide undersaturated during mantle melting and en route to the upper crust. This interpretation is consistent with the fact that most of the ~2.45 Ga Fennoscandian layered intrusions contain PGE mineralisation. Sulphide melt saturation in the dykes and layered intrusions was mostly attained after their final emplacement, likely due to crystal fractionation.

Origin of chromitites in the Norilsk-1 intrusion (Siberian LIP) triggered by assimilation of argillaceous rocks by Cr-rich basic magma

Сhromite-rich rocks and monomineralic chromitites occur in mafic-ultramafic layered intrusions worldwide, and are economically important as they host major resources of chromium and platinum group elements. One of the acknowledged genetic concepts for such mineralization advocates bulk chromite crystallization in a response to hybridization of Cr-rich basic magma with felsic melts, derived from partial melting of the wall rocks. However, there has been lack of direct insights into this process and its details have been remaining largely obscured. In this study, we report data on chromite-rich assemblages of the Noril'sk-1 intrusion (Siberian LIP), with a particular focus on chromite-hosted multiphase inclusions. Composition of the latter is different from the rocks of the Noril'sk-1 intrusion and inherits geochemical fingerprints of the wall rock argillites and, therefore, represent snapshots of the heavily-contaminated medium of the magma-wall rock reaction front. Mineral relationships in chromite-rich breccias with fragments of the wall rocks also provide valuable insight into chromite mineralization mechanisms. Our results, along with geological evidence and data on other Noril'sk-type intrusions, indicate that bulk crystallization of chromite, aided by partial suppression of silicate crystallization, occurred in the hybrid medium during digestion of the wall rocks by ascending and emplacing basic magma. Continuous flow of the magma around outshoots of the wall rocks or through a “magmatic karst” in the wall rocks, allowed for a continuous precipitation of chromite. Concentration of chromite grains to form dense mineralization apparently occurred due to formation of chromite-rich blobs around the wall rock fragments, and selective collection and transport of chromite by fluid bubbles, which formed via degassing of wall rocks and then carried chromite and relics of the wall rocks (xenoliths) to the upper parts of the pluton. We propose that assimilation-driven formation of chromite-rich lithologies in intrusions requires: (1) sufficient Cr contents in magma, which allow for its oversaturation in Cr-spinel only by a sudden cooling and addition of SiO2, K2O or/and H2O; (2) efficient disintegration and digestion of the host rocks, releasing considerable amounts of these components to the magma, and (3) a mechanism ensuring accumulation of excess chromite within a small volume (e.g. continuous reaction of Cr-rich magma with the products of the host rocks' assimilation or/and mechanical concentration of chromite).

Hyperspectral core scanner: An effective mineral mapping tool for apatite in the Upper Zone, northern limb, Bushveld Complex

The technological advances in efficient, rapid, and non-destructive hyperspectral core logging systems for systematic mineral mapping have led to the discovery and exploitation of new mineral deposits Hyperspectral imaging in the long-wave infrared range has been recently used successfully to identify various phosphate-bearing minerals (monazite, xenotime, and britholite), with limited work on apatite associated with mafic-ultramafic layered intrusions. In this study we investigate the effectiveness of a hyperspectral imaging (HSI) system with long-wave infrared (LWIR) bandwidthsto identify apatite in the Upper Zone of the Bushveld Complex. The accuracy of the HSI results was validated by mineralogical and geochemical data. The two apatite-enriched zones detected by HSI suggesting widespread development of apatite throughout the uppermost 600 m of the Upper Zone. The lower apatite-enriched zone is approximately 40 m thick, while the upper apatite-enriched zone is about 23 m thick, consistent with previous thickness determinations by traditional logging and analytical methods. Spectral mixing observed in the response of apatite is ascribed either to the common association of apatite and olivine in these rocks, or to differences between the spatial resolution of the hyperspectral image and the size of apatite grains. The VNIR-SWIR wavelength region did not show prominent spectral features of apatite. Nonetheless, HSI in the LWIR range is effective in mapping apatite and should therefore be considered as an exploration tool. This research advances our knowledge of the reflectance spectroscopy of REE-bearing minerals, which makes it easier to detect, identify, and quantify REE-bearing silicate minerals by HSI.

Petrogenesis of the Kalka, Ewarara and Gosse Pile layered intrusions, Musgrave Province, South Australia, and implications for magmatic sulfide prospectivity

The Musgrave Province of central Australia was the focus of long-lived mantle upwelling that produced large volumes of magnesian basaltic to tholeiitic magma and their felsic derivatives. The Musgrave Province contains one of the greatest concentrations of mafic–ultramafic layered intrusions globally, grouped as the Giles intrusions. In the present paper, we study the magmatic ore potential of the Kalka, Gosse Pile and Ewarara layered intrusions located in South Australia. Ewarara and Gosse Pile appear to have relatively low potential for platnium-group element (PGE) reefs and magmatic Ni–Cu, based on lack of evident metal enrichment and the absence of a mafic–ultramafic transition zone that hosts most PGE reefs globally. However, mafic–ultramafic pipes within the intrusions that could have higher ore potential have not been studied by us. At Kalka, the mafic–ultramafic transition interval is exposed, rendering this intrusion potentially more prospective for PGE reefs. However, based on the available data, this zone appears to be barren. Instead, there are signs of PGE enrichment and metal ratio variation in the magnetite-bearing upper portion of the intrusion suggestive of undiscovered PGE reefs. This interpretation is consistent with subtle Cu–Pd enrichment of soils adjacent to the upper portion of the intrusion. KEY POINTS First assessment of magmatic ore potential of Kalka, Ewarara and Gosse Pile layered intrusions in South Australia. Kalka shows signs of PGE enrichment in upper, magnetite-bearing portion of intrusion, suggesting enhanced potential for a PGE reef. Ewarara and Gosse Pile appear to be less prospective for PGE–Ni–Cu, but picrite pipes remain unstudied.

Distribution of Co-Ni in mafic-ultramafic layered intrusion and implications for formation of magmatic deposits: A case study of the Stillwater Complex

美国Stillwater岩体是全球典型的大型层状岩体之一,由底部基底岩系、超镁铁岩系和条带状岩系组成。本文分析了该岩体不同岩石类型中橄榄石、辉石、铬铁矿及全岩的镍(Ni)、钴(Co)含量变化并揭示其控制因素。在超镁铁岩系内,橄榄石的Ni、Co含量分别为2070×10^-6~3019×10^-6和105×10^-6~205×10^-6,铬铁矿的Ni、Co含量分别为434×10^-6~1843×10^-6和221×10^-6~737×10^-6,均高于共生的斜方辉石(Ni:464×10^-6~678×10^-6;Co:31×10^-6~95×10^-6)及单斜辉石(Ni:237×10^-6~378×10^-6;Co:15×10^-6~45×10^-6)。从岩体剖面上来看,最底层岩性旋回由于尚未发生铬铁矿的堆晶,橄榄石具有较高的Co含量(173×10^-6~193×10^-6),而相应G-H岩性旋回中铬铁岩所含橄榄石的Co含量由于铬铁矿的堆晶而明显降低(125×10^-6~141×10^-6)。橄榄石的大规模结晶导致最底层岩性旋回层中的铬铁矿Ni含量(434×10^-6~558×10^-6)明显低于其它含铬铁岩的岩性旋回层(B岩性旋回层:700×10^-6~1843×10^-6;G-H岩性旋回层:641×10^-6~1206×10^-6;J-K岩性旋回层:1046×10^-6~1135×10^-6)。此外,橄榄石及铬铁矿二者的矿物比例与超镁铁岩带全岩的Co/Ni含量比值具有明显的负相关性,表明这两种矿物对熔体中Ni、Co元素的控制作用。在条带状岩系中,由于橄榄岩及铬铁矿仅在局部出现而未发生大规模堆晶作用,斜方辉石(Ni:375×10^-6~652×10^-6;Co:101×10^-6~112×10^-6)与单斜辉石(Ni:278×10^-6~384×10^-6;Co:41×10^-6~56×10^-6)的Ni、Co含量与下部岩系相比均略有增加,但全岩的Ni、Co含量(Ni:88×10^-6~947×10^-6;Co:21×10^-6~53×10^-6)相比于超镁铁岩系的Ni、Co含量(Ni:698×10^-6~1755×10^-6;Co:99×10^-6~207×10^-6)明显降低,暗示橄榄石及铬铁矿在超镁铁岩系的大规模结晶已经导致了熔体的Ni、Co亏损,从而无法发生大规模的Ni-Co硫化物矿化。而硫化物的缺失使得铂族元素(PGE)更多由铬铁矿结晶所释放出的流体汇聚并向上迁移,最终被岩体的下、中部条带状岩系中橄榄岩亚带OB-I和OB-V内饱和熔离的硫化物所捕获,形成世界级的J-M Reef和Picket Pin PGE矿化层。

The distribution of trace elements in sulfides and magnetite from the Jaguar hydrothermal nickel deposit: Exploring the link with IOA and IOCG deposits within the Carajás Mineral Province, Brazil

The Jaguar nickel deposit represents an important and unconventional discovery of hydrothermal Ni resources (58.9 Mt @ 0.95% Ni) associated with magnetite and apatite within the Carajás Mineral Province, Northern Brazil. The Jaguar deposit shares several similarities with iron oxide copper–gold (IOCG) deposits of the Carajás Mineral Province, especially regarding the structural control, and the hydrothermal alteration. Although IOCG and Kiruna-type iron oxide-apatite (IOA) deposits are closely associated in several geological provinces, this association has not yet been fully constrained in the Carajás Mineral Province. The Jaguar deposit occurs near mafic–ultramafic intrusions, along a regional fault zone, and is hosted either by granitic (northern portion) or felsic subvolcanic (southern portion) rocks. It is characterized by sub-vertical zones hosting a biotite-chlorite (Bt-Chl) hydrothermal alteration with ductile structures and disseminated sulfides (pyrite and millerite) and magnetite. These zones are overprinted by sulfide-magnetite-apatite-bearing breccias, which represent the main mineralization bodies. The sulfide assemblage in the mineralized zones is dominated by pyrite, pentlandite and millerite, with minor sphalerite and chalcopyrite.We have investigated the concentration of trace elements in the magnetite and sulfides from the host rocks, different alteration facies and the mineralization at the Jaguar deposit by LA-ICP-MS. Magnetite composition ranges from higher Al, Mn, Ti and V contents, for at least part of those in host rocks (granitic and felsic subvolcanic) and Bt-Chl alteration, into lower contents in the magnetite-apatite alteration, associated with the main Ni mineralization. Magnetite with higher Ti and V contents in host rocks is associated with discrete hydrothermal alteration pockets, have high Ni/Cr ratios and plot in high temperature hydrothermal fields in multi-element diagrams. Therefore, we support that these features are compatible with a hydrothermal origin. Anomalously high-Ni contents in magnetite point that the Neoarchean mafic–ultramafic layered intrusions of the Carajás Province represent potential Ni sources for hydrothermal remobilization. Anomalously high V contents in magnetite suggest lower fO2 conditions upon the formation of the deposit. The concentration of trace elements in magnetite associated with the Jaguar deposit is similar to those from other IOCG deposits from the Carajás Province (but with higher V and Ni contents), especially the Sossego mine in the Southern Copper Belt, indicating that it could represent a Ni-rich member of the regional-scale IOCG mineral system at Carajás. Pyrite and chalcopyrite have a composition similar to those from magmatic deposits, when using previously proposed discriminant diagrams, but this is also the case for other IOA and IOCG deposits. However, low PGE contents (Ru, Rh, Ir and Os) in pyrite from the Jaguar deposit support a hydrothermal origin. Our results highlight that the classifications provided by available discriminant diagrams are not unequivocal. We suggest that multi-element diagrams represent a complementary approach to binary plots and provide a more comprehensive classification for the use of indicator minerals.

Ancient crust-derived syenitic and A-type granitic intrusions in the Emeishan large igneous province, SW China

Within the inner zone of the Emeishan large igneous province (ELIP), SW China, there are several small syenitic and A-type granitic intrusions not spatially related to giant Fe–Ti–V-deposit-bearing mafic–ultramafic layered intrusions. The representative Heime syenitic and Huili granitic intrusions as well as their spatially associated mafic dykes were developed at ca. 260 Ma, contemporaneous with the ELIP. The Heime metaluminous syenites are K-rich and have moderate SiO2 (60.1–61.4 wt%) and high total-alkali (Na2O + K2O = 9.68–10.7 wt%) contents with high K2O/Na2O ratios (1.18 to 1.74). The Huili metaluminous granites are K-rich A-type granites with moderate–high SiO2 (62.7–68.9 wt%), high total-alkali (7.96–8.62 wt%) and Zr (324–853 ppm) contents, and high 10,000 × Ga/Al ratios (2.68–3.22). Both of them are enriched in high-field-strength elements and light rare-earth elements, with significantly negative Nb–Ta–Sr–P–Ti anomalies. They also have highly evolved whole-rock SrNd isotopic compositions (Heime syenites, (87Sr/86Sr)i = 0.7092–0.7105, εNd(t) = −7.77 to −6.09; Huili granites, (87Sr/86Sr)i = 0.7116–0.7146, εNd(t) = −6.34 to −5.32). Zircon δ18O values of the Heime syenites (5.73‰–6.40‰) are slightly higher than those of the mantle zircons, and lower than those of the Huili granites (5.98‰–7.52‰). Zircon Hf 8isotopic compositions of the Huili granites (εHf(t) = −3.30 to +0.73) are higher than those of the Heime syenites (εHf(t) = −7.08 to −2.97), possibly indicating a more contribution of mantle component. In addition, the spatially associated mafic dykes display considerably higher εNd(t) values (Heime mafic dykes, −3.95 to −3.01; Heime mafic dykes, −0.58 to −0.51) than those of the Heime syenites and Huili granites. The parental magmas of both the Heime syenites and the Huili A-type granites show a close affinity to the K-rich, metaluminous syenitic magmas. Based on the Sr–Nd–O–Hf isotopic compositions of the Heime syenites, Huili granites, and spatially associated mafic dykes, we suggest that the K-rich syenitic magmas parental to the Heime syenites were produced mainly by remelting of Paleo–Mesoproterozoic mafic meta-igneous rocks, whereas those parental to the Huili A-type granites were generated by the remelting of Paleo–Mesoproterozoic basement rocks and mixing with small amounts of coeval mantle-derived basaltic melts.

Mg–Fe Isotopes Link the Geochemical Complexity of the Coldwell Complex, Midcontinent Rift to Metasomatic Processes in the Mantle

Abstract Mafic intrusions in the Coldwell Complex have previously been interpreted as forming from a metasomatized mantle source. To build upon our understanding of this metasomatism, the Mg–Fe isotope compositions of these rocks have been determined, and variations are assessed with respect to the magmatic processes that could have occurred at different stages of their formation. The mineralized Marathon Series (δ26Mg = −0.28‰ to −0.19‰), associated metabasalt (δ26Mg = −0.24‰ to −0.23‰), and the Geordie Lake gabbro (δ26Mg = −0.31‰ to −0.22‰) are characterized by δ26Mg values that are within the range of mantle values, whereas the unmineralized Layered Series (δ26Mg = −0.2‰ to −0.05‰) is heavier than mantle. In contrast, the δ56Fe values of all the Coldwell basaltic–gabbroic rocks (δ56Fe = 0.07 ± 0.08‰) are heavier than mantle but within the range of terrestrial basalts and mafic–ultramafic layered intrusions. We propose that the Mg–Fe isotope compositions of these rocks was not significantly modified by processes such as partial melting or garnet retention/fractionation in the mantle, fractional crystallization, or contamination during ascent through the crust, as the isotope values do not correlate with proxies for these processes (e.g. La/Sm and La/Yb, Gd/Yb, MgO–CaO–TiO2, and Th/Nb and Th/La, respectively). Their isotope compositions are, therefore, proposed to reflect the compositions of their metasomatized mantle sources. We conclude that metasomatism was not caused by a carbonate melt, subduction-altered oceanic crust and sediments, or an evolved silicate melt, as these processes generate light δ26Mg, variably fractionated δ56Fe, and heavy δ56Fe values, respectively, which are not observed in our dataset for the Coldwell Complex. The agent that metasomatized the mantle beneath the Coldwell Complex was likely slab-derived fluids characterized by isotopically heavy δ26Mg and basaltic δ56Fe values. This scenario can account for the lack of Fe isotope fractionation from basaltic values in all of the Coldwell rocks. The variably heavier δ26Mg of the Layered Series (−0.20 ± 0.01‰ to −0.05 ± 0.05‰) relative to the mantle (−0.25 ± 0.07‰) suggests that the magmas for the Coldwell rocks were derived by tapping of an isotopically heterogeneous mantle source that had undergone variable degrees of metasomatism. The distinctive geochemistry of mafic sequences in the Coldwell and numerous mafic dykes located in the northeast shoulder of the Midcontinent Rift suggests the presence of a variably metasomatized mantle source beneath a large area of the rift.

Styles of Fe–Ti–V ore deposits in the Neoproterozoic layered mafic-ultramafic intrusions, south Eastern Desert of Egypt: Evidence for fractional crystallization of V-rich melts

The Korab Kansi and Abu Ghalaga Neoproterozoic mafic-ultramafic intrusions in the South Eastern Desert (SED) of Egypt host economic Fe–Ti–V oxide deposits, ∼41 million tons for Abu Ghalaga ores. The Korab Kansi deposits are composed of titanomagnetite with subordinate ilmenite layers in dunites, gabbros and troctolites. The Abu Ghalaga ores consist of hemo-ilmenite and ilmenite with subordinate magnetite and titanomagnetite lenses in norites, gabbros and anorthosites. The difference in calculated oxygen fugacity (fO2: ∼ ΔFMQ–1.24 to −3.28 for Korab Kansi ores and +0.21 to −0.3 for Abu Ghalaga ores) during subsolidus re-equilibration is consistent with different dominant iron and titanium oxides, magnetite and hematite, respectively. Ilmenite in both deposits is enriched in Nb, Ta, Zr, Hf and V, but is poor in Cr and Ni relative to coexisting magnetite, which belongs to magmatic Fe–Ti–V deposits. The Korab Kansi and Abu Ghalaga ore deposits are rich in Ti, Fe, V, Nb, Ta and Hf with subordinate S, Cu, Ga and Zn at high fO2. The Abu Ghalaga gabbroic intrusion crystallized from ferrobasaltic magmas of tholeiitic affinity at lower temperature (∼1082 °C) and pressure (5.1 kbar) than the Korab Kansi intrusion (∼1180 °C, 8.3 kbar). The Korab Kansi ore deposits mainly formed by fractional crystallization of olivine followed by in situ crystallization of Fe–Ti oxides from ferropicritic/ferrobasaltic parent melts at the floor of the magma chamber. The Abu Ghalaga ores formed as a result of more advanced fractional crystallization of ferrobasaltic parent melts. The Abu Ghalaga Fe–Ti oxides in massive ores at the base of the intrusion grew in situ, whereas most ore lenses in the middle to the top of the intrusion precipitated from immiscible oxide melts that separated from the parental magma in association with floatation of plagioclase and sulfide crystals at a late magmatic stage. Both Fe–Ti ore deposits formed from similar mantle-derived magmas but Korab Kansi ores formed as early fractionates, whereas Abu Ghalaga ores formed late. The Korab Kansi ilmenite is rich in vanadium (up to 3.8 wt% V2O5) relative to that of other magmatic deposits. Its parental melt may have formed from locally Fe–Ti–V enriched part of the Neoproterozoic mantle due to interaction with upwelling asthenosphere or a mantle plume. Controlling factors for formation of economic SED Fe–Ti–V deposits were the V-rich ferropicritic/ferrobasaltic compositions of the parental magmas, and addition of H2O to cause high magma ƒO2.

Magnetite layer formation in the Bushveld Complex of South Africa

The great economic significance of layered mafic-ultramafic intrusions like the Bushveld Complex of South Africa results from the existence within them of some layers highly concentrated in valuable elements. Here we address the origins of the Main Magnetite Layer, a globally important resource of Fe-Ti-V-rich magnetite. Previous models of in situ fractional magnetite crystallization require frequent ad hoc adjustments to the boundary conditions. An alternative model incorporating compositional convection near the top of the pile and infiltration of the pile from beneath by migrating intercumulus melt fits observations without any adjustments. Lateral variations in Cr concentration formerly held as indisputable evidence for in situ crystallization can be accommodated better by models of reactive melt infiltration from below. The choice of models has pivotal ramifications for understanding of the fundamental processes by which crystals accumulate and layers form in layered intrusions.

Petrogenesis of the Paleoproterozoic Näränkävaara layered intrusion, northern Finland, Part II: U-Pb ID-TIMS age and Sm-Nd isotope systematics

Several mafic-ultramafic layered intrusions were emplaced in the NE Fennoscandian Shield during a magmatic episode at 2.44 Ga. The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, is one of the largest ultramafic bodies in the Fennoscandian Shield, with a surface area of 25 km x 5 km and a magmatic stratigraphic thickness of ~3 km. The intrusion comprises a 1.3 km-thick peridotitic–dioritic layered series (2436 ± 5 Ma) with two peridotitic reversals, and a 1.5–2 km thick basal dunite series mainly composed of olivine adcumulates (dated here). The intrusion has been studied since the 1960’s, but several questions regarding its structure and petrogenesis remain. The basal dunite shows several lithological features typical of komatiitic rather than intrusive olivine cumulates; namely, >1 km-thick “extreme” olivine adcumulates, some showing textures with bimodal grain sizes, oscillating variations in Mg# with stratigraphic height, and poikilitic chromite. With Archean greenstone belts nearby, it was previously hypothesized that the basal dunite series could represent an Archean komatiitic wall rock to the Paleoproterozoic layered series. However, our new U-Pb ID-TIMS baddeleyite age of 2441.7 ± 0.9 Ma for the basal dunite series shows that the basal dunite and layered series of the Näränkävaara intrusion are co-genetic. New whole-rock Sm-Nd isotope data from key stratigraphic units (initial εNd at 2440 Ma of -3.5 to -1.7) indicate that the intrusion was constructed from repeated emplacement of LREE-enriched high-MgO basaltic magmas that were mantle-derived and contaminated by crust, similarly to other Fennoscandian 2.44 Ga intrusions. The parental magmas show similar compositions regardless of stratigraphic position, suggesting that most wall rock contamination and homogenization had occurred before emplacement, with in situ contamination being a relatively minor process. The open-system features of the basal dunite suggest that it may have formed (at least partly) as a feeder channel cumulate, possibly related to the ~100 km long Koillismaa-Näränkävaara Layered Igneous Complex. The Näränkävaara parental magmas show variably depleted metal ratios and could have potential for orthomagmatic mineral deposits, given the availability of S-rich wall rocks.

Sintering as a key process in the textural evolution of chromitite seams in layered mafic-ultramafic intrusions

ABSTRACT Nearly monomineralic stratiform chromitite seams of variable thickness (millimeters to meters) occur in many of the world's layered mafic-ultramafic intrusions. These seams are often associated with economically significant quantities of platinum group metals, yet the petrogenesis of these societally important materials remains enigmatic. Here we evaluate processes associated with late-magmatic (postcumulus) textural maturation of chromitite seams from four layered mafic-ultramafic intrusions of different ages and sizes. From largest to smallest, these intrusions are the ∼2060 Ma Bushveld Complex (South Africa), the ∼2710 Ma Stillwater Complex (USA), the ∼1270 Ma Muskox Intrusion (Canada), and the ∼60 Ma Rum Eastern Layered Intrusion (Scotland). Three endmember chromitite textures are described, based on chromite grain size and degree of textural equilibration: (1) coarse-grained chromite crystals (>0.40 mm) that occur in the central portions of seams and exhibit high degrees of solid-state textural equilibration; (2) fine-grained chromite crystals (0.11–0.44 mm) at the margins of seams in contact with and disseminated throughout host anorthosite or pyroxenite; and (3) fine-grained chromite crystals (0.005–0.28 mm) hosted within intra-seam orthopyroxene, clinopyroxene, and olivine oikocrysts. Crystal size distribution and spatial distribution pattern analyses are consistent with coarsening occurring through processes of textural maturation, including the sintering of grains by coalescence. We propose that textural maturation initially occurred in the supra-solidus state followed by an important stage of solid-state textural maturation and that these equilibration processes played a major role in the eventual microstructural and compositional homogeneity of the chromitite seams.

Zircon U-Pb isotopic and geochemical study of metanorites from the chromite-mineralised Bacuri Mafic-Ultramafic Complex: Insights of a Paleoarchean crust in the Amapá Block, Guyana Shield, Brazil

The age and tectonic history of the chromite-mineralised Bacuri Complex, a layered mafic–ultramafic intrusion emplaced into Archean terranes of the Guyana Shield in the Amazonian Craton, are here investigated. The stratigraphy of the Bacuri Complex consists of an Ultramafic Zone composed of interlayered metaperidotites and chromitites and two mafic zones composed of metamafic rocks (amphibolites). In this study we present whole-rock lithogeochemical data integrated with zircon U-Pb isotopic and trace-element data from amphibolites from the Mafic Zone. The investigated amphibolites consist of hornblende + plagioclase ± diopside ± biotite with whole-rock compositions like noritic rocks of Archean layered intrusions. These samples are representative of amphibolites with variable proportions of hornblende and plagioclase, interpreted as resulting from amphibolite facies metamorphism of interlayered norite and leuconorite. We suggest that progressive fractionation of a primitive parental magma led to the formation of plagioclase and orthopyroxene cumulates in the mafic zones. We found that zircons from three metanorite samples comprise euhedral crystals with oscillatory zoning (interpreted to preserve a magmatic crystallisation age), as well as variably recrystallised metamorphic zircons. Their U-Pb 207Pb/206Pb ages span over 682 million years, from 3628 Ma to 2942 Ma. Trace element data suggest that magmatic zircons precipitated from late-stage magmatic fluids at ∼ 3.34 Ga (3343 ± 3.5 Ma weighted average age, n = 48, MSWD = 1.3), which defines the Bacuri Complex as one the oldest and best-preserved layered intrusions known. We show that fluid-mediated coupled dissolution-precipitation recrystallisation during metamorphism altered the composition and ages of magmatic zircons. We could not accurately define the number and age of metamorphic events in the 2942–3379 Ma age interval of metamorphic or ‘disturbed’ zircon domains. Our results indicate that continental crust was already established in the Amazonian craton in the Paleoarchean and possibly in the Eoarchean and that it has been reworked since at least the Mesoarchean.

Petrology and geochemistry of early Permian mafic–ultramafic rocks in the Wajilitag area of the southwestern Tarim Large Igneous Province: Insights into Fe-rich magma of mantle plume activity

The majority of global Fe–Ti-oxide deposits hosted in mafic–ultramafic layered intrusions are closely related to mantle plume activity. The metallogenesis of these Fe–Ti-oxide deposits is still debated, especially with regard to their mantle source and relationship to magmatism. Here we report detailed petrology and geochemistry for early Permian (ca. 280 Ma) mafic–ultramafic igneous rocks in the Wajilitag area of the western Tarim Large Igneous Province (TLIP) to establish the relationships among mantle source, magmatism, and metallogenesis in mantle plume activity. The mafic rocks (i.e., gabbro and diabase) show alkaline affinities and OIB-like trace-element patterns. Clinopyroxenites closely related to Fe–Ti-oxide deposits were derived from the same magmatic system as that of the mafic rocks. The primary magma of the Wajilitag mafic rocks was enriched in FeOT and TiO2 and depleted in SiO2, and has high FC3MS (FeOT/CaO − 3MgO/SiO2) and Fe/Mn, indicating the involvement of Si-poor pyroxenite/eclogite in the mantle source. Primary olivines from the Wajilitag gabbro have consistently high NiO and Fe/Mn. Wajilitag mafic rocks have variable Sr–Nd–Hf isotopes owing to their derivation from a hybrid mantle source with the involvement of subducted oceanic crust and sediments. Subducted slabs within the upper mantle are inferred to have been entrained by the upwelling mantle plume and preferentially melted to produce mainly Fe-rich magma. The occurrence of an adjacent oceanic subduction event just prior to the operation of a mantle plume played a crucial role in the formation of large-scale Fe–Ti-oxide deposits in LIPs.

Paleoproterozoic emplacement and Cambrian ultrahigh-temperature metamorphism of a layered magmatic intrusion from the Central Madurai Block, southern India: From Columbia to Gondwana

The Madurai Block in the Southern Granulite Terrane (SGT) of Peninsular India is one of the largest crustal blocks within the Neoproterozoic Gondwana assembly. This block is composed of three sub-blocks: the Neoarchean Northern Madurai block, Paleoproterozoic Central Madurai block and the dominantly Neoproterozoic Southern Madurai Block. The margins of these blocks are well-known for the occurrence of ultrahigh-temperature (UHT) granulite facies rocks mostly represented by Mg-Al metasediments. Here we report a dismembered layered mafic–ultramafic intrusion occurring in association with Mg-Al granulites from the classic locality of Ganguvarpatti in the Central Madurai Block. The major rock types of the layered intrusion include spinel orthopyroxenite, garnet-bearing gabbro, gabbro and gabbroic anorthosite showing rhythmic stratification and cumulate texture. The orthopyroxene-cordierite granulite from the associated Mg-Al layer is composed of spinel, cordierite and orthopyroxene. The pyroxene in both rock units is high-Al orthopyroxene formed under UHT metamorphic conditions. Conventional thermobarometry yields near-peak metamorphic conditions of 9.5–10 kbar pressure and a minimum temperature of 980 °C. We computed P–T pseudosections and contoured for the compositional as well as modal isopleths of the major mineral phases, which yield temperature above 1000 °C. FMAS petrogenetic grid, Al-in-orthopyroxene isopleth, conventional thermobarometry and calculated pseudosection reveal a clockwise pressure–temperature (P–T) path and near isothermal decompression. The U–Pb data on zircon grains from the layered magmatic suite indicate emplacement of the protolith at ca. 2.0 Ga and the metamorphic overgrowths yield weighted 206Pb/238U mean ages ca. 520 Ma. Monazite from the garnet-bearing gabbro and Opx-Crd granulite yielded 206Pb/238U weighted mean ages of ca. 532 Ma and 523 Ma marking the timing of metamorphism. We correlate the layered intrusion to a Paleoproterozoic suprasubduction zone setting, defining the Ganguvarpatti area as part of a collisional suture assembling the Northern and Central Madurai Blocks. The Paleoproterozoic magmatism and late Neoproterozoic-Cambrian UHT metamorphism can be linked to the tectonics of the Columbia and Gondwana supercontinents.

The basal dunite of the Precambrian mafic-ultramafic N\xe4r\xe4nk\xe4vaara 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.

Paleomagnetism of a late Neoproterozoic mafic-ultramafic layered intrusion of Gebel Dahanib, South Eastern Desert (Egypt): Implications for magma evolution and timing of magnetization acquisition

We present detailed rock magnetic and paleomagnetic results for a late Neoproterozoic mafic-ultramafic layered intrusion of Gebel Dahanib, South Eastern Desert, Egypt. Gebel Dahanib intrusion (GDI) is an elliptical body with saucer or funnel shape, and was intruded into deformed country rocks at ~710 Ma. It consists of two main undeformed and unmetamorphosed sequences: I) a lower ultramafic sequence composed mainly of peridotite (dunite and lherzolite cumulate) and pyroxenite (olivine websterite and websterite cumulates). Dunite cumulate occupies the basal part, which grades upward to lherzolite and pyroxenite cumulates at the top of the ultramafic sequence; and II) an upper mafic sequence that comprises olivine gabbronorite, gabbronorite, norite, and a chilled margin of fine-grained gabbronorite. An intrusive contact was observed between ultramafic and mafic sequences, which suggests an asynchronous emplacement history. To understand the magmatic evolution of GDI and to make a new contribution to the small African Neoproterozoic paleomagnetic database, 260 oriented samples were collected from four rock units (peridotite, pyroxenite, gabbro, and fine-grained gabbro). Rock magnetic experiments combined with Fe–Ti microscopic observations and demagnetization procedures indicate that both sequences have different magnetic mineral contents and paleomagnetic components. A stable component with northwesterly declination and shallow negative inclination carried by magnetite and pyrrhotite was isolated from the ultramafic sequence (Dec = 326.6°, Inc = −28.2°, α95 = 5.5°, and k = 58.72), and the corresponding paleomagnetic pole lies at lat = 39.0°N, long = 261.5°E, A95 = 4.6°, and K = 83.13. Paleomagnetic analyses on the mafic sequence yielded northeast-directed declinations, with intermediate positive inclinations (Dec = 11.3°, Inc = 47.6°, α95 = 3.9°, and k = 103.19), and a paleomagnetic pole lat = 78.7°N, long = 96.2°E, A95 = 4.4°, and K = 81.49. We postulate that the Dahanib intrusion was emplaced in two pulses: the first includes the ultramafic assemblages at the base of the intrusion; the second magma pulse makes up the upper mafic sequence. Assessment of Egyptian paleomagnetic poles from a similar time period (~710-540 Ma) indicates that only one pole (out of 21) can be considered reliable, and also from the African dataset only five of 43 poles are of high quality. These indicate that new high-quality paleomagnetic poles are needed to refine the late Neoproterozoic paleomagnetic database for Egypt and the African continent.