Metasomatic Processes Research Articles

GENETIC TYPE AND CONDITIONS OF FORMATION OF GRANITOIDS OF THE
 KISEGACH, CHASHKOV, AND ELANCHIK MASSIFS (ILMENOGORSK
 ANTICLINORIUM)

The article provides a description of the geological structure of the Kisegach, Chashkov and Elanchik migmatite-gneiss granite massifs located in the southern and southeastern parts of the IlmenogorskSysert block. The petrographic, petrogeochemical characteristics of all varieties of their constituent rocks are given. It is shown that the Kisegach massif is composed mainly of gneissic biotite granites of phase I, much less granosyenites and porphyritic granites of phase II, which are intruded by dikes of phase III leucogranites. The Chashkov and Elanchik massifs are represented by granitoids, the formation of which occurred in two stages: migmatites and gneissic granites formed in the first stage, and biotite, muscovite, and two-mica granites formed in the second stage. Phase I granitoids belong to the potassium-sodium and potassium series and belong to the migmatite-granite formation. According to biotite-garnet and biotite-amphibole thermobarometers, it was found that the formation of gneissic granites and gneissic biotite granites of the Kisegach massif occurred at a temperature of 510–540°C as a result of metasomatic processes, and porphyritic granites crystallized from the melt at a temperature of 680–700°C in the abyssal zone on depth of about 23–24 km (Р=7.1–7.4 kbar). Subsequently, all its rocks, including monzogabbro and monzodiorite, experienced regional metamorphism under the conditions of the epidote-amphibolite facies (T=550–575°C, P=5.4–6.0 kbar). Based on the study of the garnet-biotite paragenesis, which is widely represented in the granitoids of the Elanchik and Chashkov massifs, the formation temperatures of gneissic granites (530–550°C and 510–530°C, respectively) and gneissic biotite granites (540–560°C and 520–540°C) were obtained, as well as an approximate estimate of the depth of rock formation was achieved, corresponding to the near-surface or hypabyssal zones (a few kilometers). The obtained data on the physicochemical conditions of formation of the rocks of the massifs made it possible to draw a conclusion about their metasomatic nature and exclude magmatic genesis. The obtained physicochemical characteristics repeat the parameters of metamorphism for the Precambrian stratigraphic units of the Ilmenogorsk-Sysert block, as well as the Kochkar anticlinorium (East Ural uplift). This indicates a single stage in the formation of migmatite-gneiss-granite domes within the entire Southern Urals, which apparently occurred of at the Upper Precambrian — Lower Paleozoic boundary.

Role of Volatiles in the Evolution of a Carbonatitic Melt in Peridotitic Mantle: Experimental Constraints at 6.3 GPa and 1200–1450 °C

Reconstruction of the mechanisms of carbonatitic melt evolution is extremely important for understanding metasomatic processes at the base of the continental lithospheric mantle (CLM). We have studied the interaction between garnet lherzolite and a carbonatitic melt rich in molecular CO2 and H2O in experiments at 6.3 GPa and 1200–1450 °C. The interaction with garnet lherzolite and H2O-bearing carbonatite melt leads to wehrlitization of lherzolite, without its carbonation. Introduction of molecular CO2 and H2O initiates carbonation of olivine and clinopyroxene with the formation of orthopyroxene and magnesite. Partial carbonation leads to the formation of carbonate–silicate melts that are multiphase saturated with garnet harzburgite. Upon complete carbonation of olivine already at 1200 °C, melts with 27–31 wt% SiO2 and MgO/CaO ≈ 1 are formed. At 1350–1450 °C, the interaction leads to an increase in the melt fraction and the MgO/CaO ratio to 2–4 and a decrease in the SiO2 concentration. Thus, at conditions of a thermally undisturbed CLM base, molecular CO2 and H2O dissolved in metasomatic agents, due to local carbonation of peridotite, can provide the evolution of agent composition from carbonatitic to hydrous silicic, i.e., similar to the trends reconstructed for diamond-forming high density fluids (HDFs) and genetically related proto-kimberlite melts.

Petrological features of the first Cenozoic alkaline magmatic event recorded in northwestern Uruguay, southern extreme of the Paraná basin

The Cenozoic mafic alkaline rocks found in northwestern Uruguay, in the southernmost portion of the Paraná Basin, comprise three subvolcanic plugs composed of Ne-tephrites. They cut through basalts that make up the Paraná Magmatic Province as well as part of the sandstones of the Late Jurassic–Early Cretaceous age. These mafic rocks are characterized by an aphanitic hypocrystalline texture that is occasionally porphyritic with pyroxene and olivine macrocrysts/xenocrysts. We present the first lithogeochemical results that also include the Sr–Nd–Pb isotopes to determine the petrogenetic processes. These mafic rocks are enriched in light rare earth elements (La/LuN = 23.12–28.81) with respect to chondrite, in large-ion lithophile elements (Ba, U, and La), and in high-field-strength elements, such as Ta, Nb, and Zr, with respect to primitive mantle, and have a Mg# of 0.53–0.56. The trace element geochemistry and isotopic data (low radiogenic 206Pb/204Pb and low 87Sr/86Sri between 18.315 and 18.733, 0.70397 and 0.70443, respectively) indicate that the primary magma could be the product of the melting of an enriched mantle source with the possible involvement of metasomatic processes, lacking significant contamination by the continental crust. The K–Ar (whole rock) geochronology conducted on two samples yielded ages of 63.7 ± 2.5 and 51.5 ± 1.7 Ma for this magmatic event.

Deformation and metasomatism recorded by single-grain apatite petrochronology

Abstract The timing and processes of ductile deformation and metasomatism can be documented using apatite petrochronology. We integrated microstructural, U-Pb, and geochemical analyses of apatite grains from an exhumed mylonitic shear zone in the St. Barthélémy Massif, Pyrenees, France, to understand how deformation and metasomatism are recorded by U-Pb dates and geochemical patterns. Electron backscatter diffraction (EBSD) analyses documents crystal plastic deformation characterized by low-angle boundaries (<5°) associated with dislocation creep and evidence of multiple slip systems. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb maps indicate that dates in deformed grains reflect, and are governed by, low-angle dislocation boundaries. Apatite rare earth element (REE) and U-Pb behavior is decoupled in high-grade gneiss samples, suggesting REEs record higher-temperature processes than U-Pb isotopic systems. Apatite from (ultra)mylonitic portions of the shear zone showed evidence of metasomatism, and the youngest dates constrain the age of metasomatism. Collectively, these results demonstrate that crystal plastic microstructures and fluid interactions can markedly change apatite isotopic signatures, making single-grain apatite petrochronology a powerful tool for dating and characterizing the latest major deformation and/or fluid events, which are often not captured by higher-temperature chronometers.

Diamond-destructive mantle metasomatism: Evidence from the internal and external textures of diamonds and their nitrogen defects

Metasomatic processes modify the composition of the subcratonic lithospheric mantle and can either form or destroy diamonds. The composition of these metasomatic agents is uncertain and has been mostly deduced from chemical zonation and overprints recorded by associated mantle silicates. Diamonds experience partial dissolution (resorption) during their residence in the mantle due to mantle metasomatism and later during their ascent in kimberlite magma. Diamonds, enclosed inside mantle xenoliths during the whole duration of ascent in kimberlite magma, can preserve their pre-kimberlite surface features, which record the last diamond-destructive metasomatic event to have occurred in the mantle. The geometry of diamond dissolution features acquired during mantle storage can provide information about diamond-destructive metasomatic events in the mantle. Diamond samples recovered from inside mantle xenoliths are extremely rare and mostly limited to eclogitic lithology, which suggests that variable resistance of different mantle lithologies to disintegration in kimberlite magma may affect representativity of these sample. Here we use whole diamond populations from exploration parcels and apply our earlier developed set of criteria to distinguish between kimberlitic and pre-kimberlitic surface features on diamonds. The study used diamonds (<1 to 4.5 mm size) from eight kimberlites in three localities: Orapa cluster, Botswana (BK1, AK15, and AK1 kimberlites), Ekati Mine, Northwest Territories, Canada (Grizzly, Leslie, Koala, and Misery kimberlites), and Snap Lake kimberlite dyke, Northwest Territories, Canada. The host kimberlites cover seven different volcaniclastic and coherent kimberlite lithologies, and our previous studies demonstrated a correlation between the style of kimberlitic resorption on diamonds and the host kimberlite lithology for these samples. From the total of 3256 studied diamonds, we identified 534 diamonds with pre-kimberlite surface textures. These pre-kimberlite surface textures display six distinct types, which are present in all the studied diamond parcels regardless of their geographic locality and host kimberlite lithology. The relative proportions of these types depend on the geographic locality showing linkage to a specific mantle source. We examined the relationship between the surface features on diamonds, their growth patterns revealed in cathodoluminescence (CL) images, the content and aggregation of nitrogen defects using Fourier transform infrared spectroscopy (FTIR), and nitrogen content in specific growth zones of diamonds obtained using secondary ion mass spectrometry (SIMS) for 82 Ekati diamonds.Our data show that growth step-faces develop on diamonds with complex multi-crystal cores, whereas flat-faced octahedra with simple oscillatory-zoned growth patterns derive from single growth events. Initial stages of dissolution affecting only outer growth zones develop simple serrate laminae on diamonds, while more extensive dissolution exposes more complex growth zones developing various shapes of laminae and etch features (trigons and irregular asperities). The effect of internal growth patterns on dissolution features is more profound during pre-kimberlitic than kimberlite-related resorption likely due to the greater role of defects in diamond dissolution at mantle conditions.Comparison with the results of diamond dissolution experiments shows that metasomatism by C-O-H fluid is not destructive to diamond, while carbonate-silicate melt-driven metasomatism causes diamond dissolution. Continuous change in the silicate content of silicate‑carbonate melts and temperature variations within 200 °C can explain all pre-kimberlite dissolution features observed in this study. Similar pre-kimberlite dissolution features on diamonds from both the Zimbabwe and Slave cratons suggests that these metasomatic processes are widespread and affected the mantle below the eight studied kimberlites.

First Age and Geochemical Data on Zircon from Riebeckite Granites of the Verkhnee Espe Rare Earth\u2013Rare Metal Deposit, East Kazakhstan

This paper is dedicated to the isotope-geochemical study of zircon from riebeckite granites of the Verkhnee Espe rare earth-rare metal deposit and the specification of its U–Pb age. Zircon from the Verkhnee Espe massif is peculiar in the high content of non-formula elements (up to 43000 ppm REE, up to 22000 Y, and others) and demonstrates a clearly expressed heterogeneous structure. The central and rim zones of the zircon show a “magmatic” rare-earth element (REE) distribution. The intermediate zones are characterized by a flattening of the REE patterns and an anomalous enrichment in REE, Y, Nb, and Ca. This compositional feature of the zircon may be caused by impact of fluid-saturated granite melts enriched in incompatible trace elements. The δ18О values in the zircon are 5.83–7.16‰, which generally corresponds to zircon formed from granitoid melts. The age of zircon from the Verkhnee Espe rare earth–rare metal deposit is 283 ± 3 Ma, which indicates that there is no significant age gap between granite crystallization, on the one hand, and metasomatic processes and ore generation, on the other.

Mineral Composition of Rare Element Enriched Rock Bodies from the Bükk Mts., NE Hungary

A rare earth (REE) and further rare element (Zr, Nb, Th, Ta) enrichment was found recently within the CriticEl project in 2016. Due to continuing studies, now we know about several enriched rock bodies from the SE and NE part of the Bükk Mts. The enrichment was caused by metasomatic processes, during which fluids penetrated through the rock bodies, causing chemical and mineralogical changes. The enriched rock bodies are Triassic metavolcanics and siliciclastic metasedimentary rocks. The protoliths consist of quartz, feldspars (albite and potassic feldspars), phyllosilicates (various micas and chlorites) and calcite, and in minor quantities Fe- and Ti-oxides, fluorapatite and titanite. The characteristic metasomatic minerals are monazite-(Ce), zircon and Nb-bearing Tioxide. Based on our data, micas and potassic feldspars were generated at least partly during the rare element mineralization process, while titanite, chlorite and albite were consumed. Apatite and Ti-oxides were decomposed and formed. The original mineral composition had a strong influence on the rate and mode of alteration.

Mining Waste as a Potential Additional Source of HREE and U for the European Green Deal: A Case Study of Bangka Island (Indonesia)

The European Commission has adopted the European Green Deal strategy, which aims to achieve climate neutrality in the EU by 2050. To achieve this goal, it is necessary to shift the economy toward the use of green and renewable energy. Critical raw materials (CRMs), Li, Co, REE, Te, Sc and others, are used in renewable energy sources (RES) production. The EU lacks its own CRM deposits, and additionally, the access to already identified deposits is limited, which is making the EU countries search for alternative CRM sources. One such source of CRMs may be mining waste generated on the Indonesian island of Bangka as a result of processing cassiterite-bearing sands. Studies of the mineral composition of the waste using the XRD method reveal rich contents of xenotime (0.79–17.55 wt%), monazite (1.55–21.23 wt%), zircon (1.87–64.35 wt%) and other minerals, carriers of valuable metals, such as Sn, Ti, Nb, Ta. The point mineral chemistry analyses were performed using EPMA. Xenotime is the main carrier of heavy rare earth elements (HREE), especially the “most critical” HREEs: Gd2O3 (1.42–7.16 wt%), Dy2O3 (2.28–11.21 wt%), Er2O3 (2.44–7.85 wt%), and Yb2O3 (1.71–7.10 wt%). Xenotime is characterized by a complex internal structure resulting from metasomatic processes occurring during their formation. In SEM-BSE imaging, they show zonation of internal structure, which is the effect of an HREE, Y, Si and U substitution in the crystal structure. On the other hand, thorite ThSiO4 and uranothorite (Th,U)SiO4 inclusions are present in xenotimes. The ICP-MS/ES studies of tailings reveal very high contents of HREE + Y (up to 7.58 wt%), U (up to 0.11), Th (up to 0.75 wt%) and Sc (132 ppm). A CRM source diversification is part of the strategy to ensure the security of raw materials for countries of the European Union and the green transformation of the continent. Bilateral EU–Indonesia cooperation in the geological exploration and development of primary and secondary sources may contribute to an increase in the supply of HREEs to the global market.

Mass balance of complementary metasomatic processes using isocon analysis

This study develops a method to estimate the redistribution of elements during several associated metasomatic processes simultaneously involved in the formation of a geological complex. The method is based on classical mass balancing using isocon analysis. The proposed approach is applicable if geological prerequisites indicate that (a) the metasomatic rocks inherited some components (x, y, etc.) from one of the earlier rocks of the studied complex; (b) these components have been transported by a fluid; and (c) this saturated fluid has produced several varieties of metasomatic rocks rich in components x, y, etc. In the case of the specified background, the proposed method estimates the mass proportions between the source rock and the varieties of the resulting metasomatic rocks. We present the geological profile of the processes to which the proposed method is applicable, the mathematical model of the method, examples of the application and interpretation of the results, and geological criteria to verify the obtained model results. In brief,•This study introduces a method for calculating the mass proportions between the source rock and metasomatic rocks formed from the material remobilized from the source rock;•The paper considers the applicability limits of the method, exemplifies the interpretation of the results, and shows the methods for monitoring the correctness of these results.

METASOMATIC CHANGES IN ROCKS OF SEDIMENTARY COMPLEXES IN THE CARPATHIAN OIL AND GAS PROVINCE AS AN INDICATOR OF CARBOHYDRATE MIGRATION

A field and laboratory investigation of sedimentary rocks in Carpathian oil and gas province was curried out. The theoretical and practical aspects of the peculiarities of the development of metasomatic process during the migration of fluids at the postsedimentation stage of rock formation were investigated. Based on detailed field researches, a consistent pattern of mineral aggregates’ and their crystalloid individuals developing was defined. The arguments for developing a new geochemical model of oil deposits indication in Carpathian oil and gas province were provided.

Geology and mineralogy of sapphire-rich metasomatites (sapphirites) deposit at the Portezuelo de Pajas Blancas, northern Chile: Genetic implications for unusual metasomatic processes in the Central Andes

The Portezuelo de Pajas Blancas deposit, located in northern Chile, represents a unique opportunity to constrain unusual geological processes involved in the genesis of sapphire-rich metasomatites in the central Andes. For this purpose, geological mapping was combined with conventional and high-resolution petrographic and mineralogical analysis (light microscope, SEM-EDS and automated mineralogy). Results demonstrate that this uncommon occurrence is genetically related to eight main lithological units that host variable proportions of corundum (sapphire), sillimanite, andalusite, diaspore, böhmite, dravite, magnesiofoitite, olenite, alunite, and calcite. Microtextural relationships between mineralogical assemblages demonstrate that the origin of this deposit was associated with two main genetic stages. The early stage is linked to the initial crystallisation of abundant sapphire, promoted by an extreme desilication during high-temperature prograde metasomatic processes due to the intrusion of a Cretaceous granodioritic magma body (Si-Al-rich) into basaltic rocks from the La Negra Formation (Al-rich Si-poor) through the Atacama Fault System. The late-stage corresponds to hydrothermal alteration of sapphires to böhmite and diaspore in the deep and shallow zones, respectively, where dravite and alunite were precipitated directly from these fluids. The formation of this late hydrothermal assemblage can be interpreted as an overprinting of a metasomatic retrograde system. This study demonstrates that the sapphire-bearing rocks at Portezuelo de Pajas Blancas correspond to primary metasomatic sapphirites related to middle- to high-temperature contact metasomatism at estimated pressures of ∼ 2 kbar.

Geochemical - geology characteristics implicating original sources and copper - deposit type in Kon Ra ore - field

Based on the research results on petrographic - mineralogical characteristics, tectonic structural features, geochemistry of major and trace elements of the bedrock, alternative rock, ore, soil, mineralogical geochemistry, mineral facies, inclusions, the origin of ore formation related to oxidized granite and skarnoid - typed metasomatic process in Kon Ra copper ore field have been identified. Petrological and mineral characteristics indicate the process of transitional metasomatism between the skarn and hornfels, also known as bimetasomatic stage (skarnoid deposit type). Diopxite represents the Progade skarnoid stage. Tremolite, actinolite, quartz, chlorite, magnetite, molybdenite, less of chalcopyrite, pyrrhotite, and pyrite indicate the retrogade skarnoid stage. The following is sulfide - quartz stage (major minerals include: quartz, chalcopyrite, pyrite, pyrrhotite, molybdenite). This result is also consistent with the formation temperature 210÷270 0C and the geochemical zoning of elements from intrusive blocks through the outer contact zone that contains the ore and surrounding rocks are as follows: Cu, Zn, Ca (the zone has lime-rich formations), Fe3+, Mo increases in the outer contact zone containing ore closed to acid intrusive rocks. Inversely, the ratios of Pb/Cu, Zn/Cu, and As content increased in the alteration from this zone to the outer one. In addition, uranium mineralization is associated with a later magma stage (pegmatite granite in endo-contact is high uranium radiation: U = 0.17÷0.2%, 3,420,000÷8,020,000 µR/h and contains uraninite).

Tintigny meteorite: the first Belgian achondrite

A late afternoon in February 1971, a meteorite impacted the rooftop of a house in Tintigny village in southern Belgium. Confirmed as a possible meteorite by the schoolteacher, the meteorite and its fall story did not leave the village. Finally, 46 years after the fall event, we got the opportunity to study and characterize this meteorite. In this work, we give a detailed report on its textural, mineralogical, whole-rock elemental and oxygen isotopic composition. Officially named as Tintigny, we classified it as an achondrite from howardite-eucrite-diogenite (HED) clan and more precisely a polymict eucrite. A brecciated basaltic rock believed to be originated from the surface of V-type asteroids namely the asteroid 4 Vesta. Tintigny has recorded the evidence of the impact metamorphism and metasomatism processes active on its parent body. Tintigny is one of the 39 eucrite falls known to date, and one of the 11 eucrites occurred in Europe. It is the fifth officially recognized meteorite and the first achondrite from Belgium. This report shows the importance of studying and accessing such a meteorite for further cosmochemical and planetary investigations and enriching our knowledge on the formation of HED meteorites and their parent bodi(es). In addition, it brings the attention to its importance as a scientific heritage that has to be properly understood and safeguarded for the generations of scientists, scholar, and amateurs to come.

Petrography and geochemistry preliminary study of Pucangan Serpentinite Geosite Karangsambung – Karangbolong Geopark Central Java, Indonesia

Geosite is a Geological Heritage object in the Geopark area with specific characteristics and is an integral part of the evolutionary history of forming an area. Serpentinite Pucangan is one of the Geosite located in Karangsambung – Karangbolong geopark. This study aims to find out the characteristics of petrography and geochemistry of Serpentinite Pucangan Geosite. The methods used in this study include literature studies, fieldwork research, petrography analysis, ore microscopy analysis, whole rock analysis consist of major element analysis using fusion inductively coupled plasma (Fus ICP) methods, trace element analysis, and rare earth element analysis using inductively coupled plasma mass spectrometry (ICP-MS) methods. Serpentine minerals with antigorite type dominate the petrography composition of Pucangan serpentinite with a little magnetite mineral. SiO2, MgO, and Fe2O3(t) are the major elements with the largest percentage of 37.93%, 5.78%, and 9.73% that have an agreement with the mineralogical composition of the rock that dominated by ferromagnesian minerals. This Geosite is interpreted to be formed due to the metasomatic process when the subduction process is happening in this area in the Pre-tertiary period.

Soil-sample geochemistry normalised by class membership from machine-learnt clusters of satellite and geophysics data

The primary aim of mineral exploration is to focus the search space from regional scale (1000 s km2) to prospect scale (1–10 km2), identify areas that warrant further investigation, and ultimately drill a target. Mineral deposits typically display geochemical anomalism related to the conditions of their formation, or the weathering and mass transport of sediment away from a mineral deposit. However, this anomalism can be obscured in soil-sample surveys due to background variations in host bedrock chemistry and the transformation and redistribution of original minerals during surface weathering, regolith development, and supergene processes. This contribution uses machine-learnt map products to aid in the interpretation of a regional-scale soil-sample dataset. ASTER (satellite spectral) data were combined with airborne radiometric data using Self-Organising Maps (SOM) to generate a map of regolith type. A map of bedrock geology was previously derived by the authors from airborne geophysical data using Random Forests. These two products are used to classify soil samples with an objective bedrock-geology and regolith-type classification. A z-score normalisation was implemented on the soil-sample geochemical data, using two different matrices (regolith type and bedrock geology) to remove the bias of bedrock geology and regolith type from the soil sample chemistry. The signal that remains likely reflects metasomatic processes, and associated elemental anomalism, potentially linked to mineralisation. These data can then be plotted to provide maps of (pathfinder) element anomalies, thus identifying potential exploration targets. We explore this approach to normalise geochemical data in a dataset of 9,924 soil samples collected over an area of 1,028 km2, from Kerkasha, Eritrea (Nubian Shield). The Kerkasha project is an early-stage exploration project, with a limited understanding of the local geology and mineral deposit types; however, it contains many Au and Cu prospects that have received minimal exploration work.

Beryl as indicator of metasomatic processes in the California Blue Mine topaz-beryl pegmatite and associated miarolitic pockets

Beryl occurs in a wide variety of rare-element pegmatites and has provided significant insights into their evolution. The geochemical signatures of beryl can record igneous and metasomatic processes but rarely could they be studied at the same location in both, the pegmatite body and associated miarolitic pockets. Further, the link between beryl chemistry and the petrogenesis of pegmatites has not yet been established convincingly. This study focuses on the recently discovered topaz-beryl granitic pegmatite deposit at the California Blue Mine in San Bernardino County, California, USA. This pegmatite deposit provides an exceptional opportunity to study microtextural (SEM and WDS-element maps) and major EMPA and trace element LA-ICP-MS concentrations in beryl in the pegmatite body and associated miarolitic pockets. The mineralogy of the pegmatites consists of: quartz + K-feldspar + albite (i.e., cleavelandite variety) + biotite + muscovite ± spessartine ± beryl (i.e., aquamarine variety) ± topaz, with a mineralogic zoning that includes border, wall, intermediate, and core zone. The miarolitic pockets contain gem quality beryl, topaz, and quartz crystals. Two major beryl types were distinguished including a primary (Brl I) and a metasomatic beryl (Brl II). Brl I occurs in the intermediate pegmatite zone and displays a blocky concentric crystal zoning characterized by a core enriched in Al and rims enriched in Na, Rb, K, and Sc. In contrast, Brl II is found mostly in the pegmatite core zone and at the contacts to, or within miarolitic pockets, and is characterized by a distinct enrichment in Cs and Li correlating with an enrichment in Na, B, and As and a depletion in Rb, K, Sc, and Fe. Brl II occurs as two variants including as replacement of Brl I (i.e., dissolution-precipitation microtextures in the pegmatites) and as single hydrothermal crystals (i.e., precipitation in the miarolitic pockets). The contrasting trends between Brl I and Brl II are interpreted to reflect two distinct processes affecting trace element variations in beryl including fractionation from a magma with systematic crystal core to rim zoning (decreasing Li and enrichment in Na, K, Rb, Sc, and Fe), and metasomatism with enrichment in Li, Cs, Na, B and As. An extensive beryl trace element dataset was generated in this study and further compared to beryl of the NYF (Nb-Y-F) and LCT (Li-Cs-Ta) pegmatite families worldwide. A significant link was found between beryl chemistry and the petrogenetic relationships of the pegmatites, which were used to distinguish four major geochemical beryl groups: (i) a low and a high Fe–Mg — alkali group, both displaying a positive alkali vs. Fe + Mg correlation; (ii) a low Fe–Mg — high alkali group displaying a negative alkali vs. Fe + Mg correlation; (iii) a low Li–Cs group; (iv) a high Li–Cs group. The more evolved LCT-type pegmatites generally overlap with the low Fe–Mg — high alkali and high Li–Cs groups (ii and iv). In contrast, NYF pegmatites coincide better with the trends observed for the low Li–Cs group as well as the low/high Fe–Mg — alkali beryl groups displaying a positive Fe + Mg vs. alkali correlation (i and iii). The beryl signatures from the California Blue Mine coincide with the NYF pegmatite classification but a partial overlap with the LCT pegmatites indicates a possible mixed origin or crustal contamination (i.e., hybrid pegmatites). These new insights reveal that beryl signatures provide a significant tool for interpreting the evolution of pegmatites from different geologic settings and permits distinguishing between igneous and metasomatic origins.

Electron microprobe monazite ages from a tin placer deposit on Bangka Island, Indonesia

A first attempt of Electron microprobe (EMP) based Th-U-total Pb in situ dating of monazites in tailings from a placer deposit (quartz bearing sands) from Bangka Island, Indonesia, revealed for the first time from a single locality in Indonesia an age cluster around 231 Ma as well as two single grains with ages of 1133 Ma and 1916 Ma. The monazite ages from this study agree with detrital zircon ages and whole rock isochron ages from magmatic intrusions, which were reported from various localities all over the Malaysian peninsula, Myanmar and Thailand but not yet from Bangka Island. Perhaps intrusions in the closer vicinity, which could be the source of the monazite under study, have not been discovered so far or the sedimentary catchment area of the placer deposit is much larger than assumed so far. The existence of the monazite ages investigated in this study may also raise the question of the paleogeographic position of Bangka Island and the relative timing of sedimentation and Miocene rifting. However, the present work is an excellent example for the resistance of monazite and its Th-U-Pb system to sedimentary processes and hence is a useful tool for provenance studies and source of Rare Earth Element (REEs) recovery. An important outcome of this study is that sedimentary monazite relics often provide a wide range of unrealistic ages. Such dates may be wrongly explained in terms of recrystallization along with metasomatic process, instead these ages are only a methodic artefact because monazite relics are either not thick enough for dating or show poor surfaces with numerous cracks.

The effect of fluorine on reaction-rim growth dynamics in the ternary CaO-MgO-SiO2 system

AbstractGrowth of reaction rims is mainly controlled by a change in physical parameters such as pressure and temperature, a change in the chemical composition of the system, and/or by the presence of volatiles. In particular, the effect of volatiles other than water on reaction-rim growth remains poorly understood. To accurately model metamorphic and metasomatic processes, a quantification of the effect of volatiles on reaction-rim growth dynamics is necessary but hitherto missing.In this study, reaction rims were experimentally grown in a series of piston-cylinder experiments in the ternary CaO-MgO-SiO2 system at 1000 °C and 1.5 GPa with 0–10 wt% F for 20 min. In the fluorine-free system, a rim sequence of wollastonite (Wo) | merwinite (Mer) | diopside (Di) | forsterite (Fo) | periclase (Per) formed, complying with the stable phase configuration at water-saturated conditions. As soon as 0.1 wt% F was introduced into the system, humite group minerals (HGMs) and monticellite (Mtc) appeared, resulting in the multilayer rim sequence Wo | Mer | Mtc | Fo + HGMs | Per. In experiments with fluorine concentrations ≥0.5 wt%, cuspidine (Csp) appears in the layer sequence and represents the major fluorine sink. Our data show that the addition of fluorine may stabilize the fluorine-bearing phases cuspidine and HGMs to higher temperatures, which is in agreement with previous studies (Grützner et al. 2017). However, the appearance of the nominally anhydrous minerals (NAMs) monticellite and åkermanite (Ak) at this P-T condition suggests that the addition of fluorine may also affect the stability of nominally fluorine-free minerals. This may be explained by the effect of fluorine on the Gibbs free energies of fluorine-bearing phases, which in turn affects the relative Gibbs free energies and thus the stabilities of all phases. An increase in absolute rim thickness from 11.8(21) to 105.6(22) µm (1σ standard deviations in parentheses) in fluorine free and 10 wt% F experiments, respectively, suggests that fluorine enhances absolute component mobilities and thus results in faster rim growth rates. Additionally, due to the presence of fluorine, a change in relative component mobilities results in microstructural changes such as a phase segregation of diopside and cuspidine at high-fluorine (≥3 wt% F) concentrations.These results not only imply that reaction rims may be used as a tool to infer the amount of fluorine present during metamorphic reactions but also that we need to consider the role of fluorine for a correct interpretation of the P-T-t history of metamorphic and metasomatic rocks.

Geochemical evolution of PGE-sulfide mineralization of the Khudolaz differentiated complex in the South urals: The role of R-factor and hydrothermal alteration

The article highlights the results of mineralogical and geochemical studies of disseminated Cu-Ni-PGE sulfide mineralization from three intrusions of the Khudolaz Differentiated Complex (KDC) in the South Urals. Sulfide mineral assemblages are shown to have formed across a wide temperature range (about 1100–150 °C) during the cooling of an immiscible sulfide melt and subsequent hydrothermal dissolution and redeposition of sulfides. During the solidification process, a crystallization of sulfide solid solutions (mss, iss, etc.) occurred, from which pyrrhotite, pentlandite, and chalcopyrite were subsequently exsoluted. The oxidation, substitution and dissolution of primary sulfides at the hydrothermal stage led to the formation of mineral aggregates of complex morphology, containing complete or partial pseudomorphs of secondary sulfides, silicates and oxides, as well as redeposited sulfide aggregates. According to LA ICP MS data for sulfides and the bulk trace element composition of ores, the depletion of sulfide minerals by some chalcophile and platinum-group elements (PGE) took place during their hydrothermal alteration (partial dissolution and replacement). Secondary sulfides such as pyrite and violarite largely inherited the geochemical features of the primary minerals during pseudomorphic replacement of pyrrhotite and pentlandite. The redeposited sulfides (pyrite and chalcopyrite) are primarily enriched by chalcophile trace elements. Leaching of PGEs from pyrrhotite and pentlandite took place during hydrothermal alteration. Here, the most significant loss was determined for Te, resulting in the depletion of sulfide minerals and ores in Te by about 5 times. The identified decrease of Te/Sb values in altered sulfides by a factor of 3–20 indicates an enrichment in Sb and a high Dhydrothermal fluid/sulfide partition coefficient (≫ 1). It is shown that the hydrothermal fluid was enriched in Sb. On the other hand, the S/Se value increases by a factor of ~ 1.5, demonstrating an Se mobility during metasomatic processes comparable to other chalcogenides.Based on the study of assemblages and traced evolution of the chemical composition of PGE minerals, three stages of mineralization are proposed: 1) crystallization of immiscible chalcogenide and highly fractionated sulfide melts; 2) segregation of isomorphic impurities of chalcogenides and PGE during the sulfides exsolution from sulfide solid solutions; 3) interaction of Pd leached from primary sulfide and PGE minerals with Sb-rich hydrothermal fluid.The calculated value of the R-factor (150–502) indicates a moderate productivity of the KDC intrusions for Cu-Ni sulfide mineralization, but a rather low potential for PGE. Although the degree of primary sulfide melt PGE-enrichment is not inferior to that of many Cu-Ni-PGE deposits in the world, the economic potential of the Khudolaz Complex intrusions is significantly reduced by their small sizes. At the same time, the strong depletion of primary sulfides in Ni, Cu, Pd, and Pt during hydrothermal alteration led to the enrichment of host rocks in the external contact.

Evolution of ultrapotassic volcanism on the Kaapvaal craton: deepening the orangeite versus lamproite debate

Abstract Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a c. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic of bulk rock compositions and is associated with strongly depleted Th–U–Nb–Ta contents at high Cs–Rb–Ba–K concentrations. The resultant high large ion lithophile element/high field strength element ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones. The Bushveld-intersecting orangeite dykes have strongly enriched Sr–Nd–Hf isotopic compositions (initial 87 Sr/ 86 Sr = 0.70701–0.70741; ε Nd = −10.6 to −5.8; ε Hf = −14.4 to −2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti–Nb–Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2–1.1 Ga Namaqua–Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma. Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H 2 O–CO 2 –F in the petrogeneses of these potentially diamondiferous alkaline rocks.