Mafic-ultramafic Intrusions Research Articles

New U-Pb and Sm-Nd Data and REE Distribution in Sulphides of Paleoproterozoic PGE Layered Intrusions in the Arctic Part of the Fennoscandian Shield

The U-Pb (on zircon) and Sm-Nd analysis is a popular isotope-geochronological tool for estimating the age of rocks from PGE mafic-ultramafic intrusions. Sulphides can be used to study the geochronology of ore processes as well, since they should indicate the time of ore mineralization. Gabbronorite has been sampled from the Passivaara reef at the Penikat layered intrusion (Finland) for U-Pb and Sm-Nd isotope single zircon analyses in order to separate sulphide minerals. The Sm-Nd isotope age of gabbronorite has been dated at 2426 ± 36 Ma; eNd(T) = −1.4 ± 0.4. The Sm-Nd isotope age on sulphides and rock-forming minerals reflects the crystallization time of the ore-bearing gabbronorite from the Passivaara reef of the Penikat layered intrusion. The mass-spectrometer analytical environment and modes of operation have been adjusted to detect REE in sulphide minerals on example of pyrite from the PGE Penikat layered intrusion (Finland) and chalcopyrite from the Talnakh deposit (Norilsk area, Russia) has been estimated. The total REE content in pyrite is ca. 3.5 ppm, which is enough to define the Sm-Nd age of pyrite. The study shows how to use the mineral/chondrite spectra to evaluate the accuracy of the REE analytical results on example of State Standard Sample 2463 (Russia).

Рудная минерализация участка Южносопчинский-1 (Мончегорский рудный район, Кольский полуостров)

The article presents new data on composition of the platinum group metals (PGM) mineralization of the Yuzhnosopchinsky-1 locality, in the massif of the same name. In the central part of the Kola Region this area is located in the zone of contact between two large layered mafic-ultramafic intrusions: Monchegorsky pluton and Monchetundra intrusion. There is the contact between fine-medium-grained metapyroxenites and coarse-grained metagabbronorites. At some distance from the contact, coarse-grained bodies of plagioclase-pyroxene and plagioclase-amphibole composition contain sulfide, oxide and PGM mineralization. Ore minerals in these vein-shaped bodies are represented mainly by the bornite-chalcopyrite-millerite group, in close spatial association with the magnetite-ilmenite group. Arsenides, tellurides and sulfides of palladium predominate among the PGM minerals.

Reduced CO2 Fluid as an Agent of Ore-Forming Processes: A Case Study of Dolomite-Replacement Skarns at the Yoko-Dovyren Massif

The paper presents newly obtained geochemical data on outer-contact rocks and carbonatereplacement skarns of the Yoko-Dovyren layered ultramafic-mafic intrusion in the northern Baikal area. The rocks initially contained CO 2 -rich fluid with a high oxygen fugacity (up to NNO + 3–4), which was generated by the partial decomposition of dolomite and by reactions between SiO 2 and carbonates. The skarn blue diopside is enriched in Pt (up to 0.2 ppm) and V (300 ppm), and the wollastonite zone of the skarns contains elevated Re concentrations (up to 0.4 ppm). The REE pattern of the contact-zone quartzite is identical to the REE patterns of phlogopite-bearing lherzolites from the lower contact part of the Yoko-Dovyren massif. These geochemical features of the rocks of the intrusion may be explained by the transfer and redeposition of material by reduced H 2 O-CO 2 fluid. According to thermodynamic calculations, a reaction between H 2 O-CO 2 fluid and high-Mg olivine at a subsolidus temperature of T = 950 о C and pressure P = 2 kbar should result in a decrease in the oxygen fugacity to QFM – 2 and, hence, generate much CO. According to the calculations, a low oxygen fugacity (close to QFM + 0.7) can also be maintained by pyrrhotite oxidation with H 2 O and CO 2 fluid components under cumulus P-T parameters. As a result of these reactions, the fluid should enrich in Pt extracted from magmatic sulfides, and this Pt can be redeposited in rocks, including those composing the skarn zones.

Geochemical characterisation of rock hydration processes using t-SNE

Dimensionality reduction provides a simple, two-dimensional representation of multi-element geochemical assays, which facilitates visualisation of complex data and enhances their interpretation. A recently proposed dimensionality reduction algorithm, namely t-distributed stochastic neighbour embedding (t-SNE), generates effective two-dimensional representations of a wide range of datasets based on pairwise statistical distances of the input. However, direct application to multi-element geochemical assays has been shown to produce representations which can fail to separate specimens by a desired geological property, such as state of hydration. Since t-SNE is a statistical distance-based method, these sub-optimal representations may be due to the presence of dimensions (i.e., elements) irrelevant to the desired property—an issue often termed the ‘curse of dimensionality’. To address this shortcoming, t-SNE was applied to (i) 31 elements in a geochemical assay database covering 16 000 drill core intervals intersecting the Kevitsa mafic-ultramafic intrusion (Lapland, Finland); and (ii) a subset of 11 elements capable of discriminating between unaltered and altered host rock specimens, as determined by a Random Forest classifier within a recursive feature elimination framework. The resulting representation more effectively separates altered and unaltered specimens, and we demonstrate that it produces more favourable representations than alternative well-known methods (namely, a self-organising map and principal components analysis) applied to the same dataset. We also demonstrate that the proposed t-SNE representation is applicable for re-logging of the specimens' alteration state as logged by geologists, and in particular provides visual insight into the labels suggested by a black box statistical re-logging algorithm.

Geochronology, geochemistry, and Sr–Nd–Hf isotopic compositions of mafic–ultramafic intrusions in the Niubiziliang Ni–(Cu) sulfide deposit, North Qaidam Orogenic Belt, NW China: Implications for magmatic source, geodynamic setting, and petrogenesis

Whole-rock geochemical and SrNd isotopic data, and zircon U–Pb–Hf isotopic data are reported for mafic–ultramafic intrusions in the Niubiziliang Ni–(Cu) sulfide deposit, located in the westernmost segment of the North Qaidam Orogenic Belt (NQOB), NW China. NiCu sulfide ores of economic interest are found within these intrusions, with the Niubiziliang III and IV intrusions hosting most of the Ni–(Cu) mineralization. The Niubiziiang mafic–ultramafic intrusions intrude biotite–plagioclase gneiss of the Paleoproterozoic Dakendaban Group. Zircons separated from the gabbro, plagioclase-bearing clinopyroxenite, and olivine websterite yield UPb ages of 389 ± 2, 373 ± 2, and 380 ± 2 Ma, respectively, indicating a genetic linkage with Middle–Late Devonian regional magmatism (390–370 Ma). Gabbro zircons have εHf(t) values of 4.2–10.9 with older TDM1 ages (0.86–0.59 Ga). The Niubiziliang intrusions contain many xenoliths of Paleoproterozoic biotite–plagioclase gneiss, and the intrusions have similar trace element patterns with strong enrichment in LILEs (e.g., K, Rb, and Th) and moderate depletion in HFSEs (e.g., Nb, Ta, P, and Ti). The major and trace element characteristics, and Sr–Nd–Hf isotopic compositions indicate the parental magmas originated from a metasomatized, asthenospheric mantle source that had previously been modified by subduction-related fluids. The magmas experienced significant crustal contamination both in the magma chamber and during ascent, triggering sulfur over-saturation that resulted in the deposition and enrichment of sulfides. Considering the tectonic evolution of this region, we infer that the Niubiziliang mafic–ultramafic intrusions formed in a post-collisional extensional setting related to delamination of the subcontinental lithosphere.

Geology and metamorphism of the neoproterozoic Mangabal Complex: An example of Ni[sbnd]Cu-PGE mineralized intrusion in the Goiás Magmatic Arc, central Brazil

Inserted in the context of the Goiás Magmatic Arc, the Mangabal Complex represents a partially metamorphosed mafic-ultramafic intrusion, which is part of a cluster of neoproterozoic layered bodies formed during the Brasiliano Orogeny in central Brazil. The Mangabal Complex is composed of two exposures, the northern and the southern members, which exhibit the similar igneous mineralogy, crystallization sequence and were affected by the same metamorphic conditions. The stratigraphy of the Mangabal Complex can be divided into three main zones: i. the Lower Mafic Zone (LMZ), composed of adcumulatic norite and, forming the basal portion of the southern member; it is composed of adcumulatic norite; ii. the Ultramafic Zone (UZ), characterized by dunite and harzburgite cumulates, and iii. the Upper Mafic Zone (UMZ), consisting predominantly of norite, with restricted interlayeres of feldspathic dunite-orthocumulate. Petrographic data reveal a crystallization sequence for the Mangabal Complex formed by Olivine + Cr-Spinel > Olivine + Orthopyroxene > Orthopyroxene + Plagioclase > Clinopyroxene. In the olivine cumulus, the forsterita (Fo) content (Fo 78–85) suggest a moderately primitive parental magma. The NiCu-PGE mineralization occurs as disseminated sulfides, located in the UMZ, UZ and LMZ cumulate rocks, or forming massive to net-textured bodies in the metamorphic rocks of the UMZ. The main sulphide assemblage is composed of pyrrhotite + pentlandite + chalcopyrite ± pyrite, with minor pyrite + millerite. Disseminated sulphides are present in igneous rocks as well as in the metamorphic rocks that still partially preserve their original primary texture. This fact, together with the occurrence of abundant olivine and chromium-bearing spinel cumulates, suggests a fertile magma for the Mangabal Complex. However, the highest S, Ni, Cu, and PGE contents correlate positively with and are related to amphibolite and calc-silicate rocks associated with shear zones, indicating that metamorphism and deformation processes were critical in promoting the remobilization of sulphides and their deposition along shear zones. The primary mineral assemblage was largely replaced due to high-pressure upper-amphibolite facies metamorphism. The paragenesis representing metamorphic peak conditions is composed of kyanite + orthoamphibole + hornblende, which reveals pressures of approximately 8.5 kbar and a temperature up to 750 °C. The metamorphic mineral assemblage, together with the proximity to the São Luis de Montes Belos shear zone, suggests that kyanite crystallization in the Mangabal Complex and similar layered intrusions worldwide might be linked to the development of major crustal-scale structures.

The Hudesheng mafic–ultramafic intrusions in the Oulongbuluke Block, Qinghai Province, NW China: chronology, geochemistry, isotopic systematics and tectonic implications

AbstractThe Hudesheng mafic–ultramafic intrusions are located in the Oulongbuluke Block, north of the Qaidam Block in Qinghai Province, NW China. We carried out a detailed study of the intrusions, including field observations, petrology, zircon U–Pb geochronology, Lu–Hf isotopes, bulk-rock major- and trace-element geochemistry, and mineral compositions, to provide a better understanding of their properties and the regional tectonic evolution. Zircon U–Pb dating on gabbro and pyroxenite samples yielded ages of 465 and 455 Ma, respectively. Geochemical data, in conjunction with the field observations and petrological features, suggest that the complex is Alaskan-type and the magma was derived from a depleted mantle source that was hydrous picritic basalt in composition and influenced by crustal contamination and slab-derived fluid metasomatism. Based on all the chronological, petrological, mineralogical and geochemical and regional geological data, we conclude that the palaeo-ocean closed diachronously from west to east between the Qaidam and Oulongbuluke blocks, and that the ocean in the east of the North Qaidam region closed after ∼455 Ma.

The Cu-Ni mineralization potential of the Kaimuqi mafic-ultramafic complex and the indicators for the magmatic Cu-Ni sulfide deposit exploration in the East Kunlun Orogenic Belt, Northern Qinghai-Tibet Plateau, China

The East Kunlun Orogenic Belt experienced two periods of mantle-derived magmatic activity during its post-collisional stages. One occurred during the Late Silurian-Early Devonian, and the other occurred during the Middle-Late Triassic. The Late Silurian-Early Devonian Xiarihamu mafic-ultramafic intrusions were found to be associated with a giant magmatic sulfide Ni-Co deposit discovered in 2012. The U-Pb age of websterite in the Kaimuqi mafic-ultramafic intrusion is 221.1 ± 2.3 Ma. None of the Middle-Late Triassic mafic-ultramafic intrusions in the East Kunlun Orogenic Belt contain Cu-Ni mineralization. Based on simulations by the software MELTS, we found that the crystallization process leads to a drop in the sulfur content at sulfide saturation. The mineralized Late Silurian-Early Devonian mafic-ultramafic intrusions exhibit both a broader range of m/f values and higher m/f values than the non-mineralized intrusions in the East Kunlun Orogenic Belt, which means that the mineralized intrusions experienced a long process of crystallization differentiation. This process is probably responsible for Cu-Ni mineralization in the Early Devonian Xiarihamu mafic-ultramafic intrusion. Compared to the olivine and clinopyroxene in the Xiarihamu giant Cu-Ni deposit, the Late Triassic Kaimuqi olivine contains significantly lower MgO, Fo and SiO2 contents and higher CaO and FeO contents, and the Kaimuqi clinopyroxene contains higher FeO and CaO contents. Therefore, we infer that the Kaimuqi intrusion formed from a melt with higher FeO and CaO contents and a lower SiO2 content. This melt required more sulfur to achieve sulfide saturation. This characteristic most likely hindered mineralization in the Kaimuqi mafic-ultramafic intrusion. All of the mafic-ultramafic intrusions that host Cu-Ni deposits in the East Kunlun Orogenic Belt contain lherzolite, harzburgite, and orthopyroxenite, indicating that the pyroxene in these intrusions is mainly the orthopyroxene. If an intrusion contains more clinopyroxene, it indicates that its magma contains more calcium. The magma with higher calcium content will need a higher sulfur content to achieve sulfide saturation. Although websterite occurred in the Kaimuqi intrusion, the clinopyroxene and orthopyroxene content in the websterite is 75% and 20%, respectively, and the peridotite lithofacies is wehrlite. Therefore, orthopyroxene is rare in the Kaimuqi mafic-ultramafic intrusions. Thus, the Kaimuqi mafic-ultramafic intrusion may not have had the potential to form Cu-Ni ore deposits. Finally, this paper summarizes some geological and geochemical indicators for Cu-Ni mineralization in the mafic-ultramafic intrusions in the East Kunlun Orogenic Belt. These indicators provide some enlightenment to the exploration of Cu-Ni deposits in mafic-ultramafic intrusions, especially for the geological exploration bureaus or companies, which do not have the budget for large numbers of geological dating, isotopes, major elements, and platinum group elements during the exploration evaluation process in the current global mining recession situation.

Origin of Late Triassic mafic–ultramafic intrusions in the Hongqiling Ni–Cu sulfide deposit, Northeast China: evidence from trace element and Sr–Nd isotope geochemistry

There are many Late Triassic mafic–ultramafic intrusions in the Hongqiling magmatic Ni–Cu sulfide deposit, Northeast China. Research on magma evolution leading to formation of these mafic–ultramafic intrusions is of great significance for understanding the mantle beneath Northeast China and associated Ni–Cu mineralization. A trace element study of the No. 1, 3, and 7 intrusions in the Hongqiling deposit reveals that these mafic–ultramafic intrusions are characterized by enrichment of incompatible elements, which however cannot be interpreted by subduction modification. Furthermore, model of batch partial melting of depleted mantle accompanied by upper crustal contamination can simulate the trace element patterns of these mafic–ultramafic intrusions, but partial melting of depleted mantle accompanied by lower crustal contamination model cannot work. In addition, Sr–Nd isotopic compositions of the Hongqiling No. 1, 3, and 7 intrusions also indicate that crustal contamination could have occurred mainly during the magma ascent. Consequently, a possible scenario for the magma evolution is that the primary mafic–ultramafic magma was derived from batch partial melting of a depleted mantle, and then contaminated by Cambrian–Ordovician metamorphic rocks of the Hulan Group during ascent. We conclude that the mantle source contained no significant crustal component in the Late Triassic and was also independent of substantial contribution from subducted material, and therefore the Mesozoic large-scale lithospheric delamination beneath eastern China may happen after a period of time of the Late Triassic.

Petrogenesis and mineralization of the Huangshandong mafic–ultramafic intrusion in eastern Tianshan, NW China

The Huangshandong deposit, located in the Central Asian Orogenic Belt (CAOB), is the largest Ni–Cu sulfide deposit in the eastern Tianshan and Beishan regions of China. The deposit is hosted in a postorogenic intrusion that comprises diorite, gabbrodiorite, bojite, olivine gabbro, gabbronorite, troctolite, and lherzolite. Here, we report the Permian zircon U–Pb crystallization ages of 278.5 ± 2.1 Ma, 278.1 ± 1.9 Ma, and 279.6 ± 1.9 Ma for the olivine gabbro, bojite, and gabbrodiorite, respectively, as well as geological, petrological, and geochemical data for the various lithofacies of the deposit. As a result of the edge cooling effect, the intrusion cooled rapidly from the outside inwards as follows: crystallizing diorite → gabbrodiorite → gabbro → lherzolite → gabbronorite. In addition, further tectonic activity resulted in the injection of a more primitive magma in place, which crystallized the lherzolite in the bottom and middle transitional zones of the intrusion. The crystallization sequence of the intrusion is generally as follows: olivine → orthopyroxene + labradorite → clinopyroxene + labradorite → amphibole + andesine. At the beginning of mineral crystallization, the metalliferous magma was segregated due to sulfide immiscibility and separated by gravity. With gabbronorite crystallization, the immiscible liquid sank to the base of the magma chamber and crystallized as Cu‐ and Ni‐rich sulfides. Tholeiitic and calc‐alkaline rocks are characterized by relative enrichment in large‐ion lithophile elements and depletion in high‐field‐strength elements, and values of Th/Ta (2.25–5.94), (Nb/La)N (1.88–4.39), and Th/Nb (0.29–0.82) indicate that the mantle source was composed of a dehydrated subducted plate or mantle wedge. In addition, values of Nb/U (2.64–10.68) and Ce/Pb (1.33–3.53) suggest that the magma was contaminated by the continental crust during its ascent along pre‐existing structures in the Carboniferous strata.

Compositions of olivine from the Wajilitag mafic-ultramafic intrusion of the Permian Tarim Large Igneous Province, NW China: Insights into recycled pyroxenite in a peridotite mantle source

The Tarim large igneous province (TLIP) comprises a complex suite of lithologies including voluminous ferrobasalts, although the petrogenetic relation of these rocks is still poorly understood. In this study, we report the chemical compositions of olivine in two types of olivine pyroxenites (porphyritic and coarse-grained) from the Wajilitag mafic-ultramafic intrusion in the northwestern TLIP, aiming to characterize the mantle sources of magmas. Olivine in the porphyritic olivine pyroxenites shows low 100 * Mn/Fe (∼1.3) and Mn/Zn (<13) ratios, whereas that in the coarse-grained olivine pyroxenites is characterized by markedly higher ratios of 100 * Mn/Fe (>1.9) and Mn/Zn (>14), suggesting a heterogeneous mantle source beneath the Wajilitag area. The low olivine V/Sc values (<1.5) in the two types of olivine pyroxenites reflect high oxygen fugacity conditions during magma crystallization. The maximum temperatures of crystallization of porphyritic olivine pyroxenites and olivine pyroxenites estimated based on the Al-in-olivine thermometer are 1350 °C and 1190 °C, respectively. A comparison of the results from our study in terms of mantle source and oxygen fugacity suggests less potential for metallogeny in this region, as against the Panxi area in the Emeishan Large Igneous Province.

Formation of the Lubei magmatic Ni–Cu deposit in a post-subduction setting in East Tianshan, North West China

The Lubei Ni–Cu sulfide ore deposit with 7.8 Mt sulfide ores at an average grade of 0.88 wt% Ni and 0.5 wt% Cu, is associated with a mafic–ultramafic intrusion in the western part of the East Tianshan in Xinjiang Uygur Autonomous Region, NW China. The primary mineralization is hosted by ultramafic rocks which are part of a 2.1 km2 mafic–ultramafic intrusion located within the Kanggurtag fault zone. In this paper, we intend to probe the ore-formation processes for the Lubei Ni–Cu sulfide ore deposit and discuss key factors that control the ore formation processes in a post-subduction setting. The Lubei primary magmas were derived from 13 to 17 % partial melting of a hydrous mantle containing ∼0.29 wt% H2O estimated using the silicate-liquid model pMELTS. The Lubei sulfide ores are characterized by low platinum group elements (PGE) and high mantle-normalized Pd/Ir ratios, and disseminated sulfide ores have higher Cu, Ni and PGE tenors than massive and vein sulfide ores indicative of crystal fractionation of Fe-rich monosulfide solid solution. The PGE-depleted Lubei sulfides possibly resulted from early sulfide segregation from the Lubei primary magmas as indicated by high Cu/Pd ratios (up to 106), and the proportion of early sulfide losses was 0.02 wt% which has depleted the Lubei parental magmas and later fractionated sulfides in chalcophile elements. This depletion would been compensated at different levels by later sulfide–magma interaction when the parental magmas reached in situ sulfide saturation, with an R factor (mass ratio of silicate to sulfide liquids) of 100–1000. The formation of the observed Lubei disseminated sulfide ores was associated with 5–15 vol% magma crystallization with the main silicate minerals being spinel and olivine, as modeled using the silicate-liquid model MELTS. The δ34S values of the Lubei sulfides vary over a narrow range of −0.3 to +1.8‰, indicating a mantle origin for the sulfur. The key factor that controls the formation of magmatic Ni–Cu deposits in post-subduction settings is a metasomatized mantle accompanied by activation of translithospheric faults during post-collisional extension. This mechanism alternatively interprets the widespread magmatic Ni–Cu deposits in the southern Central Asian Orogenic Belt and implies translithospheric fault as a potential target for Ni–Cu exploration.

Platinum-Group Mineral Occurrences and Platinum-Group Elemental Geochemistry of the Xiadong Alaskan-Type Complex in the Southern Central Asian Orogenic Belt

Alaskan-type complexes commonly contain primary platinum-group element (PGE) alloys and lack base-metal sulfides in their dunite and chromite-bearing rocks. They could therefore host PGE deposits with rare sulfide mineralization. A detailed scanning electron microscope investigation on dunites from the Xiadong Alaskan-type complex in the southern Central Asian Orogenic Belt revealed: various occurrences of platinum-group minerals (PGMs) that are dominated by inclusions in chromite grains containing abundant Ru, Os, S and a small amount of Pd and Te, indicating that they mainly formed prior to or simultaneously with the crystallization of the host minerals; A few Os–Ir–Rurich phases with iridium/platinum-group element (IPGE) alloy, anduoite (Ru,Ir,Ni)(As,S)2−x and irarsite (IrAsS) were observed in chromite fractures, and as laurite (RuS2) in clinopyroxene, which was likely related to late-stage hydrothermal alteration. The rocks in the Xiadong complex display large PGE variations with ∑PGE of 0.38–112 ppb. The dunite has the highest PGE concentrations (8.69–112 ppb), which is consistent with the presence of PGMs. Hornblende clinopyroxenite, hornblendite and hornblende gabbro were all depleted in PGEs, indicating that PGMs were likely already present at an early phase of magma and were mostly collected afterward in dunites during magma differentiation. Compared with the regional mafic–ultramafic intrusions in Eastern Tianshan, the Xiadong complex show overall higher average PGE concentration. This is consistent with the positive PGE anomalies revealed by regional geochemical surveys. The Xiadong complex, therefore, has potential for PGE exploration.

Sub-arc mantle heterogeneity in oxygen isotopes: evidence from Permian mafic–ultramafic intrusions in the Central Asian Orogenic Belt

Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ18O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~ 5.3‰) and are apparently at odds with the mantle-like eNd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like eHf(t) values from 5.1 to 11.9 and crust-like δ18O values from 7.6 to 10.1‰. The observed increase of δ18O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ18O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (> 90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.

Mg–Sr–Nd isotopic constraints on the genesis of the giant Jinchuan Ni–Cu–(PGE) sulfide deposit, NW China

Magmatic sulfide deposits are principal resources of nickel and platinum-group elements (PGE) in the world. These deposits form as the result of the concentration of immiscible sulfide-liquid droplets segregated from mafic or ultramafic magma. Sulfide saturation in the magmas is commonly linked to contamination by the continental crust. The Jinchuan Ni–Cu–(PGE) deposit is the largest single magmatic sulfide deposit in the world. It is hosted by a small mafic–ultramafic intrusion in the North China Craton. The Jinchuan mafic–ultramafic intrusion intruded Paleoproterozoic marbles, gneisses, and granitic migmatites. This paper reports the magnesium, strontium and neodymium isotopic compositions of the intrusion and the country rocks. Olivine, pyroxene and whole rocks of the lherzolites from the Jinchuan intrusion have δ26Mg values ranging from −0.40‰ to −0.26‰, with a weighted mean value of −0.31 ± 0.06‰ (2SD), which is slightly lighter than expected in a mantle-derived magma. The marble country rocks have relatively light δ26Mg values varying from −2.00‰ to −1.17‰, with a weighted mean value of −1.53 ± 0.02‰ (2SD). The variations are attributed to the interaction between basaltic magma and marble country rocks as the principal mechanism which generated the low Mg isotopic signatures in the Jinchuan deposit. The (87Sr/86Sr)i and εNd values of the lherzolites vary from 0.706615 to 0.714612 and from −8.29 to −6.13, respectively. The marble country rocks have relatively high (87Sr/86Sr)i ratios (0.713662 to 0.740069) and low εNd values (−25.43 to −12.41). The negative Nb anomalies in all samples from the intrusion indicate that the parental magma underwent crustal contamination. Mg–Sr–Nd isotopic mixing calculations indicate that the Jinchuan intrusion underwent 10–20% bulk contamination involving the lower continental crust, followed by <10% bulk contamination by marbles. Our petrological and geochemical investigation of the Jinchuan intrusion provides evidence that a combination of the two processes led to the formation of the ore deposit. The assimilation of lower crustal rocks reduced the sulfur solubility; the assimilation of marble country rocks made the magma more oxidized and hence decreased the sulfur solubility further. The results from this study show that Mg isotopes are useful in the study of carbonate assimilation by mafic magma, and marble assimilation by basaltic magma is an important mechanism to produce sulfide saturation in the genesis of Ni–Cu–PGE sulfide deposits.

Geochronological, mineralogical and geochemical studies of sulfide mineralization in the Podong mafic-ultramafic intrusion in northern Xinjiang, western China

The Podong mafic-ultramafic intrusion is one of many Ni-Cu sulfide-mineralized intrusions in the Beishan-Tianshan Orogenic Belt in northern Xinjiang, western China. The temporal and lithological variations of the Ni-Cu mineralized intrusions in Beishan are much less constrained than those in Tianshan. This paper provides the first set of geochronological and geochemical data for the Podong magmatic sulfide deposit that is present in the Beishan region. The host intrusion is composed of lherzolite, troctolite, gabbronorite and gabbro. Ni-Cu mineralization occurs as disseminated sulfides in gabbronorite (± olivine). The zircon U-Pb age of gabbronorite is 270 ± 2.7 Ma, identical to the zircon U-Pb age of the nearby Poyi magmatic sulfide deposit that is hosted in troctolite and ultramafic rocks. These ages together define a new episode of Ni metallogeny in the Beishan region, which is ∼10 Ma younger than that in the Tianshan region to the north. The Fo contents of olivine in the Podong mafic-ultramafic intrusion are from 76 to 87 mol%, lower than the values for the nearby Poyi mafic-ultramafic intrusion. The Podong mafic-ultramafic rocks are characterized by εNd(t) values from −0.61 to +1.92, low to elevated initial 87Sr/86Sr ratios (0.7055–0.7090), and pronounced negative Nb-Ta anomalies. The Sr-Nd isotope data indicate that the parental magma of the Podong intrusion experienced 0–13 wt% crustal contamination, which cannot fully account for the observed negative Nb-Ta anomalies in the rocks. Sulfur isotope compositions of the Podong deposit are similar to those of Pre-Permian sedimentary rocks in the area as well as typical mantle values. The results support the premise that fractional crystallization and assimilation of siliceous crustal materials may have played a key role in triggering sulfide saturation in the Podong magma, although the involvement of external sulfur cannot be completely ruled out. Like other Permian magmatic sulfide deposits in the Beishan-Tianshan Orogenic Belt, the PGE tenors of bulk sulfides in the Podong deposit are extremely low, indicating a severely PGE-depleted parental magma. The Cu/Pd ratios of the Podong magmatic sulfide deposit are significantly higher than the mantle values, consistent with previous sulfide segregation at depth. The results from the Podong magmatic sulfide deposit confirm that widespread Ni metallogeny in a convergent tectonic zone is very protracted and the mineralized lithologies are highly variable, which have significant implications for regional Ni-Cu exploration.

Rochas intrusivas básicas no sudeste do Cinturão Dom Feliciano, RS: petrografia e geoquímica das associações I e II

Intrusões dioríticas, gabroicas e outras rochas máficas-ultramáficas registram tanto os processos de extração e transporte de magmas do manto, resultando na acresção vertical de material à crosta, quanto os processos de interação entre esses dois componentes. Recentemente, essas rochas têm sido apontadas como importantes marcadores petrogenéticos, quando associadas ao magmatismo intermediário a ácido, em ambientes orogênicos a pós-colisionais. Na porção sudeste do Cinturão Dom Feliciano, no Rio Grande do Sul, constituído majoritariamente por granitoides, ocorrem intrusões dioríticas e gabroicas de pequeno porte e caráter dominantemente básico, na região compreendida entre Pinheiro Machado e Pedro Osório. Este artigo apresenta a caracterização geológica, mineralógica, petrográfica e geoquímica dessas rochas, propondo assim a separação de duas associações principais. A Associação I (AI), constituída por rochas gabroicas, comumente cumuladas e por vezes estratificadas, engloba os corpos do Passo da Fabiana, Passo da Olaria, Arroio Santa Fé e Desvio Herval. A Associação II (AII), diorítica a gabroica, compreende rochas maciças a localmente foliadas, nas ocorrências do Alto Alegre, Passo dos Machados e Campo Bonito. As duas associações são subalcalinas, com afinidade cálcio-alcalina médio-K (AI) e médio a alto-K (AII), com altos teores de alumina (Al2O3 &gt; 17%), mesmo para os termos não cumulados. Diferenças quanto aos elementos maiores (CaO, Mg# e álcalis), menores e traços (P2O5, Zr, Nb, Y, total e padrões de ETR) referendam a separação proposta. As assinaturas magnéticas sugerem o controle no posicionamento das intrusões por descontinuidades litosféricas, que teriam possibilitado a ascensão e a colocação desses magmas em níveis superiores da crosta. Processos petrogenéticos envolvendo a participação de líquidos derivados do manto em reservatórios supracrustais e a sua evolução nas deep crustal hot zones são propostos.

Permian bimodal magmatism in the southern margin of the Central Asian Orogenic Belt, Beishan, Xinjiang, NW China: Petrogenesis and implication for post-subduction crustal growth

Previous studies show that Permian bimodal magmatism is widespread in the Tianshan region, the southern part of the Central Asian Orogenic Belt in northern Xinjiang, NW China. We have found that such bimodal magmatism extended farther to the south in the Beishan region. Our new zircon U-Pb age data show that the dolerite and granite dykes in the Poyi area of the region were emplaced at ~270 Ma, >20 myr after subduction ended in the region. The dated dolerite dyke is characterized by pronounced negative Nb-Ta anomalies, high εNd(t) from +4.3 to +5.1 and low initial 87Sr/86Sr ratios from 0.7051 to 0.7060 for whole rocks, plus high εHf(t) from +8.96 to +13.0 and elevated δ18O from 5.8 to 6.5‰ for zircon crystals, which indicate a subduction-modified mantle source and moderate degrees of crustal contamination (<10 wt%). Whole-rock 1000Zn/Fe, Mn/Zn, Dy/Yb and La/Yb ratios of the Poyi dolerite dyke are consistent with a peridotite-dominant mantle source. Between two possible causes of mantle partial melting, such as mantle plume impingement and lithospheric delamination coupled with asthenosphere upwelling, we favor the latter because existing data indicate that the Permian mafic-ultramafic intrusive rocks in the region were emplaced over a long period (>25 myr) and show no age progression in a single direction, which are inconsistent with the results of mantle plume magmatism. Whole rocks from the Permian granite dyke in the Poyi area are characterized by strong negative Nb-Ta anomalies, negative εNd(t) from −2.4 to −1.6 and high initial 87Sr/86Sr from 0.7080 to 0.7088. Magmatic zircon crystals from the granite dyke have εHf(t) from +0.86 to +5.64 and δ18O from 6.5 to 7.2‰. The isotope compositions of the granite dyke are within the ranges of the pre-Permian crustal rocks in the region, indicating a predominantly crustal source. Based on these data, we propose that partial melting in the crust was induced by underplating of mantle-derived mafic magmas, some of which were emplaced in the upper parts of the crust to form the broadly coeval dolerite dykes and mafic-ultramafic intrusions. The results from this study support the view that post-subduction bimodal magmatism is important for continental crustal growth and differentiation.

Timing, geochemistry and tectonic setting of Ni-Cu sulfide-associated intrusions of the Halls Creek Orogen, Western Australia

The Lamboo Province of the East Kimberley region of Western Australia contains a record of crustal and tectonic evolution from ca. 1912 to 1808 Ma. In this interval, current models indicate an ocean basin was systematically closed from ca. 1880 Ma until collision of the Kimberley (Western Zone) and North Australia Cratons (Eastern Zone) at the ca. 1830 Ma Halls Creek Orogeny. Within this orogen, the Central Zone has two proposed origins: (1) an intra-oceanic island-arc; and (2) an exotic continental fragment. In this study, we investigated the origin of Ni-Cu mineralized mafic-ultramafic intrusions found in the Central Zone. We performed U-Pb SHRIMP geochronology on 21 samples, together with O-isotopes in zircon and whole-rock geochemistry, to understand the formation of the intrusions and their host mineralization within the tectonic evolution of the Lamboo Province. Geochronology demonstrates that the majority of intrusions are synchronous at ca. 1848–1838 Ma (ca. 1844 ± 2 Ma), and were intruded into the <1869 Ma Tickalara metasedimentary package (with 1895–2810 Ma detrital zircons). The Ni-Cu mineralized Copernicus pyroxenite was dated at 1823 Ma. Whole-rock geochemistry indicates a subduction origin for the mafic-ultramafic magmatism, with trace element data trending with modern continental-arc magmas, and lacking distinct assimilation trends with the Tickalara metasedimentary host-rock. Oxygen isotope data (δ18O ca. 5.6–12.8‰), supports this interpretation. The occurrence of more mantle-derived compositions, in early metamafic dykes, suggests a setting for the Central Zone that included both mantle- and arc-derived magmas. This, together with evidence for syn-magmatic high-temperature metamorphism, suggests a continental back-arc setting in the Central Zone at 1845 Ma. This interpretation, as well as the Archean continental zircon detritus identified in the Tickalara metasediments, indicates the Central Zone is a continental fragment rather than an island arc. Subsequently, this study demonstrates that Paleoproterozoic continental arcs are a prospective setting for orthomagmatic Ni-Cu-PGE systems.

Temporal variations in the mantle source beneath the Eastern Tianshan nickel belt and implications for Ni–Cu mineralization potential

Many small mafic–ultramafic intrusions are found in the Eastern Tianshan Ni belt along the southern margin of the Central Asian Orogenic Belt (CAOB). The Ni–Cu mineralized Hulu intrusion is located in the eastern part of the Jueluotage area of the Eastern Tianshan Ni belt. This mafic–ultramafic intrusion comprises a shallow layered sequence and deeper dike-like unit. Precise zircon U–Pb dating yielded ages of 381.0 ± 1.3 and 377.0 ± 3.0 Ma for sulfide-bearing samples from the layered unit and 388.6 ± 2.7 Ma for a mineralized gabbro from the dike-like unit. 176Hf/177Hf values of zircons from the layered unit range from 0.282846 to 0.283165 and correspond to εHf(t) values of +10.6 to +20.4. δ18O values for these zircons range from +4.8‰ to +5.5‰. Similarly, zircons from the sulfide-bearing gabbro of the dike-like unit have εHf(t) and δ18O values from +13.0 to +18.2 and +5.0‰ to +5.6‰, respectively. Normalized SiO2, Al2O3, TiO2, and Na2O + K2O contents increase with decreasing MgO content, whereas the FeOtotal content is positively correlated with MgO, which indicates that the different lithologies resulted from the fractional crystallization of a common parental magma. Furthermore, the mafic–ultramafic intrusions and their calculated parental magmas in the Jueluotage area have similar rare earth element (REE) patterns with slight enrichment of the light REE relative to heavy REE. All of the rocks and estimated parental magmas exhibit depletions in high-field-strength elements (HFSE; e.g., Nb, Ta, and Zr) and enrichments in large-ion lithophile elements (LILE; e.g., U). The pronounced light REE and LILE enrichments and HFSE depletions of the mafic–ultramafic intrusions and calculated parental magmas are indicative of a subduction-modified mantle source. Clinopyroxenes in the Hulu intrusion have variable Al2O3 (4.39–7.32 wt%) and low TiO2 (0.33–0.67 wt%) contents, which are comparable to typical Alaskan-type complexes that were emplaced in subduction environments. In a TiO2 versus Alz (AlIV × 100/2) diagram, data for the Devonian mafic–ultramafic intrusions in the Jueluotage area fall within the fields of Alaskan-type complexes, whereas data for most of the Permian intrusions fall outside of these fields. Systematic in situ zircon O and Hf isotope analyses from the Devonian mafic–ultramafic intrusions in the Jueluotage area exhibit higher δ18O and lower εHf(t) values than those of the Permian intrusions, which may be explained by a change from a Devonian fluid- to Permian fluid- and melt-modified mantle source. The mafic–ultramafic intrusions that were emplaced in both subduction and post-collisional extensional environments in the Eastern Tianshan Ni belt were favorable hosts to magmatic Ni–Cu deposits, despite their different mantle sources.