Ultramafic Bodies Research Articles

A story of olivine from the McIvor Hill complex (Tasmania, Australia): Clues to the origin of the Avebury metasomatic Ni sulfide deposit

Magmatic differentiation and/or assimilation and related segregation of immiscible sulfide liquid are generally believed to be critical processes in the formation of the majority of orthomagmatic Ni sulfide deposits. In recent years, a new class of Ni sulfide deposits formed by metasomatic and/or hydrothermal modification of peridotites has been recognized. The serpentinite-hosted Avebury Ni sulfide deposit (Tasmania, Australia), the largest known non-magmatic sulfide deposit, provides an unprecedented opportunity to understand sources of metals and fluids responsible for this style of economic mineralization. Our study shows that serpentinization of the Ni-bearing olivine in the Cambrian peridotites of the McIvor Hill complex was followed by metasomatic transformation assisted by heat and fluids supplied by the nearby Late Devonian granite intrusion. The role of the above in the formation of an economic concentration of Ni sulfides is supported by (1) abundant Ni-Fe alloys and sulfides associated with serpentinization of peridotitic olivine, (2) metasomatic olivine containing inclusions of serpentine and metalliferous brines, and (3) the Late Devonian age of the Ni sulfide deposit. The Avebury metasomatic olivine is Ni-depleted and enriched in Mn relative to olivine of similar Fo content in nearby unmineralized peridotites, and to olivine in subduction-related mafic magmas generally. The unusual minor element chemistry of olivine is matched by a unique set of olivine-hosted multiphase inclusions composed of fibrous Mg-silicates and various Na-, K-, Fe-, Ca-, Mn-, and Ba-bearing chlorides/hydrochlorides, sulfides, arsenides magnetite, REE minerals, and Fe-Ni alloys. Peridotite whole-rock Sr-Nd-Pb isotope data and U-Pb dating of metasomatic titanite support earlier suggestions that Ni mineralization is temporally and genetically related with the intrusion of the nearby 360 Ma Heemskirk Granite. It appears that the multiphase inclusions in metasomatic olivine demonstrate chemical signatures of both in situ serpentinites (entrapped alloys, sulfides, arsenides, and magnetite) and distal fluids (enrichment in Pb, Bi, Sn, Sb, Sr, Ba, Rb, Cs, and Ce). We propose that magmatic olivine in large ultramafic bodies provides almost infinite Ni to replacive serpentinites and constitutes a major reservoir of disseminated Ni mineralization. In the case of Avebury Ni was locally redistributed from olivine in the Cambrian peridotites to mainly Fe-Ni alloys and sulfides during serpentinization in the early Paleozoic. In the Devonian reheating and interaction with a fluid in the contact aureole of the Heemskirk Granite led to de-serpentinization and formation of metasomatic high-Mn, low-Ni olivine with inclusions of serpentine and entrapped alloys, sulfides, arsenides, and magnetite, and metalliferous brines rich in granitic elements. Nickel released from serpentinite in this process was re-deposited near the margins of the peridotite to form the Avebury Ni orebody. Our model of serpentinization-related release of Ni from magmatic olivine, in situ precipitation of metallic, sulfide, and arsenide Ni-minerals, and their redistribution and recrystallization in hydrothermal conditions represents an alternative to Ni remobilization from magmatic sulfides.

Zonation of the Newry Igneous Complex, Northern Ireland, based on geochemical and geophysical data

The Late Caledonian Newry Igneous Complex (NIC), Northern Ireland, comprises three largely granodioritic plutons, together with an intermediate–ultramafic body at its northeastern end. New whole-rock geochemical data, petrological classifications, and published data, including recent Tellus aeromagnetic and radiometric results, have been used to establish 15 distinct zones across the four bodies of the NIC. These become broadly younger to the southwest of the complex and toward the centres of individual plutons. In places, zones are defined by both current compositional data (geochemistry and petrology) and Tellus results. This is particularly clear at the eastern edge of the NIC, where a thorium-elevated airborne radiometric signature occurs alongside distinct concentrations of various elements from geochemistry. However, in the northeastern-most pluton of the NIC, a prominent ring-shaped aeromagnetic anomaly occurs independent of any observed surface compositional variation, and thus the zones in this area are defined by aeromagnetic data only. The origins of this and other aeromagnetic anomalies are as yet undetermined, although in places, these closely correspond to facies at the surface. The derived zonation for the NIC supports incremental emplacement of the complex as separate, distinct magma pulses. Each pulse is thought to have originated from the same fractionally crystallising source that periodically underwent mixing with more basic magma.

Widespread tungsten isotope anomalies and W mobility in crustal and mantle rocks of the Eoarchean Saglek Block, northern Labrador, Canada: Implications for early Earth processes and W recycling

Well-resolved 182W isotope anomalies, relative to the present mantle, in Hadean–Archean terrestrial rocks have been interpreted to reflect the effects of variable late accretion and early mantle differentiation processes. To further explore these early Earth processes, we have carried out W concentration and isotopic measurements of Eoarchean ultramafic rocks, including lithospheric mantle rocks, meta-komatiites, a layered ultramafic body and associated crustal gneisses and amphibolites from the Uivak gneiss terrane of the Saglek Block, northern Labrador, Canada. These analyses are augmented by in situ W concentration measurements of individual phases in order to examine the major hosts of W in these rocks. Although the W budget in some rocks can be largely explained by a combination of their major phases, W in other rocks is hosted mainly in secondary grain-boundary assemblages, as well as in cryptic, unidentified W-bearing ‘nugget’ minerals. Whole rock W concentrations in the ultramafic rocks show unexpected enrichments relative, to elements with similar incompatibilities. By contrast, W concentrations are low in the Uivak gneisses. These data, along with the in situ W concentration data, suggest metamorphic transport/re-distribution of W from the regional felsic rocks, the Uivak gneiss precursors, to the spatially associated ultramafic rocks.All but one sample from the lithologically varied Eoarchean Saglek suite is characterized by generally uniform ∼+11 ppm enrichments in 182W relative to Earth's modern mantle. Modeling shows that the W isotopic enrichments in the ultramafic rocks were primarily inherited from the surrounding 182W-rich felsic precursor rocks, and that the W isotopic composition of the original ultramafic rocks cannot be determined. The observed W isotopic composition of mafic to ultramafic rocks in intimate contact with ancient crust should be viewed with caution in order to plate constraints on the early Hf–W isotopic evolution of the Earth's mantle with regard to late accretionary processes. Although 182W anomalies can be erased via mixing in the convective mantle, recycling of 182W-rich crustal rocks into the mantle can produce new mantle sources with anomalous W isotopic compositions that can be tapped at much later times and, hence, this process should be considered as a mechanism for the generation of 182W-rich rocks at any subsequent time in Earth history.

Floresta and Bodocó Mafic–Ultramafic Complexes, western Borborema Province, Brazil: Geochemical and isotope constraints for evolution of a Neoproterozoic arc environment and retro-eclogitic hosted Ti-mineralization

The retrograde high-pressure metamorphosed Floresta and Bodocó mafic–ultramafic complexes (FMUC and BMUC, respectively) occur as clusters of small bodies, lenses and boudins with close association with granitic, gneissic rocks and metasediments in the southern limits of the Alto Pajeú and Piancó–Alto Brígida domains, Borborema Province, northeast Brazil. Ultramafic lithologies including metadunites, olivine cumulate rocks, metapyroxenites, chromitites and massive ilmenomagnetitites showing evidence of cumulus textures and mafic members include garnet amphibolites and coarse metahornblendites. BMUC rocks include Cr–Fe–Ti chromitites in the Fazenda Esperança deposit, and FMUC rocks show important Fe–Ti–V economic mineralization in the mines of the Serrote das Pedras Pretas and Lagoa dos Angicos and the Riacho da Posse deposit in development (54 Mt @13% Ti). The whole-rock and mineral geochemistry of these complexes are akin to arc cumulate rocks, particularly Alaskan-type complexes, which are widely considered to have formed above subduction zones.A coarse metahornblendite from the Serrote das Pedras Pretas mine was dated by U–Pb zircon through laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). Information obtained from the cores of igneous zircons represent an upper intercept age of crystallization at 1024±18Ma with 4.2 mean square weighted deviation (MSWD), whereas zircon exhibiting coarse rims with low Th/U at 0.008–0.038 have concordia ages of 685±17 and 625±6Ma. According to their zircon textures, these ages may be representative of a high-grade eclogitic or progressive granulitic/retro-eclogitic stage during the Pan-African–Brasiliano event. Additionally, the age of the igneous protolith, 1028±63Ma, is well marked by a Sm–Nd isochron based on 12 samples.Positive ɛNd(t) values between 0.01 and 4.84 suggest a depleted mantle or a source close to the chondritic reservoir for the mafic–ultramafic bodies studied. The TDM model ages between 1.1 and 1.6Ga are consistent with a juvenile Mesoproterozoic source or mixing between Paleoproterozoic and Neoproterozoic sources. The dispersion of initial ratios of 87Sr/86Sr at 0.7016–0.7090 and the high fractionation of light rare earth elements (LREE) relative to heavy rare earth elements (HREE) suggest a depleted mantle source metasomatized by subduction-related melts/fluids.The mafic–ultramafic complexes, volcanic and sedimentary rocks, and some granitic intrusions with post-collisional signature after 975Ma constitute a proto to marginal arc-back-arc basin system that evolved between 1025 and 975Ma. However, FMUC and BMUC which are currently arc cumulate rocks were originally picrite melts in a suprasubduction ophiolite zone.The economic deposits of Fe–Ti associated with retro-eclogitic and ultramafic members of the FMUC and enclosed within the late Ediacaran granite can be linked in part by early accumulation under magmatic conditions, secondary enrichment of rutile up to 12% in the high-pressure (HP) stage, and hydrothermal interactions during granite emplacement.

A secondary precious and base metal mineralization in chromitites linked to the development of a Paleozoic accretionary complex in Central Chile

Platinum-group element (PGE) and gold inclusions are usually present in peridotites and chromitite deposits associated with ophiolites. Here, we present the first detailed study of the mineralogy of precious metals in ultramafic rocks hosted in the Paleozoic Coastal Accretionary Complex of Central Chile. In these ultramafic rocks the mineralization of precious metals is associated with small meter-size pods and veins of massive chromitite hosted in serpentinite-filled shear zones. Crystallographic orientation maps of single chromite grains, obtained using the Electron-Backscattered Secondary Diffraction technique, allow us to identify two types of chromite in the precious-metal bearing chromitites: (1) Type A chromite, characterized by an average misorientation per grain of ≤2° and chemically homogeneous cores surrounded by a porous rim with abundant inclusions of chlorite, and (2) Type B chromite, which exhibits higher degrees of misorientation (2–8°) and porosity, and abundant silicate inclusions, but a relatively homogeneous chemical composition. In situ analyses using EMPA and LA-ICP-MS for major, minor and trace elements indicate that composition of the magmatic chromite is only preserved in the cores of Type A chromite grains. Core to rim chemical trends in these Type A chromites are characterized by a progressive increase of the Cr# with a decrease of the Mg#, loss of Al and addition of Fe2+ in the porous rim. The observed changes in the microstructure and chemistry of chromite are associated with the infiltration of external fluids through shear zones filled with antigorite (±talc) developed in partly serpentinized peridotites (i.e., olivine–lizardite dunites). Thermodynamic calculations using the phase equilibria relations in the system Cr2O3–MgO–FeO–Al2O3–SiO2–H2O (CrMFASH) indicate that Fe2+-rich porous chromite+chlorite replaced the original assemblage chromite+olivine in the chromitite while prograde antigorite was formed. According to our results this transformation occurred at ~510–560°C when external fluids penetrated the ultramafic/chromitite bodies through shear zones. These temperatures are slightly higher than estimated for the metamorphic peak in the host metapelitic rocks (i.e., ~420°C at 9.3kbar), suggesting that a hotter ultramafic body was captured by the metasediments of the accretionary prism during their exhumation through subduction channel. Chlorite geothermometry yielded a wide range of lower temperature from 430 to 188°C, for chlorite present in the porous chromite rims. These results are in agreement with the retrograde overprint under greenchist-facies metamorphism conditions recorded by metapelitic host rocks and minor volcanogenic massive sulphide deposits in the area (300–400°C, ~3–4kbar). We suggest that although initially decoupled, the chromitite-bearing ultramafic rocks and their metasedimentary host undergone a common metamorphic PT pathway of exhumation during the formation and evolution of the subduction-related accretionary complex.The chromitites contain appreciable amounts of the platinum-group elements (up to 347ppb total) and gold (up to 24ppb), present as inclusions of platinum-group minerals (PGM) and alloys as well as native gold. The PGM identified include native osmium, laurite (RuS2), irarsite (IrAsS), osarsite (OsAsS), omeiite (OsAs2), Pt–Fe alloy (possibly isoferroplatinum) and a suite of inadequately identified phases such as PtSb (possibly stumpflite), PdHg (possibly potarite), RhS, Ir–Ni and Ir–Ni–Ru compounds. Only a few grains of osmium and laurite were identified in unaltered cores of chromite and therefore considered as magmatic in origin formed during the high-T event of chomite crystallisation in the upper mantle. The other PGM were located in the porous chromite associated with chlorite or base-metal minerals (BMM) that often fill the pores of this altered chromite or are intergrowth with antigorite in the host serpentinized ultramafic rock. The assemblage of BMM identified in the studied rocks include sulphides [millerite (NiS), polydymite (Ni3S4), violarite (FeNi2S4), galena (PbS), sphalerite (ZnS), chalcocite (CuS)], arsenides [(orcelite (Ni5−xAs2) and maucherite (Ni11As8)], the sulpharsenide gersdorfitte (NiAsS), and native bismuth. The irregular shape of several PGM grains observed in porous chromite suggest disequilibrium, whereas others exhibit perfectly developed crystal faces with the associated secondary silicate or base-metal mineral suggesting neoformation of PGMs in situ from metamorphic fluids. We suggest that the origin of these PGM inclusions is magmatic, but some grains were reworked in situ when metalloid (i.e., As, Sb, Pb, Zn and Hg)-rich fluids released from metasediments penetrated the ultramafic rocks through active shear zones, once the ultramafic bodies became tectonically mixed with the host metasedimentary host rocks. During this event, gold sourced from the (meta)sediments was also precipitated within chromitites and serpentinites.

Sr-Nd-Os-S isotope and PGE geochemistry of the Xiarihamu magmatic sulfide deposit in the Qinghai–Tibet plateau, China

The newly discovered Xiarihamu Ni-Cu deposit is located in the Eastern Kunlun orogenic belt in the northern part of the Qinghai–Tibet plateau, western China. It is the largest magmatic Ni-Cu sulfide deposit found thus far in an arc setting worldwide and ranks second in China in terms of total Ni resources. Fe-Ni-Cu sulfide mineralization occurs in a small ultramafic body that is part of a larger mafic-ultramafic complex formed by protracted Silurian-Early Devonian basaltic magmatism. The mineralized ultramafic body is composed predominantly of lherzolite and olivine websterite, with minor dunite, websterite and orthopyroxenite. Here we report new PGE (platinum group element) data and the results of a new, integrated Sr-Nd-Os-S isotope study. The initial concentrations of Rh and Pd in the parental magma are estimated to be 0.014 ppb and 0.24 ppb, respectively, which are more than one order of magnitude lower than those in undepleted mantle-derived magmas such as many continental picrites. The observed PGE depletions in the Xiarihamu parental magma are attributed to sulfide retention in the source mantle, because the degree of partial melting required to generate the Xiarihamu primary magma was not high enough for a magma of that composition to dissolve all sulfides in the source. The (87Sr/86Sr) i ratios and eNd (t) of the Xiarihamu host rocks range from 0.7062 to 0.7105 and from −1.97 to −5.74, respectively, indicating 5–30 wt% crustal contamination in the Xiarihamu magma. These data also reveal that the source mantle for the Xiarihamu magma is isotopically (Sr-Nd) more enriched than that for the average Cenozoic arc basalt. The γOs(t) and δ34S values of sulfide ores from the Xiarihamu deposit range from 78 to 1393 and from 2 to 6‰, respectively. These values clearly indicate addition of crustal Os and S to the Xiarihamu parental magma. Metal tenors such as Ni and Rh are inversely correlated with γOs(t) and δ34S values. This indicates that mixing between different pulses of magma with different isotope compositions took place during magma emplacement at Xiarihamu, and that external S played a critical role in the formation of the Xiarihamu magmatic Ni-Cu sulfide deposit.

The impact of early sulfur saturation and calc-crustal contamination on ore-forming process in the Posan mafic–ultramafic complex: Derived from the shallow depleted mantle, Beishan region, NW China

The Posan ultramafic intrusion is one of the ~280Ma Pobei mafic–ultramafic complexes located in the Beishan rift, on the northeastern margin of the Tarim Craton. Given that three finished drill holes reveal less economic mineralization, it is necessary to understand the ore potential of this little ultramafic intrusion. Detailed fieldwork shows that it is comprised of five stages of magma events. Among them, stages I, II and III are the mafic bodies dominated by gabbro, gabbronorite and olivine gabbro, and stages IV and V are the ultramafic bodies characterized by layered intrusions. Theoretically, the Posan ultramafic intrusion meets the first requirement of ore mineralization because of its high MgO contents (14.76%) of the parental magma. Moreover, the low Ni content (less than 1900ppm) in olivine and the paragenesis of sulfides and spinel demonstrate that the Posan intrusion experienced early sulfur saturation before or during the fractional crystallization of spinel and olivine. This part of the sulfide, which may contain Ni–Cu or PGE mineralization, deserves to be the focus of future work. In stage IV, the continuous decrease of Ni contents in olivine with the fractional crystallization, the calc-crustal contamination (irregular distributed plagioclase, high Th/Yb ratios, Nb and Zr depletion and K, Rb, Ba, U, Th and Pb enrichment) instead of sulfur-rich crustal contamination, and the high level of oxygen fugacity (+2.1<fO2QFM<+3.0) result in no obvious disseminated sulfides created in this stage. A clinopyroxene TiO2-Alz diagram confirms that the Beishan mafic–ultramafic complexes were formed in a rift-related circumstance rather than the arc-related cumulus. Low TiO2/Yb and Nb/Yb ratios reflect that the Beishan complexes were derived from the shallow depleted mantle without garnet. Considering the temporal and spatial relationship of the Tarim large igneous province and the large numbers of mafic–ultramafic intrusions in the Beishan region, it is reasonable to speculate that the Beishan mafic–ultramafic complexes, including the Posan intrusion (275.8±2.7Ma), have a genetic affiliation with, but were not directly formed by, the early Permian Tarim Plume.

A Comprehensive View from Geophysical Signatures over Chitradurga Schist Belt, Karnataka

Abstract Geology of Chitradurga Schist Belt (CSB) from Gadag to Srirangapatna, as inferred from the gravity and magnetic data analysis and supportive evidences from digitized Airborne Total count map is presented in this paper. Significant inferences have emerged from the integrated geophysical analysis coupled with geological information. Pattern, continuity and intensity of anomalies indicate that CSB can be viewed as three sectors; northern (Gadag), central (Chitdradurga) and southern (Nagamangala). Qualitatively geophysical signatures are diagnostic for the purpose of broad classification of the area into different geological domains such as granite-gneissic (gravity lows, fluctuating magnetic values and moderate radiometric counts), metavolcanic (gravity highs, magnetic highs and radiometric lows), metasedimentary (moderate gravity low, low magnetic values and moderate radiometric values) and granitic (gravity low, magnetic high and radiometric high) regions. Images prepared from gravity, magnetic and radiometric values indicated possibility of an additional arm of the schist belt from Huliyar to Hosdurga between the known Kibbanahalli and Chitradurga arms of CSB. From the gravity image it appears that the eastern margin of CSB may be encompassing Bukkapatna granite in the central sector of CSB. Gravity low closures and gravity high gradient between the granites of Chitradurga and those near Jagalur suggest different nature of geological continuity. Structural features, particularly the sheared contacts of metavolanics with metasedimentaries are demarcated based on gravity gradients. Based on these inferred features coupled with known geological controls of mineralization, a few target areas for gold / base metal exploration are identified. A major structural feature affecting the strike of arms of CSB may be the possible regions for emplacement of ultramafic bodies as observed near Sasival and Ghattishosalli. This in turn is significant for mineral exploration in Chitradurga Schist belt. Gravity and magnetic profiles drawn across CSB for every 15’ latitude indicated crustal architecture beneath CSB. Significant rise is noticed in the background gravity field from the arms of CSB to the main schist belt suggesting that the Bababudan volcanics have generated from different depths compared to the Chitradurga volcanics. Further, long wavelength magnetic anomaly coinciding with the eastern margin of schist belt suggests the differences in nature of crust on either side. Frequency analysis of gravity and magnetic anomalies and the modeling indicated the depth persistence of geological units.

Evolution of high-pressure mafic granulites and pelitic gneisses from NE Madagascar: Tectonic implications

The occurrence of high-pressure mafic–ultramafic bodies within major shear zones is one of the indicators of paleo-subduction. In mafic granulites of the Andriamena complex (north-eastern Madagascar) we document unusual textures including garnet–clinopyroxene–quartz coronas that formed after the breakdown of orthopyroxene–plagioclase–ilmenite. Textural evidence and isochemical phase diagram calculations in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2 system indicate a pressure–temperature (P–T) evolution from an isothermal (780°C) pressure up to c. 24kbar to decompression and cooling. Such a P–T trajectory is typically attained in a subduction zone setting where a gabbroic/ultramafic complex is subducted and later exhumed to the present crustal level during oceanic closure and final continental collision. The present results suggest that the presence of such deeply subducted rocks of the Andriamena complex is related to formation of the Betsimisaraka suture. LA-ICPMS U–Pb zircon dating of pelitic gneisses from the Betsimisaraka suture yields low Th/U ratios and protolith ages ranging from 2535 to 2625Ma. A granitic gneiss from the Alaotra complex yields a zircon crystallization age of ca. 818Ma and Th/U ratios vary from 1.08 to 2.09. K–Ar dating of muscovite and biotite from biotite–kyanite–sillimanite gneiss and garnet–biotite gneiss yields age of 486±9Ma and 459±9Ma respectively. We have estimated regional crustal thicknesses in NE Madagascar using a flexural inversion technique, which indicates the presence of an anomalously thick crust (c. 43km) beneath the Antananarivo block. This result is consistent with the present concept that subduction beneath the Antananarivo block resulted in a more competent and thicker crust. The textural data, thermodynamic model, and geophysical evidence together provide a new insight to the subduction history, crustal thickening and evolution of the high-pressure Andriamena complex and its link to the terminal formation of the Betsimisaraka suture in north-eastern Madagascar.

Neoarchean arc magmatism followed by high-temperature, high-pressure metamorphism in the Nilgiri Block, southern India

The Nilgiri Block, southern India is an exhumed lower crust formed through arc magmatic processes in the Neoarchean. The main lithologies in this terrane include charnockites, gneisses, volcanic tuff, metasediments, banded iron formation and mafic–ultramafic bodies. Mafic–ultramafic rocks are present towards the northern and central part of the Nilgiri Block. We examine the evolution of these mafic granulites/metagabbros by phase diagram modeling and U–Pb sensitive high resolution ion microprobe (SHRIMP) dating. They consist of a garnet–clinopyroxene–plagioclase–hornblende–ilmenite±orthopyroxene±rutile assemblage. Garnet and clinopyroxene form major constituents with labradorite and orthopyroxene as the main mineral inclusions. Labradorite, identified using Raman analysis, shows typical peaks at 508cm−1, 479cm−1, 287cm−1 and 177cm−1. It is stable along with orthopyroxene towards the low-pressure high-temperature region of the granulite facies (M1 stage). Subsequently, orthopyroxene reacted with plagioclase to form the peak garnet+clinopyroxene+rutile assemblage (M2 stage). The final stage is represented by amphibolite facies–hornblende and plagioclase–rim around the garnet–clinopyroxene assemblage (M3 stage). Phase diagram modeling shows that these mafic granulites followed an anticlockwise P–T–t path during their evolution. The initial high-temperature metamorphism (M1 stage) was at 850–900°C and ~9kbar followed by high-pressure granulite facies metamorphism (M2 stage) at 850–900°C and 14–15kbar. U–Pb isotope studies of zircons using SHRIMP revealed late Neoarchean to early paleoproterozoic ages of crystallization and metamorphism respectively. The age data shows that these mafic granulites have undergone arc magmatism at ca. 2539.2±3Ma and high-temperature, high-pressure metamorphism at ca. 2458.9±8.6Ma. Thus our results suggests a late Neoarchean arc magmatism followed by early paleoproterozoic high-temperature, high-pressure granulite facies metamorphism due to the crustal thickening and suturing of the Nilgiri Block onto the Dharwar Craton.

Effects of hydrous alteration on the distribution of base metals and platinum group elements within the Kevitsa magmatic nickel sulphide deposit

Platinum and nickel are commonly assumed to be immobile in most conditions, especially during low temperature hydrothermal alteration. However, only a small number of studies have rigorously tested this assumption. The Ni–Cu–(PGE) sulphide ore body hosted by the Kevitsa intrusion, northern Finland, provides a natural laboratory to study the behaviour of base metals and platinum group elements (PGE) during low temperature alteration. This ca. 2060Ma mafic–ultramafic intrusion, located in the Central Lapland greenstone belt, hosts disseminated Ni–Cu–(PGE) sulphide mineralisation in the middle part of the main ultramafic body. The mineralisation, which contains a range of Ni, Cu and PGE grades, is affected by three main alterations (serpentinisation, amphibolitisation and epidotisation), and is cross cut by various types of veins. The effect of the circulation of hydrothermal fluids on the distribution of base metals and PGE was studied at two different scales. Interrogation of an extensive deposit-wide assay database provided information on the deposit-scale (kilometre scale) effect of these different alteration styles, and a detailed study, involving laboratory X-ray fluorescence (XRF), portable XRF and micro-XRF mapping, of drill-core samples containing cm-scale cross-cutting veins provided information on the small scale (centimetre to decimetre scale) remobilisation of base metals and PGEs. Results show that the hydration and carbonation of the Kevitsa mineralised mafic–ultramafic intrusion did not significantly affect the distribution of Ni and PGE at scales larger than a few mm, and that Cu and Au are the only metals that are affected by small to large scale remobilisation from centimetre to kilometre scale.

Banded sulfide-magnetite ores of Mauk copper massive sulfide deposit, Central Urals: Composition and genesis

The results of investigation of metamorphosed sulfide-magnetite ores from the Mauk deposit located within the Main Ural Fault at the junction of Tagil and Magnitogorsk massive sulfide zones are discussed. The ore-hosting sequence comprises metamorphic rocks formed from basalt, carbonaceous and carbonaceous-cherty siltstone, and lenticular serpentinized ultramafic bodies. The ores of the deposit are represented by banded varieties and less frequent breccia. The clastic origin of the banded ore is indicated by load casts at the bottom of sulfide beds, alternation of sulfide and barren beds, and the truncation of the growth zones of pyrite crystals. Pyrite, pyrrhotite, chalcopyrite, sphalerite, and magnetite are the major minerals of the banded ores. The internal structure of the listed minerals testifies to the deep metamorphic recrystallization of primary hydrothermal-sedimentary ores accompanied with deformation. Cubanite, pyrrhotite, mackinawite, greigite, and gold are enclosed in metacrysts of pyrite, magnetite, and chalcopyrite. The accessory minerals of the Pb-Bi-Te, Bi-Te, and Ag-Te systems as well as uraninite have been found at the Mauk deposit for the first time. Magnetite predominantly replaces pyrite and less frequently chalcopyrite, pyrrhotite, and gangue minerals. It was established that the major carriers of As and Co are crystals of metamorphic pyrite. Chalcopyrite is the major carrier of Zn, Sn, Te, Pb, Bi, and Ag. Admixture of Fe and Cu is typical of sphalerite, and Se and Ni are characteristic of pyrrhotite. Ti, V, Mn, Sb, As, Ba, and U are concentrated in magnetite. The banded ores of the Mauk deposit are suggested as having been transformed in several stages: diagenesis, anadiagenesis, epidiagenesis (t 500°C).

Interactions between the shallow wedge mantle and subducted slab ~ the Shiraga ultramafic body, Kochi prefecture ~

Interactions between the shallow wedge mantle and subducted slab ~ the Shiraga ultramafic body, Kochi prefecture ~

Geochronology, petrology and Hf–S isotope geochemistry of the newly-discovered Xiarihamu magmatic Ni–Cu sulfide deposit in the Qinghai–Tibet plateau, western China

This paper reports the first set of data for the newly-discovered Xiarihamu magmatic Ni–Cu sulfide deposit in the Eastern Kunlun Paleozoic arc terrane which is located in the northern part of the Qinghai–Tibet plateau. An on-going drilling campaign reveals ~100million tons of sulfide mineralization with the average grade of 0.8wt.% Ni and 0.1wt.% Cu for the deposit. This makes the Xiarihamu deposit one of the 20 largest magmatic Ni–Cu sulfide deposits in the world and the largest ever found in arc settings. The deposit is hosted in a small ultramafic body intruding older gabbroic and metamorphic rocks. New zircon U–Pb isotope age data reveal that the ultramafic body (411.6±2.4Ma) is ~20Ma younger than the host gabbroic intrusion (431.3±2.1Ma). The ultramafic body is composed predominantly of lherzolite and olivine websterite, with minor dunite, websterite and orthopyroxenite. Mineralization mainly occurs as sub-horizontal to gently dipping (<30°) disseminated sulfide zones that are generally concordant with the lithological structure of the ultramafic intrusion. The lateral extension and thickness of individual mineralized zones are up to ~200m and ~100m, respectively. Sulfide mineral assemblages are composed of pyrrhotite, pentlandite and chalcopyrite. The Xiarihamu ultramafic rocks show light REE enrichments and pronounced negative Nb anomalies, plus significant Ca-depletion in olivine (<700ppm Ca), which are characteristic of many arc basalts in the world. Olivine crystals in the Xiarihamu ultramafic rocks have relatively primitive compositions, with Fo contents up to 90mol%, close to the mantle value. The contrasting Ni contents of olivine crystals with similar Fo contents from different sulfide-mineralized zones in a single drill core indicate that at least two pulses of sulfide-laden magma with different Ni compositions were involved in the development of the deposit. Estimated parental magma for the Xiarihamu lherzolites contains 52.4wt.% SiO2 and 9.8wt.% MgO, which are within the ranges of boninites worldwide, supporting the interpretation that the Xiarihamu sulfide ore-bearing ultramafic intrusion is the product of subduction-related boninitic magmatism. The εHf (t) values of zircon crystals from the Xiarihamu ultramafic rocks vary from 1 to 5, indicating minor crustal contamination of the parent magma. The involvement of external sulfur in the genesis of the Xiarihamu magmatic sulfide deposit is evident from higher δ34S values of sulfide ores (3.5 to 6.8‰) than typical mantle value (0±2‰), although the actual contaminants are yet to be identified.

Composition, melting and evolution of the upper mantle beneath the Jurassic Pindos ocean inferred by ophiolitic ultramafic rocks in East Othris, Greece

Large ultramafic bodies of the East Othris ophiolite in Central Greece consist of serpentinites, of harzburgite precursors, as well as serpentinized lherzolites, which have been intruded by thin dykes of olivine-rich and olivine-poor pyroxenites. They represent parts of partially altered upper mantle wedge rocks in a Mid-Late Jurassic intraoceanic subduction setting of the Pindos microocean, a western strand of the Tethyan oceanic realm. Serpentinization and rodingitization occurred during their exhumation toward the fore-arc oceanic region and accretionary prism through a subduction channel. Petrography and geochemistry show that protoliths of most serpentinites and serpentinized peridotites are harzburgites, while few are more fertile lherzolites. Petrogenetic modeling reveals that the former harzburgites correspond to highly depleted residual mantle peridotites, which formed after moderate degrees (~13–20 %) of hydrous partial melting, whereas lherzolites, being closely related to the ophiolitic mantle peridotites of West Othris, resulted after lower partial melting degrees (~7–10 %). Mineral chemistry and geochemical data from pyroxenites imply that they have been derived after crystallization of a subduction-related IAT hydrous magma that formed after moderate partial melting degrees (~14–19 %), quite similar to those that produced the harzburgites. Melting processes for the East Othris mantle peridotites occurred in the spinel-stability field, at estimated equilibrium temperatures ranging between 900 and 1,050 °C and pressures between 1.4 and 1.7 GPa, in a rather highly oxidized environment. It is estimated that the primary IAT magma, formed under relatively high temperatures with liquidus temperature at ~1,260 °C and mantle potential temperature at ~1,372 °C. Cooling rates of the shallow mantle beneath the Pindos oceanic basin, from its Mid-Triassic rift/drift phase and the subsequently developed Mid-Late Triassic short-lived intraoceanic subduction, to the Mid-Late Jurassic main subduction phase, are estimated at ~0.7 and ~1.6 °C/Ma, respectively, with the latter being considered as unusually high.

阿武隈変成帯中に露出する沈み込み帯域オフィオライト断片の岩石学

The Abukuma plateau, which is located along the Pacific coast of northeastern Japan, is composed mainly of Cretaceous granitic rocks and regional metamorphic rocks. In the central part of the plateau, the Gosaisho metamorphic rock series in the east overthrust onto the Takanuki metamorphic rock series in the west. The Gosaisho series is mostly composed of mafic and siliceous rocks, and the Takanuki series is mainly composed of pelitic-psammitic rocks. In the Gosaisho series, many small ultramafic bodies are present in the areas adjacent to the Takanuki series. These ultramafic rocks are affected by contact metamorphism of the Cretaceous granitic rocks in various degrees, but their protoliths are discerned as mantle peridotites and ultramafic cumulates based on their bulk rock chemistry. The ultramafic cumulates are accompanied by metagabbros. At Mount Otsube, mantle peridotites occupy the foot of the mountain and cumulates comprise its top part. The rock assemblage and geological relationship indicates that the ultramafic bodies in this area are fragments of the lower part of an ophiolite. Bulk rock chemistry of the mantle peridotite as well as its olivine and spinel chemistry suggest that the mantle peridotite is highly depleted and of island arc origin. In view of the spinel chemistry, the ultramafic rocks might be independent of the Gosaisho-Takanuki metamorphic rocks and be fragments of the early Paleozoic Hayachine-Miyamori ophiolite. However, the ultramafic cumulates are much richer in Fe (~ Fo70) than those in other ophiolites in Japan. This dismembered ophiolite, which we call Furudono ophiolite, may be a new type of the Paleozoic ophiolites in Japan.

A geobotanical investigation of the Koedoesfontein Complex, Vredefort Dome, South Africa

The Vredefort Dome (impact structure) in South-Africa hosts several unique peralkaline to ultramafic intrusive bodies that were emplaced into its core and collar. Because of the broad difference in mineral composition, between the intrusions as well as the abundant siliciclastic country rocks, abrupt transitions in soil chemical characteristics are produced that can ultimately affect the floristic and physiognomic characteristics of the associated vegetation in the dome. Despite various efforts to study the ultramafic-adapted vegetation of southern Africa, little is known about the effect these particular intrusions have on the soil and floristical components of the dome. The aim of the present study was to investigate the geobotany of one of the ultramafic-peralkaline complexes in the dome, namely the Koedoesfontein Complex. The complex hosts several different intrusions that collectively form a mineralogical gradient ranging from mafic- to felsic-dominant silicates. Three primary components (geology, soil and woody vegetation) were sampled along this gradient and analysed to determine the effect of these intrusions on the residual soil chemistry, and the floristics and physiognomy of the supported vegetation type. Weathering of the peralkaline to ultramafic lithologies greatly affected the soil’s chemical properties and related species composition and physiognomy. Major chemical variation in the soil, including the total chromium and nickel content, is correlated with the difference in abundance of sodium-, calcium-, iron- and magnesium-rich silicate minerals. Significant variations in woody species composition (particularly across Senegalia and Vachellia genera) have been established between soils originating from ultramafic–mafic, intermediate and felsic rock types and can be utilised as indicators for different geological substrates within the dome.

Ophiolitic rocks of the Indo-Myanmar Ranges, NE India: relicts of an inverted and tectonically imbricated hyper-extended continental margin basin?

Abstract The Indo-Myanmar Ranges (IMR) of NE India are host to various ophiolitic rocks, including metamorphosed Alpine-type harzburgite and lherzolite. Compared to abyssal peridotites of normal oceanic lithosphere, these ultramafic rocks are enriched in trace and rare earth elements. Spilitic pillow lavas along with mafic dykes and sills locally intruded into the serpentinized ultramafic rocks and associated pelagic sediments exhibit alkaline compositional affinities. Ophiolite formation and emplacement were by a process analogous to that described for mantle exhumation in hyper-extended continental margin settings and ophiolites in parts of the European Alps, involving very slow passive continental margin rifting accompanied by slow upwelling or extensional unroofing of the subcontinental upper mantle up to the seafloor. Preliminary palaeomagnetic measurements conducted on ultramafic rocks within the IMR ophiolite belt give a virtual geomagnetic pole (VGP) at 47° N, 045° E for thermal demagnetization (TDM) measurements and 33° N, 013° E for the alternating field demagnetization (AfD) measurements, requiring an anticlockwise rotation of the ultramafic bodies by 14° during the subduction process. The original trend of the spreading axis of the ophiolites was probably NE–SW, with spreading directed NW–SE. Computation of palaeolatitude of the ultramafic rocks gives an average value of 24.67°. Comparison between the palaeolatitude and the present latitude of the sample sites provides a mere latitudinal shift of less than 1°. Field studies, combined with an analysis of structural and tectonic features in the IMR, suggest a generalized WNW–ESE (east–west) compression and NNE–SSW (north–south) extension contradictory to the NNE–SSW contraction indicated by seismic data. Area balancing techniques employed along sections orientated perpendicular to regional tectonic strike in the IMR reveal systematic variations in the amount of crustal shortening, with a maximum of approximately 60% recorded in the Nagaland–Manipur segment along 25.644° N, 93.826° E–25.076° N, 95.897° E. The amount of shortening gradually decreases away from the axis of maximum shortening and on both sides. Calculations of relative plate motion based on rotation vectors given by different workers for various plate pairs represented in the region reveal that the interaction between the Indian and Myanmar plates can ideally produce the structural and tectonic features of this range. Dextral shear coupled to oblique subduction of the Indian Plate below the Myanmar Plate can best explain all of the structural and tectonic features present in the IMR.

Petrogenesis and geotectonics of the Mikame ultramafic body, western Shikoku, Japan

Petrogenesis and geotectonics of the Mikame ultramafic body, western Shikoku, Japan

Zoned chromite records multiple metamorphic episodes in the Calzadilla de los Barros ultramafic bodies (SW Iberian peninsula)

Podiform chromitites occurring in the ultramafic bodies of Calzadilla de los Barros, in the Ossa-Morena zone of the Iberian Massif (SW Iberian Peninsula), were deformed and metamorphosed together with their host rocks, leading to the development of variably complex patterns of zoning in chromite grains. These patterns consist of cores with variable composition surrounded by thin rims of porous chromite. Two types of zoned chromite are observed in chromitites: (1) crystals with zoned cores characterised by progressive Mg# [=Mg/(Mg + Fe 2+ )] decrease from inner to outer core, surrounded by Fe 2 O 3 -poor, porous rims, and (2) crystals showing the opposite chemical trend in cores (progressive Mg# increase from inner to outer core) and Fe 2 O 3 -rich porous rims. Mg-rich chlorite is the only silicate mineral forming the matrix of all chromitites and filling most pores in chromite rims. Accessory chromites in dunites show complete transformation to ferrian chromite and Cr-rich magnetite. However, some of them exhibit complex chemical and textural zoning with three concentric zones (from inner to outer core and to inner rim) characterised by progressive Mg# increase and Cr# [=Cr/(Cr + Al)] decrease, surrounded by an outer rim with almost the same composition as the inner core. Some chromite grains from massive chromitites (defined here as higher than 85 vol.% chromite) still preserve inner core compositions unaffected by metamorphic transformation. These inner cores show high-Al composition (Cr# = 0.48–0.52) with Mg# ranging from 0.65 to 0.70. These compositions resemble those reported for Al-rich, podiform chromitites in ophiolites elsewhere and indicate that chromitites from Calzadilla de los Barros derived from a MORB melt type in a MORB or supra-subduction zone setting. The chemical and textural variations in zoned chromite from chromitites and dunites can be interpreted in terms of a multistage process characterised by two episodes of retrograde metamorphism separated by a high-temperature heating event. This interpretation suggests a correlation with the tectono-metamorphic evolution of the Neoproterozoic basement of the Ossa-Morena zone.