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Gold-platinum mineralization was discovered in ultramafic–mafic rocks of the Ariadne massif of the Sikhote-Alin orogenic belt. In this article, we show that ultramafic (peridotite and pyroxenite) and mafic (ilmenite and hornblende gabbro) rocks form a single series with gradual accumulation of rare and rare earth elements of more differentiated varieties. New age determinations for zircon from ilmenite gabbro are given (164 ± 0.48 Ma). We distinguished four varieties of gold differing in the Au, Ag, Cu, and Hg contents. Platinum-bearing minerals are isoferroplatinum and cuprite. The isotopic composition of sulfides from ultramafic rocks suggests a mantle source, while sulfides from ilmenite gabbros are characterized by isotopically light sulfur, indicating the possible involvement of crustal rocks in ore genesis. Carbon isotope analysis of the carbon-bearing matter of mafic–ultramafic rocks showed the presence of mantle fluids and isotopically light biogenic matter from sedimentary rocks. The data indicate the influence of mantle and crustal processes on the formation of ore-bearing mafic and ultramafic intrusions. [ABSTRACT FROM AUTHOR]

A simple method was developed for the determination of REEs, Zr, Hf, Th and U in ultramafic rocks by inductively coupled plasma‐mass spectrometry with a combination of RE and UTEVA extraction resins for their separation and pre‐concentration. Ultramafic rocks were digested with HNO3‐HF‐HClO4 and finally turned into 11 mol l−1 HCl solutions together with H3BO3 to remove insoluble fluorides. The removal of matrix elements was achieved during the loading procedure. Following this, REEs on RE resin, and Zr, Hf, Th and U on UTEVA resin were eluted with 10 ml of 0.24 mol l−1 HCl, with recoveries better than 94.4%. This method was validated using reference materials JP‐1, DTS‐2B, OKUM, UB‐N, MUH‐1 and DZΣ‐2, and the measurement results for target analytes were comparable to literature values, indicating its applicability to the determination of REEs, Zr, Hf, Th and U at ultra‐trace level in ultramafic rocks. [ABSTRACT FROM AUTHOR]

Serpentinization is an important geochemical process producing natural hydrogen (H 2), and mafic and ultramafic serpentinized rocks, typically occurring in ophiolites and peridotite massifs, are major targets for H 2 exploration. Understanding the volumes of H 2 stored in these rocks is a key step towards establishing the potential of sepentinized systems to host H 2 sources and reservoirs. Nonetheless, published data on the concentration of H 2 directly extracted from serpentinized rocks are extremely scarce. This work provides a first dataset of H 2 extracted by planetary ball mill from 58 rock samples from different ophiolites in Greece, peridotites of the Tekirova ophiolite in Turkey, and chromitites and surrounding talc schist from an Archean-Paleoproterozoic greenstone belt in Brazil. The highest H 2 concentrations, from 0.1 mL/kg (considered as a threshold above which artificial H 2 by milling is negligible) to 16.2 mL/kg, were found in serpentinized harzburgites or dunites, and in chromitites hosted within serpentinized peridotites. The H 2 content of other mafic rocks that are not in contact or influenced by a serpentinized system, mostly basalts, is below 0.1 mL/kg. Serpentinized peridotites represent the logical source rocks of H 2 (i.e., hydrogen in peridotites is generally autochtonous) and chromitites, lacking significant olivine and serpentinized minerals, may host H 2 derived by the surrounding serpentinized rocks (hydrogen in chromitites is mostly allochtonous). The H 2 dataset of this work can be used as a reference for further studies on the capacity of ophiolitic rocks to produce and host natural hydrogen. • First dataset on H 2 concentration in mL/kg in different ophiolitic rocks. • Serpentinized harzburgites/dunites and chromitites host the highest H 2 concentrations. • Hypothesis of autochtonous hydrogen in peridotites, allochtonous hydrogen in chromitites. [ABSTRACT FROM AUTHOR]

The global transition to a low-carbon energy future has ignited interest in hydrogen as a clean and versatile energy carrier. NW Saudi Arabia contains vast tracts of mantle-dominated Neoproterozoic ophiolites overlain by a sedimentary cover in the north, NE and east. Our research explores the untapped potential of Saudi Arabia's ophiolite serpentinites as a possible source of hydrogen generation through serpentinization and related processes. H 2 is commonly derived from serpentinization, which primarily involves the hydration of minerals rich in iron and magnesium (e.g., olivine), resulting in the formation of serpentine and H 2. Serpentinites are formed by reaction between fluids and mantle rocks, and hydrogen and methane are released in extreme reducing conditions. Under strongly reducing conditions, the formation of magnetite generates hydrogen during serpentinization, following reactions such as: 3Fe 2 SiO 4 + 2H 2 O = 2Fe 3 O 4 +3SiO 2 +2H 2. (1) We have conducted a comprehensive assessment of serpentinites in the Arabian Shield, employing the Geographic Information System (GIS) to locate Arabian ophiolites with the largest ultramafic segments. Ophiolites studied include Jabal Ess, Jabal Wask, Bi'r Tululah, and Jabal Tharwah. After ophiolite identification, we delineated fractures and measured fracture density to provide insight into the potential of serpentinite fractures on the amount of possible hydrogen seepage, or natural accumulation in case of an overlying seal. Our goal is to do spatial analysis that can be a guide to the most favorable environment for geologic H 2 exploration; as such, this does not however, provide an estimate of the total H 2 storage capacity. A complimentary H 2 anomaly survey however, will ground-truth the results of this study. • The Arabian Nubian Shield is comprised of numerous serpentinite-dominated ophiolites. • The possibility of natural hydrogen accumulations within these ophiolite massifs are indicated by the permeable zones of fracture density maps. • The shield is overlain by a sedimentary cover in the east which may act as an effective seal. [ABSTRACT FROM AUTHOR]

The findings of platinum group metal mineralization (PGM) and the distribution of platinum group elements (PGE) in lherzolites of the Northern Kraka massif are described. The total contents of PGE are approximately two orders of magnitude lower than those in chondrite and are close to pyrolite, relative to which the studied lherzolites are enriched in Pd and depleted in Ru. In segregations of PGMs, the presence of all PGEs (except rhodium) in various proportions was established. All found grains are divided into three contrasting types: the refractory triad Os–Ir–Ru, essentially platinum with the participation of Pd, and Cu–Pd. Almost all found PGM grains are localized either in the peripheral parts of grains of altered sulfides (heazlewoodite, pentlandite) or in the silicate matrix in the immediate vicinity of sulfide segregations. Based on the mineral associations and PGE distribution, a probable genesis of segregations has been suggested. Associations of Cu–Pd and Pd–Pt(+Cu) composition most likely formed during the crystallization of sulfides from the extracted partial melts. This is indicated by their close association with clinopyroxenes and the presence of relatively fusible platinoids and copper. The associations of Pt–Ir and Os–Ir–Ru(+Pt) composition are most likely restitic, formed in place of primary mantle sulfides as a result of extraction of more fusible elements and further desulphurization. The isolation of platinoids as their own mineral phases is associated with the influence of superimposed low-temperature processes – subsolidus redistribution during cooling and subsequent serpentinization.

Data on the morphology, composition, textural and structural features of chromite deposits of the Ufaley ultramafic massif are presented. The mineralogical and compositional features of the host ultramafic rocks allow us to interpret them as depleted restite from partial melting of mantle peridotites. Relatively wide variations in the composition of ore-forming chromian spinel grains (#Cr 0.6–0.8) and noticeable metamorphism of disseminated ores with replacement of chromite by Cr-magnetite are noted. It is assumed that chromitite bodies were initially formed under the conditions of the upper mantle by a rheomorphic mechanism, and then their structural and geochemical transformation took place in the collisional setting of the upper part of the crust. Flattened bodies of disseminated chromitites have been preserved near competent gabbroid blocks, while other deposits have been transformed into lenses and podiform bodies of densely disseminated and massive ores of smaller size. The “cold tectonics” of the crustal stage led to the disintegration of deposits and the simultaneous local enrichment of deformed chromitite bodies.

In the paper data of morphology, textural and structural features of chromitites from deposits of southeast part of Kempirsay massif (South Urals, Kazakhstan) are summarized. It is showed that formation of unique chromium deposits is closely related with formation processes of wall dunite-harzburgite association and that chromitite localization occur abidingly in olivine monomineralic rock – dunite. Superimposed low-T processes altered primary mineralogical composition of wall peridotites completely but these affected weakly their structure on the micro and macro scale. Mesh serpentine replaced olivine and pyroxene grains but pseudomorphosis of both are survive. Addition, significant displacements of mineral aggregates in the massive peridotite blocks are not observed and it allow to study textural and structural characteristic of chromitites and primary wall ultramafic rocks. We have found some major features of building of ore-bearing associations as follow: (i) increasing chromite grain size according to increasing concentration of chromite, (ii) widespread of deformational structures – ore folding and boudinage, extrusion of solid dunite into massive chromitite, break of ore veinlets. We have performed retrospective analysis of papers about Kempirsay chromitite which in present day are not available for wide readers. Based on this analysis and our observations, we propose a modified dynamic model of chromitite formation as result rheomorphic differentiation of upper mantle matter during its upwelling from deep zone of rift structure with later transformation in the upper mantle of fore-arc setting.

The Seiland Igneous Province (SIP) is a large province of mafic and ultramafic (UM) complexes interpreted to be relics of a giant plumbing system feeding the Ediacaran Central Iapetus Magmatic Province. The Reinfjord Ultramafic Complex (RUC) is one of the four major ultramafic complexes of the SIP. The RUC has a younger dunite core surrounded by wehrlite and lherzolite embedded in country rocks consisting of layered gabbros with sub‐horizontal layering and metamorphosed sedimentary rocks. Here, we develop a 3D subsurface model for the RUC using high‐resolution magnetic and gravity data and extensive petrophysical measurements from oriented surface samples and drill core samples. Our model indicates that the RUC narrows in depth, extending a minimum of 1.4 km below sea level, and plunges eastwards below the country rock. This model allows us to decipher the lithologic heterogeneities, and the depth and lateral extent of ultramafic rocks, which we interpret in the context of the geologic history of the area. The RUC is spatially separated from other UM complexes of the SIP and the result of this study indicates a smaller depth extent. Combining these findings with the previously reported distribution of the SIP rocks based on the regional gravity data, we propose that the uplift of the crustal block hosting the RUC is larger than for ultramafic complexes in the northwestern part of the SIP. Plain Language Summary: Worldwide, mafic and ultramafic intrusions are considered potential hosts for economic Ni‐Cu and platinum group elements. Magma‐conduit systems are particularly interesting for such mineralization, because they provide an ideal environment for the formation of large economic sulfide occurrences such as the Norilsk Talnakh deposits. We investigate one of the ultramafic complexes of a large magmatic province considered to represent a deep root zone where transient melts passed en route to upper crustal levels. The RUC is an ideal magma‐conduit system for research of large igneous provinces. Here, we construct a subsurface model of the ultramafic complex using gravity and geomagnetic field data constrained by petrophysical data sets from rock samples. We identified the different mafic and ultramafic units, and the alteration of rocks linked to later tectonic events. The localization and the estimation of relative volumes of altered and unaltered mafic and ultramafic rocks provide information about the geodynamic history and the ore potential of these rocks. Key Points: The modeled structure of the ultramafic complex resulted in a minimum volume of 6.5 km3 and an eastward plunging root with depth of 1.4 kmModeling results suggest localized serpentinization of ultramafic rocks with shallow to negative natural remanent magnetization (NRM) inclination and high Königsberger ratioA new proposed uplift history for the Seiland Igneous Province indicating the southern part as a deeper section of the continental crust [ABSTRACT FROM AUTHOR]

The redox state of chromium ores of the main industrially significant types, developed in ultramafic rocks of the Rai-Iz–Voykar complex of the Polar Urals, was studied. The chromitites occurring in various geological settings—dunite–harzburgite complex and large dunite bodies metamorphosed to different degrees—have been investigated. For the first time, oxygen fugacity and temperature of the formation of chromium ores, based on olivine–spinel equilibrium, were studied in more than representative samples from the Rai-Iz and Voikar–Synya massifs. The Fe/(Fe + Mg) of minerals increases linearly from chromitites to their host ultramafic rocks at each of the studied objects. The temperature of olivine–spinel equilibrium in chromitites varies within 550–800°С. The oxygen fugacity in aluminous chromitites averages FMQ +0.5–1.5 log units; in medium chromium, FMQ +1.5–2.5 log units; in high-chromium ones, more than +2.5 log units relative to the FMQ buffer. The oxygen fugacity in densely disseminated chromitites is 0.5–1 log units higher than in poorly disseminated and rarely disseminated ones. The T–fO2 values correspond to the crustal conditions and are close to those established in the metaultramafic rocks of the studied massifs. The data obtained may indicate that the concentration of the ore component and the formation of chromium ore deposits occur not only in mantle or lower crustal conditions, characterized by fO2 values close to the FMQ buffer, but also as a result of ultramafic crustal metamorphism occurring in an oxidizing environment. [ABSTRACT FROM AUTHOR]

Metasomatic zircons in ultramafic rocks are crucial in tracking orogenic processes. Metasomatic zircons are conceived to form by local zircon saturation resulting from effective Zr transport during metamorphic events and incomplete Zr accommodation in mantle minerals. However, the accommodation capacity of Zr in mantle minerals remains unknown, hampering our understanding of zircon genesis in ultramafic rocks. Here we performed experiments at 2.0−6.0 GPa and 1050−1200 °C and determined Zr content (solubility) in the mantle minerals at zircon saturation (ZCZS). The results show that ZCZS values for the main mantle minerals are olivine (ol, 3.68 ± 0.27 ppm)

Lacustrine strata-bound magnesite deposits associated with Alpine-type ultramafic rocks are hydrothermal in origin. The magnesite ores of the Kamado deposit are unconformably underlain by mid-Jurassic marine carbonate and ultramafic rocks of the Bangong-Nujiang ophiolite suite and are in fault contact with hanging wall rocks composed of siliceous sinter. Three types of cryptocrystalline magnesite ores can be identified in Kamado: (1) strata-bound massive magnesites, representing the main ore type in the upper part; (2) banded ores in the lower part; and (3) some vein and stockwork ore in the ultramafic wall rocks. Integrated scanning electron microscopy, C–O isotope analysis, and geochemical analyses were carried out on the Kamado deposit. The results indicate that: (1) the orebody is composed of magnesite, with accessory minerals of aragonite, opal, and chromite; (2) the siliceous sinter and relatively high B (32.0–68.1 ppm) and Li (14.7–23.4 ppm) contents of the magnesite ores reflect long-term spring activity in Kamado; (3) the light carbon (δ13CV-PDB: −4.7 ± 0.3‰ to −4.1 ± 0.6‰) and oxygen isotopic compositions (δ18OV-SMOW: +12.3 ± 0.3 to +16.3 ± 0.1‰) of the stockwork ores in the foot wall rocks indicated that the carbon in fractures in the ultramafic rocks is from a mixture of marine carbonate and oxidized organic-rich sedimentary rocks, reflecting a typical "Kraubath-type" magnesite deposit; and (4) the relatively heavy carbon isotopic (δ13CV-PDB: +8.7 ± 0.4‰ to +8.8 ± 0.3‰) composition of the banded magnesite ores in the lower segment may have formed from heavy CO2 generated by anaerobic fermentation in the lakebed. Additionally, the carbon isotopic (δ13CV-PDB: +7.3 ± 0.3‰ to +7.7 ± 0.7‰) composition of the massive magnesite ores in the upper segment indicates a decline in the participation of anaerobic fermentation. As this economically valuable deposit is of the strata-bound massive ore type, Kamado can be classified as a lacustrine hydrothermal-sedimentary magnesite deposit, formed by continuous spring activities under salt lakes on the Tibetan Plateau, with the Mg mainly being contributed by nearby ultramafic rocks and the carbon mainly being sourced from atmosphere-lake water exchange, with minor amounts from marine carbonate strata. [ABSTRACT FROM AUTHOR]

The Bundelkhand craton in India preserves important records of archean geological evolution, where several ultramafic rocks belonging to the Babina Greenstone Belt (BGB) occur as isolated and oval shaped bodies. These rocks are composed of olivine, orthopyroxene, amphiboles, and serpentine along with accessory mineral phases like chromian spinel and ilmenite. Here we present the major and trace element geochemistry of these ultramafic rocks that are characterised by low SiO2 (45.16–49.00 wt%), high MgO (24.41–29.15 wt%) and moderate Fe2O3 (5.82–9.95 wt%) with high Ni (1164–1674 ppm), Cr (1532–3477 ppm) and Cu (14.7–39.5 ppm) suggesting primary magmatic nature. The rocks show low rare earth element (REE) content (ΣREE 2.1–3.5 ppm) with depleted LREE pattern and flat to slightly fractionated HREE pattern similar to abyssal peridotite signature. The Nb/Yb ratio ranges between 0.01 to 0.20 (average = 0.03), similar to that of N-MORB, suggesting magma derivation from a depleted mantle source, further substantiated by the Th/Yb vs. Ta/Yb plot. Trace elements like Ta and Pb show positive spikes, whereas La, Nb, Pr and Ce show depleted nature. The rocks generally have low platinum group elements (PGE) content (

The heterogenous mineralogy of ultramafic deposits hosting mining operations makes it challenging to accurately determine the waste rock's mineral carbonation potential (MCP). Additionally, the significantly higher carbonation capabilities of olivine than serpentine add to the difficulty. To address this issue, in this work, a new and unique tool called the MCP calculator was developed as a Microsoft ExcelTM spreadsheet to accurately determine the amount of anthropogenic CO2 that a given rock mass can sequester through mineral carbonation. The program estimates the modal mineral abundance of ultramafic rocks to aid in MCP estimation. This tool is designed to be cost-effective and tailored for use by the mining industry, utilising abundant lithogeochemical data to evaluate their deposit as a potential substrate for industrial mineral carbonation operations. The paper introduces the MCP calculator, outlines a framework for developing the MCP parameter, and presents an example of its application. The calculator is specific to the mineral assemblage investigated at the Turnagain ultramafic complex in northern British Columbia but can be adjusted to study comparable deposits. The paper acknowledges that using waste rock in a mineral carbonation operation requires economic and practical decisions beyond the scope of the research. [ABSTRACT FROM AUTHOR]

Ultramafic rocks of the Yarlung-Tsangpo ophiolites (lherzolites, harzburgites, dunites and pyroxenites) have been interpreted as lithospheric mantle relics of the Cretaceous Neo-Tethys Ocean (NTO). Studying these rocks could provide key constraints on the NTO's evolution and invaluable information on sub-ridge mantle heterogeneity. The Xiugugabu ophiolite crops out in the western segment of the Yarlung-Tsangpo Suture Zone (YTSZ) and is dominantly composed of mantle components with minor crustal material. Here we report a comprehensive field, petrological and geochemical study of the ultramafic rocks from this ophiolitic massif. The lherzolites and harzburgites are less refractory than the Xigaze ultramafic rocks in the middle part of the YTSZ. Petrology and simple models are consistent with 10–20% fractional melting from an anhydrous, depleted mantle source, within the spinel and garnet stability fields. Part of dunites (Dunite-II) and pyroxenites formed during pervasive melt impregnation into host peridotites and record the modal and geochemical effects of post-melting melt–rock interactions. The discrepancy of light rare earth element (LREE) contents between whole rocks and their two pyroxene components in peridotites indicates that they have trapped interstitial melts along grain boundaries at a post-melting stage. These melts were potentially derived from shallow sources in spinel domains. Bulk rock osmium isotopic compositions suggest that the pyroxenites and part of the dunites are highly radiogenic. These enriched Os signatures are likely imposed by percolating melts derived from mantle sources hybridized by ancient recycled crustal materials. When comparing with results from previous studies of other YTSZ peridotites with ultra-depleted nature and unradiogenic Os isotopic compositions, we conclude that the sub-ridge mantle beneath the Neo-Tethyan Ocean was heterogeneous, with not only unradiogenic but also highly radiogenic mantle domains. [ABSTRACT FROM AUTHOR]

Platinum group minerals (PGMs) belonging to laurite (RuS2) - erlichmanite (OsS2) solid solution series and Os-Ir-Ru alloys generally occur as minute inclusions (<10 μm) in both stratiform and podiform chromitite deposits. In a recent study of chromitite samples from Bangur chromite deposit in the Boula-Nuasahi ultramafic complex, Odisha, India, we found a relatively coarsegrained (~30 μm) laurite crystal in addition to other PGMs. This laurite occurs as an included crystal within a ferritchromite grain and has a chemical composition (wt%): Ru ~ 43-45, Os ~ 12-15, S ~ 35, Ir ~ 2-3 and As ~ 0.2, as measured by electron probe micro-analyzer. Due to the high osmium content, it is named as Oslaurite. The mode of occurrence, texture, structure and chemical composition of laurite support a magmatic origin for this mineral that might have crystallized at a temperature slightly lower than 1200°C. The unusual and complex oscillatory zoning (i.e. Os-rich and Os-poor bands) observed within the laurite could have developed below 1200°C (but above 1000°C) in a late magmatic stage due to local fluctuations in the surrounding thermodynamic conditions, primarily the sulphur fugacity. [ABSTRACT FROM AUTHOR]

Dikes of Paleoproterozoic ultramafic rocks from the Kun-Manie area were studied within the southeastern framing of the Siberian Platform. At the end of the Paleoproterozoic, this area was spanned by the processes of overthrust faultings, the latest of which were accompanied by the emplacement of mafic-ultramafic dikes and sills that control the distribution of Cu–Ni–PGE sulfide mineralization. The petrographic texture of ultramafic dikes suggests their emplacement in tectonically active settings. The elevated concentrations of LREE relative to HREE coupled with other data allowed us to ascribe them to high-Mg ultramafic rocks of the picrite–tholeiite series, which were derived from picrobasaltic rather than from ultramafic magma. The REE level and normalized patterns in ultramafic rocks indicate the involvement of sufficiently enriched sources in the generation of primary melts, which produced rocks that radically differed from peridotites of ophiolite associations and typical komatiite melts. In terms of trace-element patterns and contents of indicator elements (Nb, Yb and Th), they are close to an E-MORB type "enriched" mantle source, but differ in the lower REE concentrations. The Zr/Y–Nb/Y relations in the ultramafic rocks suggest a nonplume source of their primary melts in the upper depleted mantle. Rocks derived from such a melt are characteristically enriched in fluid components (S, Cl, F, As, Te, Se, and H2O), which is evident in the presence of hydroxyl-rich minerals: amphiboles and phlogopite of igneous origin. The ore-forming system evolved from Ni-bearing at the early magmatic stage during the formation of disseminated mineralization to Cu-rich at the postmagmatic stage. The geological data indicate that it was formed during the collision between the Paleoproterozoic island arc and the microcontinent. The combination of subduction and intraplate geochemical characteristics in magmatic rocks is characteristic of a transform margin as a result of the oceanic (Upper Maya) and continental (Uchur) microplates sliding horizontally relative to each other. [ABSTRACT FROM AUTHOR]

The composition and age of subalkaline dolerites, localized in chromite-bearing ultramafic rocks of the Rai-Iz massif, were studied. Dolerites are traced along the tectonic zones inside ultramafic rocks and are not observed beyond their boundaries, spatially tending to deposits and occurrences of chromite ores. The studied dolerites are characterized by high concentrations of alkalis, TiO2, P2O5, enrichment in large-ion lithophilic elements and Th; the amount of REE varies from 103.3 to 164.2 ppm; (La/Sm)n = 1.3–1.7. The age of the subalkaline dolerites was determined by the method 40Ar/39Ar (402.7 ± 3.8 Ma). At this time, ultramafic rocks of most of the Rai-Iz massif were metamorphically processed with the formation of high-chromium chromitites and with the isolation of a vein series of subalkaline dolerites in the discharge zones. [ABSTRACT FROM AUTHOR]

Lacustrine strata-bound magnesite deposits associated with Alpine-type ultramafic rocks are hydrothermal in origin. The magnesite ores of the Kamado deposit are unconformably underlain by mid-Jurassic marine carbonate and ultramafic rocks of the Bangong-Nujiang ophiolite suite and are in fault contact with hanging wall rocks composed of siliceous sinter. Three types of cryptocrystalline magnesite ores can be identified in Kamado: (1) strata-bound massive magnesites, representing the main ore type in the upper part; (2) banded ores in the lower part; and (3) some vein and stockwork ore in the ultramafic wall rocks. Integrated scanning electron microscopy, C–O isotope analysis, and geochemical analyses were carried out on the Kamado deposit. The results indicate that: (1) the orebody is composed of magnesite, with accessory minerals of aragonite, opal, and chromite; (2) the siliceous sinter and relatively high B (32.0–68.1 ppm) and Li (14.7–23.4 ppm) contents of the magnesite ores reflect long-term spring activity in Kamado; (3) the light carbon (δ13CV-PDB: −4.7 ± 0.3‰ to −4.1 ± 0.6‰) and oxygen isotopic compositions (δ18OV-SMOW: +12.3 ± 0.3 to +16.3 ± 0.1‰) of the stockwork ores in the foot wall rocks indicated that the carbon in fractures in the ultramafic rocks is from a mixture of marine carbonate and oxidized organic-rich sedimentary rocks, reflecting a typical “Kraubath-type” magnesite deposit; and (4) the relatively heavy carbon isotopic (δ13CV-PDB: +8.7 ± 0.4‰ to +8.8 ± 0.3‰) composition of the banded magnesite ores in the lower segment may have formed from heavy CO2 generated by anaerobic fermentation in the lakebed. Additionally, the carbon isotopic (δ13CV-PDB: +7.3 ± 0.3‰ to +7.7 ± 0.7‰) composition of the massive magnesite ores in the upper segment indicates a decline in the participation of anaerobic fermentation. As this economically valuable deposit is of the strata-bound massive ore type, Kamado can be classified as a lacustrine hydrothermal-sedimentary magnesite deposit, formed by continuous spring activities under salt lakes on the Tibetan Plateau, with the Mg mainly being contributed by nearby ultramafic rocks and the carbon mainly being sourced from atmosphere-lake water exchange, with minor amounts from marine carbonate strata.

Abstract: We describe the new species ognitite, NiBiTe, and a Co-rich variety of maucherite, hitherto unreported; both were discovered in the Ognit ultramafic complex of Neoproterozoic age in Eastern Sayans, Russia. The mean composition of ognitite (n = 7) is: Ni 17.05, Fe 0.07, Cu 0.14, Pd 0.14, Te 32.53, Bi 49.64, total 99.57 wt.%, corresponding to: (Ni1.11Cu0.008Fe0.005Pd0.005)Σ1.13Bi0.90Te0.97 (Σ atoms = 3 apfu). Ognitite is trigonal, space group P3m1 [R1 = 0.0276 for 81 reflections with Fo > 4σ(Fo)]. The unit-cell parameters derived from the single-crystal X-ray diffraction data are: a = 3.928(1) Å, c = 5.385(1) Å and V = 71.95(4) Å3, with Z = 1. The c:a ratio is 1.37. The powder X-ray diffraction data obtained on the same fragment used for the single-crystal study are: a = 3.9332(4) Å, c = 5.3920(6) Å and V = 72.24(1) Å3. Ognitite exhibits the brucite-type structure with edge-sharing NiTe3Bi3 octahedra forming sheets parallel to (0001). It is related to melonite, but is distinct compositionally by the extreme Bi-enrichment (melonite and its synthetic analogue contain

The origins of early Paleozoic orogen in South China have two different models: subduction model and intra‐continental model. Here we report two new identification of ~440 Ma arc‐related ultramafic intrusions in Tingzifan (TZF) and Fomuting (FMT) along Jiangshan‐Shaoxing fault (JSF) in South China, respectively. The Silurian ultramafic intrusions are composed of olivine pyroxenite, the SiO2, MgO and TiO2 contents of olivine pyroxenites are 39.67–41.25 wt%, 28.98–31.38 wt% and 0.23–0.51 wt%, respectively. The geochemical compositions of the olivines, clinopyroxenes and hornblendes suggest an arc‐related environment for these intrusions. As for the whole‐rock trace elements, the ultramafic intrusions contain low total rare earth element (REE) contents (27.59–34.26 μg/g) and high field strength elements (HFSEs), such as Nb, Zr, Hf, Ti, and are systematically enriched in large ion lithophile elements and light rare earth elements (LREEs). Trace element compositions share most features of Alaskan‐type ultramafic‐mafic intrusions. Isotopically, the TZF and FMT ultramafic intrusions are characterized by negative Zircons εHf(t) values (0.38–7.54). Combined with their whole‐rock and mineral chemistry as well as zircon Hf isotope, we suggest that the Alaskan‐type TZF and FMT pyroxenite were formed at the root of the continental arc by underplating and fractional crystallization of mafic magma which derived from subduction metasomatized mantle source. Thus, we proposed that the early Paleozoic ultramafic–mafic along Jiangshan‐Shaoxing fault were most likely related to early Paleozoic subduction of the Paleo‐South China Ocean between Cathaysia and Yangtze blocks, arguing that the origins of early Paleozoic orogen in the South China Block is a typical subduction‐accretionary collisional‐type orogenic belt rather than an intraplate belt. [ABSTRACT FROM AUTHOR]

The current research to study the petrography, mineralogy and geochemical relationships of the ultramafic rocks in the Galalah area within Erbil Governorate, at the Unstable Shelf in the Imbricate Zone, to determine the origin of the ultramafic rocks. Dunite and Harzburgite are the most abundant rock bodies in the study area, and they represent most of the outer surface exposure of the ultramafic rock aggregates. The dunite rocks are characterized by pale brown color on the altered surface that is broken into olive green and dark green lithic. Dunite represents a homogenous lithologic feature that mainly consists predominately of olivine with accessory spinel and traces of orthopyroxene. While the harzburgite appears as a massive rock body consisting of olivine, orthopyroxene, and clinopyroxene. These rocks are mainly characterized by alteration to serpentinite, with accessory chromian spinel, secondary magnetite, chlorite, and talc. The studied ultramafic rocks are predominately rich in ferromagnesian minerals of less silica content, and other major elements affected by serpentinization are causing the deviation of elements. They have a strong decrease in sodium and calcium oxides relative to MgO and verified the silica and alumina with MgO and a slight increase in iron oxide. The low values of TiO2 in dunite and harzburgite are characteristic property of the Alpine type ophiolites during the primary process of the mantle in its upper part, before the tectonic emplacement of the ultramafic rocks, and also due to partial melting degree for the asthenospheric mantle. The MREE depletion relative to HREE and LREE may be attributed to the hornblende absence and the presence of olivine and pyroxenes minerals and/or to the source rock features. The U-shaped patterns of REE and their ratios are typical properties of ophiolitic ultramafic rocks, which are compatible with the supra-subduction zone. During the partial melting, the negative anomalies of Nb in ultramafic rocks with sequestration of the Nb concentration by sphene and ilmenite in various types of the source rocks have also been attributed to the crust contamination. These are also characterized by island arc conditions, and its depletion in the studied rocks may indicate the fore-arc tectonic environments. [ABSTRACT FROM AUTHOR]

Rock-forming olivines from mantle ultramafic rocks of Kraka massifs have been studied by several methods, including microstructural analysis, scanning electron microscopy, and LA-ICP-MS. In all studied peridotite and dunite samples, olivine demonstrates a distinct crystallographic preferred orientation (CPO) due to the rock formation under high-temperature plastic flow conditions. Olivine from wallrock dunites at podiform chromitite deposits is substantially depleted in Al, as compared to olivine from peridotites and thin dunite veinlets in the dunite–peridotite complex. Olivine aggregates from primary dunites contain numerous newly formed high-Al Cr-spinel segregations as thin needles and rods, turning into morphologically more complicated forms as they become larger. They are interpreted as resulting from such processes as segregation of trace elements, coalescence, and spheroidization of newly formed Cr-spinel grains induced by plastic deformation of olivine. [ABSTRACT FROM AUTHOR]

The results of detailed study of the platinum-group minerals (PGM) from explosive breccias of the Poperechnoe Fe–Mn deposit (the Lesser Khingan Range, Russian Far East) are presented. Native PGMs are dominated by isoferroplatinum; rutheniridosmine, native iridium, platinum, and osmium are less common. Micron-sized segregations of laurite, bowieite, сuproiridsite, cuprorhodsite, hollingworthite, as well as new mineral phases of (Ir,Rh,Os)7(S,As)13, (Rh,Ir,Ru)7(S,As)13, and Pd3(Sb,As) were recognized as microinclusions in isoferroplatinum and on the surface of its grains. It is shown that the studied PGMs are derived from rocks of ultramafic formations: (1) vein pyroxenites, harzburgites, and dunites of metamorphic and cumulative complexes of the most depleted peridotite varieties of the suprasubduction wedge of island-arc ophiolites and (2) vein pyroxenites of cumulative high-pressure ultramafic complexes of the basement of the ensialic island arc and products of evolution of suprasubduction mantle melts. In terms of the contents of platinoids, the compositions of isoferroplatinum from explosive breccias are divided into four groups: group I: isoferroplatinum of fluid-metamorphogenic genesis from harzburgite, group II: isoferroplatinum of fluid-metamorphogenic genesis from dunites and magmatogenic-fluid-metasomatic genesis from vein pyroxenites; group III: isoferroplatinum of magmatogenic genesis from chromitites of dunites of the cumulative complex; and group IV: isoferroplatinum with an elevated Pd content, which is likely derived from the melt formed by explosive breccias. Three scenarios of PGM occurrence in the fluid-saturated andesite–dacite melt of explosive breccias at the Poperechnoe deposit are discussed: (1) directly from early ultramafic complexes in suprasubduction settings; (2) from the reservoir of ancient platinum placer deposits; and (3) from a mantle wedge above the Mesozoic subduction zone during the generation of initial island-arc melts. The first two petrogenetic models suggest "rejuvenation" of the 190Pt–4He age of isoferroplatinum grains as a result of the thermal effect of the andesite–dacite melt to the age of 125 ± 21 Ma. The third geodynamic scenario suggests that the Lower Cretaceous age of isoferroplatinum marks the processes of metamorphogenic-metasomatic transformation of the dunite–harzburgite mantle wedge and probably corresponds to the age of the subduction magmatism at the Poperechnoe Deposit in particular and within the Lesser Khingan terrane as a whole. PGM associations in andesite–dacite breccias of the Poperechnoe Deposit are a new type of potentially meaningful noble-metal mineralization in the Russian Far East, while explosive breccias themselves (fluidoliths) can serve as a criterion to search for lode and placer platinoid deposits of volcanogenic-explosive genesis in the territory of the Russian Federation. [ABSTRACT FROM AUTHOR]

When plate tectonics initiated remains uncertain, partly because many signals interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant‐lid tectonics. One such signal involves the petrogenesis of early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt (Greenland), some phaneritic ultramafic rocks have been dominantly interpreted as subduction‐related, tectonically‐exhumed mantle slices or cumulates. Here, we compared Eoarchean phaneritic ultramafic rocks from the Isua supracrustal belt with mantle peridotites, cumulates, and phaneritic ultramafic samples from the Paleoarchean East Pilbara Terrane (Australia), which is widely interpreted to have formed in non‐plate tectonic settings. Our findings show that Pilbara samples have cumulate and polygonal textures, melt‐enriched trace element patterns, relative enrichment of Os, Ir, and Ru versus Pt and Pd, and chromite‐spinel with variable TiO2 and Mg#, and relatively consistent Cr#. Both, new and existing data show that cumulates and mantle rocks potentially have similar whole‐rock geochemical characteristics, deformation fabrics, and alteration features. Geochemical modeling results indicate that Isua and Pilbara ultramafic rocks have interacted with low‐Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magmatism. Such melts cannot have interacted with a mantle wedge. Correspondingly, geochemical compositions and rock textures suggest that Isua and Pilbara ultramafic rocks are not tectonically‐exhumed mantle peridotites, but are cumulates that experienced metasomatism by fluids and co‐genetic melts. Because such rocks could have formed in either plate or non‐plate tectonic settings, they cannot be used to differentiate early Earth tectonic settings. Plain Language Summary: Earth's rigid outer shell is broken into rigid pieces that move relative to each other. These motions are generally understood to reflect plate tectonics. However, the origins of plate tectonics are poorly understood. This contribution focuses on an aspect of this problem: the lack of consensus concerning when plate tectonics started. We examine some of the oldest rocks on Earth that researchers have speculated record plate tectonic processes: ultramafic rocks from the ≥3.7 billion‐years‐old Isua supracrustal belt of southwestern Greenland. A leading hypothesis suggests that these rocks are slices of mantle emplaced by plate tectonic deformation. We test the viability of an alternative hypothesis that they may have crystallized from magmas at shallow crustal levels, which does not require a plate tectonics setting. Specifically, we compare new and published mineral and chemical features of the Isua ultramafic rocks with similar rocks from known crustal and mantle settings, including new data from similarly ancient terrane in northwestern Australia that is widely considered to be non‐plate tectonic. Results show that each feature of the Isua ultramafic rocks is consistent with crustal crystallization. Therefore, these ultramafic rocks do not constrain early plate tectonics, which could have developed later. Key Points: Isua and Pilbara ultramafic rocks interacted with low Pt and Pd lavas from deep, potentially reduced mantle and/or magma chambersIsua and Pilbara ultramafic rocks are geochemically similar and can be interpreted as crustal cumulatesIsua and Pilbara ultramafic rocks can form in either plate or non‐plate tectonic settings, thus do not require >3.2 Ga subduction [ABSTRACT FROM AUTHOR]

The formation of copper-rich cumulates at the base of arc crusts has been proposed as a key process modulating the geochemical evolution of the continental crust and the genesis of giant ore deposits of copper. Despite the importance of these phenomena, the degree to which the lower crustal evolution of magmatic systems is influenced by open-system interaction and the assimilation of pre-existing crustal materials remains unclear. To tackle this issue, we provide direct isotopic constraints on the evolution of deep magmatic systems in arcs by measuring the osmium and oxygen isotope composition of hydrous copper-rich (∼730 μg·g−1 Cu) ultramafic cumulates formed at the base of the Acadian orogen (∼40 km deep) in the New England Appalachians (northeastern USA). The radiogenic 187Os/188Os initial ratios (ranging from 0.31 to 0.67) and the elevated δ18O values (8.97 ± 0.42 ‰ for orthopyroxene; 9.25 ± 0.26 ‰ for phlogopite) suggest a significant role of open-system magmatic differentiation, involving crustal assimilation, in the formation of these cumulates. Modeling of the 187Os/188Os and δ18O composition of the cumulates suggests that the observed isotopic compositions result from the initial evolution of parental magmas under sulfide-undersaturated conditions, followed by saturation after approximately 15% to 20% of assimilation and fractional crystallization progression. These results suggest that the assimilation of crustal material led to a drop in the magmatic system's f O2 (∼ΔFMQ < −1), triggering sulfide segregation and the formation of copper-rich cumulates. Our findings align with the hypothesis that magma-crust interactions can lead to the formation of lower crustal domains enriched with Cu, which may constitute a pre-stage in the formation of some porphyry copper deposits, particularly in collisional orogens. • Osmium and oxygen isotope compositions suggest significant crustal assimilation in forming copper-rich sub-arc cumulates. • Isotopic modeling indicates sulfide saturation during assimilation–fractional crystallization. • Crustal assimilation may be a necessary step for generating lower crustal domains enriched in copper. • Despite extensive assimilation, the cumulate line of descent is predominantly controlled by fractional crystallization. [ABSTRACT FROM AUTHOR]

[Display omitted] • Tourmaline orbicules hosted by ultramafic cumulates are first reported in SW Greenland. • These tourmaline orbicules result from a complex metasomatic process. • Ultramafic cumulates are highly susceptible to the late selective silicification. The Neoarchean Storø Supracrustal Belt in SW Greenland comprises a sequence of mature quartzite, metapelite, amphibolite, and ultramafic rocks that underwent amphibolite facies metamorphism during the amalgamation of the Mesoarchean Akia Terrane and the Eoarchean Færingehavn Terrane. In this belt, tourmaline is found in a transition zone between ultramafic and metapelitic rocks, but also occurs as orbicules within the ultramafic rocks. These tourmaline orbicules hosted by ultramafic rocks are reported for the first time in the North Atlantic craton, thus indicating a unique formation mechanism. We conducted a comprehensive examination of the petrology, whole-rock and mineral chemistry, and oxygen isotope compositions from borehole samples in the Storø Supracrustal Belt, to elucidate the metasomatic events associated with the formation of the orbicular tourmalines. The Storø ultramafic rocks have high MgO, Cr, and Ni contents, with low abundances of REE and HFSE, and preserve a typical cumulate texture. These features are similar to those of ultramafic cumulates found in other Archean supracrustal belts, suggesting a cumulate origin for the Storø ultramafic rocks. Furthermore, the morphology and composition of the tourmaline orbicules within these cumulates indicate that they originated from melts with high boron and high water concentrations that infiltrated the ultramafic rocks. The main factor influencing the morphology of the tourmaline orbicules is the silicification of the ultramafic rocks, driven by their lower chemical potential of SiO 2 compared to the surrounding rocks. This silicification process, in combination with compositional variations of cumulates during fractional crystallization, has contributed to the geochemical diversity observed in Archean ultramafic rocks. Thus, it is crucial to understand the effects of such selective metasomatism on Archean ultramafic rocks, as this will facilitate the extraction of original information preserved in the early rock record. [ABSTRACT FROM AUTHOR]

Formation of ultramafic cumulate rocks at the arc root serves as a key process to understanding the deep evolution of arc magma and crustal growth. However, the arc root cumulates are easy to delaminate due to their high density, making limited field exposure. Here, we conducted comprehensive petrographic observations and geochemical analysis of typical ultramafic rocks (peridotites and pyroxenites) in an ophiolitic mélange from the central Beishan orogenic belt in the southern Central Asian Orogenic Belt (CAOB). Petrographic evidence shows that these rocks exhibit characteristic adcumulate and mesocumulate textures with intercumulus minerals such as olivine, orthopyroxene, and clinopyroxene. The mineral characteristics of ultramafic rocks, including olivine and orthopyroxene with lower NiO (0.10–0.33 wt% and 0.03–0.09 wt%, respectively) and Mg# (81–87 and 86–88, respectively), clinopyroxene with higher CaO (19.0–24.7 wt%), and spinel with lower Mg# (0–49), are distinct from those of mantle rocks but resemble cumulates. Both whole rock and calculated equilibrium melt exhibit the compositions of island arc magma characterized by enrichment in large ion lithophile elements (e.g., Cs, U, Pb, and Sr) and depletion in high field strength elements (e.g., Nb, Ta, Zr, Hf, and Ti). The covariation of high Mg# and low SiO 2 in whole rocks is akin to typical arc root cumulates from classic island arcs (e.g., Kohistan and Talkeetna). The positive zircon ε Hf (t) values (+13 to +16) and calculated crystallization pressure of 0.9–1.1 GPa (∼30–40 km) support the formation of ultramafic cumulates in a juvenile island arc setting. These findings imply that most ultramafic rocks in the central Beishan orogenic belt are arc root cumulates instead of the mantle portion of an ophiolitic mélange. Fractional crystallization models suggest that arc root cumulate formation may have influenced the evolution of intermediate-felsic rocks in this area, and that the differentiation of hydrous arc magma played a crucial role in large-scale crustal growth in the southern CAOB during the early Paleozoic. • Ultramafic rocks show cumulate textures and island arc geochemical features. • These rocks are arc-root cumulates, not the mantle portion of an ophiolitic complex. • The crystallization of cumulates promotes intermediate-felsic rock formation. • Hydrous arc magma differentiation has a critical role in crustal growth in the CAOB. [ABSTRACT FROM AUTHOR]

The heterogenous mineralogy of ultramafic deposits hosting mining operations makes it challenging to accurately determine the waste rock’s mineral carbonation potential (MCP). Additionally, the significantly higher carbonation capabilities of olivine than serpentine add to the difficulty. To address this issue, in this work, a new and unique tool called the MCP calculator was developed as a Microsoft ExcelTM spreadsheet to accurately determine the amount of anthropogenic CO2 that a given rock mass can sequester through mineral carbonation. The program estimates the modal mineral abundance of ultramafic rocks to aid in MCP estimation. This tool is designed to be cost-effective and tailored for use by the mining industry, utilising abundant lithogeochemical data to evaluate their deposit as a potential substrate for industrial mineral carbonation operations. The paper introduces the MCP calculator, outlines a framework for developing the MCP parameter, and presents an example of its application. The calculator is specific to the mineral assemblage investigated at the Turnagain ultramafic complex in northern British Columbia but can be adjusted to study comparable deposits. The paper acknowledges that using waste rock in a mineral carbonation operation requires economic and practical decisions beyond the scope of the research.

Abstract When plate tectonics initiated remains uncertain, partly because many signals interpreted as diagnostic of plate tectonics can be alternatively explained via hot stagnant‐lid tectonics. One such signal involves the petrogenesis of early Archean phaneritic ultramafic rocks. In the Eoarchean Isua supracrustal belt (Greenland), some phaneritic ultramafic rocks have been dominantly interpreted as subduction‐related, tectonically‐exhumed mantle slices or cumulates. Here, we compared Eoarchean phaneritic ultramafic rocks from the Isua supracrustal belt with mantle peridotites, cumulates, and phaneritic ultramafic samples from the Paleoarchean East Pilbara Terrane (Australia), which is widely interpreted to have formed in non‐plate tectonic settings. Our findings show that Pilbara samples have cumulate and polygonal textures, melt‐enriched trace element patterns, relative enrichment of Os, Ir, and Ru versus Pt and Pd, and chromite‐spinel with variable TiO2 and Mg#, and relatively consistent Cr#. Both, new and existing data show that cumulates and mantle rocks potentially have similar whole‐rock geochemical characteristics, deformation fabrics, and alteration features. Geochemical modeling results indicate that Isua and Pilbara ultramafic rocks have interacted with low‐Pt and Pd melts generated by sequestration of Pd and Pt into sulphide and/or alloy during magmatism. Such melts cannot have interacted with a mantle wedge. Correspondingly, geochemical compositions and rock textures suggest that Isua and Pilbara ultramafic rocks are not tectonically‐exhumed mantle peridotites, but are cumulates that experienced metasomatism by fluids and co‐genetic melts. Because such rocks could have formed in either plate or non‐plate tectonic settings, they cannot be used to differentiate early Earth tectonic settings.

In this study, we use Cr-spinel as an efficient indicator to evaluate the oxygen fugacity evolution of the Xiarihamu Ni-Cu deposit and the Shitoukengde non-mineralized intrusion. Oxygen fugacity is calculated using an olivine-spinel oxybarometer, with spinel Fe3+/ΣFe ratios determined by a secondary standard calibration method using an electron microprobe. Cr-spinel Fe3+/ΣFe ratios of the Xiarihamu Ni-Cu deposit vary from 0.32 ± 0.09 to 0.12 ± 0.01, corresponding to magma fO2 values ranging from ΔQFM+2.2 ± 1.0 to ΔQFM-0.6 ± 0.2. By contrast, those of the Shitoukengde mafic-ultramafic intrusion increase from 0.07 ± 0.02 to 0.23 ± 0.04, corresponding to magma fO2 varying from ΔQFM-1.3 ± 0.3 to ΔQFM+1.0 ± 0.5. A positive correlation between fO2 and Cr-spinel Fe3+/ΣFe ratios suggests that the Cr-spinel Fe3+/ΣFe ratios can be used as an indicator for magma fO2. The high fO2 (QFM+2.2) of the harzburgite in the Xiarihamu Ni-Cu deposit suggests that the most primitive magma was characterized by relatively oxidized conditions, and then became reduced during magmatic evolution, causing S saturation and sulfide segregation to form the Xiarihamu Ni-Cu deposit. The evolution trend of the magma fO2 can be reasonably explained by metasomatism in mantle source by subduction-related fluid and addition of external reduced sulfur from country gneisses (1.08–1.14 wt% S) during crustal processes. Conversely, the primitive magma of the Shitoukengde intrusion was reduced and gradually became oxidized (from QFM-1.3 to QFM+1.0) during crystallization. Fractional crystallization of large amounts of Cr-spinel can reasonably explain the increasing magma fO2 during magmatic evolution, which would hamper sulfide precipitation in the Shitoukengde intrusion. We propose that the temporal evolution of oxygen fugacity of the mantle-derived magma can be used as one of the indicators for evaluating metallogenic potential of Ni-Cu sulfide deposits and the reduction processes from mantle source to shallow crust play an important role in the genesis of magmatic Ni-Cu sulfide deposits. [ABSTRACT FROM AUTHOR]

The results of 190Pt–4He dating of placer-forming platinum-group minerals (PPMs) from the placer deposits of Mokhovoi Creek, its tributaries, and eluvial and proluvial deposits of the peneplain surface of dunites of the Chad alkaline-ultramafic massif (Aldan Shield, Ayan-Maya district of Khabarovsk krai) are presented. 190Pt and 4He were measured in 27 samples. The statistical processing of the mineralogical and geochemical data allows us to separate three 190Pt–4He age isochrones (Ma): 123 ± 6; 113 ± 6; and 107 ± 6, which is in good agreement with the geological observations and the mineralogy of the platinum group elements (PGEs) of the Chad massif. The range of 190Pt–4He age isochrones indicates that platinum-metal ore formation occurred over 16 Myr. The geological and mineralogical data and the 190Pt–4He isochrones of different ages allow us to conclude that the platinum-metal ore formation in the Chad massif was polycyclic during the Early Cretaceous. A comparison of the 190Pt–4He ages of the PPMs of the Chad and Kondyor alkaline-ultramafic massifs indicates that there are temporal features between the platinum ore formation and the formation of these plutons during the Mesozoic tectonic–magmatic activation of the Aldan Shield. [ABSTRACT FROM AUTHOR]

The ophiolitic rocks, island arc metasediments, and granitic rocks are encountered in the study of the Allaqi province. The present study concentrated on ultramafic rocks and their various forms of hydrothermal alteration. Ophiolites commonly occur as dismembered rock sequences and have different types of hydrothermal alteration products. Ophiolites contain the essential type as serpentinites and talc-carbonate rocks, listwaenites, and amphibolites along shear zones and faults. Listwaenite is one of the most significant rocks in this area because it contains gold. The whole serpentinite rocks are affected by regional metamorphosis into greenschist facies. Geochemically, Neoproterozoic serpentinites fall into SSZ ophiolites, alpine-type peridotite, formed in the forearc peridotite tectonic environment. Regardless, the listwaenite and talc-carbonate rocks revealed that they trend to silica, carbonate, and talc. Listwaenites are classified into two types: carbonate-rich listwaenites and silica–carbonate-rich listwaenites. Magnetite, chromite, galena, and gold are the most typical ores recorded in this research.

Abstract Background The New Guinean archipelago has been shaped by millions of years of plate tectonic activity combined with long-term fluctuations in climate and sea level. These processes combined with New Guinea’s location at the tectonic junction between the Australian and Pacific plates are inherently linked to the evolution of its rich endemic biota. With the advent of molecular phylogenetics and an increasing amount of geological data, the field of New Guinean biogeography begins to be reinvigorated. Results We inferred a comprehensive dated molecular phylogeny of endemic diving beetles to test historical hypotheses pertaining to the evolution of the New Guinean biota. We used geospatial analysis techniques to compare our phylogenetic results with a newly developed geological terrane map of New Guinea as well as the altitudinal and geographic range of species ( https://arcg.is/189zmz ). Our divergence time estimations indicate a crown age (early diversification) for New Guinea Exocelina beetles in the mid-Miocene ca. 17 Ma, when the New Guinean orogeny was at an early stage. Geographic and geological ancestral state reconstructions suggest an origin of Exocelina ancestors on the eastern part of the New Guinean central range on basement rocks (with a shared affinity with the Australian Plate). Our results do not support the hypothesis of ancestors migrating to the northern margin of the Australian Plate from Pacific terranes that incrementally accreted to New Guinea over time. However, our analyses support to some extent a scenario in which Exocelina ancestors would have been able to colonize back and forth between the amalgamated Australian and Pacific terranes from the Miocene onwards. Our reconstructions also do not support an origin on ultramafic or ophiolite rocks that have been colonized much later in the evolution of the radiation. Macroevolutionary analyses do not support the hypothesis of heterogeneous diversification rates throughout the evolution of this radiation, suggesting instead a continuous slowdown in speciation. Conclusions Overall, our geospatial analysis approach to investigate the links between the location and evolution of New Guinea’s biota with the underlying geology sheds a new light on the patterns and processes of lineage diversification in this exceedingly diverse region of the planet.

[Display omitted] • A Ni-bearing laterite profile was newly found in the eastern Sulawesi. • The lateritization was resulted from in-situ weathering of ultramafic rocks. • Nickel mainly occurs in serpentine and Fe oxy-hydroxides. • Nickel enrichment was related to secondary adsorption in the saprolitic layer. • Lithology played a critical role in controlling lateritization and Ni enrichment. The Southeast Asia is one of the most important producers of Ni-laterite deposits in the world, attracting more and more attentions in recent years. However, the lateritization processes as well as the enrichment mechanisms of Ni have not been well constrained. Recently, a Ni-rich laterite profile (KPP) was newly found in the Kelurahan Pondidrha area, Sulawesi island, Indonesia. To better understand the lateritization and Ni enrichment mechanisms, in this study, detailed mineralogical and geochemical analyses were conducted on the KPP. The KPP can be divided into bedrock, saprolitic and lateritic layers from bottom to top. The bedrock is serpentinized harzburgite mainly composed of olivine, serpentine and pyroxene, while the saprolitic layer mainly consists of Fe oxides, olivine, serpentine and pyroxene, and the lateritic layer is dominated by quartz and Fe oxy-hydroxides. Nickel is mainly hosted by serpentine in the bedrock and saprolitic layers, but by oxy-hydroxides in the lateritic layer. From the bedrock to the lateritic layer, whole-rock MgO and CaO contents decrease continuously while TFe 2 O 3 contents increase, with TiO 2 /Al 2 O 3 , Zr/Hf and Pd/Pt ratios keeping stable. Combined with whole-rock pH value deceasing from the bedrock to the weathering products, the above results indicate the decomposition of silicate minerals and the formation of Fe oxides/oxy-hydroxides through hydrolysis during in-situ weathering. This led to the release of Ni and its adsorption by secondary minerals (e.g., Fe oxy-hydroxides in the lateritic layer and serpentine in the saprolitic layer). Nickel shows the highest contents in the saprolitic layer rather than the more weathered lateritic layer, indicating the decoupling between the lateritization and the Ni enrichment degrees. This could be due to the enhanced leaching of Ni from the surface under the tropical rainforest climate and the secondary adsorption in the deeper levels. These Ni enrichment features are commonly observed in the Mg-silicate type laterites, which are distinct from the oxide-type showing highest Ni enrichment in the lateritic layer, indicating that lithology played a critical role in controlling the lateritization and Ni enrichment. [ABSTRACT FROM AUTHOR]

Olivine in cumulates from the Early Jurassic Polaris Alaskan-type ultramafic-mafic intrusion in the North American Cordillera displays a wide range of textures and compositions produced in a dynamic crystallization environment as part of a transcrustal arc magmatic system. Major, minor, and trace element analyses of olivine were determined using combined electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to assess the utility of olivine as a petrogenetic indicator in ultramafic intrusions. High forsterite contents (>Fo 87) of olivine from dunite, olivine wehrlite, and wehrlite indicate that the parent melts were primitive in composition and that they may represent intrusive analogues to extrusive arc picrites found elsewhere in the Cordillera. Forsterite-Ni systematics (Fo 92–80 , Ni = 3000–300 ppm) show fractional crystallization-controlled evolution from dunite to olivine wehrlite to wehrlite through olivine clinopyroxenite. Low-Ni olivine (Fo 84–80 , 500–300 ppm) in magnetite-olivine clinopyroxenite indicates the effects of sulfide melt saturation and Ni-depletion of the residual magma. Entrainment of high-Fo olivine in evolved melts and mixing between fractionated and primitive magmas explains olivine compositions with Ni concentrations (1000–3000 ppm) in excess of those predicted by fractional crystallization models. Extensive subsolidus diffusion of Mg, Fe, Ni, Mn, Ca, Cr, Al, and Li occurred during crystallization and cooling of olivine. Uniform concentrations of Mg, Fe, Ni, and Mn in individual samples and across crystals of olivine indicate complete equilibration in the consolidating cumulate pile. Calcium, Cr, and Al from Polaris olivine (e.g., typically <1000 ppm Ca, <100 ppm Cr and Al), and from plutonic olivine in general, are depleted relative to volcanic olivine as the result of diffusional re-equilibration with interstitial melt and other co-crystallizing phases (Cr-spinel, clinopyroxene). Ratios of first-row transition elements (Mn, Zn, Fe, Co, Ni) in olivine were modified during intracrustal processes (e.g., fractional crystallization, diffusion), however, ratios from the most primitive olivine are indicative of primary mantle signals. Lithium concentrations in olivine (2–12 ppm) show evidence for subsolidus diffusive re-equilibration with Li-enriched melts. Ratios of V/Sc (0.03–0.5) are consistent with relatively oxidized parent magmas and arc mantle sources to Polaris and Alaskan-type intrusions. The Polaris parent magmas ascended through the uppermost mantle and lowermost crust on sufficiently short timescales to avoid significant magmatic differentiation prior to emplacement at a depth of ≤15 km. The findings of this microanalytical study highlight the utility of olivine as a petrogenetic recorder of primary magmatic and high-temperature diffusional processes in ultramafic plutonic rocks and contribute to our understanding of the evolution of primitive arc magmas. • Olivine chemistry is investigated in the Polaris intrusion, British Columbia. • Olivine records the primitive nature of Alaskan-type parent magmas. • Open-system fractional crystallization/magma mixing are dominant magmatic processes. • Plutonic olivine experiences extensive re-equilibration and subsolidus diffusion. • Primitive olivine in arcs can be used to infer mantle sources and redox state. [ABSTRACT FROM AUTHOR]

The Mogok metamorphic belt (MMB) in central Myanmar is well known for its complex tectonics, magmatism, and metamorphism in the framework of Tethyan subduction and India–Asia collision. It is also world renowned due to the gemstone‐class rubies and sapphires. Identification and discrimination of those petrological units in this region therefore are important for understanding the gemstone mineralization. Ultramafic massifs also crop out together with the gemstone‐bearing metamorphic rocks in the MMB. It is still poorly constrained about their origin (i.e., mantle peridotites of Jurassic–Cretaceous ophiolites or cumulate rocks) and their relationship with the gemstone generation. Here, we report petrological and geochemical data for the Mogok ultramafic rocks. Petrographic observations show that they have typical cumulate textures, with a crystallization order of olivine/spinel‐orthopyroxene‐clinopyroxene. They have low whole‐rock Al2O3 and CaO, and extremely high spinel Cr# (=Cr/[Cr + Al]; up to 0.86). Clinopyroxenes of these rocks show light rare earth element enriched patterns when normalized to CI chondrite. The results suggest that the Mogok ultramafic rocks have different compositions, both whole‐rock and mineral, from mantle peridotites of Myanmar ophiolites, such as the Kalaymyo and Myitkyina ophiolites, but resemble typical Alaskan‐type ultramafic cumulates. Therefore, the Mogok ultramafic rocks are cumulates generated by high‐pressure crystallization of hydrous arc melts, probably during the subduction of the Tethyan oceanic slab. We argue that the emplacement of these arc melts may have provided additional heat for the earlier magmatic rocks and regional high‐temperature metamorphism, although there is a need to further constrain the ages of these rocks. This, along with magmatism and metamorphism during post‐collisional extension, probably collectively contributed to the generation of world‐class coloured gemstone mineralization. [ABSTRACT FROM AUTHOR]

Recognizing controlling factors of groundwater chemistry in the ophiolite region of the southeastern area of Kermanshah is the aim of this study. The findings reveal that some samples' calcium, magnesium, and sodium absorption risk is higher than the standard range. Besides, statistical relationships of the data were investigated to recognize the key factors controlling water chemistry. Pearson's correlation was used to determine the elements with the same source. Saturation index (SI) and water–rock interaction helped us find the important minerals in reaction with water. Besides, the map of the spatial distribution of heavy elements was applied to prove the elements with a common source. According to these ties, the important factors controlling groundwater chemistry of the region are dissolution of gypsum as the common source of Ca, Sr, and S; dissolution and weathering of Cr-spinel mineral in peridotites as the common source of Si, Mg, and Cr; dissolution of existing Fe–Mg olivine and pyroxene in peridotite silicates as the common source of Mg and Ni, and dissolution of chalcedony, barite, and calcite as three minerals with the highest dissolution in water–rock interaction of groundwater samples. [ABSTRACT FROM AUTHOR]

Unusual deposits formed by corundum are described from two separate pegmatitic veins in the Hafafit-Nugrus area of the South Eastern Desert of Egypt. Other related minerals are described too. The vein-type of corundum at Locality 1 (Vein 1) crosscuts through exposed rocks of an ophiolitic mélange, whereas at Locality 2 (Vein 2), it crosscuts through exposed serpentinised ultramafic rocks. The main minerals in Vein 1 are plagioclase, corundum, grossular, phlogopite, muscovite and clinochlore, while almandine, xenotime-(Y), allanite-(Ce), zircon, Cr-rich spinel, apatite, titanite, fergusonite-(Y), meta-ankoleite, U-rich thorite (uranothorite), carbonate and illite are the accessories. In Vein 2, the main minerals are plagioclase, corundum, phlogopite and chlorite, while clinochlore, euxenite-(Y), Nb-rich rutile, almandine, xenotime-(Y), allanite-(Ce), zircon, spinel, apatite, titanite, kasolite, dickite, illite, carbonate, antigorite and talc are accessories. The two types of corundum veins differ in their concentrations of Th, U, Zr, Nb, Ta, REE, Y and Li in whole-rock compositions. Field observations, mineralogy and chemical analyses of samples from the two veins of corundum deposits, as well as concentrations of chromophore elements in corundum crystals, suggest metasomatic origins. The present study suggests that the two types of corundum veins formed at different ages from different residual magmas that underwent in-situ hybridisation with the host rocks.

The microstructure and composition of accessory chromian spinels from peridotite and ore-forming chromian spinels from disseminated ores in dunite near the boundary between the mantle and crustal ophiolite sequences are studied. Application of the method of backscattered electron diffraction (EBSD) demonstrated an existence of microtextural heterogeneity of individuals, expressed in a domain (subgrain) structure, which is caused by plastic deformation and recrystallization of mineral grains. It is shown that the formation and growth of new chromian spinel grains proceeded synkinematically in the solid state due to the segregation of aluminum and chromium in the composition of rock-forming silicates. It is concluded that the microstructural patterns of chromitite were formed under the influence of two simultaneous processes: (1) a decrease in the grain size via translational sliding and dynamic recrystallization and/or (2) the growth of recrystallized grains and their aggregation. [ABSTRACT FROM AUTHOR]

The relevance of the research. Numerous intrusive mafic-ultramafic bodies have been established in the Western Sayan. Their formational affiliation and metallogenic specialization have not yet been fully clarified and constantly attract attention of both industrial and scientific organizations in relation to their metallogenic specialization in chromium, nickel, copper, and precious metals. To clarify their formation affiliation, we obtained the original results of a petrological study of the mafic-ultramafic Kizir-Burluksky massif, which allow us to identify the conditions for its formation, subsequent transformation during long-term evolution at the upper mantle – earth crust levels. Objects: mafic-ultramafic Kizir-Burluksky massif. It is located in the north-eastern part of the Western Sayan and, together with the Kalninsky and Ergaksky massifs, is an integral part of the potentially chromite-bearing massifs of the Amylsky ore region. Methods. Petrographic studies of ultramafic rocks were carried out with polarizing microscope AxioScope-40 Carl Zeiss. The chemical composition of silicate and ore minerals was determined by the method of X-ray spectrum microanalysis using a scanning electron microscope Tescan Vega II LMU equipped with an energy-dispersive spectrometer Oxford INCA Energy 350 and a wave-dispersive spectrometer Oxford INCA Wave 700. Results. According to a petrological study, the Kizir-Burluksky massif is represented mainly by extremely depleted dunites, formed in the upper mantle, at the levels of which they underwent intense high-temperature plastic deformations by intracrystalline translational slip with a subordinate role of syntectonic recrystallization. In crustal conditions, under the influence of later of gabbroid intrusives, the dunites underwent a high-temperature metasomatism with the formation of rocks of the verlite-clinopyroxenite association.

Abstract We conducted triaxial friction tests at hydrothermal conditions (25°C–350°C) on gouges of peridotite and its principal mineral constituents olivine and orthopyroxene. Pore‐fluid chemistry was varied by the use of peridotite, granite, or quartzite driving blocks (representing wall rock) housing the gouge layer. Samples sheared at slow rates initially strengthen to a peak value, and then weaken toward a residual strength. The transition is accompanied by a change from velocity‐weakening to velocity‐strengthening behavior marked by a series of small stress drops. The extent of weakening varies with the ultramafic mineralogy and with the chemical environment established by the driving block lithology. The strengths of olivine and olivine‐rich peridotite gouges decrease substantially (to μ ∼ 0.25–0.30), and that of orthopyroxene to a lesser extent, at temperatures ≥200°C when sheared between crustal driving blocks. Less weakening is observed in the peridotite‐block experiments; the minimum strength of the peridotite gouges (μ ∼ 0.5) occurs at 250°C, the temperature at which olivine hydration rates are near their maximum in ultramafic rocks. The strength reductions in all experiments are attributed to solution‐transfer (pressure solution) processes that come to predominate over cataclastic mechanisms during shear. The lower pH of fluids in contact with silica‐saturated crustal rocks enhances the weakening of olivine‐rich gouges. In these short‐duration experiments, secondary phyllosilicate mineral growth was of a limited extent and varied with gouge and wall‐rock mineralogy and with temperature. Over geologic time spans, however, the alteration assemblages will assume an increasingly important role in fault‐zone behavior.

The paper reports first comprehensive geological, petrographic, mineralogical, and geochemical data on one of the world's oldest Tiksheozero ultramafic‒alkaline‒carbonatite complex (~1.99 Ga), which belongs to the Mid-Paleoproterozoic igneous province of the Baltic Shield. The complex was formed in three intrusive phases. The first phase is composed of the low-alkali mafic‒ultramafic rocks: dunites, wehrlites, clinopyroxenites, and gabbro. The rocks of the second phase are alkaline ultramafic rocks represented mainly by jacupirangites (alkaline clinopyroxenites) and foidolites (melteigites, ijoliltes, and urtites), with subordinate olivinites, alkaline gabbro, and nepheline syenites. The third intrusive phase is made up of carbonatites. Geochemical and mineralogical data indicate that all three phases were derived from different primary melts. It is shown that the nepheline syenites were obtained by fractionation of foidolites. A model of formation of such complexes through decompressional melting of mantle plume head enriched in carbonate fluid is proposed. [ABSTRACT FROM AUTHOR]