Ultramafic Units Research Articles

Ecological and Human Health Risk Assessment Based on Stream Sediments from Coastal Oecusse (Timor)

Timor Island is located in a geologically complex region strongly affected by the collision of the Australian margin with the Banda volcanic arc. In Oecusse, an enclave of East Timor in the western part of Timor, crop out several lithological units of the Banda Terrane that are associated with the obduction of oceanic crust and upper mantle on the Australian continental crust. This study reports the geochemistry of stream sediments from the coastal region of the Oecusse enclave, where the Banda Terrane is best represented, employing statistical analyses to discern the sources of metal(oid)s and assessing ecological and health risks. Arsenic, Cr, and Ni are the elements with higher potential ecological risk factors. The potential ecological risk index (PERI), which combines single indexes of ecological risk factors for multiple elements, is very high in a stream sourced by the Lolotoi-Mutis Complex. Significant risks for human health were found for As (sourced by the Lolotoi-Multis Metamorphic Complex and basalts of the Barique Formation), Mn (sourced by the Maubisse and Barique formations), and V (sourced by the Manamas Formation). The highest values of hazard index (HI), however, were determined with Cr, in particular for children (HI higher than 10 in 12 sediments sourced by ultramafic units and their covering sedimentary units). This investigation shows that high geogenic concentrations of several elements, particularly those derived from the oceanic crust and the upper mantle, raise significant cancer and non-carcinogenic risks.

Co, Sc, and Ni enrichment in the Cagdianao nickel laterite deposit, Dinagat Islands, Philippines

Abstract Nickel laterites are soils that are derived from the weathering of mafic and ultramafic rocks. These deposits are being extracted mostly for their economic Ni and Fe content. However, in recent studies, the potential of nickel laterites to hold significant amounts of critical metals such as Sc and Co are already being investigated. In the Philippines, nickel laterite deposits form through the weathering of ophiolitic units. One of these is the Cagdianao nickel laterite deposit which was derived from the weathering of the ultramafic units of the Dinagat Ophiolite Complex in Dinagat Island, Surigao Del Norte, Philippines. The bedrock of the Dinagat Ophiolite Complex is dominantly comprised of harzburgite. This harzburgite bedrock is overlain by a nickel laterite profile consisting of horizons of Mg- and Si-rich rocky saprolite, earthy saprolite, Fe-rich limonite, and an Al-rich topsoil. Drillcores and field exposures of the nickel laterite profile were observed, sampled, and subjected to petrography, whole rock geochemistry (XRF and ICPMS), and mineral chemistry (LA-ICPMS) analyses. In terms of major elements, Fe is dominant in the limonite zone (up to 59.76 wt.%). Mg and Si are more enriched in the rocky saprolite layer (up to 28.41 and 37.41 wt.%, respectively). In terms of the critical minerals, Ni is more enriched in the saprolite zone (up to 29936 ppm) while Co and Sc are more enriched in the limonite zone (up to 1080 and 86 ppm, respectively). Elemental mapping of the bedrock samples reveals the strong affinity of Ni to olivine and serpentine. Sc was revealed to have a greater affinity with orthopyroxenes. Lastly, Co is observed to have a slight affinity with olivine. These results show a relative enrichment of Co in the profile, particularly in the limonite. Co within the laterite profile also has a very strong affinity with MnO. Similarly, the highest concentration values for Sc were also observed in the limonite. It also exhibits an affinity for Fe2O3 and Al2O3. Lastly, Ni was observed to have a strong affinity with olivine and serpentine minerals in the bedrock and the rocky saprolite horizons. Within the deposit, Ni is the main commodity while Co, Sc, and REEs can be considered as by-products.

A comparison of PCA and ICA in geochemical pattern recognition of soil data: The case of Cyprus

Multivariate analysis of soil geochemistry is a powerful tool for differentiating lithological units and detecting geochemical dispersion halos related to mineralization or contamination. While univariate analysis can effectively identify lithological units with pronounced variations, it may fail to differentiate between subtler variations in lithologies. Traditional multivariate techniques such as principal component analysis (PCA) have limitations, including difficulties in understanding the individual contributions of each variable and an inability to work with non-Gaussian data. Independent component analysis (ICA) has emerged as a potential alternative, as it can effectively identify independent components of non-Gaussian data. In this study, we compared the effectiveness of PCA and ICA in relating multivariate soil geochemistry to parent lithology using the Soil Geochemical Atlas of Cyprus and associated digital geological maps. Both PCA and ICA were able to differentiate between the ultramafic units within the Troodos Ophiolite (TO) and the Circum-Troodos Sedimentary Succession (CTSS). However, ICA was more effective than PCA in identifying pillow lavas, providing a clear separation in the scores for IC4 and IC5. Furthermore, both PCA and ICA were able to separate the sheeted dykes from the cumulate mafic units within the TO. The gabbro unit is closely defined by IC2 scores. In contrast, PCA failed to provide factors that effectively delineated the Mamonia Terrane from other units, especially the TO, while ICA was able to provide a distinct separation in IC4 and IC5 scores. Separation between the CTSS and Quaternary units was weakly observed in IC2 scores. These findings demonstrate that there is a difference in the effectiveness of PCA and ICA in identifying different lithological units and emphasize the need for a careful selection of multivariate methods to differentiate between subtle differences in soil geochemistry relating to variations in parent lithology.

Applied geochemistry for the discrimination between mafic and ultramafic rocks in Cu-Ni-PGE-bearing layered complexes: A case study at the la Jovita-Las Águilas belt, Sierra Grande de San Luis, Argentina

The Virorco intrusive body is comprised in La Jovita-Las Águilas mafic-ultramafic belt, Sierra Grande de San Luis, Argentina. The ultramafic rocks of this belt carry a base metal sulphide mineralization and platinum group minerals. Macroscopic description of mafic and ultramafic rocks, done by the mining exploration companies, can not differentiate the mineral percentages. Therefore, considering the geochemical composition of the already classified mafic-ultramafic rocks of surface samples from the mentioned belt, we established compositional ranges of geochemical parameters (MgO, Al2O3, Na2O, Sr and Cr) for both mafic and ultramafic rocks. Linking the geochemical data of drill cores made by Castillian Resources Corp. company over the Virorco body, with the mentioned ranges, it was possible to discriminate between mafic and ultramafic rocks present in such drill cores. This provides a useful prospecting tool to identify the mafic and ultramafic units that carry the mineralization and to correlate mineralized units from the different drill cores. This method can be applied, not only on the bodies of the mentioned belt, but in other similar deposits elsewhere. The methodology proposed in the present contribution was validated by the Machine Learning algorithm Random Forrest with a 91% success rate.

Anatomy of an obducted ultramafic unit (Tiébaghi Massif – Peridotite Nappe – New Caledonia): Polyphase brittle tectonics constrained by fault-slip data and crack seal mineralogy

ABSTRACT New data collected in the mining areas of the Tiébaghi Massif, a klippe of the Peridotite Nappe located in the north of New Caledonia, are presented. Data were collected in open cast nickel mine pits and chromite underground works, notably along a 1.2-km long tunnel which penetrates the deep geological structure of the massif. Surface and sub-surface geology, microtectonic observations and kinematic analysis of fracture planes, Electrical Resistivity Tomography, micron-scale Raman spectroscopy and Transmission Electron Microscopy were used to characterise the different tectonic regimes and associated mineralisations. Three stages of deformation and their kinematic axes are identified. Fractures coated with antigorite-tremolite minerals are associated to supra-subduction melt intrusions in a compressional regime characterised mainly by thrust and transcurrent faults. They are followed by fractures coated with polygonal serpentine formed in an extensional regime. The two former mineral phases are commonly superimposed on the same plane. Third-stage consist of fractures associated with supergene minerals deposited in an extensional regime, associated with gravity driven normal faulting. Fractures associated with this supergene stage have reactivated the former hypogene network.

Application of Satellite Remote Sensing, UAV-Geological Mapping, and Machine Learning Methods in the Exploration of Podiform Chromite Deposits

The irregular and sporadic occurrence of chromite pods in podiform chromite deposits (PCD), especially in mountainous terranes with rough topography, necessitates finding innovative methods for reconnaissance and prospecting. This research combines several remote sensing methods to discriminate the highly serpentinized peridotites hosting chromite pods from the other barren ultramafic and mafic cumulates. The case study is the area of the Sabzevar Ophiolite (NE Iran), which hosts several known chromite and other mineral deposits. The integration of satellite images [e.g., Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite sensor, Landsat series, and Sentinel-2] coupled with change detection, band rationing, and target detection algorithms [including the Spectral Angle Mapper (SAM)] were used to distinguish potential lithological units hosting chromites. Results have been verified by an initial on-field checking and compared with the high-resolution (GSD ~6 cm) orthomosaic images obtained by the processing of photographs taken from an Unmanned Aerial Vehicle (UAV) at a promising area of 35 km2. The combination of visual interpretation and supervised classification by machine learning methods [Support Vector Machine (SVM)] yielded the production of a geological map, in which the lithological units and structures are outlined, including the crust-mantle transition zone units, mafic cumulates, crosscutting dykes, and mantle sequences. The validation of the results was performed through a second phase, made up of field mapping, sampling, chemical analysis, and microscopic studies, leading to the discovery of new chromite occurrences and mineralized zones. All ultramafic units were classified into four groups based on the degree of serpentinization, represented by the intensity of their average spectral reflectance. Based on their presumed protolith, the highly serpentinized ultramafics and serpentinites were classified into two main categories (dunite or harzburgite). The serpentinite with probable dunitic protolith, discriminated for a peculiar Fe-rich Ni-bearing lateritic crust, is more productive for chromite prospecting. This is particularly true at the contact with mafic dykes, akin to some worldwide chromite deposits. The results of our work highlight the potential of multi-scale satellite and UAV-based remote sensing to find footprints of some chromite mineral deposits.

Geochemistry of Layered Ultramafic Rocks in J.C. Pura Schist Belt, Dharwar Craton, Karnataka, India

The Jayachamarajapura schist belt in western Dharwar craton, southern India, is predominantly an ultramafics dominant terrain. These rocks have been extensively metamorphosed and altered to serpentinite. The komatiite nature of ultramafics is conspicuous. In most of the areas of the belt these ultramafics are massive in nature. However, some of the ultramafic units show layered nature. But, their outcrops are encompassed within the massive komatiitic bodies. These komatiitic ultramafics are predominantly Mg-rich in nature. The layered rocks are also Mg-rich, and their field setting and geochemistry suggest their intermittent occurrence as sills, during the differentiation of peridotitic magma. The layered rocks, which have been intensely serpentinisation show homogenous nature. They are almost wholly made of serpentine with occasional relics of pyroxene. Secondary carbonate mineral is often noticed. Their higher MgO content indicates Mg-rich ultramafic magmatism during Archaean orogeny.

Mapping of Nickel Laterite Resources Using Geographical Information Systems (Sig), Case Study Koninis Region, Central Sulawesi Province

The Map Banggai geology sheet especially Bunta has ultramafic rocks so it needs to be mapping the laterite nickel resources. This study aims to determine the spread, determine the levels of mineral elements contained and determine the estimated nickel laterite resource in the research area. The research method was carried out by taking field data and testing the sample levels then analyzed using the application of Geographic Information Systems (GIS). The results showed that the distribution of laterite nickel was found in the ultramafic unit area with elevations between 110 - 170 m which extended from north to south of the research area with an area of ​​29.25 ha, levels of Ni elements for Cut Off Grade (COG) were 1.6% and Fe 34.29% with a thickness of 3 m and an estimated nickel laterite (hypothetical) resource of 1,140,750 tons.

Polyphase tectonic reworking of serpentinites and chlorite‐tremolite‐talc rocks (SW Spain) from the subduction forearc to intracontinental emplacement

AbstractThe petrostructural and geochronological study of a poorly known ultramafic unit from SW Spain (Badajoz–Córdoba belt) combined with previous structural data permits disclosure of a history of metasomatism, tectono‐metamorphism, reworking and isotopic resetting related to a poly‐orogenic evolution in different geodynamic scenarios. The heterogeneous ultramafic unit studied contains antigorite‐serpentinites and metasomatized ultramafic rocks (chlorite‐talc schists, tremolite‐talc‐chlorite rocks and magnesio‐hornblende‐chlorite rocks). Mantle‐wedge serpentinization was followed by Si and Al pre‐ to syn‐metamorphic/tectonic metasomatism in a subduction realm. Petrofabrics of selected lithologies reveal variable syn‐metamorphic crystal‐plastic deformation and recrystallization (assisted by other mechanisms) under relative high pressure, concomitant with the conditions recorded by neighbouring tectonic units that were later intruded by Ordovician granites. The resultant ensemble was reworked and isotopically reset much later in an intracontinental ductile shear zone. Syn‐ to late‐tectonic apatite from chlorite‐talc schists provides an anchored Tera–Wassenburg isochron radiometric age of 342.8 ± 12.2 Ma that provides evidence for the decoupling between isotopic systems and microstructures. The results are discussed from a twofold perspective: with regard to the likely tectonic context of this ophiolite (the current analogue of the Mariana forearc) and with regard to regional geological implications.

Magnetic on-time transient electromagnetic method (MoTEM) for imaging magnetic susceptibility

SUMMARY We present the magnetic on-time transient electromagnetic (MoTEM) method for obtaining information about subsurface magnetic susceptibility from transient electromagnetic (TEM) measurements. The method uses the primary field produced by a TEM system as the inducing magnetic field and extracts from the measured late on-time response the magnetic responses produced by the subsurface magnetic susceptibility. The method assumes that the transmitter waveform is long enough to allow for the dissipation of eddy currents until the electric conductivity effect becomes negligible towards the end of the on-time transmission and that the measured magnetic field or its time derivative is dominantly the response to magnetic susceptibility. Using numerical simulations, we study the dissipation of the field produced by the conductivity anomaly for different times of the transmitter waveform and demonstrate that the signal becomes dominated by the susceptibility anomaly (MoTEM signal) at late on-time for both moving and fixed-loop sources. We present observational evidence for the presence of MoTEM responses by showing that they can be identified in field data measured from a fixed-loop ground TEM survey over the ultramafic units in the Raglan nickel belt. Our study lays the foundation for the MoTEM method, demonstrates that it can be realized with existing TEM acquisition systems, and provides the guidance for new system development. The MoTEM response could enable the mapping and imaging of subsurface susceptibility from the airborne EM data and, therefore, has the potential to double the information content in such data by imaging both conductivity and magnetic susceptibility.

Opening and closure of Cadomian peri-Gondwanan oceans: age and evolution of the Mérida Ophiolite (SW Iberia)

ABSTRACT In the SW Iberian Massif, the Ossa-Morena Complex contains a stack of units of different origin and tectonothermal evolution. Individual terranes of the complex record a Cadomian history, traditionally interpreted in relation to the dynamics of a peri-Gondwanan volcanic arc. The interpretation of the Cadomian terranes is a key issue in the reconstruction of the Ediacaran margin of Gondwana in NW Africa. In the Mérida Massif, a mafic – ultramafic unit stacked between two units of continental or arc affinity is interpreted as a Cadomian ophiolite, the Mérida Ophiolite. Within it, mafic lithologies dated at c. 596 Ma (U-Pb in zircon) have calc-alkaline or tholeiitic affinity, indicating generation in a supra-subduction zone setting. The ophiolite shows a main event of Cadomian metamorphism (c. 590 Ma, U-Pb garnet dating) with a prograde P-T path and peak conditions at c. 700°C and 8 kbar. The mafic rocks show Nd isotopic compositions clearly different from those characteristics of the mafic rocks formed in the axial zone of the volcanic arc. Hence, the Mérida Ophiolite probably represents oceanic lithosphere of a back-arc basin, opened between the volcanic arc and the outer margin of Gondwana. Closure of the back-arc occurred during collision of the volcanic arc with the margin of Gondwana (starting at c. 590 Ma), which caused rapid accretion of the back-arc oceanic lithosphere beneath the arc and prograde metamorphism typical of subduction inception. The Mérida Ophiolite was finally obducted above the continental margin of Gondwana, presently preserved in gneissic unit piled beneath the ophiolite. Cadomian evolution of this peri-Gondwanan arc section continued with new tectonothermal events dated in the range 560–540 Ma that alternated with episodes of intense magmatic activity and preceded the collapse of the magmatic arc and its erosive levelling, followed by the generation of a new episode of intense magmatic activity in the arc and the unconformable deposition of the Ediacaran-Cambrian Malcocinado Formation.

Tectonic setting and mineralisation potential of the Cowley Ophiolite Complex, north Queensland

Northeast Queensland contains multiple slices of mafic–ultramafic units, strung out along regional faults that mark major tectonic boundaries. One such complex is the Cowley Ophiolite Complex, which is situated along the Russell-Mulgrave Fault. The Cowley Ophiolite Complex is a differentiated mafic–ultramafic complex composed of gabbro, chlorite schist, anthophyllite schist and serpentinite. We interpret that the alteration assemblages observed throughout the Cowley Ophiolite Complex reflect an amphibolite facies metasomatic overprint within a supra-subduction zone setting. This interpretation is consistent with geochemical discrimination of the gabbro, and chromite grains from the anthophyllite schist. The Cowley Ophiolite Complex records a higher metamorphic grade than the surrounding Hodgkinson Formation, and we interpret this to reflect the allochthonous structural setting of the complex, positioned along an ancient subduction margin. This subduction margin is represented today by the Russell-Mulgrave Fault. Metasomatism and emplacement of the complex probably pre-dated deposition of the Mossman Orogen’s active margin successions. Our interpretation of an active subduction complex, which pre-dated the formation of the Mossman Orogen, suggests that the Russell-Mulgrave Fault is a Paleozoic, continental suture zone. The Cowley Ophiolite Complex presents little indication of economic mineralisation; however, much of the complex remains unexplored. KEY POINTS The Cowley Ophiolite Complex is a differentiated, mafic–ultramafic complex formed within a supra-subduction zone setting. Emplacement of the Cowley Ophiolite Complex, along the Russell-Mulgrave Fault, pre-dated the formation of the Hodgkinson Province. The Russell-Mulgrave Fault represents an ancient subduction margin, and is interpreted as a Paleozoic, continental suture zone.

Downdip Development of the Ni-Cu-PGE-Bearing Mafic to Ultramafic Uitkomst Complex, Mpumalanga Province, South Africa

The about 2055-Ma-old mafic to ultramafic Uitkomst Complex in the Mpumalanga Province of South Africa hosts the low-grade-large-tonnage Ni-Cu-PGE deposit, Nkomati. The complex is regarded to represent a satellite to the Bushveld Complex and a feeder to an eroded magmatic reservoir in the southeast. Aeromagnetic surveys and previous drilling indicated an overall northwestern-downdip extension of the complex, but the question is to what extent and in which expression can the complete intrusion be found under cover in the northwest? Answering this, a mineralogical, geochemical and geochronological investigation of a borehole intersection of the whole complex at Little Mamre was carried out, using petrography, XRF, EPMA and LA-ICP-MS U–Pb analyses of zircons for age determination. Although the total thickness of the rock units is larger than to the southeast, emplacement, litho- and mineral chemistry trends, expression of alteration mineralogy and style of sulphide mineralisation are similar. The amount of sulphide mineralisation is on average less than in the southeast. The upper ultramafic unit contains, more frequently, pegmatoidal sections, and the Chromitiferous Harzburgite unit has less massive chromitite layers than the southeastern parts of the complex, whereas the gabbro(-norite) units contain more interstitial liquid with late-stage minerals. The findings confirm that the anvil-shaped intrusion in cross section continues with increased thickness towards northwest at a shallow dip; although approaching the deeper part of the igneous reservoir, mineral compositions are partially more evolved. The overall mineralogical consistency downdip supports a situation of multiple magma replenishment along a flat-lying, northwest–southeast trending conduit, resulting in an evolved cumulus mineral assemblage in the upper part.

Comparative Study on the Origin and Characteristics of Chinese (Manas) and Russian (East Sayan) Green Nephrites

Green nephrites are widely pursued for their mild texture and vivid color. In recent years, many Russian green nephrites appeared in China (the world’s largest nephrite market) and competed with the Chinese Manas green nephrites, which are traditionally highly valued. In this study, we compared the appearance, mineralogy and geochemical features (with EPMA and LA-ICP-MS) of Chinese (Manas) and Russian (East Sayan) green nephrites to objectively characterize and distinguish between these two nephrites. Chinese (Manas) and Russian (East Sayan) green nephrites are mined from serpentinized ultramafic units in the northern Tian Shan and East Sayan orogen, respectively. In terms of appearance, the Manas green nephrites are slightly bluish or grayish, whilst their East Sayan counterparts are brighter (duck-egg cyan). The Manas nephrites commonly have a caramel color, crumple structure, characteristic white globules and sinuous veins, green stains and yellow–green veins, together with a local fibrous structure. The East Sayan green nephrites are more transparent, with a gentler fine texture, uniform color, many black spots and a few green spots. Some green nephrites from the Arahushun mine of East Sayan have an ice-like appearance. Microscopic petrography and EPMA analysis indicate that both the Manas and East Sayan green nephrites comprise mainly tremolite with minor actinolite. Minor minerals in the Manas samples include chromite, chlorite-group minerals, and uvarovite; whilst those in the East Sayan samples include actinolite, chromite, chlorite-group minerals, and bornite. Bornite is not found in any other sources of green nephrite, and thus is characteristic of Russian (East Sayan) green nephrites. LA-ICP-MS trace element data in their amphiboles and Single-Factor Analysis of Variance (ANOVA) results suggest that the differences in Cr, Zn, Y, Ba, and Sr contents and values of δEu, Eu/Sm, (La/Yb)N, (La/Sm)N, (Gd/Yb)N, ∑HREE, ∑LREE/∑HREE between the two nephrites are present, and can be used as their origin trace.

Ferrotorryweiserite, Rh5Fe10S16, a New Mineral Species from the Sisim Placer Zone, Eastern Sayans, Russia, and the Torryweiserite–Ferrotorryweiserite Series

Ferrotorryweiserite, Rh5Fe10S16, occurs as small grains (≤20 µm) among droplet-like inclusions (up to 50 μm in diameter) of platinum-group minerals (PGM), in association with oberthürite or Rh-bearing pentlandite, laurite, and a Pt-Pd-Fe alloy (likely isoferroplatinum and Fe-Pd-enriched platinum), hosted by placer grains of Os-Ir alloy (≤0.5 mm) in the River Ko deposit. The latter is a part of the Sisim placer zone, which is likely derived from ultramafic units of the Lysanskiy layered complex, southern Krasnoyarskiy kray, Russia. The mineral is opaque, gray to brownish gray in reflected light, very weakly bireflectant, not pleochroic to weakly pleochroic (grayish to light brown tints), and weakly anisotropic. The calculated density is 5.93 g·cm–3. Mean results (and ranges) of four WDS analyses are: Ir 18.68 (15.55–21.96), Rh 18.34 (16.32–20.32), Pt 0.64 (0.19–1.14), Ru 0.03 (0.00–0.13), Os 0.07 (0.02–0.17), Fe 14.14 (13.63–14.64), Ni 13.63 (12.58–14.66), Cu 4.97 (3.42–6.41), Co 0.09 (0.07–0.11), S 29.06 (28.48–29.44), and total 99.66 wt.%. They correspond to the following formula calculated for a total of 31 atoms per formula unit: (Rh3.16Ir1.72Pt0.06Ru0.01Os0.01)Σ4.95(Fe4.48Ni4.11Cu1.38Co0.03)Σ10.00S16.05. The results of synchrotron micro-Laue diffraction studies indicate that ferrotorryweiserite is trigonal; its probable space group is R3¯m (#166) based on its Ni-analog, torryweiserite. The unit-cell parameters refined from 177 reflections are a = 7.069 (2) Å, c = 34.286 (11) Å, V = 1484 (1) Å3, and Z = 3. The c:a ratio is 4.8502. The strongest eight peaks in the X-ray diffraction pattern derived from results of micro-Laue diffraction study [d in Å(hkil)(I)] are 2.7950 (202¯5) (100); 5.7143 (0006) (60); 1.7671 (224¯0) (44.4); 3.0486 (202¯1) (39.4); 5.7650 (101¯2) (38.6); 2.5956 (202¯7) (37.8); 3.0058 (112¯6) (36.5); and 1.5029 (42¯ 2¯12) (35.3). Ferrotorryweiserite and the associated PGM crystallized from microvolumes of residual melt at late stages of crystallization of grains of Os- and Ir-dominant alloys occurred in lode zones of chromitites of the Lysanskiy layered complex. In a particular case, the residual melt is disposed peripherally around a core containing a disequilibrium association of magnesian olivine (Fo72.9–75.6) and albite (Ab81.6–86.4), with the development of skeletal crystals of titaniferous augite: Wo40.8–43.2En26.5–29.3Fs20.3–22.6Aeg6.9–9.5 (2.82–3.12 wt.% TiO2). Ferrotorryweiserite represents the Fe-dominant analog of torryweiserite. We also report occurrences of ferrotorryweiserite in the Marathon deposit, Coldwell Complex, Ontario, Canada, and infer the existence of the torryweiserite–ferrotorryweiserite solid solution in other deposits and complexes.

High temperature fluid-rock interaction recorded in a serpentinized wehrlite from eastern Singhbhum Craton, India: Evidence from mineralogy, geochemistry and in situ trace elements of clinopyroxene

A N-S trending, lenticular, and dismembered ultramafic unit is found emplaced within the multiply deformed metapelites of the North Singhbhum Mobile Belt, belonging to the Singhbhum Craton, eastern India. Their occurrence in an orogenic belt, lack of evidences of intrusive activity and close proximity to the Singhbhum Shear Zone suggest emplacement by thrust control. These rocks have experienced extensive serpentinization which has obscured and obliterated some of the features of the protolith. Relict areas of the Bangriposi wehrlite rocks indicate that the protolith might have been a cumulate clinopyroxene-dunite/wehrlite from an oceanic magma chamber. Detailed petrography and mineralogy reveal numerous metasomatic features which are separate from the serpentinization event: (i)Very high Ni-content of the relict olivines which does not correlate with Fo contents, (ii) Presence of secondary clinopyroxene (Cpx2) rims around primary olivine and replacing primary clinopyroxene (Cpx1), which have major and trace element compositions similar to clinopyroxenes from diopsidites of crustal origin (iii) Occurrence of secondary calcic hornblendes as tiny blebs within the Cpx2 rims (iv) Occurrence of secondary Mg-chlorite as interstitial patches between olivine rimmed by Cpx2 and inter-mixed with antigorite in the matrix, (v) Modifications of original Cr-spinels to ferri-chromite cores rimmed by magnetite. These features indicate these rocks have experienced fluid-rock interaction at temperatures above the serpentinization process. Primary Cpx1 grains exhibit elevated LREE compositions and can either reflect effects of cryptic metasomatism or crystallization from an enriched melt. Both groups of clinopyroxenes exhibit LILE enrichment and HFSE depletion. P-T estimates from Cpx1 and Cpx2 yield values of ~1200 °C and 0.5–1.2 GPa. The calcic hornblendes provide more realistic estimates of ~1000 °C and ~ 1.0 GPa, which suggest that the Cpx2 + hornblende assemblage formed during high temperature fluid-rock interaction. Presence of interstitial chlorite, chlorite+antigorite in the matrix and Cr-spinel recrystallization suggests successively decreasing temperatures during the fluid-rock interaction episode. Modeled trace element characteristics of fluids which are supposed to be in equilibrium with Cpx2 show similarities with high-temperature, ore-forming basinal brines. Similar basinal brines formed Cu-U ores in the Singhbhum Shear Zone and was likely the metasomatic agent of the Bangriposi wehrlite rocks. These rocks were originally cumulate ultramafic units from an oceanic crustal magma chamber, which experienced prolonged fluid-rock interaction in a retrogressive regime, until its emplacement as a dismembered unit in an orogenic belt.

Spatial distribution of ultrahigh-temperature granulites of the Highland Complex of Sri Lanka: Lowermost continental crust above an ultrahot palaeo-Moho

The spatial distribution of isolated ultra-high-temperature (UHT) granulites in Sri Lanka corresponds to the exposure level of the deepest granulites of the Highland Complex (HC). Their spatial distribution, and that of ultramafic bodies in the HC, defines a relatively thin (several km) lowermost crustal layer that has been folded into a N-plunging asymmetric fold (F3 in our scheme), Z-shaped looking N. We postulate that the UHT granulites represent suitable lithologies within this lowermost crustal layer above an ultrahot palaeo-Moho. Prior to D3, this layer had already been imbricated and intensely folded with the HT granulites and with ultramafic units after peak metamorphism, leading to a thick package of HT granulites with isolated UHT assemblages. This concept has some important implications for granulite terrains in general: (i) UHT granulites elsewhere may also represent a thin zone above an ultrahot palaeo-Moho and overlain by HT granulite terrains, and their spatial relationships may have been complicated by ductile deformation during early cooling; (ii) many other HT granulite terrains may be underlain by a similar zone of UHT granulites, and, if so, new discoveries of UHT terrains may be expected.

Petrogenesis and Ni-Cu exploration potential of Devonian mafic–ultramafic intrusions in the southern part of the Central Asian Orogenic Belt, NW China: constraints from zircon O isotopes and whole-rock Sr-Nd isotopes

ABSTRACT Weakly disseminated sulphide mineralization is present in the outcrops of several mafic–ultramafic intrusions in the Beishan region of the Central Asian Orogenic Belt in western China, but the potential of these intrusions to host economic sulphide ores at depths is not known. A better understanding of the genetic control on the mineralization may help to answer this important question. With this in mind, we have selected two of them that were emplaced during the Devonian, a period when subduction-related mafic magmatism undoubtedly occurred in the region, for a geochemical study. They are the Miaomiaojing and Guaishishan mafic–ultramafic intrusions. Our new zircon U-Pb dating indicates that they were emplaced at 377.5 ± 5.3 and 358.6 ± 3.9 Ma, respectively. Lherzolite, olivine websterite, troctolite and gabbro are the main rock types for both intrusions. Disseminated sulphide mineralization is concentrated in the ultramafic units of both intrusions. The whole-rock samples are characterized by light REE enrichments, pronounced negative Nb-Ta anomalies, and variable εNd(t) values from −3.54 to +2.17. Zircons from the intrusions have elevated δ18O values (average value: 7.48 ± 0.09 ‰) compared to that of the mantle. The trace element and isotope data support the interpretation that the parental magmas for the Devonian mafic–ultramafic intrusions are contaminated arc mafic magmas. Petrological modelling reveals that fractional crystallization together with crustal assimilation may have played an important role in triggering the sulphide saturation in the magmas. The Ni tenors (in recalculated 100% sulphide) for the sulphide-mineralized samples from both intrusions are similar and up to 6.7 wt% Ni and 2.8 wt% Cu, comparable to the metal tenor of most economic Ni-Cu deposits in the CAOB, implying that samples containing higher amounts of sulphides would become Ni-Cu ores. The olivine Fo and Ni contents from sulphide-bearing ultramafic rocks show a negative correlation, consistent with sub-solid Fe-Ni exchange between olivine and sulphide melt. Together with the similar mineralization characteristics between these sulphide-bearing intrusions and the coeval Heishan Ni-Cu deposit, we suggest that these intrusions have the potential to host Ni-Cu sulphide ores at depths, which could be further tested by geophysical survey followed by drilling.

Geology of Ali Khanzai Block of Zhob Ophiolite, Balochistan, Pakistan

Abstract The Zhob Ophiolite is divided into Naweoba, Omzha and Ali Khanzai blocks. Ali Khanzai Block is further divided into ultramafic, mafic, and lava units which are surrounded by sedimentary rocks successions. The ultramafic unit contains ultramafic tectonic and ultramafic cumulate, mafic rock unit consists of foliated and layered gabbros and mafic dykes are doleritic in composition. Volcanic–volcaniclastic–pelagic rocks unit consists of thick volcanic pillow basalt, hyaloclastite, bedded chert, pelagic limestone and hemipelagic mudstone. The metamorphic sole rocks are tectonically high distorted and dismembered, comprising of amphibolite and greenschist facies. They might have formed through the process of early intra-oceanic obduction of the ophiolite. All units make thrusted contacts and are highly deformed. Petrography and geochemical studies divide the Ali Khanzai Block into rock types such as gabbro, olivine gabbro, dolerite, basalt, basaltic andesite and basaltic trachy-andesite, chert, mudstone, and limestone, dunite, serpentinite, harzburgite, and wehrlite. Mafic dyke intrusions crosscut mantle rocks of block. The mantle rocks are altered, deformed, and deeply weathered, maybe residual melting of enriched mantle peridotite. The contact relationship of dolerite dykes with peridotite indicta that they are late magmatic intrusions. The Crustal gabbros are partially chloritic and sericitic and occur within mantle peridotite,, they may have formed from fractional crystallization in a magma chamber. The volcanic-volcaniclastic-pelagic sediments surround mantle and crustal rock units. It might be a mélange in nature is much like that of the Bagh Complex found beneath the Muslim Bagh Ophiolite, and other ophiolites around the world.

The geology, geochemistry, and petrogenesis of the Huckleberry Cu-Ni-PGE prospect in the Labrador Trough, Canada: Perspectives for regional prospectivity

The Palaeoproterozoic Huckleberry Cu-Ni-(PGE) prospect in the Labrador Trough, northern Québec, represents a ~ 400-m-thick, out-of-sequence sill complex that comprises a ~200-m-thick glomeroporphyritic gabbro, intruded in its centre by a ~200-m-thick differentiated gabbro-peridotite sill and in its gabbroic footwall, several thinner (<30 m) ultramafic sills. Globular sulphides are present at the base of the sill complex, whereas disseminated to net-textured sulphides occur in the ultramafic units (Cu/Ni = 0.1–0.8) as well as their gabbroic footwall (Cu/Ni = 1–3). The glomeroporphyritic gabbro sill stack (MgO ~4 wt%, TiO2 ~0.6 wt%, Na2O + K2O ~2–3%, Eu/Eu* ~ 1.2, Anplg ~70–60) comprises several sills characterised by sharp changes in size and abundance of plagioclase glomerocrysts. We hypothesise that the glomerocrysts represent remobilised crystal mushes that were dislodged from a floatation cumulate in a staging chamber during episodic expulsion of magma. In the central gabbro-peridotite sill, mineral compositions (Foolv ~75–75, Mg#opx ≈ 78–68, Anplg ≈ 78–70) and whole-rock data (MgO ≈ 22 – 27%, TiO2 ≈ 0.4%) suggest that the parent magma was an olivine-saturated basalt containing 8–9 wt% MgO. Whole-rock geochemical data further suggests that the parent magmas did not undergo any significant contamination (La/SmN < 2, S/Se < 4000). The Cu/Pd values of ultramafic units (>10,000) suggest sulphide melt saturation was attained before their final emplacement in magma feeder conduits or staging chambers at relatively low R factors (1000–5000). Downward decreasing concentrations of chalcophile elements in the drill cores suggests that sulphide melt percolated downward from the ultramafic cumulate units into the glomeroporphyritic gabbro footwall. We propose that the footwall ultramafic sills represent downward injections of olivine and sulphide melt from the overlying gabbro-peridotite sill, which mechanically concentrated high volumes of sulphide in narrow sills.