Mafic-ultramafic Intrusions Research Articles

Monomineralic anorthosites in layered intrusions are indicators of the magma chamber replenishment by plagioclase-only-saturated melts

The formation of some Earth’s monomineralic igneous rocks appears to be prohibited by constraints imposed by liquidus phase-equilibria on evolution of mantle-derived magmas. Yet, these rocks exist as stratiform layers in many mafic-ultramafic intrusions. One conspicuous example is monomineralic anorthosites in the Bushveld Complex that occur as stratiform layers up to hundreds of kilometres in length. Such monomineralic anorthosites appear to require parental melts saturated in plagioclase only but where and how to produce these melts remains a contentious issue. Here we argue that they are likely sourced from deep-seated magma reservoirs. In response to pressure reduction, these ascending melts become first superheated and then saturated in plagioclase after stalling and cooling in shallow-level chambers. Adcumulus growth of plagioclase from such melts at the chamber floor results in the formation of monomineralic anorthosites. We propose that stratiform layers of monomineralic anorthosites in layered intrusions are products of the chamber replenishment by melts whose saturation in plagioclase as a single liquidus phase is triggered by their transcrustal ascent towards the Earth’s surface.

Geochronology and petrogenesis of the Late Silurian Shitoukengde mafic–ultramafic intrusion, NW China: Implications for the tectonic setting and magmatic Ni-Cu mineralization in the East Kunlun Orogenic Belt

ABSTRACT The Shitoukengde mafic–ultramafic intrusion, located in the East Kunlun Orogenic Belt, is spatially and temporally associated with the Xiarihamu Ni-Cu sulphide deposit, the second-largest Ni deposit in China. This intrusion comprises mainly lherzolite, harzburgite, olivine websterite, and gabbroic rocks. The Shitoukengde mafic–ultramafic rocks are enriched in light rare earth elements (REE) relative to heavy REE and show pronounced negative Nb–Ta anomalies. Magmatic zircon crystals of the studied samples have high δ18O values (6.5 to 8.0‰) with a weighted mean value of 7.3‰ that is higher than the typical mantle value (~5.3‰). They are characterized by high εHf(t) values (–2.0 to 9.2) and variable εNd(t) values (–4.46 to 2.83). Olivine grains from the ultramafic rocks have high forsterite (Fo) values of 79.3–86.8 and variable Ni contents of 746–3347 ppm. Our calculations indicate that parental magma of the Shitoukengde intrusion had a high-Mg basaltic composition with 10.9 wt% MgO and 9.8 wt% FeOT. Fractional crystallization and significant crustal contamination triggered S saturation and sulphide enrichment in the magma. Zircon LA-ICPMS and SIMS U–Pb analyses reveal the consistent Silurian ages of 420–424 Ma for the gabbronorite and Ol websterite, which is coeval with the Xiarihamu Ni-Cu deposit. Based on these data and the regional tectonic setting, we suggest that the Shitoukengde mafic-ultramafic rocks formed in the post-collisional, extensional environment. The cessation of subduction may have resulted in break-off of the dense subducted slab, and triggered ascent of the hot convective asthenospheric mantle. The mantle wedge was modified by the subduction-related fluids and these enriched mantle components were incorporated into the ascending asthenospheric melts at relatively shallow depths, generating the high-Mg basaltic magmas that formed the compositionally variable mafic-ultramafic rocks (e.g., Shitoukengde and Xiarihamu intrusions) in the Eastern Kunlun Orogenic belt.

Petrogenesis of the ~740 Korab Kansi mafic-ultramafic intrusion, South Eastern Desert of Egypt: Evidence of Ti-rich ferropicritic magmatism

The Neoproterozoic Korab Kansi mafic-ultramafic intrusion is one of the largest (100 km2) intrusions in the Southern Eastern Desert of Egypt. The intrusion consists of Fe-Ti-bearing dunite layers, amphibole peridotites, pyroxenites, troctolites, olivine gabbros, gabbronorites, pyroxene gabbros and pyroxene-hornblende gabbros, and also hosts significant Fe-Ti deposits, mainly as titanomagnetite-ilmenite. These lithologies show rhythmic layers and intrusive contacts against the surrounding granites and ophiolitic-island arc assemblages. The wide ranges of olivine forsterite contents (Fo67.9-85.7), clinopyroxene Mg# (0.57–0.95), amphibole Mg# (0.47–0.88), and plagioclase compositions (An85.8-40.9) indicate the role of fractional crystallization in the evolution from ultramafic to mafic rock types. Clinopyroxene (Cpx) has high REE contents (2–30 times chondrite) with depleted LREE relative to HREE, like those crystallized from ferropicritic melts generated in an island-arc setting. Melts in equilibrium with Cpx also resemble ferropicrites crystallized from olivine-rich mantle melts. Cpx chemistry and its host rock compositions have affinities to tholeiitic and calc-alkaline magma types. Compositions of mafic-ultramafic rocks are depleted in HFSE (e.g. Nb, Ta, Zr, Th and U) relative to LILE (e.g. Li, Rb, Ba, Pb and Sr) due to the addition of subduction-related hydrous fluids (rich in LILE) to the mantle source, suggesting an island-arc setting. Fine-grained olivine gabbros may represent quenched melts approximating the primary magma compositions because they are typically similar in assemblage and chemistry as well as in whole-rock chemistry to ferropicrites. We suggest that the Korab Kansi intrusion crystallized at temperatures ranging from ~700 to 1100 °C from ferropicritic magma derived from melting of metasomatized mantle at <5 Kbar. These hydrous ferropicritic melts were generated in the deep mantle and evolved by fractional crystallization under high ƒO2 at relatively shallow depth. Fractionation formed calc-alkaline magmas during the maturation of an island arc system, reflecting the role of subduction-related fluids. The interaction of metasomatized lithosphere with upwelling asthenospheric melts produced the Fe and Ti-rich ferropicritic parental melts that are responsible for precipitating large quantities of Fe-Ti oxide layers in the Korab Kansi mafic-ultramafic intrusion. The other factors controlling these economic Fe-Ti deposits beside parental melts are high oxygen fugacity, water content and increasing degrees of mantle partial melting. The generation of Ti-rich melts and formation of Fe-Ti deposits in few layered intrusions in Egypt possibly reflect the Neoproterozoic mantle heterogeneity in the Nubian Shield. We suggest that Cryogenian-Tonian mafic intrusions in SE Egypt can be subdivided into Alaskan-type intrusions that are enriched in PGEs whereas Korab Kansi-type layered intrusions are enriched in Fe-Ti-V deposits.

Rift-related Neoproterozoic tholeiitic layered mafic intrusions at northern Yangtze Block, South China: Mineral chemistry evidence

Neoproterozoic layered mafic intrusions occur along the northern margin of the Yangtze Block in South China. In this paper, we report the chemical compositions of clinopyroxene, plagioclase, and olivine from two major mafic intrusions at Bijigou (BJG) and Wangjiangshan (WJS), to constrain the parental magma characteristics and geodynamic setting. The olivine and plagioclase compositions indicate that the parental magma for BJG and WJS was an evolved tholeiitic magma with high FeOT/MgO ratios. An origin of arc magmatism is not supported by the following mineralogical and petrological observations: (1) the strong rift-affinity indicated by olivine, plagioclase and clinopyroxene geochemistry, (2) the absence of high anorthite in the cumulate rocks, (3) the paucity of abundant cumulus hornblende crystals, and (4) the lack of voluminous intermediate rocks in the rock associations. We propose that the BJG and WJS mafic-ultramafic intrusions formed under a rift extensional setting during 820–780 Ma. The subduction process ceased before 820 Ma and did not directly contribute to the generation of the BJG and WJS mafic-ultramafic intrusions. The arc-like NbTa depleted trace element pattern in the parental magma is inherited from the mantle metasomatized by earlier subduction processes.

Parental magma, magmatic stratigraphy, and reef-type PGE enrichment of the 2.44-Ga mafic-ultramafic N\xe4r\xe4nk\xe4vaara 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.

Desulphurisation, chromite alteration, and bulk rock PGE redistribution in massive chromitite due to hydrothermal overprint of the Panton Intrusion, east Kimberley, Western Australia

The Central and Western Zones of the Halls Creek Orogen (HCO) of the East Kimberley region of Western Australia host various Paleoproterozoic layered mafic–ultramafic intrusions. The 1856 ± 2 Ma layered mafic–ultramafic Panton Intrusion of the Central Zone contains a reef-type platinum-group element (PGE) mineralisation associated with metamorphosed chromitite horizons. Here we investigate the compositional variation of chromite within the A chromitite layer, or Main Chromitite Layer (MCL), base metal sulphides (BMS), and the associated PGE distribution. Electron-microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data reveal complex chemical zoning of the chromite reflecting post-magmatic sub-solidus re-equilibration and fluid–rock interaction during metamorphism of the host rocks. The sulphur isotopic composition of the BMS (δ34S: −1.67 to 2.89‰) suggests negligible contamination of the parental melt by crustal sulphur and thus favours BMS formation due to fractional crystallisation. Furthermore, pentlandite compositions indicate the highest ƒS2 in the centre of the MCL, which coincides with a considerably heavier sulphur isotope composition compared to pentlandite in the surrounding ultramafic host rock. Alteration of the BMS during amphibolite facies hydration of the Panton Intrusion resulted in the formation of secondary bornite and digenite from the chalcopyrite as well as desulphurisation of pyrrhotite and pentlandite to secondary magnetite. The bulk rock PGE concentrations are controlled by the distribution of platinum-group minerals (PGM) due to the low PGE concentrations in BMS. PGM are present as inclusions in chromite, suggesting that they formed under sulphur-undersaturated conditions. Additional PGM are present within the interstitial serpentine-chlorite matrix suggesting that they precipitated during fluid-rock interaction and hydration of the silicate matrix. Likely sources for the remobilised PGE include PGM inclusions in altered rim zones of magmatic chromite and PGE released due to desulphurisation of BMS despite their low PGE concentrations.

Multidisciplinary study of a complex magmatic system: The Savannah Ni-Cu-Co Camp, Western Australia

The Savannah (formerly Sally Malay) Ni-Cu-Co Camp is composed of a group of mineralised intrusions (Savannah and Savannah North) and petrologically similar mafic-ultramafic intrusions without known mineralisation (Sub-chamber D, Dave Hill, and Wilson’s Creek). The results of a wide range of analytical techniques (geochemical, geochronological and geophysical) are integrated to gain a better understanding of the geometry of the Savannah mineralised magmatic intrusions and the timing of events. Magnetic anisotropy results suggest that Savannah and Savannah North have both been affected by similar deformation events, implying near-synchronous intrusion. However, Savannah and Savannah North show evidence of fundamentally different styles of deformation (strong folding at Savannah, as opposed to gentle tilting at Savannah North). This discrepancy can be reconciled with a resolvable variation in age, with Savannah North being the younger by ca. 0.8–2.5 Ma, as well as difference in the geometry and associate rheological properties of the intrusions: the larger more equant Savannah North intrusion may have been more resistant to deformation than the thinner elongated Savannah body. Isotopic and geochemical data on the mafic-ultramafic intrusions of the Savannah Ni-Cu-Co Camp indicate that the entire intrusive suite, with the exception of the early mafic granulite dykes, was derived from a similar parent magma of basaltic composition with likely continental arc affinity. Results of sulfide tenor calculations combined with mineral chemistry and in-situ S isotope data provide insight into ore genetic processes: in both the Savannah and Savannah North intrusions tenors are low, with median values in semi-massive ores of 3.8% Ni and 0.9% Cu in 100% sulfide for Savannah and 2.7% Ni and 1.0% Cu for Savannah North. PGE tenors are similar and extremely low in both deposits, ranging from below 5 ppb to 200 ppb for Pt (median 6 ppb) and from 40 ppb to 480 ppb (median 110) for Pd, with Pd/Ir of 25–160 and Pd/Pt of 3–50 (10th and 90th percentiles). For both Savannah and Savannah North, S isotopic analyses indicate a limited amount of incorporation of crustal S, and the contaminant does not appear to have been the immediately adjacent Tickalara Metamorphics, implying either a deeper crustal contaminant, or a component of subduction-derived crustal S in the mantle source. Finally, olivine records a history of postcumulus re-equilibration with trapped silicate melt and cumulus sulfide liquid, giving rise to distinctive compositional trends. The Savannah Ni-Cu-Co Camp provides further evidence that arc and collisional environments are conducive to the formation of intrusion-hosted orthomagmatic Ni-Cu-Co deposits. The multidisciplinary approach that we describe here allowed us to unravel the petrological, structural, and mineralisation history of a cryptic but prospective region, and provided insights to help evaluate the prospectivity of neighbouring intrusions. This approach can be used in a wide range of settings to resolve critical factors such as S source, timing of intrusion and mineralisation, and structural controls.

Major deposits and occurrences of platinum ores of the Kola belt layered intrusions (Kola Peninsula, Russia)

Research subject. This paper presents data characterizing major occurrences and deposits of platinum group elements (PGE) of Kola belt layered mafic-ultramafic intrusions (Kola Peninsula).Materials and methods. The study was based on extensive experimental data collected by the author over a long-term period and a comprehensive analysis of published and archive literature pertaining to this field.Results. Certificates (so-called passports) were compiled for all major PGE objects in theKola Peninsula, which include diverse characterization parameters (geological, structural, isotope-geochemical, mineralogical). The formation of PGE mineralization was compared with both the age characteristics of large layered complexes and the sequence of intrusion of individual magmatic phases.Conclusion. The proposed parameters can be used to determine specific features of PGE objects, thus allowing identification of objects promising in terms of industrial development. The obtained results demonstrate the importance of taking into account these specific features during mineral deposit exploitation.

Complexity of characterizing mafic-ultramafic remnants of Archean layered complexes in high-grade terranes: The scenario from Motloutse Complex, southwestern Zimbabwe craton

Mafic-to ultramafic-rocks of Archean layered complexes in high-grade terranes typically occur as inclusions closely associated with supracrustal rocks in granitoid gneiss. The reconstruction of the original stratigraphy and sequence of events related to Archean layered complexes is complicated by the fragmental occurrence and effect of overprint event(s). The case study presented here highlight these issues from the Motloutse Complex terrane, southwestern Zimbabwe Craton. Peridotite-pyroxenite and gabbro-leucogabbro-anorthosite form separate bodies closely associated with amphibolite in the Lechana area. They occur as inclusions within ∼2.65 Ga tonalite gneiss. Amphibolite is massive, has a stratiform occurrence and is folded, with gabbro-leucogabbro-anorthosite intruding along the fold limbs. Rare amphibolite xenoliths occur in gabbro-leucogabbro-anorthosite and peridotite-pyroxenite. Modal and cyclic layering is well preserved in the mafic and ultramafic intrusive bodies. Anatectic and alkali-metasomatic overprints, the later related to shear deformation, locally affected the Lechana rocks. The late stage events are contemporaneous and is dated by the ∼2.0 Ga (2013 ± 18 Ma) U-Pb zircon overgrowth age. Amphibolite exhibit prominent granoblastic polygonal fabric defined by amphibole-plagioclase. Gabbro-leucogabbro-anorthosite are locally gneissic. The different mafic rocks are variably affected by overprint events, and contain garnet related to anatectic overprint and epidote related to metasomatic overprint. Peridotite-pyroxenite exhibits no metamorphic re-equilibration textures, but are serpentinized and host widespread magnetite-rich layers. Amphibolite and associated gabbro have conspicuous sulphide concentration (up to 5 modal%), and include pyrrhotite, chalcopyrite (replaced by covellite), pyrite and pentlandite. No magnetite or sulphide concentration occur with leucogabbro-anorthosite. Whole-rock geochemical characteristics of the amphibolites are similar to tholeiitic basalts, but that of the peridotite-pyroxenite and gabbro-leucogabbro-anorthosite indicate different lineage. The suggested model considers amphibolite as a stratiform meta-basalt, which was folded during an earlier deformation event. Based on the presence of meta-basalt (amphibolite) roof pendants on top of both mafic and ultramafic intrusions, it is argued that the peridotite-pyroxenite and gabbro-leucogabbro-anorthosite were emplaced into Archean oceanic crust. The mafic-ultramafic intrusions could either be part of a single Mesoarchean layered complex or represent two different suites. In view of the restricted occurrence, lack of interlayering, different pyroxene affinities (clinopyroxene in gabbro and orthopyroxene in peridotite-pyroxenite) and wide compositional gap between mafic and ultramafic intrusions, we favour the later scenario. Peridotite-pyroxenite, interpreted as part of a Mesoarchean layered ultramafic complex, intruded the mafic volcanics. Layered gabbro-leucogabbro-anorthosite also intruded into the basaltic rock, separate from the ultramafic complex. The intrusion of protolith of the tonalite gneiss then dismembered the mafic and ultramafic sequences. The results of the present study highlight the need for detailed evaluation of spatially associated mafic-ultramafic rock remnants in high-grade terranes; they need not be genetically related but could be part of different magmatic episodes. Such exercise has implications in properly understanding the geodynamic setting of formation of these Archean upper mantle-derived rocks; the current arguments polarised between two end-member settings – plate margin versus intraplate setting.

The ~500 Ma Asaji ultramafic–mafic intrusion in Kyushu, southwest Japan: Implications for boninitic magmatism in a late Cambrian nascent arc

It is thought that the continental crust that ultimately became Japan (i.e., proto-Japan) formed during the early Cambrian near the margin of the South China block following its separation from the Australian block. However, the onset of timing of subduction and details of the magmatic evolution of the region are poorly understood, due mainly to the sporadic occurrence of Cambrian rocks. A high-temperature metamorphic complex is distributed throughout eastern Kyushu island, Japan. At the northeastern end of the complex, a narrow zone of ultramafic–mafic rocks includes a late Cambrian ophiolitic assemblage, namely the Asaji ultramafic–mafic intrusion. The ultramafic cumulates in the intrusion consist of harzburgite (spinel–olivine–orthopyroxene), lherzolite–olivine websterite (olivine–orthopyroxene–clinopyroxene ± spinel), and websterite (orthopyroxene–clinopyroxene ± olivine), which formed in that order during differentiation of a parental magma. The chemical compositions of clinopyroxene and spinel from the cumulates show strongly depleted features (e.g., spinel Cr# [= atomic Cr/(Cr + Al)] ≤ 0.91 and TiO2 < 0.3 wt%, and clinopyroxene YbN < 2) and comparable to those of spinel in boninites. In addition, the calculated trace element composition of melts in equilibrium with clinopyroxene from the ultramafic rocks closely resembles the composition of the North Tongan boninite. These petrological and geochemical features suggest that the Asaji ultramafic–mafic intrusion consists of cumulates from a boninitic magma. The presence of boninitic magmatism suggests that the Asaji ultramafic–mafic intrusion was the result of subduction initiation and the infiltration of water into hot asthenosphere during the formation of nascent arc crust.

The Role of Siliceous High-Magnesium Basalts during the Formation of a Neoproterozoic Mafic-Ultramafic Intrusion in the Tarim Craton (China)

The Xingdi No.2 intrusion in the Kuluktag Block of northeastern Tarim Craton, which intrudes into the Palaeoproterozoic basement with an exposed area of ca. 12 km2, is an orthopyroxene-rich mafic-ultramafic intrusion in a continental rift setting. It consists of gabbros, pyroxenites, and peridotites, and exhibits a crystallization sequence of the principal rock-forming minerals from olivine, orthopyroxene, clinopyroxene, to plagioclase. The gabbros show a concordant SHRIMP U-Pb zircon age of 752 ± 5.4 Ma. In addition, the olivine grains have forsterite content values of 78–85 mole% and mostly contain low NiO, MnO, CaO, and Cr2O3. The rocks are relatively enriched in large ion lithophile elements and LREE, and depleted in HSFE, have non-radiogenic Pb, low εNd (t) values (−2.8 to −23), initial 87Sr/86Sr ratios (0.7059–0.7130). It could be concluded that the rocks represent an analogue of siliceous high magnesium basaltic magma originated by the partial melting of a hydrous and enriched subcontinental lithospheric mantle and contaminated by the continental crust.

Barium Minerals Barite and Chlorinedominant Ferrokinoshitalite $${\mathbf{BaFe}}{{_{3}^{{2 + }}}^{{}}}[{\mathbf{C}}{{{\mathbf{l}}}_{{\mathbf{2}}}}/{\mathbf{A}}{{{\mathbf{l}}}_{{\mathbf{2}}}}{\mathbf{S}}{{{\mathbf{i}}}_{{\mathbf{2}}}}{{{\mathbf{O}}}_{{{\mathbf{10}}}}}]$$ in Plagioperidotites of the Yoko-Dovyren Intrusion, Northern Baikal Area: Products of Epigenetic Low-Grade Metamorphism

The plagioperidotites of the Yoko-Dovyren ultramafic–mafic intrusion, which were affected by low-grade metamorphism (LGM) to the prehnite–pumpellyite facies (PPF), show evidence of Ba, Cl, and Sr mobilization. The background Ba and Sr contents in the plagioperidotites are 36–313 ppm (at an average of 130 ppm) and 25–169 ppm (average 86 ppm), respectively, at Ba/Sr = 0.5–4 (average 1.5). The Ba-bearing minerals are phlogopite and plagioclase, and the Cl-bearing ones are chloroferrisadanagite in alumochromite-hosted inclusions, late magmatic phlogopite, K–Cl ferropargassite and Cl-apatite in the margins of sulfide pockets. In the course of the low-grade metamorphism, these minerals were replaced by chlorites, tremolite, and diopside, which contain very little Ba and Cl. In plagioperidotite domains containing Ba‑bearing minerals, these rocks contain 348–518 ppm Ba and 4–6 ppm Sr, which indicates that Ba was redistributed and Sr was removed. The metamorphic Ba minerals are Sr-free barite and Ba–Fe–Cl mica: Cl-dominant ferrokinochitalite. They coexist with minerals of the rodingite association: hydrogarnet, tremolite, diopside, chlorite, antigorite, magnetite, and hydroxylapatite. The ferrokinoshitalite replaces phlogopite, plagioclase, and sulfides, contains up to 21 wt % BaO, 31% FeO, and 11% Cl, and has a Fe mole fraction f = 75.8–90.5. The composition of a ferrokinochitalite individual riches in Cl corresponds to the formula $${{{\text{(B}}{{{\text{a}}}_{{{\text{0}}{\text{.83}}}}}{{{\text{K}}}_{{{\text{0}}{\text{.16}}}}}{\text{)}}}_{{{\text{0}}{\text{.99}}}}}{{{\text{(Fe}}_{{2.63}}^{{2 + }}{\text{M}}{{{\text{g}}}_{{{\text{0}}{\text{.28}}}}}{\text{Fe}}_{{0.04}}^{{3 + }}{\text{A}}{{{\text{l}}}_{{{\text{0}}{\text{.02}}}}}{\text{C}}{{{\text{r}}}_{{{\text{0}}{\text{.01}}}}}{\text{M}}{{{\text{n}}}_{{{\text{0}}{\text{.01}}}}}{\text{)}}}_{{\text{3}}}}{\text{[(C}}{{{\text{l}}}_{{{\text{1}}{\text{.86}}}}}{\text{O}}{{{\text{H}}}_{{{\text{0}}{\text{.12}}}}}{{{\text{S}}}_{{{\text{0}}{\text{.02}}}}}{{{\text{)}}}_{{\text{2}}}}{\text{/A}}{{{\text{l}}}_{{{\text{1}}{\text{.86}}}}}{\text{S}}{{{\text{i}}}_{{{\text{2}}{\text{.14}}}}}{{{\text{O}}}_{{{\text{10}}}}}{\text{]}}$$ . Because ferrokinochitalite from the Yoko-Dovyren Massif is Cl-dominated, it is a new mineral species.

Delineating ore-forming rock using a frequency domain controlled-source electromagnetic method

Several magmatic Ni-Cu deposits associated with Permian-Triassic mafic-ultramafic intrusions have been discovered during the last three decades in the southern part of the Central Asian Orogenic Belt, the northeastern margin of Tarim, and the margin of the North China Craton. The accurate delineations of the large resistive rock masses are crucial for locating potential ore bodies when they are located beneath rock masse. The controlled-source electromagnetic method (CSEM) is an important tool for investigating resistivity contrast in earth subsurface. In this study, we present a comprehensive example of using frequency domain CSEM survey to delineate the resistive rock masses that are related to the forming of orebody. The horizontal electric field, which is more sensitive to resistive target than the other components, is used in our study. We first built a geophysical model of the survey area based on prior confirmed geophysical and geological information. Then, both forward and inverse modeling were performed in order to assess the detectability of the resistive ore-forming rock masses in the model. Finally, this study acquired field data in the Kalatongke Ni-Cu Deposit, which is located in the Kalatongke District of the southern section of the Central Asian Orogenic Belt, and is specifically part of China’s southern Altai Orogenic Belt. The resistive ore-forming rock masses were successfully identified during the survey process and further verified by the drilling results. It was observed that both the synthetic and field examples indicated that the adopted frequency domain CSEM method successfully measured the electrical fields, and could be utilized to explore deeply buried resistive targets in future prospecting activities.

Multiple sulfur isotopes reveal a possible non-crustal source of sulfur for the Bushveld Province, southern Africa

Abstract The source of sulfur for sulfide mineralization is a major question for the origin of platinum group element deposits such as the Rustenburg Layered Suite (RLS) of the Bushveld Complex and the nearby Waterberg Project (WP; a large palladium-dominant deposit) in southern Africa. Both deposits are mafic-ultramafic intrusions associated with the ca. 2.06 Ga Bushveld magmatism but are hosted in distinct country rocks. This contrast allows a critical assessment of the contribution upper crustal assimilation provides to sulfide mineralization, and refinement of our understanding of sources of mass-independent fractionated sulfur (MIF-S) to these intrusions. The WP has a signature of anomalous sulfur (average Δ33S = 0.113‰ ± 0.016‰, 1 s.d.), similar to the RLS (avgerage Δ33S = 0.137‰ ± 0.025‰, 1 s.d.). There is no evidence for influence of host rock as a source of anomalous sulfur. The lack of a significant variation of Δ33S values within the WP stratigraphy, and the distinct upper continental crust into which the WP magmas would have been emplaced, shows that addition of upper crustal sulfur is not necessary for PGE formation. This suggests that contamination of WP and RLS magmas with a surface-derived component of Archean age occurred at depth, prior to emplacement.

Paleoproterozoic Mafic-Ultramafic Magmatism in the Northern Borborema Province, Northeast Brazil: Tectonic Setting and Potential for Deposits

Mafic-ultramafic intrusions form a 32-km-long NE-SW-trending array within Paleoproterozoic arc-related calc-alkaline plutonic rocks of the Rio Piranhas terrane in the Borborema Province, Northeast Brazil. The mafic-ultramafic intrusions consist of wehrlite, clinopyroxenite, and troctolite. They outcrop as small elongated bodies hosted by orthogneiss and migmatites associated with minor amphibolite and supracrustal rocks. Primary magmatic features predominate in the core of the intrusions, whereas metamorphic minerals and textures characterize their outer zones. The composition of cumulus olivine from different intrusions ranges from Fo80.6 to Fo67.6 (forsterite), indicating moderately primitive parental magmas. The compositional range of cumulus plagioclase coexisting with cumulus olivine in troctolites is characterized by high anorthite content (An79.9–An89.0). This feature, together with interstitial magmatic amphibole in troctolites, is characteristic of gabbroic intrusions in subduction-related magmatic arcs. U-Pb zircon ages indicate that mafic-ultramafic intrusions crystallized at ca. 2190 ± 5 Ma. U-Pb zircon ages of host orthogneiss indicate that they crystallized from calc-alkaline felsic magmas at ca. 2220–2223 Ma, thus providing only ∼30 My for the reported events of felsic and mafic plutonism. U-Pb zircon ages of host rocks include older ages (2.47 and 2.69 Ga) indicating that older crustal components are associated with the 2.23 Ga calc-alkaline magmatism. The presence of older crustal components is consistent with arc-related features, indicated by geochemical data and variable εNd(t) values in the mafic-ultramafic intrusions. The tholeiitic and calc-alkaline magmatism is correlated with the ca. 2.15–2.25 Ga magmatic arc reported in previous studies of the Rio Piranhas terrane. The mafic-ultramafic magmatism is interpreted as having originated in a magmatic arc, possibly resulting from partial melting of a mantle wedge above the subduction zone in late stages of the orogeny and suggests a new window of opportunity for exploration of Ni-Cu–platinum group element deposits.

Zoned orthopyroxenes in the Ni-Co sulfide ore-bearing Xiarihamu mafic-ultramafic intrusion in northern Tibetan Plateau, China: Implications for multiple magma replenishments

The Xiarihamu mafic-ultramafic intrusion is situated in the East Kunlun Orogenic Belt (E-KOB), northern Tibetan Plateau, China, with Ni-Co sulfide mineralization mainly in orthopyroxenite and olivine orthopyroxenite. Whole-rock and mineral compositions show cyclic variations in drill-hole intersections through the Xiarihamu mafic-ultramafic intrusion. Microbeam X-ray fluorescence (XRF) mapping shows reverse zoning (Cr-poor core, Cr-rich rim) and oscillatory zoning (Cr-poor core, Cr-rich mantle and Cr-poor rim) in orthopyroxene of olivine orthopyroxenite and orthopyroxenite. Zoned orthopyroxenes are euhedral or subhedral, and there are relatively sharp interfaces between zones with distinct Cr contents. Crystal-size distribution (CSD) curves of orthopyroxenes in olivine orthopyroxenite show concave-upward curvature at size larger than 3.0 mm, and larger orthopyroxenes commonly display zoning, implying a secondary recrystallization and grain growth of larger orthopyroxenes. Quantitative analyses show that Cr and enstatite contents of cores are relatively lower, but incompatible element (e.g., Ti, Zr, Ga and Sr) contents of cores are higher than rims in reverse zoned orthopyroxenes, and chemical profiles of unzoned orthopyroxenes resemble those of rims (but different from those of cores) in reverse zoned orthopyroxenes.Chemical profiles of orthopyroxenes indicate that Cr zoning is not due to the change in Cr partitioning behavior between orthopyroxene and silicate melt, which shows significant temperature and redox dependence. Textural feature and crystal-size distribution of orthopyroxenes suggest the formation of Cr-rich zones attests to the multi-stage growth in the process of magma replenishment, rather than a reactive liquid flow model or in-situ trapped liquid overgrowth. Orthopyroxene zoning, together with compositional variation and intrusive relationship between olivine orthopyroxenite and orthopyroxenite imply that the Xiarihamu mafic-ultramafic intrusion was formed by multiple magma replenishments with magma varying roughly from a more evolved to a more primitive composition.

Derivation of the Early Carboniferous Wedgeport pluton by crystal fractionation of a mafic parental magma: a rare case of an A-type granite within the Meguma terrane (Nova Scotia, Canada)

AbstractThe Cambrian–Ordovician metasedimentary rocks of the Meguma terrane (Canadian Appalachians) were extensively intruded by silicic plutons during Middle Devonian to Early Carboniferous times. Syn-plutonic but volumetrically minor mafic-ultramafic intrusions were also emplaced. In most localities, the silicic plutons and mafic-ultramafic intrusions do not appear to be petrogenetically related and are likely derived from different sources. The Attwoods Brook gabbronorite of SW Nova Scotia yielded an in situ zircon weighted-mean 206Pb–238U age of 357.9 ± 3.3 Ma that is within the uncertainty of the age of the neighbouring Wedgeport pluton (357 ± 1 Ma). The Wedgeport pluton is a rare example of a mantle-derived, peraluminous A-type granite within the Meguma terrane. The similar ages and Nd isotopes of the Attwoods Brook gabbronorite (εNd(t) = +1.1 to +4.0) and Wedgeport pluton (εNd(t) = +2.1 to +3.3) suggest the two intrusions are petrogenetically related. Fractional crystallization modelling demonstrates that a parental magma similar to the Attwoods Brook gabbronorite can produce residual silicic liquids that resemble the granites of the Wedgeport pluton, indicating that they could be members of the same intrusive complex. The emplacement of the gabbronorite and Wedgeport pluton occurred during a period of tensional plate stress that was contemporaneous with rifting of the Maritimes Basin that produced the Fountain Lake continental flood basalts and A-type granites of the Cobequid Highlands within the Avalon terrane. It is possible that the Early Carboniferous rocks of SW Nova Scotia are related to the rifted-related magmatism within the Maritimes Basin.

280–310 Ma rift-related basaltic magmatism in northern Baoshan, SW China: Implications for Gondwana reconstruction and mineral exploration

The temporal and spatial variations of Late Paleozoic basaltic lavas in Baoshan, the northern part of a Gondwana-derived micro-continental block called Sibumasu, are important for Gondwana reconstruction. Magmatic zircon crystals from three selected dolerite dykes in the Baoshan region yield U-Pb ages from ~295 to ~310 Ma. These new ages, together with previous zircon U-Pb ages for this type of rock, define a protracted (~30 myr) episode of basaltic magmatism from ~310 to 280 Ma in a small area of this region, which is inconsistent with the typical temporal-spatial distribution of mantle plume magmatism. The trace element compositions of the Baoshan dolerite dykes and associated lavas are similar to arc basalts as well as continental flood basalts worldwide, showing light REE enrichments and negative Nb-Ta anomalies. Mixing calculations using the Sr-Nd-Hf isotope data of the Baoshan mafic rocks indicate that their intriguing trace element characteristics can be explained by contamination of mantle-derived magmas with crustal materials. Our new data, together with the lack of Late Paleozoic are-related calcalkaline rocks and granitoids in the Sibumasu block and other contemporaneous Gondwana-derived micro-continental blocks, strongly support the premise that the 310–280 Ma basalts and dolerites in the Baoshan region are the products of continental rift-related magmatism rather than arc magmatism. Based on the temporal correlation of the 310–280 Ma rift-related magmatism in several related Gondwana-derived micro-continents (Sibumasu, South Qiangtang, Lhasa and Himalaya), plus other independent constraints such as paleoclimate biotas and paleolatitudes from the literature, we provide an improved model for the configuration of the Gondwana supercontinent in the Early Permian. Based on the results from this study, we conclude that further investment in the exploration of magmatic Ni-Cu sulfide deposits associated with the 310–280 Ma mafic-ultramafic intrusions in northern Baoshan and the other related Gondwana-derived micro-continental blocks is warranted.

A review of the types and ore mechanism of the cobalt deposits

<p indent=0mm>Cobalt is an important strategic metal of major industrial countries. In order to invoke the importance of the cobalt deposits to the strategic industries in China, this paper aims to introduce the different types of the cobalt deposits in the worldwide, to review the ore mechanism of the cobalt deposits and to propose the research directions of the future studies for the cobalt deposits. Global cobalt resources can be grouped into two deposit types: terrestrial and seafloor deposits. The major terrestrial cobalt deposits include sediment-hosted stratiform copper-cobalt, nickel-cobalt laterite, and magmatic sulfide copper-nickel deposits. Other sources include black shales hosted, skarn and replacement, metasedimentary rocks hosted, iron oxide copper-gold deposits, volcanic massive sulfides, polymetallic veins (five-element veins and other vein types), and Mississippi Valley-Type lead-zinc deposits. Seafloor cobalt-related deposits are mainly iron-manganese-cobalt nodules and crusts, which are hardly explored and productive. The detailed information of the major cobalt deposits is also listed, presenting the tonnage and grade of various types of the cobalt deposits in China. The polymetallic veins and black shales hosted types have the highest grade, and both types should be paid more attentions during the mineral exploration. For the geochemical behavior of the cobalt during the magmatic and hydrothermal environment, the partition coefficient of cobalt in the process of mantle melting under different tectonic background and water contents are relatively stable. Also, during the fractional crystallization of ultramafic-mafic magmas in the magmatic sulfide deposits, cobalt has a slight preference for olivine relative to basalt. In hydrothermal ore system, aqueous Cobalt(II) chloride complexes play a crucial role in cobalt transport and deposition, and their solubility has a close relationship with the temperature and salinity of the fluids. The debate on the genesis of various cobalt-rich deposits can be considered as the debate on the evaluation between diagenetic and superimposed processes in the ore formation. Besides some polymetallic veins, black-shale hosted, Mississippi Valley-Type lead-zinc, and iron oxide copper-gold deposits, the process of cobalt enrichment has close relationships with the ultramafic-mafic intrusions or volcanic during the magmatic processes, hydrothermal processes, or weathering. Moreover, the multiple-stage hydrothermal processes are also critical to cobalt enrichment. We suggest that the spatial-temporal distribution of different cobalt deposits in China and how their formations are related to the tectonic evolution of the craton and orogenic belt should be investigated immediately to determine the controlling geological factors in different cobalt deposits. Also, the thermodynamic properties of different cobalt-bearing speciation under different physico-chemical environments should be identified. The relationship between geodynamic processes and formation of cobalt-rich deposits needs to be considered. Finally, great attention should be paid to the study of marine iron-manganese-cobalt nodules and crusts and their proper mining technology.

Chemistry of chromium spinel in high-Mg rocks from the Morungava Intrusion, Cretaceous Paraná Igneous Province, southernmost Brazil

Mafic-ultramafic intrusions within continental flood basalt terrains are frequently associated with Cu-Ni-PGE mineralization. This study aims to constrain the petrogenesis of early crystal phases in a promising exploration target, the Morungava mafic-ultramafic intrusion that is associated with the Parana flood basalts. Therefore, we analyzed the composition of chromiferous spinel and associated olivine of ultramafic rocks from the sill-like intrusion. Samples were selected from different drill core intervals with high MgO and Cr contents. Two generations of Cr-spinel and olivine were identified in olivine gabbronorite and wehrlite rocks from the intrusion. The first type (Spl1) is enclosed in the core of high-Mg idiomorphic olivine crystals (Ol1) and has higher Cr2O3 (28–35 wt.%), Al2O3 (13–33 wt.%), MgO (12.6–14.6 wt.%), and lower TiO2 (0.5–0.8 wt.%) compared to the second type of spinel (Spl2). Spl2 occurs in the interstitial space between olivine and clinopyroxene and contains higher TiO2 (2.0–15.8 wt.%) and lower Al2O3 (5.2–10.5 wt.%), Cr2O3 (10–25 wt.%), and MgO (2.8–7.6 wt.%) contents. Geothermometric calculations using high-Mg idiomorphic Ol1 - Spl1 pairs resulted in temperatures below the corresponding solidus, indicating subsolidus exchange between these minerals and accounting for the Cr-Al trend in Spl1. The high Mg-contents, normal zoning and high but decreasing Cr contents in Ol1 are consistent with crystallization at an early stage during progressive fractional crystallization. Fractionation of olivine, Cr-spinel, clinopyroxene, and plagioclase deceased Mg, Al, and Cr, and increased Fe and Ti in the residual melt. The compositional hiatus with the low-Mg olivine (Ol2) and especially with Spl2 crystals most likely reflects crystallization from an evolved interstitial liquid at a time where Ol1-Spl1 were relatively isolated from diffusional interaction, in addition to solvus processes in spinel. The Fe-Ti- trend in Spl2 likely reflects varying degrees of evolution of the interstitial melts. The Morungava intrusion thus records a complex history of extensive reactions that started with fractionating Ol1-Spl1 in a magma chamber that experienced periodic magma addition, and finished in trapped, interstitial, in situ intercumulus liquids (Ol2-Spl2). The setting, host rocks, and geochemical characteristics of the Morungava intrusion are reminiscent of the Cu-Ni-PGE mineralized Noril’sk-Talnakh ultramafic complex, confirming the inferred mineralization potential.