New data on the Pb isotope composition of Noril’sk sulfide ores

Doklady Earth Sciences -     

Upstairs-downstairs: supercontinents and large igneous provinces,are they related?

There is a correlation of global large igneous province (LIP) events with zircon age peaks at 2700, 2500, 2100, 1900, 1750, 1100, and 600 and also probably at 3450, 3000, 2000, and 300 Ma. Power spectral analyses of LIP event distributions suggest important periodicities at 250, 150, 100, 50, and 25 million years with weaker periodicities at 70–80, 45, and 18–20 Ma. The 25 million year periodicity is important only in the last 300 million years. Some LIP events are associated with granite-forming (zircon-producing) events and others are not, and LIP events at 1900 and 600 Ma correlate with peaks in craton collision frequency. LIP age peaks are associated with supercontinent rifting or breakup, but not dispersal, at 2450–2400, 2200, 1380, 1280, 800–750, and ≤200 Ma, and with supercontinent assembly at 1750 and 600 Ma. LIP peaks at 2700 and 2500 Ma and the valley between these peaks span the time of Neoarchaean supercraton assemblies. These observations are consistent with plume generation in the deep mantle operating independently of the supercontinent cycle and being controlled by lower-mantle and core-mantle boundary thermochemical dynamics. Two processes whereby plumes can impact continental assembly and breakup are (1) plumes may rise beneath supercontinents and initiate supercontinent breakup, and (2) plume ascent may increase the frequency of craton collisions and the rate of crustal growth by accelerating subduction.     

Sources and formation conditions of sulfide-silicate magmas in the Noril’sk district

Geology, tectonomagmatic reactivation of the Noril??sk district, as well as stratigraphy and geochemistry of the volcanic sequence are considered. Sources and formation mechanism of ore-bearing magma and the scope of ore formation are discussed. The Permian-Triassic flood-basalt magmatism of the Noril??sk district developed in part of the Siberian Platform with Archean-Paleoproterozoic basement broken into blocks and overlapped by a sedimentary cover up to 13 km thick and a volcanic sequence reaching 3.7 km in thickness. The geophysical data show that remnants of the subducted ancient oceanic crust exist in the mantle and fragments of transitional magma chambers and conduits are retained at different levels of the Earth??s crust. The cyclic tectonomagmatic evolution of the territory was characterized by alternation of extension with intense volcanic activity and compression accompanied by waning of volcanic eruptions. The early rifting, transitional stage, and late dispersed spreading are distinguished. The associations of volcanic (lavas and tuffs) and intrusive rocks were formed during each stage. The volcanic sequence is subdivided into 11 formations. The intrusions of the Talnakh and Noril??sk ore fields are distinguished by two-level structure with the Upper Noril??sk ore-bearing intrusions above and the Lower Noril??sk barren intrusions below. Two types of primary magmas differ in geochemistry of lavas and intrusions: (1) OIB-type high-Ti magma (iv, sv, gd formations of the first stage from bottom to top) and (2) low-Ti magma (hk, tk, nd formations of the second stage and mr-mk formations of the third stage). The nd formation depleted in ore elements and the ore-bearing cumulus composed of silicate and sulfide melts in combination with early silicate minerals and chromite are products of the fractionation of the primary low-Ti magma. As follows from geochemical parameters, intrusions of the Lower Noril??sk type are comagmatic to the evolved lavas of the nd₃ subformation, whereas intrusions of the Upper Noril??sk type are comagmatic to the lavas of the mr-mk formations. Geochemical similarity with volcanic rocks provides evidence for the composition of the initial magma and the time of intrusion emplacement. The ore-bearing intrusions of the Upper Noril??sk type were formed at the onset of the third stage, when the primitive low-Ti magma similar to the lavas of mr-mk formations in composition was emplaced. When intruding, this melt captured and transported ore-bearing cumulus (drops of sulfide melt, early olivine and chromite grains) into the magma chamber. Separate portions of sulfide liquid were involved into movement as a self-dependent intrusive subphase during formation of the Talnakh and Kharaelakh intrusions. An extremal effect of pressure on sulfur concentration in fluid-bearing and sulfide-saturated mafic magmas has been established in experiments to be P = 1?2 GPa. In this interval of pressure, the S concentration in sulfide-saturated magmas increases in the following sequence: dry magma ??(H₂O + CO₂)-bearing magma <H₂O-bearing magma. In the regions of low (<0.3 GPa) and high (>2.5 GPa) pressures, the S contents (0.1?C0.2 wt %) are commensurable. The extremal baric relationship of S concentration in fluid-bearing and sulfide-saturated mafic magmas may be important for the formation of ore-bearing magmas. The calculation results show that the amount of sulfides in the known deposits does not exceed 2% of geological resources of the sulfides separated from the flood basalts. Therefore, the chance of discovery of new deposits remains rather high. Proceeding from the conditions of ore-bearing magma formation and geological setting of the known deposits, criteria for recognition of potentially ore-bearing areas are proposed and such areas are outlined.     

Geochemical aspects of the assimilation of host rocks by basaltic magmas during the formation of Noril’sk Cu-Ni ores

In models for the genesis of the Noril’sk Pt-Cu-Ni ore deposits, much importance is attached to the processes of assimilation of host rocks by basaltic melts. This idea is based on unusual relations between the silicate and sulfide constituents of this type of ore deposits and also on the heavy sulfur isotopic composition of the sulfide ores. The reason for this unusual composition is thought to be the assimilation of anhydrite from the host rocks. However, no other factors able to influence this process have ever been analyzed in the literature. We were the first to thoroughly analyze the inner structure of contact aureoles of the intrusions hosted in various rocks: the Maslovsky intrusion in Early Triassic basalts of the Ivakinsky and Nadezhdinsky formations and the Talnakh intrusion in Devonian anhydrite-bearing carbonate-terrigenous rocks. The distributions of trace elements, the ⁸⁷Sr/⁸⁶Sr isotopic ratio, and Sm and Nd isotopes indicate that host rocks were either not assimilated at all, or their effect is perceptible only within a very narrow (1 m) zone in the eastern apophyse in the southern portion of the Maslovsky intrusion. The Sr, Nd, and particularly, Pb isotopic composition indicate that the anhydrite could not be the source of isotopically heavy sulfur for sulfides at Noril’sk deposits. The ores of the Maslovsky and Talnakh intrusions have similar sulfur isotopic composition of their sulfides (the maximum δ³⁴S values of these sulfides reach +10.8 and +14.2‰, respectively), in spite of the significant differences in the rocks hosting these intrusions. Our newly obtained data indicate that assimilation was insignificant and could not affect the origin of the ores.     

Maslovsky ore deposit of Noril’sk region: Morphology and internal structure of a layered intrusion

The Maslovsky ore deposit is an interesting but poorly explored geological locality in the Noril’sk ore region. Interpretations of the morphology and internal structure of the intrusion, which hosts PGE and copper-nickel deposits, are provided in this study. The complicated morphology of the Maslovsky intrusion enables us to recognize two layered gabbro-dolerite bodies, which are connected by a feeder dyke. Their internal structure testifies to the similarities and differences in these intrusive bodies. The results of this study have potential for further petrological and geochemical research.     

Ultramafic–mafic igneous complexes of the Precambrian East Siberian metallogenic province (southern framing of the Siberian craton): age,composition, origin,and ore potential

Based on new data on the age, mineralogy, and geochemistry of ultramafic–mafic complexes in the Precambrian structures of the southern periphery of the Siberian Platform, the East Siberian metallogenic (PGE–Cu–Ni) province is recognized. It includes the Yenisei Ridge, Precambrian Kan uplift, Alkhadyr terrane with the adjacent structures of the Biryusa block, and northern Baikal region (Yoko-Dovyren and other massifs of the Baikal–Patom basin). We have established that the U–Pb and Ar–Ar ages of ore-bearing complexes of dunite–peridotite– pyroxenite–gabbro association correspond to the Late Riphean (728–710 Ma). The mineralogical and geochemical similarity of ore-bearing complexes in different areas testifies to their genetic entity. All parental melts were similar in composition to picrites. The calculation results and the PGE enrichment of rocks and ores show high degrees of melting of the mantle source, which agrees with the plume model of formation of the ore-magmatic system. The recognized province is similar in the type of magmatism and time of its occurrence to the Franklin LIP in northern Canada. It is one of the highly promising ore districts of East Siberia.     

Time series analysis of mantle cycles Part Ⅱ:The geologic record in zircons,large igneous provinces and mantle lithosphere

Igneous and detrital zircons have six major U/Pb isotopic age peaks in common(2700 Ma,1875 Ma.1045 Ma,625 Ma,265 Ma and 90 Ma).For igneous rocks,each age peak is comprised of subpeaks with distinct geographic distributions and a subpeak age range per age peak ≤100 Myr.There are eight major LIP age peaks(found on≥10 crustal provinces)of which only four are in common to major detrital zircon age peaks(2715 Ma,1875 Ma,825 Ma,90 Ma).Of the whole-rock Re depletion ages,58% have correspo nding detrital zircon age peaks and 55% have corresponding LIP age peaks.Ten age pea ks are fou nd in common to igneous zircon,detrital zircon,LIP,and Re depletion age time series(3225 Ma,2875 Ma,2145 Ma,2085 Ma,1985 Ma,1785 Ma,1455 Ma,1175 Ma,825 Ma,and 90 Ma).and these are very robust peaks on a global scale as recorded in both crustal and mantle rocks.About 50% of the age peaks in each of these time series correspond to predicted peaks in a 94-Myr mantle cycle,including four of the ten peaks in common to all four time series(2875 Ma,1785 Ma,825 Ma and 90 Ma).Age peak widths and subpeak ranges per age peak suggest that mantle events responsible for age peaks are100 Myr and many50 Myr in duration.Age peak geographic distributions show three populations(≤1000 Ma,2500-1000 Ma,2500 Ma),with the number of new provinces in which age peaks are represented decreasing with time within each population.The breaks between the populations(at 2.5 Ga and 1 Ga)fall near the onsets of two transitions in Earth history.The First Transition may represent a change from stagnant-lid tectonics into plate tectonics and the Second Transition,the onset of subduction of continental crust.The major factor controlling geographic distribution of age peaks is the changing locations of orogeny.Before ～2 Ga,age subpeaks and peaks are housed in orogens within or around the edges of crustal provinces,mostly in accretionary orogens.but beginning at 1.9 Ga,collisional orogens become more important.The coincidence in duration between magmatic flare-ups in Phanerozoic arcs and duration of age subpeaks(10-30 Myr)is consiste nt with subpeaks representing periods of enhanced arcrelated magmatism.probably caused by increased subduction flux.The correlation of isotopic age peaks between time series supports a cause and effect relationship between mantle plume activity,continental magma production at convergent margins,and crustal deformation.Correlation of over half of the detrital zircon age peaks(and six of the nine major peaks)with Re depletion age peaks supports an interpretation of the zircon peaks as crustal growth rather than selective preservation peaks.     

Differentiated Mafic–Ultramafic Intrusions of the Kruglogorsky Type in the Noril’sk Area: Petrology and Ore Potential

Intrusions of the Kruglogorsky type are an integral part of magmatic formations in the Noril’sk area. The marginal portions of these intrusions are composed of microdolerite, dolerite, and contact gabbrodolerite. The central parts of the intrusions consist of leucogabbro and of olivine-free, olivine-bearing, and olivine gabbro-dolerite. Leucogabbro is a characteristic rock of this type of intrusions and sometimes composes up to half of the thicknesses of the rock units. The rocks with plagioporphyritic textures are widespread. Olivine-free, olivine-bearing, and olivine gabbro-dolerite occur as horizons with indistinct boundaries, which are unevenly distributed over the vertical sections of the lithological units. The olivine is the most magnesian (Fo_89-64) and richest in Ni (up to 0.23 wt % NiO) in the olivine gabbro-dolerite. The clinopyroxene is represented by augite (Fs_12-29). The rock-forming minerals are typically zoned. The Sr isotopic composition of the rocks (calculated for an age of 250 Ma) varies within a considerable range (⁸⁷Sr/⁸⁶Sr = 0.705972–0.708006), due to metasomatic alterations. The variations in the Nd and Sr isotopic composition of the Kruglogorsky intrusion are close to those in rocks of the Noril’sk-type ore-bearing intrusions. The olivine-bearing and taxitic gabbro-dolerite host Pt–Cu–Ni ore mineralization, which are of economic value for disseminated ores of the Talnakh area.     

Structure and geochemical characteristics of trap rocks from the Noril’sk Trough,Northwestern Siberian craton

Modern analytical methods (XRF and ICP-MS) were employed for the first time to investigate the geochemical characteristics of rocks from the Noril’sk Trough. It differs from other folded structures of the region in the presence of massifs with high-grade Pt-Cu-Ni ores at relatively high levels in the section of the platform cover, in the middle part of the tuff-lava sequence. This provides an opportunity of directaions of observe geologic relationships between the extrusive and intrusive rocks of the Siberian Traps and more reliably test the possibility of their comagmatic origin, because other intrusions with massive ores occur at a deeper stratigraphic level in the Devonian sequences (Kharaerlakhsky and Talnakhsky massifs in the Kharaerlakhsky Trough). A comparison of the volcanic sections of the Noril’sk Trough and other parts of the region suggests that the tectonic structures were initiated before or simultaneously with trap development. By the example of the sill of the Maslovsky intrusion, it was shown that the ore-bearing gabbro-dolerites were formed in post-Nadezhdinsky time. The geochemical characteristics of the rocks of the Noril’sk 1 and Maslovsky intrusions were compared with those of the supposedly comagmatic rocks of the Gudchikhinsky, Tuklonsky, Nadezhdinsky, and Morongovsky formations. It was found that the Gudchikhinsky picritic basalts show higher Gd/Yb values compared with the picritic gabbro-dolerites (on average, 2.5 and 1.5, respectively) and higher nickel and calcium contents in olivine (0.44 and 0.22 wt % NiO and 0.30 and 0.12 wt % CaO for Fo₈₂, respectively). The high-magnesium rocks of the Tuklonsky Formation are depleted in U and enriched in Eu relative to their intrusive analogues and also differ with respect to NiO and CaO contents in olivine (0.11 and 0.21 wt % NiO and 0.21 and 0.12 wt % CaO for Fo₇₈, respectively). The rocks of the Nadezhdinsky Formation show strongly enriched incompatible trace element patterns, especially for LREE. Although the Morongovsky Formation is geochemically similar to the intrusions of the Noril’sk complex, it shows a lower weighted mean MgO content (6–7 wt % as compared with 11–12 wt % for the gabbro-dolerites). Thus, it was shown by the example of the geologic setting and compositional features of the Maslovsky intrusion that the massifs with high-grade mineralization are not analogues of volcanic complexes, but were produced by an independent magmatic stage.     

Oxygen Isotope Composition of Zircons from the Talnakh Economic Intrusion of the Noril’sk Province: First Data

Doklady Earth Sciences - This study presents the first results of oxygen isotope analyses (δ18O) collected on zircons from the Talnakh economic intrusion within the Noril’sk province....     

Geochemistry and geochronology of the mafic lavas from the southeastern Ethiopian rift (the East African Rift System): assessment of models on magma sources,plume–lithosphere interaction and plume evolution

Major and trace element and isotopic ratios (Sr, Nd and Pb) are presented for mafic lavas (MgO > 4 wt%) from the southwestern Yabello region (southern Ethiopia) in the vicinity of the East African Rift System (EARS). New K/Ar dating results confirm three magmatic periods of activity in the region: (1) Miocene (12.3–10.5 Ma) alkali basalts and hawaiites, (2) Pliocene (4.7–3.6 Ma) tholeiitic basalts, and (3) Recent (1.9–0.3 Ma) basanite-dominant alkaline lavas. Trace element and isotopic characteristics of the Miocene and Quaternary lavas bear a close similarity to ocean island basalts that derived from HIMU-type sublithospheric source. The Pliocene basalts have higher Ba/Nb, La/Nb, Zr/Nb and ⁸⁷Sr/⁸⁶Sr (0.70395–0.70417) and less radiogenic Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.12–18.27) relative to the Miocene and Quaternary lavas, indicative of significant contribution from enriched subcontinental lithospheric mantle in their sources. Intermittent upwelling of hot mantle plume in at least two cycles can explain the magmatic evolution in the southern Ethiopian region. Although plumes have been originated from a common and deeper superplume extending from the core–mantle boundary, the diversity of plume components during the Miocene and Quaternary reflects heterogeneity of secondary plumes at shallower levels connected to the African superplume, which have evolved to more homogeneous source.     

Geochronology and geochemistry of the Nantianwan mafic–ultramafic complex,Emeishan large igneous province: metallogenesis of magmatic Ni–Cu sulphide deposits and geodynamic setting

The Nantianwan mafic–ultramafic complex is situated in the northwest part of the Panxi district, southwest China. It consists predominantly of gabbros, gabbronorites, and lherzolites. LA–ICP–MS U–Pb zircon dating of the gabbronorites yields an age of 259.7 ± 0.6 million years, consistent with the ages of other mafic–ultramafic intrusions in the Emeishan large igneous province (ELIP). Gabbronorites and lherzolites host Cu–Ni sulphide ores. Cumulus texture is pronounced in these rocks, containing magnesium-rich olivine (up to 81.4% forsterite). SiO₂ contents of the lherzolites range from 42.93 to 44.18 wt.%, whereas those of the gabbronorites vary between 44.89 and 52.76 wt.%. Analysed samples have low rare earth element (REE) contents (23.22–30.16 ppm for lherzolites and 25.21–61.05 ppm for gabbronorites). Both lherzolites and gabbronorites have similar chondrite-normalized REE patterns, suggesting that they are comagmatic. All samples are slightly enriched in large ion lithophile elements (LILEs, e.g. Rb, Ba, and Sr) relative to high field strength elements (HFSEs, e.g. Nb, Ta, and Ti), very similar to those of ocean island basalts (OIBs). The presence of cumulus textures and geochemical signatures indicates that fractional crystallization played an important role in the petrogenesis of these rocks. Initial (⁸⁷Sr/⁸⁶Sr) _t (t?=?260 Ma) ratios and ?_Nd(t) values of the mafic–ultramafic suite vary from 0.70542 to 0.70763, and??0.4 to 1.7, respectively. Compared to the Cu–Ni-bearing Baimazhai and Limahe intrusions in the ELIP, which were considerably contaminated by variable crustal materials, the Nantianwan complex exhibits much lower (⁸⁷Sr/⁸⁶Sr) _t . Their ?_Nd(t) versus (Th/Nb)_PM ratios also indicate that the ore-bearing magmas did not undergo significant crustal contamination. In combination with (Tb/Yb)_PM versus (Yb/Sm)_PM modelling, we infer that the magmas originated from an incompatible elements-enriched spinel-facies lherzolite that itself formed by interaction between the Emeishan plume and the lithospheric mantle. Most plots of NiO versus Fo contents of olivine suggest that sulphides are separated from the parental magma by liquid immiscibility, which is also supported by bulk-rock Cu/Zr ratios of the lherzolites (7.04–102.67) and gabbronorites (0.88–5.56). We suggest that the gabbronorites and lherzolites experienced undersaturation to oversaturation of sulphur; the latter may be due to fractional crystallization in a high-level magma chamber, accounting for the sulphide segregation.     

Structural controls on the primary distribution of mafic–ultramafic intrusions containing Ni–Cu–Co–(PGE) sulfide mineralization in the roots of large igneous provinces

Deposits of Ni–Cu–Co–(PGE) sulfide often occur in association with small differentiated intrusions that reside within local transtensional spaces in strike-slip fault zones. These faults often develop in response to incipient rifting of the crust and the development of large igneous provinces due to far-field stresses generated by plume-induced continental drift. We review the geology of a number of large and small nickel sulfide deposits and the associated intrusions, and show that the geometry of the host intrusion and localization of the mineral zones can be classified into three main groups. Further, we show that the morphology of each is controlled by space created in response to deformation on structures.One group of intrusions has the plan shape of an asymmetric rhomboid with the long axis sub-parallel to a fault zone, and contacts which have often been structurally modified during and/or after emplacement of the magma. The typical cross section is a downward-closing cone shape with curved walls and often a dyke-like keel at the base. This morphology is found in the Ovoid and Discovery Hill Zones of the Voisey's Bay Deposit (Canada), the Jinchuan, Huangshan, Huangshandong, Hongqiling, Limahe, Qingquanshan, and Jingbulake (Qingbulake) Intrusions in China, and the Eagle and Eagle's Nest deposits in the USA and Canada, respectively.A second group of deposits is associated with conduits within dyke and sheet-like intrusions; these deposits are often associated with discontinuities in the dyke which were created in response to structural controls during emplacement. Examples include the Discovery Hill Deposit and the Reid Brook Zone of the Voisey's Bay Intrusion, where there are plunging domains of thicker dyke which control the mineralization inside the dyke, and thin discontinuous segments of the dyke which are associated with structurally controlled mineralization in the surrounding country rock gneisses. The Oktyabrysk, Taimyrsk, Komsomolsk, and Gluboky Deposits in the Noril'sk Region of Russia are localized at the base of thicker parts of the Kharaelakh Intrusion which appear to be a conduit that follows synformal features in the country rocks located west of the Noril'sk–Kharaelakh Fault. Other examples of dyke-like bodies with both variation in width and the development of discontinuities are the Copper Cliff and Worthington Offset Dykes which radiate away from the Sudbury Igneous Complex (Canada). The distribution of ore bodies in these Sudbury Offset Dykes is principally controlled by variations in the thickness of the dyke, interpreted to reflect the presence of conduits within the dyke.A third group of mineralized intrusions located within structural corridors have the geometry of oblate tubes; examples include Kalatongke in China, Northeastern Talnakh and Noril'sk 1 in Russia, Babel–Nebo in Australia, and Nkomati in South Africa. Sometimes these oblate tube-like intrusions form in bridging structures between larger intrusions hosted in the more significant structures. Examples include the Tamarack Intrusion in Minnesota, USA, and the Current Lake Complex in Ontario, Canada, both of which contain magmatic Ni–Cu sulfide mineralization.In all of these deposits, the intrusions appear to be open system magma pathways, and so the term “chonolith” can be applied to describe them as a group. All of these intrusions are characterized by a high ratio of sulfide/silicate; there are 1–3 orders of magnitude more sulfide in the intrusion than the magma contained in the intrusion is capable of dissolving. The formation of these deposits is considered to have taken place in open system magma conduits. It is possible that the metal tenor of the sulfides were upgraded by equilibration of successive batches of silicate magma passing through the conduit, and equilibrating with a stationary pool of magmatic sulfide. At Voisey's Bay there appears little doubt that the sulfides were injected through a conduit dyke into higher level magma chambers. A similar model has been proposed for the formation of the deposits at Jinchuan and Noril'sk–Kharaelakh. Economically significant nickel sulfide deposits that tend to be high in Ni tenor, are often related to the late injection of magma that form distinct parts of the intrusion, and the localization of mineralization tends to be related to changes in the geometry of the magma chamber. Strongly deformed and metamorphosed komatiite-associated deposits (e.g. Pechenga, Thompson, and the Yilgarn komatiite associations) appear to be the remains of open system magma conduits which are now represented by segmented and boudinaged ultramafic bodies as a result of more than 4 phases of post-emplacement deformation.LIP activity at craton margins has long been recognized as a key control on the genesis of magmatic sulfide deposits; we show that the principal regional controls of strike-slip tectonics underpin the local geometry of the intrusions, and we provide an explanation for why so many of the global nickel sulfide ore deposits are associated with intrusions that share common morphologies and characteristics. This model provides a framework for more detailed structural investigations of nickel sulfide deposits, and it is a predictive framework for mineral exploration.     

Geochemical studies and petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm (trending N-S to NNW-SSE) from eastern Dharwar craton,India: implications for Paleoproterozoic large igneous provinces and supercraton superia

The Archean eastern Dharwar craton is transacted by at least four major Proterozoic mafic dyke swarms. We present geochemical data for the ~2.21–2.22 Ga N-S to NNW-SSE trending Kunigal mafic dyke swarm of the eastern Dharwar craton to address its petrogenesis and formation of large igneous province as well as spatial link to supercontinent history. It has a strike span of about 200 km; one dyke of this swarm runs ~300 km along the western margin of the Closepet granite. Texture and mineral compositions classify them as dolerite and olivine dolerite. They show compositions of high-iron tholeiites, high-magnesian tholeiites or picrites. Geochemical characteristics of the sampled dykes suggest their co-genetic nature and show variation from primitive (Mg#; as high as ~76) to evolved (differentiated) nature. Although geochemical characteristics indicate possibility of minor crustal contamination, they show their derivation from an uncontaminated mantle melt. These mafic dykes are probably evolved from a sub-alkaline basaltic magma generated by ~20 % batch melting of a depleted lherzolite mantle source and about 15–30 % olivine fractionation. Paleoproterozoic (~2.21–2.22 Ga) mafic magmatism is recognized globally as dyke swarms or gabbroic sill complexes in the Superior, Slave, North Atlantic, Fennoscandian and Pilbara cratons. Possible Paleoproterozoic Dharwar–Superior–North-Atlantic–Slave correlations are constrained with implications for the configuration of supercraton Superia.     

Geochronology–geochemistry of the Cida bimodal intrusive complex,central Emeishan large igneous province,southwest China: petrogenesis and plume–lithosphere interaction

The Cida complex is situated in the Panxi region and is predominantly composed of mafic-ultramafic and syenitic rock units; minor amounts of intermediate rocks occupy the contact zone between the two major rock types. The intermediate unit is mineralogically heterogeneous and typically exhibits a mottled structure. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb zircon dating shows that the mafic-ultramafic rocks and syenitic rocks formed almost coevally (243 ± 0.77 Ma and 240.5 ± 0.76 Ma, respectively). These ages may represent the end phase of the Emeishan large igneous province (ELIP) magmatism. Most of these three rock types possess alkaline and metaluminous affinities. The mafic-ultramafic, syenitic, and intermediate units have K₂O + Na₂O contents of 1.85–5.16, 6.55–10.46, and 9.55–11.54 wt.%, and SiO₂ contents of 40.06–46.70, 61.74–68.54, and 51.57–54.13 wt.%, respectively. The mafic-ultramafic unit displays ocean-island basalt (OIB)-like primitive-mantle-normalized incompatible element patterns, coupled with low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7048–0.7064), positive ?_Nd(t) (0.32–2.23), and zircon ?_Hf(t) (4.53–14.17) values, consistent with a mafic plume-head origin, whereas one exceptional sample with negative ?_Nd(t) (–0.22) can be interpreted as due to the involvement of considerable amounts of enriched subcontinental lithospheric mantle. The relatively low (La/Yb) _N ratios (3.40–7.69) reflect a spinel-facies lherzolite source. The syenitic unit is characterized by enrichment in large ion lithophile elements (e.g. Rb, K, Pb) and light rare earth elements (LREEs), relative to high field strength elements (e.g. Nb, Ta, P, Ti) and heavy rare earth elements (HREEs), respectively. These features, together with their metaluminous affinities, low SiO₂ contents, lower initial ⁸⁷Sr/⁸⁶Sr ratios (0.7043), positive ?_Nd(t) (0.18), and zircon ?_Hf(t) (2.63–10.09) values as well as modelling of REEs, can be plausibly explained by crustal partial melting of juvenile basic materials beneath the Yangtze Block. In contrast, the field, petrographic observations, and geochemical signatures (e.g. the linear correlations between FeO* and MgO, K/Ba and Rb/Ba ratios) suggest that the intermediate unit may have resulted from magma mixing between the syenitic and basaltic magmas that in turn had evolved from a parental mafic-ultramafic liquid. Thus, the formation of the Cida complex can be attributed to the plume–lithosphere interaction plus partial melting of juvenile basic lower crust in response to heating of underplated plume-derived basaltic magma.     

Geological Relationships Between the Intrusions,Country Rocks,and Ni-Cu-PGE Sulfides of the Kharaelakh Intrusion,Noril′sk Region:Implications for the Roles of Sulfide Differentiation and Metasomatism in Their Genesis

The Ni-Cu-platinum group element sulfide ore deposits of the Kharaelakh Intrusion,Noril′sk Region,Siberia,represent a large concentration of sulfides associated with a small differentiated intrusion formed at the edge of the Siberian Craton in the roots of the Siberian Trap flood basalt.The deposit is associated with an intrusion that occupies a flanking periclinal structure adjacent to the Noril′sk-Kharaelakh Fault.The intrusion is strongly differentiated and comprises taxitic gabbrodolerites,picritic gabbrodolerites,and gabbrodolerites within the main body which in turn forms a chonolith within a sheet-like intrusion that extends laterally to form extensive undifferentiated sills of gabbrodolerite.The intrusion substantially replaces the stratigraphy of the country rocks,and although it appears to have exploited the axis of structures developed in response to transtension,the intrusion has created space by both mechanical dilation of stratigraphy and magmatic replacement of pre-existing sedimentary rocks.The frontal lobes of the main intrusion have complex apophyses of gabrodolerite on a range of scales that demonstrate replacement of the sedimentary rocks and link to the development of an extensive metamorphic halo in the country rocks.This halo is much narrower over the main body of the intrusion,and these observations have implications for the thermal history of the intrusion.Mg-skarns and breccias are developed in the roof of the main body of the intrusion.Within the intrusion,the taxitic rocks contain vesicles and the blebby sulfides developed in the picritic and taxitic gabbrodolerites appear to have a linkage to volatile phases.Cuprous sulfide mineralization developed at the roof of the Kharaelakh Intrusion is associated with metamorphosed and skarn-bearing country rocks,and appears to have been generated by a combination of sulfide fractionation and associated metasomatism.The geological relationships appear consistent with a chonolith model for the development of the differentiated intrusion and mineralization,but the extent of metasmorphism of the country rocks appears to be related to the unusual thickness of gabbrodolerite apophyses at the flanks of the intrusion rather than metamorphism produced by the passage of mafic magma through the intrusion.Variations in disseminated sulfide compositions and metasomatic textures in the skarns are described,and a model is proposed which balances traditional views on the evolution of the magma conduits with the impact of magmatic fluids transported through the magma column(i.e.transmagmatic fluids).The importance of structures in controlling the nature of the conduit,and the resultant small intrusions with excess sulfide is a feature of many other Ni-Cu sulfide deposits including Voisey′s Bay,and it is suggested that the sulfides are more likely to have beentransported from depth into their final resting place rather than developed by in-situ equilibration of sulfide with fresh magma in the chonolith.     

Mineralogy, geochemistry and stratigraphy of the Maslovsky Pt–Cu–Ni sulfide deposit, Noril’sk Region, Russia

We report new data on the stratigraphy, mineralogy and geochemistry of the rocks and ores of the Maslovsky Pt–Cu–Ni sulfide deposit which is thought to be the southwestern extension of the Noril’sk 1 intrusion. Variations in the Ta/Nb ratio of the gabbro-dolerites hosting the sulfide mineralization and the compositions of their pyroxene and olivine indicate that these rocks were produced by two discrete magmatic pulses, which gave rise to the Northern and Southern Maslovsky intrusions that together host the Maslovsky deposit. The Northern intrusion is located inside the Tungusska sandstones and basalt of the Ivakinsky Formation. The Southern intrusion cuts through all of the lower units of the Siberian Trap tuff-lavas, including the Lower Nadezhdinsky Formation; demonstrating that the ore-bearing intrusions of the Noril’sk Complex post-date that unit. Rocks in both intrusions have low TiO₂ and elevated MgO contents (average mean TiO₂ <1 and MgO?=?12?wt.%) that are more primitive than the lavas of the Upper Formations of the Siberian Traps which suggests that the ore-bearing intrusions result from a separate magmatic event. Unusually high concentrations of both HREE (Dy+Yb+Er+Lu) and Y (up to 1.2 and 2.1?ppm, respectively) occur in olivines (Fo_79.5 and 0.25% NiO) from picritic and taxitic gabbro-dolerites with disseminated sulfide mineralization. Thus accumulation of HREE, Y and Ni in the melts is correlated with the mineral potential of the intrusions. The TiO₂ concentration in pyroxene has a strong negative correlation with the Mg# of both host mineral and Mg# of host rock. Sulfides from the Northern Maslovsky intrusion are predominantly chalcopyrite–pyrrhotite–pentlandite with subordinate and minor amounts of cubanite, bornite and millerite and a diverse assemblage of rare precious metal minerals including native metals (Au, Ag and Pd), Sn–Pd–Pt–Bi–Pb compounds and Fe–Pt alloys. Sulfides from the Southern Maslovsky intrusion have δ ³⁴S?=?5–6‰ up to 10.8‰ in two samples whereas the country rock basalt have δ ³⁴S?=?3–4‰, implying there was no in situ assimilation of surrounding rocks by magmas.