Ultramafic Part Research Articles

A polygenetic origin for the Sikhoran ultramafic-mafic complex in South Iran

The nature and geodynamic setting of the Sikhoran ultramafic–mafic complex, southwest Iran, is debated, with multiple competing theories having been proposed in recent years. The Sikhoran ultramafic–mafic complex is located in the southern part of the Sanandaj-Sirjan metamorphic-magmatic zone (SSMMZ) and is comprised of porphyroclastic, transitional and layered ultramafic-mafic sequences of Carboniferous age. These units are cut by isotropic gabbros, diabasic and pegmatoid gabbroic dikes with ages ranging from Permian to the Cretaceous. These bodies also intrude into Upper Paleozoic metamorphic rocks, where they induced high-temperature contact metamorphism during the Late Carboniferous. Here we present new textural descriptions, mineral and whole-rock geochemical analyses, and the results of zircon UPb dating of magmatic mafic rocks and metamorphic host rocks from this region in order to reconstruct the petrogenesis of the Sikhoran ultramafic–mafic complex. Zircon UPb ages from layered gabbros indicate Late Carboniferous (320.8 ± 6.4 Ma) crystallization, which is similar to the age of the Upper Paleozoic host gneisses (334.6 ± 4.9 Ma). By contrast, isotropic gabbros that cross-cut the complex have a crystallization age of 178.3 ± 2.3 Ma, and anatectic quartz diorite/plagiogranite produced from partial melting of hosting amphibolite in contact with the gabbros yielded Late Triassic-Early Jurassic ages (187.2 ± 2.6 Ma). These data suggest a polygenetic origin and a range of tectonic settings for various parts of the ultramafic-mafic complex. Geochemical modeling shows that mantle plume-related melting (∼5–40%) of metasomatized mantle could have formed the ultramafic-mafic layered cumulate complex during the Late Carboniferous. A geochemical transition from island arc tholeiite (IAT) isotropic gabbros to cross-cutting enriched mid-ocean ridge basalts (E-MORB)–diabasic dikes occurred between the Late Triassic-Early Jurassic to the Late Cretaceous. The mantle-derived ultramafic parts of the Sikhoran ultramafic-mafic complex have similarities with other known ultramafic-mafic mantle plumes (e.g., Tinaqulllo, Lherz, and Ronda), and its layered ultramafic-mafic segment has similarity with layered intrusions (e.g., Pulur complex in Turkey). The highest observed crystallization temperature in porphyroclastic, transitional and layered ultramafic mantle sections of ∼1140 °C is considered as the mantle potential temperature, since the highest crystallization temperatures are observed in the most forsteritic olivine crystals. Based on amphibole chemistry, the crystallization conditions of transitional to layered ultramafic rocks at 900–1005 °C occurred at high H2O contents in a melt between nickel–nickel oxide (NNO) −8.5 and NNO −9.0, which lies along the NNO−2 buffer. These results are consistent with a model in which the opening of the Zagros Neotethys Oceanic basin in the Esfandagheh region initiated in the Late Carboniferous, alongside the formation of an extensional environment that allowed ascent of a subcontinental mantle plume. Subduction initiation of oceanic lithosphere began during the Late Triassic-Early Jurassic and ended in the Late Cretaceous.

The B isotopic signature of serpentine from obducted ophiolites: Mixing of fluids and tectonic implications

Obducted ophiolites expose oceanic lithosphere slivers in suture zones. The few previous studies focusing specifically on the serpentinization of the ultramafic part of ophiolites mostly concluded that serpentinization was driven by seawater. Recent developments in B isotope geochemistry have enabled the tracking of fluid source(s) in serpentinites, suggesting δ11B values above +10‰ mostly reflect seawater hydration and δ11B values below +10‰ mostly reflect hydration by subducted crust-derived metamorphic fluids. In this study, we further investigate the fluid sources recorded by ophiolitic serpentinites by presenting B isotopic geochemistry for seventeen samples from well-characterized ophiolites in Guatemala, Iran, and Brazil. The samples from Guatemala and Iran are from fossil Supra Subduction Zones (SSZ), while samples from Brazil represent a fossil Ocean-Continent Transition (OCT). Most of the samples display the pseudomorphic mesh matrix and bastite microtexture typical of serpentinites. Nine of the samples are either totally (both microtextures) or partially (one microtexture only) out of isotopic equilibrium, with δ11B variations of up to 18‰ within individual samples/textures. Among the samples with a narrow δ11B range (i.e., at isotopic equilibrium), those from a fossil OCT ophiolite have δ11B ranging from +3.8 to +23.2‰, which is broadly consistent with serpentinization by seawater. In contrast, serpentinites from fossil SSZ ophiolite have δ11B straddling 0‰, ranging from −7.7 to +13.5‰. These data indicate that seawater alone cannot be responsible for serpentinization of these ultramafic rocks, but rather either subducted crust-derived metamorphic fluids, or a mixture of seawater and subducted crust-derived metamorphic fluids. This interpretation agrees with the recent suggestions that SSZ ophiolites possibly represent fossil forearc lithosphere.

Multi-stage serpentinization of ultramafic rocks in the Manlay Ophiolite, southern Mongolia

Serpentinization of ultramafic rocks in ophiolites is key to understanding the global cycle of elements and changes in the physical properties of lithospheric mantle. Mongolia, a central part of the Central Asian Orogenic Belt (CAOB), contains numerous ophiolite complexes, but the metamorphism of ultramafic rocks in these ophiolites has been little studied. Here we present the results of our study of the serpentinization of an ultramafic body in the Manlay Ophiolite, southern Mongolia. The ultramafic rocks were completely serpentinized, and no relics of olivine or orthopyroxene were found. The composition of Cr-spinels [Mg# = Mg/(Mg + Fe2+) = 0.54 and Cr# = Cr/(Cr + Al) = 0.56] and the bulk rock chemistry (Mg/Si = 1.21–1.24 and Al/Si < 0.018) of the serpentinites indicate their origin from a fore-arc setting. Lizardite occurs in the cores and rims of mesh texture (Mg# = 0.97) and chrysotile is found in various occurrences, including in bastite (Mg# = 0.95), mesh cores (Mg# = 0.92), mesh rims (Mg# = 0.96), and later-stage large veins (Mg# = 0.94). The presence of lizardite and chrysotile and the absence of antigorite suggests low-temperature serpentinization (<300 °C). The lack of brucite in the serpentinites implies infiltration of the ultramafic rocks of the Manlay Ophiolite by Si-rich fluids. Based on microtextures and mineral chemistry, the serpentinization of the ultramafic rocks in the Manlay Ophiolite took place in three stages: (1) replacement of olivine by lizardite, (2) chrysotile formation (bastite) after orthopyroxene and as a replacement of relics of olivine, and (3) the development of veins of chrysotile that cut across all previous textures. The complex texture of the serpentinites in the Manlay Ophiolite indicates multiple stages of fluid infiltration into the ultramafic parts of these ophiolites in southern Mongolia and the CAOB.

Late Cretaceous changes in oceanic currents and sediment sources in the eastern Tethys: insights from Nd isotopes and clay mineralogy

The Late Cretaceous is marked by geodynamical changes including Africa-Eurasia convergence that resulted in the narrowing of the Tethys Ocean and in ophiolite obduction along the southern margin of the various continental blocks in eastern Tethys. Geochemical and mineralogical analyses were performed on the Shahneshin section (Zagros Basin - eastern Tethys), to explore the consequences of this evolution on oceanic circulation, and to estimate the role of ophiolite weathering that can impact climate through atmospheric CO2 drawdown. [Ni] and [Cr] sharply increase in the uppermost Campanian – Maastrichtian interval and are not coeval with an increase in the Nd isotope composition (ɛNd) of the detrital fraction of the sediments. This is interpreted as reflecting weathering of the mantellic, ultramafic part of the ophiolite sequence at that time, implying that ophiolites were exposed on the continents as soon as the Coniacian in the vicinity of the Zagros Basin. Hence their weathering could have contributed to the Late Cretaceous climatic cooling. Clay mineralogy reveals an alternation of kaolinite-rich humid periods (Coniacian/Santonian and late early Campanian to latest Campanian) and kaolinite-depleted arid periods (early Campanian and the latest Campanian to late Maastrichtian). The ɛNd(t) of bathyal waters (carbonate leached fraction) appears quite radiogenic, in the range of -3.4 to -5.2 ɛ-units, about 2 to 4 ɛ-units higher than the detrital fraction. Changes in the ɛNd(t) of detrital material delivered to the Zagros Basin appear to control the ɛNd(t) of the local bottom waters from the Coniacian to the middle Campanian. By contrast, decoupling of detrital ɛNd(t) from that of the local bottom waters from the middle Campanian to the Maastrichtian points to increasing fluxes of westward flowing Pacific radiogenic water masses into the eastern Tethys at that time. These results support an intensification of the Tethyan Circumglobal Current, likely related to the narrowing of the Tethyan seaway.

Sill Intrusion into Pyroxenitic Mush and the Development of the Lower–Upper Critical Zone Boundary of the Bushveld Complex: Implications for the Origin of Stratiform Anorthosites and Chromitites in Layered Intrusions

Abstract Layered mafic–ultramafic intrusions are the fossilized remnants of magmatic plumbing systems and provide excellent natural laboratories to investigate the processes of magma differentiation and solidification. The Rustenburg Layered Suite is the plutonic mafic–ultramafic part of the Bushveld Complex of South Africa and it has traditionally been assumed to have formed from an upwardly aggrading (and in-sequence) crystal pile in a melt-dominated chamber. In this study, we present field and petrological observations, complemented with detailed plagioclase mineral chemistry [molar An, light rare earth elements (LREE) and strontium isotopes] for the first stratiform anorthosite layer (MG3F anorthosite) at the Lower–Upper Critical Zone boundary (LCZ–UCZ) in the eastern limb of the Bushveld Complex. We use these data to test the overarching paradigm of a melt-dominated chamber for the magmatic evolution of the Rustenburg Layered Suite. The MG3F anorthosite is immediately overlain by the MG3 chromitite and both are surrounded by pyroxenite. A distinctive ‘egg-box’ structure, consisting of round pyroxenite blocks mantled by chromitite, marks the LCZ–UCZ boundary, and represents an erosional disconformity at the base of the MG3F anorthosite. The MG3F anorthosite is laterally continuous for hundreds of kilometers in the eastern limb. In the northern–central sector of the eastern limb, the 1·5 m thick MG3F anorthosite is characterized by non-cotectic proportions of foliated plagioclase and chromite chains that lie parallel to the foliation. The MG3F anorthosite is divisible into two sub-layers on the basis of (1) a compositional break in plagioclase molar An, LREE and strontium isotope composition and (2) a peak in chromite mode (up to 12 vol%). In the lower half of the layer plagioclase LREE concentrations increase upward, molar An shows a marginal decrease upward and strontium isotopes are relatively homogeneous (87Sr/86Sr2·06Ga 0·7056–0·7057). In the upper half of the layer, plagioclase LREE concentrations decrease upward, molar An shows a marginal increase upward and strontium isotopes show strong inter- and intra-grain variability (87Sr/86Sr2·06Ga 0·7053–0·7064). Strontium isotopes in interstitial plagioclase in the immediate footwall and hanging-wall pyroxenites show similar 87Sr/86Sr2·06Ga values to the MG3F anorthosite and decrease with distance from the MG3F anorthosite. In the southern sector of the eastern limb, the 4 m thick MG3F anorthosite exhibits identical stratigraphic compositional trends in terms of molar An in plagioclase. We infer that the MG3F anorthosite formed by two successive sill-like injections of magma into a resident viscoplastic pyroxenitic crystal mush. An initial pulse of plagioclase-saturated melt underwent in situ fractional crystallization, manifested as upwardly decreasing molar An and upwardly increasing LREE in plagioclase in the lower half of the MG3F anorthosite. Sill intrusion caused deformation of the viscoplastic pyroxenite mush and vortices of superheated liquid generated by frictional viscous heating caused disaggregation of the footwall pyroxenitic mush. Disaggregated blocks of pyroxenitic mush reacted with the superheated liquid (a hybrid chromite-saturated melt) to produce chromite-rich rims at the base of the MG3F anorthosite (egg-box structure). A second sill-like injection of magma then entered the chamber that halted in situ crystallization. This sill was a plagioclase slurry that contained isotopically distinct plagioclase laths compared with those present in the previous sill. The upward increase in molar An of plagioclase, and decreasing LREE, may be explained by the slurry becoming more primitive in melt composition with time. The second sill also caused mush disaggregation and renewed the production of a hybrid chromite-saturated melt. Chromite crystals were then mobilized and injected as slurries at the interface between the sill and resident mush towards the back of the flow, culminating in the development of the MG3 chromitite. Our model for the development of the Lower–Upper Critical Zone boundary questions the existence of a melt-dominated chamber and it has implications for the origin of stratiform anorthosites (and chromitites) in crustal magma chambers.

Differentiated Mafic–Ultramafic Intrusions of the Zubovsky Type in the Norilsk Area: Petrochemistry, Geochemistry, and Ore Potential

Intrusions of the Zubovsky type (1) are commonly dominated by troctolite in their ultramafic parts and have an insignificant number (if any) of ultramafic units; (2) occur among olivine and olivine-bearing gabbro-dolerites and often show evidence of a number of accumulation of cumulus olivine, (3) have weakly pronounced (if any) taxitic gabbrodolerite units; (4) include widespread leucocratic hybrid-metasomatic rocks and hybrid quartz- and cordierite-bearing norites and gabbronorites at contacts with the host aluminosilicate rocks; and (5) generally have a low ore content, with the prevalence of disseminated ore units among troctolites and ultramafic rocks, and almost complete absence of massive sulfide ores. The paper is the first to present data on the composition and Nd–Sr–Cu–S isotope-geochemical characteristics of rocks and sulfide ores from the Zubovsky-type intrusions in the Norilsk area. The silicate material of the Zub-Marksheider and Vologochan intrusions is characterized by similar Nd–Sr isotopic parameters: eNd = –1.2 ± 0.5, (87Sr/86Sri) = 0.70685 ± 0.00127, and eNd = 1.2 ± 0.8, (87Sr/86Sri) = 0.70634 ± 0.00068, respectively. These parameters are close to those of intrusions accompanied by economic ore mineralization in the Norilsk area. The whole-rock samples generally have a more ‘radiogenic’ Nd isotopic composition than that of the pyroxene and plagioclase and less ‘radiogenic’ than that of the olivine. The highest eNd value (ca. 5.3) of the olivine indicates that depleted mantle (DM) material was involved in the magma-generating processes. Copper–sulfur isotope data for sulfide ores of the Zub-Marksheider and Vologochan intrusions are remarkably different: δ34S = 1.3 ± 2.3‰, δ65Cu = –0.1 ± 0.15‰ and δ34S = 7.0 ± 1.2‰, δ65Cu = –0.7 ± 0.3‰, respectively. The Cu isotope composition of the sulfide mineralization of both intrusions is close to that of massive and disseminated sulfide ores from the Talnakh deposit in the Norilsk area. The sulfur isotope data suggest that the host rocks had little effect on sulfide mineralization. The identified Nd–Sr–S–Cu isotope-geochemical parameters of the rocks and ores are an important indicator of the potential for rich PGE–Cu–Ni sulfide ores.

Occurrence of Graphite-Like Carbon in Podiform Chromitites of Greece and Its Genetic Significance

The role of post-magmatic processes in the composition of chromitites hosted in ophiolite complexes, the origin of super-reduced phases, and factors controlling the carbon recycling in a supra-subduction zone environment are still unclear. The present contribution compiles the first scanning electron microscope/energy-dispersive (SEM/EDS) data on graphite-like amorphous carbon, with geochemical and mineral chemistry data, from chromitites of the Skyros, Othrys, Pindos, and Veria ophiolites (Greece). The aim of this study was the delineation of potential relationships between the modified composition of chromite and the role of redox conditions, during the long-term evolution of chromitites in a supra-subduction zone environment. Chromitites are characterized by a strong brittle (cataclastic) texture and the presence of phases indicative of super-reducing phases, such as Fe–Ni–Cr-alloys, awaruite (Ni3Fe), and heazlewoodite (Ni3S2). Carbon-bearing assemblages are better revealed on Au-coated unpolished sections. Graphite occurs in association with hydrous silicates (chlorite, serpentine) and Fe2+-chromite, as inclusions in chromite, filling cracks within chromite, or as nodule-like graphite aggregates. X-ray spectra of graphite–silicate aggregates showed the presence of C, Si, Mg, Al, O in variable proportions, and occasionally K and Ca. The extremely low fO2 during serpentinization facilitated the occurrence of methane in microfractures of chromitites, the precipitation of super-reducing phases (metal alloys, awaruite, heazlewoodite), and graphite. In addition, although the origin of Fe–Cu–Ni-sulfides in ultramafic parts of ophiolite complexes is still unclear, in the case of the Othrys chromitites, potential reduction-induced sulfide and/or carbon saturation may drive formation of sulfide ores and graphite-bearing chromitites. The presented data on chromitites covering a wide range in platinum-group element (PGE) content, from less than 100 ppb in the Othrys to 25 ppm ΣPGE in the Veria ores, showed similarity in the abundance of graphite-like carbon. The lack of any relationship between graphite (and probably methane) and the PGE content may be related to the occurrence of the (Ru–Os–Ir) minerals in chromitites, which occur mostly as oxides/hydroxides, and to lesser amounts of laurite, with pure Ru instead activating the stable CO2 molecule and reducing it to methane (experimental data from literature).

Whole-rock and mineral compositional constraints on the magmatic evolution of the Ni-Cu-(PGE) sulfide ore-bearing Kevitsa intrusion, northern Finland

The ~2.06Ga mafic-ultramafic Kevitsa intrusion is located in the Central Lapland greenstone belt. The lower ultramafic part of the intrusion hosts a large disseminated Ni-Cu-(PGE) sulfide deposit with Ni tenors ranging widely from <4wt% (uneconomic false ore and contact mineralization) to 4–7wt% (regular ore) and up to 40wt% (Ni-PGE ore). The stratigraphy of the ultramafic cumulates is divided into the basal pyroxenite–gabbro (Basal series), olivine pyroxenite (OLPX), pyroxenite, and plagioclase-bearing (ol) websterite (pOLWB), of which the latter occurs together with minor microgabbros in the ore-bearing domain of the intrusion. Around the ore domain, the ultramafic cumulate succession records a simple lithological stratigraphy and modest and predictable variations in whole-rock and mineral compositions. The ore-bearing domain, in contrast, is characterized by a complex internal architecture, variations in whole-rock and mineral compositions, and the presence of numerous inclusions and xenoliths.The OLPXs are mainly composed of cumulus olivine (Fo77–89) and clinopyroxene (Mg#81–92) with variable amounts of oikocrystic orthopyroxene (Mg#79–84). They comprise the bulk of the ultramafic cumulates and are the dominant host rocks to the sulfide ore. The host rocks to the regular and false ore type are mineralogically and compositionally similar (Fo~80–83, mostly) and show mildly LREE-enriched REE patterns (CeN/YbN~2), characteristic for the bulk of the Kevitsa ultramafic cumulates. The abundance of orthopyroxene and magnetite is lowest in the host rocks to the Ni-PGE ore type, being in line with the mineral compositions of the silicates, which are the most primitive in the intrusion. However, it contrasts with the LREE-enriched nature of the ore type (CeN/YbN~7), indicating significant involvement of crustal material in the magma.The contrasting intrusive stratigraphy in the different parts of the intrusion likely reflects different emplacement histories. It is proposed that the Kevitsa magma chamber was initially filled by stable continuous flow (“single” input) of basaltic magma followed by differentiation in an at least nearly closed system. In the following stage, new magma pulses were repeatedly emplaced into the interior of the intrusion in a dynamic (open) system forming the sulfide ore bodies. To gain the peculiar compositional and mineralogical characteristics of the Ni-PGE ore type, the related magma probably interacted with different country rocks en route to the Kevitsa magma chamber.

Peridotitic minerals of the Nidar Ophiolite in the NW Himalaya: sourced from the depth of the mantle transition zone and above

Abstract The Nidar Ophiolite Complex (NOC) consists of a c. 10 km thick ophiolite suite in the NW Himalaya, India. The c. 7 km thick lower ultramafic part of the ophiolite body terminates against the Tso Morari Crystallines, which represent the leading edge of the Indian continental margin. Mineral inclusions from the peridotites in the lower ultramafic part of the NOC were studied, including C 2 /c clinoenstatite, disordered coesite and high-pressure Mg 2 SiO 4 (probably β-Mg 2 SiO 4 ). These minerals, found in two lherzolite bodies from the ophiolite's mantle section, were characterized by laser Raman spectroscopy and electron micro-probe analysis. Textural evidence supporting decompression from an ultra-high-pressure condition was also observed, such as Cr spinel exsolution needles in olivine crystals. The systematic mineral phase transitions of coesite→quartz, high-pressure clinoenstatite→orthoenstatite and β-Mg 2 SiO 4 →Cr spinel exsolution needles in olivine suggest that the mantle section of the Nidar Ophiolite evolved from the deep mantle beneath a palaeo-spreading centre. The phase stabilities of these high-pressure minerals require derivation from the depth of the mantle transition zone (410–660 km). A transport mechanism for these minerals is suggested via dunite channels along a mantle adiabat in the focused convective flow below the spreading centre. This mechanism brought these deep mantle phases into the ultramafic part of the NOC. These observations suggest that some part of the mantle section of the NOC in the NW Himalaya originated in a mid-ocean ridge setting. Supplementary material: Representative mineral chemical compositions of the lherzolites (1M1 and 1NU27) and the host channelized dunites are available at http://www.geolsoc.org.uk/SUP18836 .

Petrological Evolution of the Upper Mantle Beneath the Southern Sanandaj-Sirjan Zone: Evidence from Kuhshah Peridotite Massif, Southeast Iran

The Kuhshah ultramafic complex is located in the south-east of Sanandaj-Sirjan metamorphic zone, near the probable remnants of Neotethys plate in Iran. It consists of highly depleted harzburgites, dunites, chromitite bands and altered gabbros. The ultramafic parts have been intruded by numerous clinopyroxenite dykes and veins. In the harzburgites, there are different generations of olivine, orthopyroxene and spinel. The chemical composition of the first generation minerals indicates that the harzburgites are depleted in incompatible elements, but the other generations show melt/rock interaction features which can be found in a suprasubduction zone setting. During subduction of the Neotethys plate beneath the Sanandaj-Sirjan zone, a back arc basin developed between the Sanandaj-Sirjan arc and the central Iranian microcontinent. Whole rock and mineral chemistry, specially, cromespinels, show that the harzburgites formed underneath this basin. After depletion, ascending melts with boninitic compositions reacted with these peridotites to form dunites and chromitite bands. The updoming and decompression of the mantle in the next stage, led to remelting at deeper levels and development of clinopyroxenite parental melts which intruded into the uppermost mantle. Then, the new basin closed and the Kuhshah peridotites were emplaced in present position.

AGE OF THE BIRD RIVER SILL, SOUTHEASTERN MANITOBA, CANADA, WITH IMPLICATIONS FOR THE SECULAR VARIATION OF LAYERED INTRUSION-HOSTED STRATIFORM CHROMITE MINERALIZATION

The Archean Bird River sill, a 20-km-long and up to 800-m-thick mafic-ultramafic layered intrusion in the Bird River greenstone belt of southeastern Manitoba, Canada, contains significant resources of chromium and nickel-copper, and locally anomalous concentrations of platinum group elements. Stratiform chromitite, displaying both remarkable lateral continuity and local irregularities due to synmagmatic disruption, occurs in up to six main intervals over a thickness of ~60 m in the lower ultramafic part of the intrusion. Ni-Cu sulfide mineralization is hosted in ultramafic rocks at the base of, or just below, the sill (e.g., past-producing Maskwa-Dumbarton mines). In this study, we report a U-Pb age (chemical abrasion-thermal ionization mass spectrometry) for a leucogabbro from the Chrome property of 2743.0 ± 0.5 Ma (n = 8 single grains of zircon), which is interpreted as the age of crystallization of the sill and defines the timing of the associated Cr-Ni-Cu ± platinum group element (PGE) mineralization. Other Neoarchean mafic-ultramafic intrusions containing economic concentrations of chromite and/or Ni-Cu-(PGEs) occur in the northwestern part of the Superior province in the Canadian Shield (e.g., McFaulds Lake, Big Trout Lake, Puddy Lake, Shebandowan) and share broadly similar geologic relationships. The age of intrusion-hosted stratiform chromite mineralization worldwide, excluding ophiolite-hosted chromitites, ranges from Archean to Paleoproterozoic, which is comparable to that for komatiite-related Ni deposits. This temporal restriction is consistent with both deposit types requiring the involvement of high-MgO, Cr-rich parent magmas produced during large degrees of mantle melting early in Earth history.

Detection of chromite bearing mineralized zones in Abdasht ophiolite complex using ASTER and ETM+ remote sensing data

Podiform chromite ore deposits in ultramafic parts of ophiolite rock complexes can be detected using remote sensing data. This study focuses on the discrimination of chromite bearing mineralized zones using Landsat TM and Advance Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite data in Abdasht ophiolite complex, south of Iran. Several image processing methods, including Log residual, Decorellation Stretch, Band ratio and Mixture-Tuned Matched-Filtering (MTMF) have been evaluated for lithological mapping using Landsat ETM and ASTER data. The outcome showed that TIR band ratios of ASTER can discriminate quartzite, carbonate and mafic–ultramafic rocks in the ophiolite complex. Log residual, Decorollation Stretch and MTMF methods were more capable than previous published ASTER methods specifically for lithological mapping at a regional scale. New geological map of Abdasht region was produced based on the interpretation of ASTER image processing results and field verification. Consequently, the proposed methods demonstrated the ability of ASTER and Landsat ETM data to provide information for detecting chromite host rock (serpentinized dunites) that is valuable for chromite prospecting in study area. Additionally, the techniques used in this study are subtle for exploration geologist and mine engineering to identify high-potential chromite-bearing zones in the ophiolite complex, minimizing costly and time-consuming field works.

The Eldivan ophiolite and volcanic rocks in the İzmir–Ankara–Erzincan suture zone, Northern Turkey: Geochronology, whole-rock geochemical and Nd–Sr–Pb isotope characteristics

Gabbros and dolerite dikes of the Eldivan ophiolite and basaltic volcanic rocks of the ophiolitic mélange in the central part of the İzmir–Ankara–Erzincan (IAE) suture zone were investigated for their 40Ar/39Ar age and whole-rock-major-trace element and Sr–Nd–Pb isotope compositions. Based on geological and geochemical characteristics basaltic volcanic rocks in the ophiolitic mélange are subdivided into two groups (Groups I and II) with ocean island basalts or enriched mid-ocean ridge basalt characteristics, respectively. Gabbros and dolerite dikes of the Eldivan ophiolite (Groups III and IV) have instead geochemical compositions indicative of a subduction-related environment. The volcanic rocks of Group I have 87Sr/86Sr(i) between 0.7037 and 0.7044, ƐNd(i)–DM of −4.5 to −5.6, and 206Pb/204Pb(i) ranging between 18.35 and 18.75. Group II volcanic rocks have higher 87Sr/86Sr(i) values (0.7049–0.7055), ƐNd(i)–DM ranging between −5.4 and −6.0, and 206Pb/204Pb(i) between 18.14 and 18.62. The Nd isotopic signatures and 207Pb/204Pb(i) values of the volcanic rocks of both groups point to a different source with respect to those of the Eldivan ophiolite. The low 206Pb/204Pb(i) values relative to the ophiolitic rocks seem to exclude a significant contribution from a HIMU reservoir, whereas the 207Pb/204Pb(i) values slightly above the NHRL might indicate some contribution from an EM2-type reservoir. Gabbros (Group III) of the Eldivan ophiolite and dolerite dikes (Group IV) cross-cutting the ultramafic part of the ophiolite show 87Sr/86Sr(i) between 0.7038 and 0.7053, ƐNd(i)–DM from −2 to −3.6 and 206Pb/204Pb(i) between 18.10 and 18.80. The gabbros yield ca. 150Ma 40Ar/39Ar amphibole-plateau ages, which, together with the geochemical data, indicate that they were produced above subducted oceanic lithosphere in the IAE ocean domain in Late Jurassic times. Therefore, the Eldivan ophiolite in the IAE suture zone constitutes a link between the Hellenide–Dinaride ophiolite belts to the west and the Armenian–Iranian ophiolites to the east.

THE MINERALIZATION POTENTIAL OF THE BURAKOVSKY LAYERED INTRUSION, KARELIA, RUSSIA

The 2,430 Ma Burakovsky layered complex, located in Karelia, Russia, is one of the largest layered Fennoscandian intrusions. It consists, from the base to the top, of a thick ultramafic part dominated by dunite and peridotite overlain by a layered mafic part that consists of alternating layers of pyroxenite, wehrlite and gabbro (Transitional Banded Zone, TBZ), and a thick sequence of gabbroic rocks [Gabbronorite Zone (GNZ), Pigeonite Gabbronorite Zone (PGNZ), Magnetite Gabbronorite–Diorite zone (MGDZ), which predominates. The Main Chromite Horizon, located at the interface between the ultramafic part and the layered series, hosts laurite and Os–Ir alloy grains observed as tiny inclusions in chromite crystals. The Transitional Banded Zone and Gabbronorite Zone are characterized by disseminated sulfide blebs hosted by clinopyroxene and plagioclase. The blebs consist of a chalcopyrite – pyrrhotite – pentlandite assemblage locally associated with PGM found as discrete grains in sulfides, in clinopyroxene, but more commonly around the margin of sulfide grains. Platinum-group minerals consist of dominant Pd and Pt arsenides and sulfarsenides and Pt–(Pd–Bi) tellurides (sperrylite, platarsite, palladoarsenide, kotulskite, moncheite) with minor amounts of native Pt. The expression of the mineralization is consistent with the late crystallization of an immiscible semimetal-rich liquid that forms discrete PGM grains around sulfide blebs and in veinlets injected in the adjacent silicates. The PGM were also observed in association with secondary phases, dominated by epidote replacing primary base-metal sulfides. This feature, commonly described in other layered intrusions, is here mainly observed around the platinum-group minerals – base-metal sulfides association hosted by plagioclase. The existence of compositional heterogeneities in primary plagioclase likely acts as a target for the development of alteration.

Serpentinization of the oceanic lithosphere and some geochemical consequences: Constraints from the Leka Ophiolite Complex, Norway

Serpentinization of ultramafic rocks is an important process which modifies the petrophysical and geochemical properties of the affected rock. The Leka Ophiolite Complex (LOC) represents a part of the oceanic lithosphere which has been extensively serpentinized at the ocean floor. The ultramafic lithologies of the LOC preserve the history of hydration processes that took place during ocean-floor metamorphism over a wide range of decreasing temperatures. Serpentinization leads to the formation of less dense phases which results in a significant increase in bulk volume of the affected rocks. This change in density and volume of the ultramafic lithologies does not occur simultaneously in all lithologies and results in deformation of the surrounding rocks. The deformation is particularly evident in mm to dm thick, fractured and altered orthopyroxenite dykes in a dunite matrix. The fracturing process of the altered orthopyroxenite is driven by the reaction-assisted volume changes occurring in the dunites. Chemical and textural evidence show that major elements like Mg, Si and Al are essentially redistributed within the rock during serpentinization on an outcrop scale. However, there is abundant evidence for the mobility of Ca, Na, Fe and Mn at grain-size to regional scale. Alteration of primary clinopyroxenes to serpentine and clinochlore is a Ca-releasing reaction and contributes significantly to the mobilization of Ca in the fluids. This results in rodingitization of the crustal layer and also in the replacement of primary clinopyroxene, orthopyroxene and olivine by secondary diopside. Fe and Mn are mobilized simultaneously during the dissolution and subsequent precipitation of minerals throughout the serpentinization process. Olivine, in particular, is affected during the transport of iron and manganese. Metamorphic olivine has wide range of chemical compositions with Mg# as low as 0.68 and MnO contents of up to 1.5 wt.%. The fluid chemistry from high-temperature, ultramafic-hosted vent sites like Rainbow and Logatchev has elevated amounts of Fe and Mn suggesting that Fe and Mn could be transported out of the system as the degree of serpentinization increases. Therefore, serpentinization of the ultramafic part of the oceanic lithosphere may play an important role in constraining the global ocean chemical budget as well as ocean-floor mineral deposits.

PLATINUM-GROUP MINERALS FROM THE MECHANIC SETTLEMENT PLUTON, SOUTHERN NEW BRUNSWICK, CANADA

The upper Neoproterozoic Mechanic Settlement Pluton, located in the Avalon terrane, southern New Brunswick, Canada, consists of alternating lenses of ultramafic through mafic to intermediate rocks. Known platinum-group element (PGE) mineralization occurs mostly in the ultramafic parts of the intrusion in association with Cu–Ni sulfides. The platinum-group minerals (PGM) are mainly Pd-dominated Pd–Te–Bi minerals; sobolevskite (PdBi) is most abundant, with rarer kotulskite, michenerite, moncheite, and froodite. Additional minerals identified are braggite, vysotskite, sperrylite, stillwaterite, hollingworthite, plumbo-palladinite, Pt–Fe alloy, sopcheite, and paolovite. Sperrylite, Pt–Fe alloy, and braggite are the main carriers of Pt; Ir-, Os-, or Ru-based PGM were not encountered. Gold and Au–Ag alloy are also associated with PGM. Most of the PGM occur as inclusions in silicate minerals (olivine, clinopyroxene and, less commonly, amphibole and phlogopite), or in sulfides (chalcopyrite, pentlandite, pyrrhotite and, less commonly, bornite and millerite), but also occur interstitially to the silicate and sulfide minerals. The size of the PGM ranges from <1 μm to 100 × 60 μm. Textural and mineralogical evidence suggests that some PGM are magmatic, but the majority are of subsolidus origin, as indicated also by the low thermal stability of the bismuthotellurides. The Cu–Ni–PGM mineralization is attributed to the formation of an immiscible sulfide liquid and subsequent sulfide and PGM crystallization.

Multistage evolution of the Jijal ultramafic–mafic complex (Kohistan, N Pakistan): Implications for building the roots of island arcs

The ultramafic–mafic complexes located at the base of obducted paleo-island arcs are commonly interpreted as evidence for intra-crustal fractionation of primitive (high-Mg#) mantle melts. The present study, realized on the Jijal mafic–ultramafic basal section of the Cretaceous Kohistan oceanic arc (N Pakistan), discards a crystal fractionation model from a single parental magma to take into account the geochemical and isotope variations observed between the plutonic crust and the underlying ultramafic section. The basal ultramafic rocks, i.e. dunites, wehrlites and clinopyroxenites, show high Mg#, extremely depleted REE patterns, no marked HFSE anomalies and high LILE enrichment. Cpx and leached whole rocks from this unit yield restricted ε Nd values (+ 5.5 to + 6.8), a large ε Sr range (− 5.9 to + 8.8) and mainly radiogenic 207Pb/ 204Pb ratios. The overlying mafic section, i.e. gabbroic rocks, displays low Mg# (46 < Mg# < 54), enriched REE patterns and marked HFSE anomalies. These plutonic rocks have also very homogeneous Sr and Pb isotopic compositions defining a restricted domain, which does not overlap that of the ultramafic samples. Moreover, Cpx from the basal ultramafics yield an Sm–Nd isochron at 117 ± 7 Ma interpreted as dating crystallization of the ultramafic section. Together, these results indicate that the mafic rocks on one hand and the ultramafic rocks on the other hand, originate from separate sources with specific emplacement underneath the subduction zone. A lherzolite lens, collected from the top of the mafic section, plots on the early Cretaceous Sm–Nd isochron and therefore yields the same 143Nd/ 144Nd initial ratio. This suggests that the ultramafic part of the lower Kohistan arc complex sampled the initiation stages of the subduction with production of magmas with boninite-like features. At this stage, the mantle wedge has been already modified by percolation of the first slab-derived fluids. On such a scenario the lherzolite lens would correspond to the lithospheric mantle underneath the arc before its initiation. A multi-stage history is proposed for the evolution for the Kohistan arc through ∼ 30 Ma. Stage #1 reflects the spontaneous initiation of subduction associated with extensive boninitic affinity magmatism, stages #2 and #3 are related to arc building by tholeiitic melts and, a fourth step (stage #4) corresponds to the intra-crustal differentiation.

Neotethyan ophiolitic rocks of the Anatolides of NW Turkey and comparison with Tauride ophiolites

The I2zmir–Ankara–Erzincan zone in Turkey is part of the Anatolide belt. In NW Anatolia, near Kütahya, the ophiolitic rocks within the I2zmir–Ankara–Erzincan suture zone have a tholeiitic character and oceanic back-arc chemistry. Amphibolites form a metamorphic sole to part of the ophiolite in the Kütahya region and yield an 40 Ar/ 39 Ar age of 93 ± 2 Ma, interpreted to record high-grade metamorphism and intra-oceanic thrusting of pre-existing ocean crust. Gabbros and dykes yield 40 Ar/ 39 Ar ages of 94 ± 13 Ma and 48 ± 12 Ma. The first is suggested to be the ophiolite formation age, and the second a regional metamorphic event. The ophiolitic rocks of the Kütahya region are composite and record several events in the evolution of a Neotethyan seaway situated in north–central Anatolia and farther east. They are interpreted to have formed in a small ocean basin marginal to the subducting I2zmir–Ankara–Erzincan ocean. The Kütahya ophiolites of the Anatolide belt are similar in age to the Tauride ophiolites, and the sub-ophiolite metamorphic rocks and dykes in the ultramafic parts of the ophiolites in the Anatolide–Tauride Block formed at the same intraoceanic subduction zone within the same Neotethyan ocean.

Potarite (Pd-Hg) in thermally metamorphosed dunite from the Inazumi-yama ultramafic complex, southwestern Japan: an implication for the behaviour of mercury in PGE mineralization in peridotite

Abstract Pd-rich PGM (platinum-group minerals) have been found in sulphide-bearing dunites and harzburgites from the Inazumi-yama ultramafic complex in southwestern Japan. In the dunite, potarite (PdHg0.85–0.88) is the most abundant PGM but other associated PGM include stibiopalladinite (Pd5Sb2) and rare sperrylite (PtAs2) and Pd-rich alloys. A Pd telluride has been found in the harzburgite. The PGM are enclosed usually by pentlandite-heazlewoodite composite grains and, rarely, by altered chrome-spinel. These minerals are characteristic of an ultramafic assemblage but they are accompanied by ubiquitous galena and minor sphalerite not usually associated with these ultramafic assemblages. The ultramafic part of this complex has been thermally metamorphosed (olivine-talc zone) within the contact aureole of an adjacent granite. The PGM, sulphides and altered spinel are all intergrown with antigorite and/or chlorite, indicating a metamorphic overprint on the primary igneous mineralogy. The Pd/Pt ratios of 9 suggest a process of hydrothermal concentration for the Pd and it is proposed that the Pd has been remobilized and reconcentrated by hydrothermal solutions derived from the granitic magma which reacted with the Pd concentrated in the primary magmatic sulphides. Mercury may have been leached from surrounding sediments by the hydrothermal solutions. It is possible that potarite and the other PGM formed at temperatures of up to 650°C and are likely to be found in other thermally metamorphosed ultramafic rocks showing a depleted character (spinel Cr/[Cr+Al]&gt;0.7), common in orogenic belts.

Pervasive magnesium loss by marine weathering of peridotite

Bulk abyssal peridotites have lower MgO and higher SiO 2 than comparable continental peridotites. If these low MgO SiO 2 ratios were a primary feature of the oceanic mantle, a major re-evaluation of the concentrations of these elements in the bulk silicate Earth would be necessary. Alteration mechanisms documented until now in abyssal peridotites (amphibolite facies metamorphism and serpentinization) are nearly isochemical (besides water), and thus do not explain such widespread compositional differences. We investigate here the possibility that the low MgO SiO 2 ratios are due to a different type of alteration. We have measured bulk compositions in a suite of abyssal peridotites and compared these to their primary compositions, measured where possible or estimated. It is clear that the compositional differences between continental and abyssal peridotites are an alteration phenomenon. It is likely that they are not a result of serpentinization, but of pervasive weathering, below about 150°C (the temperature below which seawater is undersaturated with Mg-rich minerals), and at water/rock ratios between 10 3 and 10 5. The average dredged abyssal peridotite appears to have lost about 5 wt% MgO. Peridotite is an important component of the ocean crust. Its chemical behavior during alteration thus has a large impact on global mass budgets for many elements. A preliminary estimate of the magnitude of Mg loss from the ultramafic part of the ocean crust shows the maximum contribution to the oceans (10 12 moles Mg/y) to be significant, nearly 85% of the yearly Mg flux from rivers. Such an input of Mg to the oceans from peridotites would require that Mg-based estimates of the total hydrothermal flux be revised upward.