Country Rock Gneisses Research Articles

Palaeo-climate and -topography of the continental orogen: Theoretical inversion with initial oxygen isotopes of ancient meteoric water

ABSTRACT Ancient environments have been mostly reconstructed with exogenous records, yet the potential constraints from endogenous archives were less emphasised. It has been well known that the outer- and inner-spheres of the planetary Earth are naturally linked and/or interplayed each other among geospheres. As stable isotopes of the meteoric water are globally dependent upon precipitating environments, rocks and/or minerals hydrothermally altered by the meteoric water can thus imprint environmental information of continental settings. These valuable clues, however, have been intuitively and/or qualitatively inferred up to now. On the basis of an innovative procedure recently proposed for dealing with thermodynamic re-equilibration of oxygen isotopes between constituent minerals and water from fossil hydrothermal systems, ancient meteoric waters are theoretically inverted from the early Cretaceous post-collisional granitoid and Triassic gneissic country rocks across the Dabie orogen in central-eastern China. The initial oxygen isotopes of ancient meteoric water (i.e., $\delta ^{18}O_W^i$ value hereafter) range from −11.01 ± 0.43 (one standard deviation, 1SD) to −7.61 ± 0.07‰ in this study, yet systematically and/or statistically deviating from modern local precipitation. These imply that either palaeoclimate could be colder than the present at least during the early Cretaceous or palaeoaltimetry has geographically varied across the Dabie orogen since the Triassic. Moreover, the lifetime of fossil hydrothermal systems is kinetically quantified to less than 1.2 million years (Myr) for the concurrent lowering of oxygen isotopes of hydrothermally altered rock-forming minerals through the surface-reaction oxygen exchange with ancient meteoric waters herein. Our results thus suggest that palaeoenvironments of the continental orogen can be scientifically and methodologically unearthed from endogenous archives and theoretical inversion of $\delta ^{18}O_W^i$ values can be quantitatively applied beyond the Dabie orogen.

Zircon constraints on gold enrichment in a high-silica porphyry, Yixingzhai region, north of the Trans-North China Orogen

The Hewan granite porphyry is a newly discovered, highly evolved Au deposit in north China. Two types magmatic zircon are found in the porphyry. Type I is CL-dark and only occurs in the core of some zircons. Type II is CL-bright and mantles Type I zircon or occurs as individual grains. The two Types are close in age, i.e. 138.0 ± 2.2 Ma for Type I and 135.9 ± 2.1 Ma for Type II. Type I zircon is enriched in Y, U, Th, Nb, Ta, P, REE, while Type II zircon is relatively depleted in these elements, which reflects a reverse in the magma evolution. Whole-rock geochemistry and the presence of xenocrystic zircons (2522 ± 18 Ma), combined with εHf(t) (−20.2 to −22.3) and δ18O (5.3 to 6.6 ‰) for Type I zircons, suggests that the Hewan granite porphyry was mainly derived from the melting of the surrounding older, country rock gneiss. The higher εHf(t) (−16.5 to −19.4) and lower δ18O (5.0 to 6.0‰) of Type II zircon indicates that Type II zircon formed during infiltration of a later fluid. This later fluid also had a higher oxygen fugacity (Ce/Ce* Z = 248 to 3663) and a higher H2O activity. We propose that the Hewan granite porphyry originated from the shallow melting of regional gneisses due to heat from underplating mafic magmas related to a subduction event. Type I zircons, which formed during this event where later mantled or totally replaced by Type II zircon due to an influx of Cl-, S-, and Au-bearing fluids from these crystallizing underplating mafic magmas. The high F and pH in this shallow magma promoted decoupling of AuCl− in the upwelling fluids, which allowed for the deposition of elemental Au.

Paradoxically lowered oxygen isotopes of hydrothermally altered minerals by an evolved magmatic water

It has been well known that the influxing meteoric water can hydrothermally lower oxygen and hydrogen isotopes of rocks and/or minerals during continental magmatic or metamorphic processes in certain appropriate cases. Its opposite, however, is not implicitly true and needs independent testing. In terms of a novel procedure recently proposed for dealing with thermodynamic re-equilibration of oxygen isotopes between constituent minerals and water from fossil hydrothermal systems, the initial oxygen isotopes of water ({updelta }^{18}{text{O}}_{text{W}}^{text{i}}) are theoretically inverted from the early Cretaceous post-collisional granitoids and Triassic gneissic country rock across the Dabie orogen in central-eastern China. Despite ancient meteoric waters with low {updelta }^{18}{text{O}}_{text{W}}^{text{i}} value down to − 11.01 ± 0.43‰ (one standard deviation, 1SD), oxygen isotopes of hydrothermally altered rock-forming minerals from a granitoid were unexpectedly but concurrently lowered by an evolved magmatic water with mildly high {updelta }^{18}{text{O}}_{text{W}}^{text{i}} value of 2.81 ± 0.05‰ at 375 °C with a water/rock (W/R)c ratio of 1.78 ± 0.20 for the closed system. The lifetime of fossil hydrothermal systems studied herein is kinetically constrained to no more than 1.2 million years (Myr) via surface-reaction oxygen exchange in the late-stage of continental magmatism or metamorphism. Thereby, caution should be paid when lowered oxygen isotopes of hydrothermally altered rocks and/or minerals were intuitively and/or empirically inferred from the external infiltration of the purely meteoric water with a low {updelta }^{18}{text{O}}_{text{W}}^{text{i}} value alone.

Neoproterozoic HP granulite and its tectonic implication for the East Kunlun Orogen, northern Tibetan Plateau

The P–T path of high-pressure (HP) granulite is a sensitive indicator providing metamorphic evidence on the convergence process and on the evolutionary history of orogenic belts during the supercontinent circles. The Wulonggou mafic granulites in the East Kunlun Orogen are intercalated as lenses/boudins within the dominant amphibolite facies basement rocks. Based on microstructures and mineralogical observations, three mineral assemblage generations can be recognized in the granulite samples: (1) inclusions within garnet and amphibole comprising cpx + pl + amp + qtz + rt; (2) the matrix assemblage grt + cpx + pl + amp + qtz + rt ± bt ± ilm; and (3) a retrogressive assemblage consisting of kfs + pl + qtz + amp coronas around garnet. Phase equilibrium modeling and conventional thermobarometric calculations reveal that the Wulonggou granulite has experienced (1) first HP granulite facies metamorphism at > 12 kbar and 810–840 °C, (2) dominant HP granulite facies stage of 800–860 °C at 10.7–12.3 kbar, as well as (3) a retrograde stage of 620–650 °C and 6.7–8.1 kbar, implying a clockwise P–T path. Zircon U–Pb geochronological results indicate that the mafic granulites and the gneissic country rocks yield similar Neoproterozoic metamorphic ages of 939–941 Ma. The granulite also yields a zircon U–Pb age of 444 ± 7 Ma and amphibole 40Ar/39Ar plateau age of 433 ± 1 Ma, suggesting an Early Paleozoic metamorphic overprint at ca. 440 Ma. Together with our previous studies on the contemporaneous Neoproterozoic magmatism and metamorphism, all the above lines of evidence constrain that the Wulonggou mafic granulite has experienced high pressure granulite facies metamorphism at ca. 940 Ma, suggesting a Neoproterozoic collisional orogeny in the East Kunlun Orogen, which has been overprinted by the Early Palaeozoic orogenic processes.

Theoretical inversion of the fossil hydrothermal systems with oxygen isotopes of constituent minerals partially re-equilibrated with externally infiltrated fluids

ABSTRACTWhile the external infiltration of water has been identified from modern geothermal and/or fossil hydrothermal systems through stable isotopes, the physicochemical boundary conditions like the initial oxygen isotopes of water $( {{\rm \delta }^{ 18}{\rm O}_{\rm W}^{\rm i} } ) $ and rock as well as alteration temperature were implicitly presumed or empirically estimated by the conventional forward modelling. In terms of a novel procedure proposed to deal with partial re-equilibration of oxygen isotopes between constituent minerals and water, the externally infiltrated meteoric and magmatic water are theoretically inverted from the early Cretaceous post-collisional granitoid and intruded Triassic gneissic country rock across the Dabie orogen in central-eastern China. The meteoric water with a $ {{\rm \delta }^{ 18}{\rm O}_{\rm W}^{\rm i} } $ value of −11.01 ‰ was externally infiltrated with a granitoid and thermodynamically re-equilibrated with rock-forming minerals at 140°C with a minimum water/rock (W/R)o ratio around 1.10 for an open system. The lifetime of this meteoric hydrothermal system is kinetically constrained less than 0.7 million years (Myr) via modelling of surface reaction oxygen exchange. A gneissic country rock, however, was externally infiltrated by a magmatic water with $ {{\rm \delta }^{ 18}{\rm O}_{\rm W}^{\rm i} } $ value of 4.21 ‰ at 340°C with a (W/R)o ratio of 1.23, and this magmatic hydrothermal system could last no more than 12 thousand years (Kyr) to rapidly re-equilibrate with rock-forming minerals. Nevertheless, the external infiltration of water can be theoretically inverted with oxygen isotopes of re-equilibrated rock-forming minerals, and the ancient hydrothermal systems driven by magmatism or metamorphism within continental orogens worldwide can be reliably quantified.

Field observations, magnetic fabrics and microstructures evidences of syn-kinematic emplacement of the Numba granitic pluton (western Cameroon domain)

The N–S elongated Pan-African Numba granitic pluton (NGP), hosted in banded biotite hornblende gneiss and amphibolite country-rocks, belongs to the Western Domain of the Pan-African Belt of Central African in Cameroon. It consists of biotite hornblende granite and fine-grained biotite granite crosscut by gabbroic dykes. The western border of the granite is mylonitized. Structures and microstructures recorded in granite reveal magmatic, submagmatic and high- to moderate- temperature solid-state deformations. Magnetic characterisation of the NGP reveals its ferromagnetic behaviour due to the presence of multi-domain automorphic or xenomorphic pure magnetite grains. Field and AMS measurements testify that the study area recorded three deformation events: (1) a D1 event of ENE-WSW to E-W flattening deformation; (2) a D2 event of sinistral top-towards south sense of shear movement characterised by low to moderate dips (i) NE-SW strike foliation in country-rocks; (ii) NNE-SSW to NE-SW strike of magmatic foliation marked by the preferred orientation of K-feldspar megacrysts in granite and (iii) low plunges NE-SW mineral stretching lineation in biotite hornblende gneiss parallel to the axes of SW vergence of "S" shape F2 folds and (3) a D3 event of dextral top-towards east sense of shear movement that developed narrow NNE-SSW strike mylonite corridor. The NGP emplaced in a fracture within an amphibo-gneissic country-rock and has recorded opposite sinistral top-towards south (D2 Pan-African event) to dextral top-towards east senses of shear movements (D3 Pan-African event). It can be inferred that the emplacement and the deformation of the NGP was initiated during regional D2 deformation (dated at 613-585 Ma) when its magnetic fabric was acquired, and it was completely achieved during regional D3 deformation (dated ca. 585-540 Ma). Tj vs. Pj diagram associated with Flinn diagram suggests an emplacement in dominant flattening strain regime with a considerable shear component. The different data recorded in the NGP imply that its emplacement mechanism points to a ballooning. Indeed, the eastern domain of NGP with elliptical shape represents the inner part of the architecture while its western domain represents the outer part. The moderate to high dips observed in the inner part of the elliptical domain suggest the probable location of a feeder zone, and the lineation pointing away from the centre of the intrusion in the outer part may be developed related to the magmatic flow. This concentricity is easily associated with internal microstructures that show moderate-temperature solid-state to submagmatic and magmatic microstructures from the outside of the pluton towards the centre. The magma was then emplaced by ballooning in a gneissic country-rock that was subjected to sinistral shearing. Afterwards, the deformation characterised by a dextral shearing contributed to a generation of a NNE-SSW mylonitic band at the western border of the pluton.

Mineralogy and geochemistry of the Morro dos Seis Lagos siderite carbonatite, Amazonas, Brazil

The Morro dos Seis Lagos niobium rare earth element, Ti-bearing lateritic deposit (Amazonas, Brazil) is derived from a primary siderite carbonatite. The complex is the only example of a Nb deposit in which Nb-rich rutile is the main Nb ore mineral. Apart from the laterites, at the current level of exposure the complex consists only of siderite carbonatite; silicate rocks are absent. Three types of siderite carbonatite are recognized: (1) a brecciated and oxidized core siderite carbonatite consisting of up to 95 vol% siderite together with: hematite; pyrochlore; Nb-brookite; Ti-maghemite; and thorobastnäsite; (2) a REE- and P-rich variety of the core siderite carbonatite consisting of siderite (up to 95 vol%), hematite, minor pyrochlore, monazite and bastnäsite; (3) a border hydrothermal siderite carbonatite with ~70 vol% siderite, barite (~15 vol%), gorceixite (~7 vol%) and minor rhabdophane and pyrochlore. The country rock gneiss in which the carbonatite was emplaced was affected by potassic fenitization, with the formation of phlogopite and orthoclase together with monazite, fluorapatite and bastnäsite. The siderite carbonatites exhibit a wide variation of δ13C (−5.39‰ to −1.40‰), accompanied by a significant variation in δ18O (17.13‰ to 31.33‰), especially in the REE-rich core siderite carbonatite, and are explained as due to the presence of both H2O and CO2 in the magma. The core siderite carbonatite is the richest in Fe (48.64–70.85 wt% Fe2O3) and the poorest in Ca (up 0.82 wt% CaO) example of a siderite carbonatite yet recognized The ferrocarbonatite has significant contents of Mn, Ba, Th, Pb and LREE, and a very high Nb (up to 7667 ppm) content due to the presence of Nb-brookite. The substitution 3Ti4+ = Fe2+ + 2Nb5+ recognized in Nb-rich brookite explains enrichment of Nb in the core siderite carbonatite and indicates formation in a reducing environment. The high Nb/Ta ratio (1408–11,459) of the carbonatite is compatible with residual liquids derived by fractional crystallization. The 87Sr/86Sr (0.70411–0.70573) and 144Nd/143Nd (0.512663–0.512715) isotopic data suggest the carbonatite is mantle-derived with essentially no crustal contamination and is younger than the maximum age of 1328 ± 58 Ma (UPb in zircon). We suggest that the Morro dos Seis Lagos carbonatite complex represents the upper-most parts of a differentiated carbonatite magmatic system, and that the siderite carbonatite is related to late-magmatic-to-carbo-hydrothermal processes.

Diamond and coesite inclusions in detrital garnet of the Saxonian Erzgebirge, Germany

Abstract Local occurrences of coesite- and diamond-bearing rocks in the central Erzgebirge (northwestern Bohemian Massif, Germany) reveal ultrahigh-pressure (UHP) metamorphic conditions during the Variscan orogeny. Although UHP metamorphism supposedly affected a wider area, implying that rocks that equilibrated under UHP conditions occur dispersed in large volumes of high-pressure country-rock gneisses, mineralogical evidence is scarce. Here we have applied the new concept of capturing the distribution and characteristics of UHP rocks by analyzing inclusions in detrital garnet. Out of 700 inclusion-bearing garnets from seven modern sand samples from creeks draining the UHP area around the Saidenbach reservoir, we detected 26 garnets containing 46 mainly monomineralic coesite inclusions and 22 garnets containing 41 diamond inclusions. Combining these results with geochemical classification of the host garnets, we show (1) that coesite-bearing rocks are common and comprise eclogites as well as felsic gneisses, (2) that small inclusion size is a necessary precondition for the preservation of monomineralic coesite, and (3) for the first time, that diamond-bearing crustal rocks can be detected by analyzing the detrital record. Our results highlight the potential of this novel application of sedimentary provenance tools to UHP research, and the necessity of looking at the micrometer scale to find evidence in the form of preserved UHP minerals.

Metallogeny and tectonomagmatic setting of Ni-Cu magmatic sulfide mineralization, number I Shitoukengde mafic-ultramafic complex, East Kunlun Orogenic Belt, NW China

The number I Shitoukengde mafic-ultramafic complex is located in the eastern part of the East Kunlun Orogenic Belt (EKOB). It comprises peridotite (dunite and lherzolite), olivine websterite, pyroxenite (websterite and clinopyroxenite), and gabbro. The Ni-Cu sulfide mineralization is hosted in the peridotite and olivine websterite. Zircon SHRIMP U-Pb isotopic analyses reveal an early Carboniferous age of 333.9 ± 4.2 Ma for the olivine websterite and a Silurian age of 424.7 ± 3.7 Ma for the gabbro. Based on these data and the regional tectonic setting, we suggest that the number I Shitoukengde ultramafic rocks formed during the opening of the Paleotethys Ocean in the East Kunlun area. This geological setting is quite different than that of the Xiarihamu Ni-Co deposit (411 Ma) in the EKOB, which formed in a post-collisional environment. The whole-rock Ni contents of the peridotite (dunite and lherzolite) and olivine websterite are negatively correlated with the FeO contents and positively correlated with the MgO and Fo contents of the olivine, indicating that fractional crystallization of the parent magma of the observed rocks did not play a role in the sulfide saturation process. The ultramafic rocks with high Ni contents, i.e., the peridotite and olivine websterite, have low (87Sr/86Sr)i values, indicating low degrees of crustal contamination. The ultramafic rocks with lower Ni concentrations are in contact with the unmineralized marble country rock. We suggest that the assimilation of marble decreased the sulfide solubility of the magmas that produced the number I Shitoukengde complex. The δ34S values of the Shitoukengde sulfide hosted in the ultramafic rocks range from 1.9 to 4.3‰, with an average value of 2.9‰, indicating that crustal sulfur was added to the magma. In the country rocks, we found minor amounts of sulfide only in the gneiss from the Jinshuikou Group. Therefore, the number I Shitoukengde ultramafic rocks were probably contaminated with some sulfur from the gneiss. We propose that the most prospective ultramafic segments of the number I Shitoukengde complex are adjacent to gneissic country rocks.

The hydrothermal alteration of carbonatite in the Fen Complex, Norway: mineralogy, geochemistry, and implications for rare-earth element resource formation

ABSTRACTThe Fen Complex in Norway consists of a ~583 Ma composite carbonatite-ijolite-pyroxenite diatreme intrusion. Locally, high grades (up to 1.6 wt.% total REE) of rare-earth elements (REE) are found in a hydrothermally altered, hematite-rich carbonatite known as rødbergite. The progressive transformation of primary igneous carbonatite to rødbergite was studied here using scanning electron microscopy and inductively coupled plasma-mass spectrometry trace-element analysis of 23 bulk samples taken along a key geological transect. A primary mineral assemblage of calcite, dolomite, apatite, pyrite, magnetite and columbite with accessory quartz, baryte, pyrochlore, fluorite and REE fluorocarbonates was found to have transformed progressively into a secondary assemblage of dolomite, Fe-dolomite, baryte, Ba-bearing phlogopite, hematite with accessory apatite, calcite, monazite-(Ce) and quartz. Textural evidence is presented for REE fluorocarbonates and apatite breaking down in igneous carbonatite, and monazite-(Ce) precipitating in rødbergite. The importance of micro-veins, interpreted as feeder fractures, containing secondary monazite and allanite, is highlighted. Textural evidence for included relics of primary apatite-rich carbonatite are also presented. These acted as a trap for monazite-(Ce) precipitation, a mechanism predicted by physical-chemical experiments. The transformation of carbonatite to rødbergite is accompanied by a 10-fold increase in REE concentrations. The highest light REE (LREE) concentrations are found in transitional vein-rich rødbergite, whereas the highest heavy REE (HREE) and Th concentrations are found within the rødbergites, suggesting partial decoupling of LREE and HREE due to the lower stability of HREE complexes in the aqueous hydrothermal fluid. The hydrothermal fluid involved in the formation of rødbergite was oxidizing and had probably interacted with country-rock gneisses. An ore deposit model for the REE-rich rødbergites is presented here which will better inform exploration strategies in the complex, and has implications for carbonatite-hosted REE resources around the world.

A low-δ18O intrusive breccia from Koegel Fontein, South Africa: Remobilisation of basement that was hydrothermally altered during global glaciation?

The Cretaceous Koegel Fontein igneous complex is situated on the west coast of South Africa, and has a high proportion of rocks with abnormally low δ18O values. The rocks with the lowest δ18O values (−5.2‰) belong to intrusive matrix-supported breccia pipes and dykes, containing a variety of clast types. The breccia rocks range in SiO2 from 44 to 68wt% and their whole-rock δ18O values vary between −5.2‰ and +1.8‰. The major and trace element composition of the breccia rocks is consistent with them containing variable proportions of clasts of Cretaceous intrusive rocks and basement gneiss and the matrix being fluidized material derived from the same source as the clasts. Based on the nature of the clasts contained in the breccia, it was emplaced just prior to intrusion of the main Rietpoort Granite at 134Ma. All components of the breccia have low δ18O value and, at least in the case of the gneiss clasts, this predates incorporation in the fluidized material.Although the early Cretaceous appears to have been a period of cold climate, it is unlikely that the δ18O values of ambient precipitation (~−10‰) would have been low enough to have generated the required 18O-depletion. The basement gneiss was probably ~2–3km below the Cretaceous surface, minimizing the possibility of interaction with isotopically unmodified meteoric water, and there is no evidence for foundered blocks of cover rocks in the breccia. There is, therefore, no evidence for downwards movement of material. We favour a model where basement gneiss interacted with extremely 18O-depleted fluid during crustal reworking at ~547Ma, a time of global glaciation. Low-δ18O metamorphic fluids produced by dehydration melting of 18O-depleted gneiss became trapped and, as the fluid pressure increased, failure of the seal resulted in explosive upwards movement of fluidized breccia. Migration was along pre-existing dykes, incorporating fragments of these dykes, as well as the country rock gneiss.

The ongoing search for the oldest rock on the Danish island of Bornholm: new U-Pb zircon ages for a quartzrich xenolith and country rock from the Svaneke Granite

Previous geochronological studies on the Danish island of Bornholm have not identified any rocks older than c. 1.46 Ga. New LA-ICP-MS U-Pb zircon ages are presented for a xenolith within, and the country rock gneiss adjacent to, the Svaneke Granite on Bornholm. The xenolith is fine-grained and quartz-rich and was likely derived from either a quartz-rich sedimentary protolith or a hydrothermally altered felsic volcanic rock. The relatively fine-grained felsic nature of the country rock gneiss and the presence of large zoned feldspars that may represent phenocrysts suggest its protolith may have been a felsic volcanic or shallow intrusive rock. A skarn-like inclusion from a nearby locality likely represents an originally carbonate sediment and is consistent with supracrustal rocks being present at least locally. Zircon data from the xenolith define an upper intercept age of 1483 ± 12 Ma (2σ, MSWD = 2.5) with a poorly defined lower intercept age of 474 ± 250 Ma, and a weighted average 207Pb/206Pb age of 1477.9 ± 4.6 Ma; both these ages are older than the host Svaneke Granite (weighted average 207Pb/206Pb age of 1465.0 ± 4.8 Ma). Zircons from the gneiss define an upper intercept age of 1477.7 ± 6.8 Ma when anchored at 0 Ma, and a weighted average 207Pb/206Pb age of 1475.4 ± 6.6 Ma which overlaps statistically with the Svaneke Granite age. These ages are currently the oldest ages determined for in situ rocks on Bornholm. Evidence for substantially older basement lithologies (e.g. 1.8 Ga as observed in southern Sweden) remains absent. The zircons display clear oscillatory zoning, have Th/U typical of magmatic zircons and in some cases preserve inherited cores, all of which suggest that the ages are robust and do not represent resetting due to incorporation within or intrusion by the Svaneke Granite. Inherited zircons are not common; they have ages (c. 1.6–1.8 Ga) that are similar to those observed in other felsic basement lithologies on Bornholm. These new results suggest that prior to intrusion of the Svaneke Granite, the upper crust on Bornholm was dominated, at least locally, by lithologies similar in composition to the currently exposed felsic basement. The protoliths to the two samples investigated here must have been buried to mid-crustal depths over a relatively short time period (c. 10 Ma) prior to intrusion of the Svaneke Granite. This suggests a dynamic tectonic environment and is consistent with evidence for broadly simultaneous magmatism and deformation in basement rocks at 1.46 Ga in southern Scandinavia and burial and metamorphism of sediments in southern Skåne.

Dual sources of water overprinting on the low zircon \u03b418O metamorphic country rocks: Disequilibrium constrained through inverse modelling of partial reequilibration

Since water is only composed of oxygen and hydrogen, δ18O and δ2H values are thus utilized to trace the origin of water(s) and quantify the water-rock interactions. While Triassic high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks across the Dabie-Sulu orogen in central-eastern China have been well documented, postcollisional magmatism driven hydrothermal systems are little known. Here we show that two sources of externally derived water interactions were revealed by oxygen isotopes for the gneissic country rocks intruded by the early Cretaceous postcollisional granitoids. Inverse modellings indicate that the degree of disequilibrium (doD) of meteoric water interactions was more evident than that of magmatic one (−65 ± 1o vs. −20 ± 2°); the partial reequilibration between quartz and alkali feldspar oxygen isotopes with magmatic water was achieved at 340 °C with a water/rock (W/R) ratio of about 1.2 for an open-hydrothermal system; two-stage meteoric water interactions were unraveled with reequilibration temperatures less than 300 °C and W/R ratios around 0.4. The lifetime of fossil magmatic hydrothermal system overprinted on the low zircon δ18O orthogneissic country rocks was estimated to maintain up to 50 thousand years (Kyr) through oxygen exchange modellings. Four-stage isotopic evolutions were proposed for the magmatic water interacted gneiss.

Mapping and discriminating the Pan-African granitoids in the Hoggar (southern Algeria) using Landsat 7 ETM+ data and airborne geophysics

This study presents a multidisciplinary approach to discriminate and map different types and generations of Pan-African granitoids in the Hoggar, southern Algeria, using remote sensing and airborne geophysics in close correlation with previous works and established geological maps. RGB (Red, Green, Blue) combinations of band ratios; principal component analysis (PCA) and image classification for Landsat 7 ETM+ (Enhanced Thematic Mapper Plus), allow spatial discrimination and mapping of granitoid rocks of the studied area (200*350 km). This area extend over four contrasted Pan-African terranes (In Tedeini, Iskel, Tefedest and Laouni terranes, the two latter belong to the LATEA (Laouni-Azrou-n-Fad-Tefedest-Egéré-Aleksod-Assodé-Issalane) metacraton. The airborne magnetic intensity provides a wide range of responses from high values (youngest granitoids) to low values (volcano-sedimentary and gneissic country-rocks). Radiometric data, displaying radioelements concentration, discriminate efficiently the late alkaline granitoids (high values), the calk-alkaline granitoids (intermediate values) and the Tonalite-Trondhjemite-Granodiorite series (low values). This study led to the establishment of a more accurate geological map where the geochemical characteristics of the Pan-African granitoids are determined, including plutons not yet studied, especially in the poorly known In Tedeini terrane, and brings new constraints for the geodynamic development of the Tuareg Shield, which includes the Hoggar.

Fluid-induced breakdown of white mica controls nitrogen transfer during fluid–rock interaction in subduction zones

ABSTRACTIn order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ15Nair (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ15N values (δ15N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ15N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ15N = + 2.1 to +3.6‰).Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.

The multi-stage tectonic evolution of the Xitieshan terrane, North Qaidam orogen, western China: From Grenville-age orogeny to early-Paleozoic ultrahigh-pressure metamorphism

The geodynamic evolution of the early Paleozoic ultrahigh-pressure metamorphic belt in North Qaidam, western China, is controversial due to ambiguous interpretations concerning the nature and ages of the eclogitic protoliths. Within this framework, we present new LA-ICP-MS U–Pb zircon ages from eclogites and their country rock gneisses from the Xitieshan terrane, located in the central part of the North Qaidam UHP metamorphic belt. Xitieshan terrane contains clearly different protolith characteristics of eclogites and as such provides a natural laboratory to investigate the geodynamic evolution of the North Qaidam UHP metamorphic terrane. LA-ICP-MS U–Pb zircon dating of three phengite-bearing eclogites and two country rock gneiss samples from the Xitieshan terrane yielded 424–427Ma and 917–920Ma ages, respectively. The age of 424–427Ma from eclogite probably reflects continental lithosphere subduction post-dating oceanic lithosphere subduction at ~440–460Ma. The 0.91–0.92Ga metamorphic ages from gneiss and associated metamorphic mineral assemblages are interpreted as evidence for the occurrence of a Grenville-age orogeny in the North Qaidam UHPM belt. Using internal microstructure, geochemistry and U–Pb ages of zircon in this study, combined with the petrological and geochemical investigations on the eclogites of previous literature’s data, three types of eclogitic protoliths are identified in the Xitieshan terrane i.e. 1) Subducted early Paleozoic oceanic crust (440–460Ma), 2) Neoproterozoic oceanic crust material emplaced onto micro-continental fragments ahead of the main, early Paleozoic, collision event (440–420Ma) and 3) Neoproterozoic mafic dikes intruded in continental fragments (rifted away from the former supercontinent Rodinia). These results demonstrate that the basement rocks of the North Qaidam terrane formed part of the former supercontinent Rodinia, attached to the Yangtze Craton and/or the Qinling microcontinent, and recorded a complex tectono-metamorphic evolution that involved Neoproterozoic and Early Paleozoic orogenies.

The Nickeliferous Archean Madziwa Igneous Complex, Northern Zimbabwe: Petrological Evolution, Magmatic Architecture, and Ore Genesis

The nickeliferous Madziwa Igneous Complex consists of several lenticular, mafic-ultramafic enclaves within late Archean gneissic terrain. The lenses of igneous rock are locally deformed and metamorphosed to lower greenschist grade. They are here interpreted to be remnants of a large, composite, magmatic structure with two main components: (1) a set of narrow (10s–100s of meters wide) dikes intruding the gneissic foliation, and (2) a large lopolith made up of leuconorite. Some dikes contain a differentiated and vertically oriented layered suite comprising a central pyroxenite layer, plus norite as both a continuous marginal layer and intermittent layers within the pyroxenite. The pyroxenite-norite suite has a preliminary U-Pb zircon age of 2684 Ma. Other dikes are made up of diorite and/or ferrodiorite; in places, the dioritic rocks also intrude the pyroxenite-norite suite. The leuconorite lopolith transects both the dikes and the gneissic country rocks, its basal contacts with the pyroxenite-norite suite varying locally from intrusive to gradational to (magmatic) erosional. Although modified in places by secondary mobilization, disseminated Ni-Cu sulfides are primarily hosted within the central (ortho- to mesocumulate) pyroxenite adjacent to internal norite layers. Whole-rock geochemical data establish the comagmatic origin of the principal rock types and indicate an Archean, D-type, basaltic source magma with ca. 8% MgO, the two dioritic rock types representing late, immiscible, silica- and Fe-Ti-P-rich derivatives of pyroxenite-norite(-leuconorite) crystallization. In addition to the major rock types, peridotites occur in several outlying dikes and may represent fractionates of a komatiitic precursor to the basalt. Geologic and geochemical evidence points to bulk assimilation of country rock gneiss by the Madziwa magma and sulfide segregation triggered by felsic contamination. These processes did not occur locally within the dikes but rather in the conduit system prior to the emplacement of the magma charged with sulfide droplets. The unusual vertical layering of the pyroxenite-norite sequence and the localization of the sulfide ores are attributed to the strong outward, cooling gradient across such narrow dikes and to large-scale, lateral movement of sulfide droplets through the solidifying, pyroxene crystal framework ahead of an advancing postcumulus plagioclase crystallization front.

Multi-element variations in olivine as geochemical signatures of Ni-Cu sulfide mineralization in mafic magma systems—examples from Voisey’s Bay and Pants Lake intrusions, Labrador, Canada

Olivine from the olivine gabbro to troctolite intrusions at Voisey’s Bay and at Pants Lake, Labrador, was analyzed for multiple elements (Ca, Sc, Mg, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, and Zr) with secondary ion mass spectrometry (SIMS). Both intrusions have similar lithologies and petrographic characteristics and are approximately coeval (1.34 and 1.32 Ga, respectively) members of the Mesoproterozoic Nain Plutonic Suite. The Voisey’s Bay intrusion hosts a producing economic Ni-Cu sulfide deposit, whereas the Pants Lake intrusion displays evidence of Ni-Cu sulfide mineralization, but, to date, a viable ore deposit has not been discovered. Olivine from both barren and mineralized lithologies was analyzed to assess the potential of the olivine composition for providing a record of silicate melt evolution and sulfide saturation related to formation of a massive sulfide deposit. Two detailed transects were sampled, using five diamond drill holes that laterally approach the basal massive sulfide in the Eastern Deeps portion of the Voisey’s Bay intrusion from the barren central part. Olivine displays distinct trace element distributions that vary coherently with host lithology and proximity to sulfide mineralization. In particular, olivine shows an increase in Fe (∼Fo80 to ∼Fo60), Mn (∼2500 to 5000 ppm), and Zn (∼280 to 700 ppm), generally coupled with a decrease in Ni (∼1600 to ∼900 ppm), Co (∼270 to ∼190 ppm), and Cr (∼110 to 45 ppm), from barren troctolite (normal troctolite—NT) and weakly mineralized troctolite (variable-textured troctolite—VTT) towards the heavily mineralized, brecciated basal succession (basal breccia sequence—BBS). The enrichment in Fe-Mn-Zn is most pronounced in samples that laterally approach, but do not directly intersect, the massive sulfide deposit at the base of the intrusion, particularly in samples from the lower variable-textured troctolite and the basal breccia. Olivine from gabbro lithologies within the basal gabbro (BG) and upper gabbro (UG) of the Pants Lake intrusion is fairly homogeneous (∼Fo60, ∼4300 ppm Mn, ∼460 ppm Zn, ∼340 ppm Ni, ∼50 ppm Cr), with a more evolved composition (∼2100 ppm Ca, ∼20 ppm Sc, ∼300 ppm Ti, ∼4 ppm Y) than the average olivine of the Voisey’s Bay intrusion (∼570 ppm Ca, ∼6 ppm Sc, ∼65 ppm Ti, ∼0.3 ppm Y). Pants Lake olivine also commonly lacks the mutually Mn-Zn-rich signature of olivine from the basal breccia at Voisey’s Bay that is characteristic of contamination of the parent magma by country rock gneiss, and there reflects a close proximity to massive sulfides. In conjunction with petrographic observation and the stratigraphic context, a multiple-element (V-Cr-Mn-Fe-Co-Ni-Zn) regression can be applied to calculate a lateral proximity of olivine to massive sulfide mineralization in the Eastern Deeps part of the Voisey’s Bay intrusion, over a distance exceeding 150 m. The compositional variations in olivine from the economic Voisey’s Bay intrusion (bimodal and primitive) and the mainly barren Pants Lake intrusion (homogeneous and evolved) provide potential as a regional-scale mineralogical fertility indicator for mafic intrusions with comparable types of sulfide mineralization.

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.

The Samapleu mafic-ultramafic intrusion and its Ni-Cu-PGE mineralization: an Eburnean (2.09 Ga) feeder dyke to the Yacouba layered complex (Man Archean craton, western Ivory Coast)

AbstractThe Yacouba layered complex intrudes the Archean (3.5–2.7 Ga) Kenema-Man craton in the Samapleu-Yorodougou area, western Ivory Coast. In Samapleu area, the complex was recognized in drill holes at three locations: Samapleu Main (SM); Samapleu Extension 1 (E1) and Yorodougou (Yo). It comprises websterites, peridotites and gabbro-norites arranged symmetrically with mafic layers at the center and ultramafic layers at both margins. The complex is inclined at 70–80° to the SE. The thickness of individual layers varies from 2 to 60 m and the total thickness is 120 to 200 m. At the E1 site, the complex extends to depths > 500 m.Contacts with the country rock gneiss are characterized by a hybrid zone that is a few meters thick and composed of plagioclase-orthopyroxene bearing metabasites, and locally (E1 site) a metamorphic assemblage of sapphirine-cordierite-sillimanite-spinel ± rutile. This assemblage is attributed to contact metamorphism during intrusion of the complex in the lower crust at a depth of about 25 km. Zircons in country rock gneisses and granulites, as well as in the hybrid facies, yield Archean ages of ~ 2.78 Ga, similar to ages reported in the Man craton. Rutiles in the hybrid zone give a U-Pb age of 2.09 Ga, which is interpreted as the age of contact metamorphism and emplacement of the intrusion.The Samapleu Main and Samapleu Extension 1 sites contain Ni and Cu sulfide deposit with reserves estimated as more than 40 million tons grading 0.25% Ni and 0.22% Cu (Sama Nickel-CI, August 2013). The Ni-Cu mineralization is composed of pentlandite, chalcopyrite, pyrrhotite and rare pyrite, which is disseminated mainly in pyroxenite or occurs as subvertical and semi-massive to massive sulfide veins. The sulfide textures range from matrix ore, net-textured, droplets or breccia textures. Zones enriched in PGM, particularly Pd, are associated with the sulfides and several chromite bands are also present. These observations suggest that an immiscible sulfide liquid formed from a parental silicate liquid and percolated through the crystal pile.The parental melt composition, determined using the Chai and Naldrett [1992] method, has a SiO2-rich mafic composition with 53% SiO2 and 10% MgO. This result, the presence of the hybrid zone, and the trace-element signature determined using the Bedard [1994] method, suggest a mantle-derived basaltic parental magma that had assimilated abundant continental crust.These observations indicate that Samapleu intrusion corresponds to a magmatic conduit of the Yacouba complex as at Jinchuan (China), Voisey’s bay (Canada), Kabanga (Tanzania) or Nkomati (South Africa).