Mineral Chemical Analyses Research Articles

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.

Qmin – A machine learning-based application for processing and analysis of mineral chemistry data

Mineral chemistry analysis is a valuable tool with many applications in mineralogy and mineral prospecting and beneficiation studies. This type of analysis can point out relevant information, such as the concentration of the chemical element of interest in the analyzed phase and, thus, the predisposition of an area for a given commodity. Therefore, considerable amounts of data have been generated, especially with the use of electron probe microanalyzers (EPMA), either for academic research or for prospecting and applied mineralogical work in the mineral industry. We have identified an efficiency gap when manually processing and analyzing mineral chemistry data, and thus, investigated the possibility that such research might benefit from the versatility of machine learning algorithms. We present Qmin, an application that assists in increasing the efficiency of processing and analysis of mineral chemistry data through automated routines. Our code benefits from a hierarchical structure of classifiers and regressors trained by a Random Forests algorithm developed on a filtered training database extracted from the GEOROC (Geochemistry of Rocks of the Oceans and Continents) repository, which is maintained by the Max Planck Institute for Chemistry. To test the robustness of our application, we applied a blind test with more than 22,000 mineral chemistry analyses compiled for diamond prospecting within the scope of the Diamante Brasil Project of the Geological Survey of Brazil. The blind test yielded a balanced classifier accuracy of ∼99% for the minerals known by Qmin. This outcome emphasizes the potential of machine learning techniques in assisting the processing and analysis of mineral chemistry data.

Melt Percolation, Melt-Rock Reaction and Oxygen Fugacity in Supra-Subduction Zone Mantle and Lower Crust from the Leka Ophiolite Complex, Norway

Abstract Samples of peridotites and pyroxenites from the mantle and lower crustal sections of the Leka Ophiolite Complex (LOC; Norway) are examined to investigate the effects of melt-rock reaction and oxygen fugacity variations in the sub-arc oceanic lithosphere. The LOC is considered to represent supra-subduction zone (SSZ) oceanic lithosphere, but also preserves evidence of pre-SSZ magmatic processes. Here we combine field and microstructural observations with mineral chemical and structural analyses of different minerals from the major lithologies of the LOC. Wehrlite and websterite bodies in both the mantle and lower crust contain clinopyroxene likely formed at a pre-SSZ stage, characterised by high Al, high Cr, low Mg crystal cores. These clinopyroxenes also exhibit low Al, low Cr, high Mg outer rims and intracrystalline dissolution surfaces, indicative of reactive melt percolation during intrusion and disruption of these lithologies by later, SSZ-related, dunite-forming magmas. Chromian-spinel compositional variations correlate with lithology; dunite-chromitite Cr-spinels are characterised by relatively uniform and high TiO2 and Al2O3, indicating formation by melt-rock reaction associated with SSZ processes. Harzburgite Cr-spinel compositions are more variable but preserve a relatively high Al2O3, low TiO2 endmember that may reflect crystallisation in a pre-SSZ oceanic spreading centre setting. An important finding of this study is that the LOC potentially preserves the petrological signature of a transition between oceanic spreading centre processes and subsequent SSZ magmatism. Single crystal Cr-spinel Fe3+/ΣFe ratios calculated on the basis of stoichiometry (from electron microprobe [EPMA] and crystal structural [X-ray diffraction; XRD] measurements) correlate variably with those calculated by point-source (single crystal) Mössbauer spectroscopy. Average sample EPMA Fe3+/ΣFe ratios overestimate or underestimate the Mössbauer-derived values for harzburgites, and always overestimate the Mössbauer Fe3+/ΣFe ratios for dunites and chromitites. The highest Fe3+/ΣFe ratios, irrespective of method of measurement, are therefore generally associated with dunites and chromitites, and yield calculated log(fO2)FMQ values of up to ~ + 1.8. While this lends support to the formation of the dunites and chromitites during SSZ-related melt percolation in the lower part of the LOC, it also suggests that these melts were not highly oxidised, compared to typical arc basalts (fO2FMQ of > + 2). This may in turn reflect the early (forearc) stage of subduction zone activity preserved by the LOC and implies that some of the arc tholeiitic and boninitic lava compositions preserved in the upper portion of the ophiolite are not genetically related to the mantle and lower crustal rocks, against which they exhibit tectonic contacts. Our new data also have implications for the use of ophiolite chromitites as recorders of mantle oxidation state through time; a global comparison suggests that the Fe3+/ΣFe signatures of ophiolite chromitites are likely to have more to do with local environmental petrogenetic conditions in sub-arc systems than large length-scale mantle chemical evolution

Mineralization styles, alteration mineralogy, and sulfur isotope geochemistry of volcanogenic massive sulfide deposits in the Shadli Metavolcanics Belt, South Eastern Desert, Egypt: Metallogenic implications

Three distinct occurrences of Zn–Cu–Pb mineralization (Um Samiuki, Hilgit, and Maaqal) in Neoproterozoic island arcs of the Shadli Metavolcanics Belt, South Eastern Desert, Egypt, have been investigated in this study. The Shadli Metavolcanics Belt erupted in two magmatic cycles, with the Older Um Samiuki Metavolcanics group getting overlain by the Younger Hamamid Metavolcanics (YHM) group. The YHM group is characterized by two distinct cycles of volcanism. The studied deposits are mainly hosted by volcanic and volcaniclastic rocks that belong to the first cycle of the YHM group. The host-rock succession was interpreted to have been formed by back-arc extensional volcanism and was then deformed and metamorphosed to the lower greenschist facies. The relationship between the mineral occurrences and host rocks remains poorly constrained and controversial. We report new field observations, paragenetic data, mineral chemistry analyses, and in situ sulfur isotopes to characterize the sulfide mineralization and to gain insights into its genesis. Two mineralization styles have been identified in these deposits. The first style in the Um Samiuki deposit is expressed by two massive mineralized bodies that are mostly stratiform. It is distinguished by sphalerite, chalcopyrite, pyrite, galena, and Ag-bearing minerals; followed by bornite and covellite in decreasing abundance. Less abundant supergene phases include secondary copper minerals and iron oxyhydroxides. The hydrothermal alteration is characterized by sericitization, chloritization, epidotization, and silicification. The second style represents disseminated and vein-type sulfides mainly encountered in Hilgit and Maaqal occurrences. It is characterized by pyrite with minor sphalerite and chalcopyrite. The δ34SV-CDT values range from + 5.44‰ to + 9.08‰ with an average of + 8.11‰ ±1.1‰ (n = 18) for massive mineralized zones and from − 2.25‰ to + 3.78‰ with an average of + 1.06‰ ±2.3‰ (n = 8) for disseminated and vein sulfides. Such a variation in the sulfur isotope signatures may imply that both mineralization styles were derived from the thermochemical sulfate reduction (TSR) of seawater and leaching of basement lithologies in varying quantities, consistent with Volcanogenic Massive Sulfide (VMS) deposits of back-arc basins. The negative δ34SV-CDT values of some sulfides from the disseminated and vein mineralization may be seen as evidence for magmatic SO2 disproportionation. The studied VMS deposits display a close spatial and genetic association with submarine felsic volcanism of the YHM group, and their temporal distribution corresponds closely with the Mozambique Ocean closure stage of the Pan-African Orogeny.

The Latowu Ultramafic Rock-Hosted Iron Mineralization in the Southeastern Arm Sulawesi, Indonesia: Characteristics, Origin, and Implication for Beneficiation

Latowu ultramafic block in the Southeastern Arm Sulawesi locally hosts elevated concentrations of Fe in addition to Ni. We investigated both host rock and mineralized samples' mineralogy and chemistry to find out mineralogical and chemical characteristics and interpret the iron mineralization process with beneficiation implications. The mineralogical nature of the samples was analyzed using optical microscopy and X-ray diffractometry (XRD) methods. The whole-rock and mineral chemistry analyses were performed using X-ray fluorescence (XRF) spectroscopy and electron probe microanalysis (EPMA) techniques. The analysis showed that the ultramafic rocks had been undergone a strong to complete serpentinization degree where lizardite appears to be the predominant mineral. Magnetite in this research comprised the principal iron-bearing mineral and functioned as discrete fine-grains and subhedral to anhedral crystals. Magnetite occurs as fragments in breccia, alteration rim in spinel, fine-grained disseminations, and micro veins. This research found that the whole-rock chemistry of an ultramafic breccia showed an elevated concentration in Fe2O3 with a grade of 28.44 wt%. Electron probe analysis of magnetite shows a wide variation of Fe ranging from 31.10 wt% to 67.20 wt%. It is interpreted that the formation of magnetite within ultramafic rocks is influenced by the hydration of primary minerals, mainly olivine. Iron is most likely released from olivine or pyroxene crystals during serpentinization, and the higher water content of serpentine promotes its mobility. It is suggested that the magnetic separation method can be potentially used to increase the Fe grade.

Mesoarchean continental intraplate volcanism and sedimentation: The case of the Simlipal basin, Singhbhum Craton, eastern India

The origin of continental intraplate basalts, their role in crust-mantle interaction, and processes of formation of the associated sedimentary rocks have remained important research problems. The poorly-studied Simlipal basin hosts a well-preserved, weakly-metamorphosed ensemble of three bands of mafic volcanic rocks intercalated with shallow marine sandstones and minor phyllites, occurring unconformably over a Paleoarchean continental basement of the Singhbhum Craton. Little is known about the age of the basin, the nature of sediment source and source weathering, petrogenesis of the volcanic rocks, and the tectonic setting of the volcano-sedimentary assemblage. To address these issues, we present whole-rock geochemical data on the Simlipal sediments and volcanic rocks, detrital zircon U-Pb ages and mineral chemical analyses. The sandstones and phyllites are characterized by high K2O/Na2O and chemical index of alteration. The source-indicating elemental ratios and REE patterns (negative Eu anomaly and flat HREE) suggest that felsic rocks, especially the granitoids resulting from shallow crustal melting, were the dominant source. The detrital zircons are mostly rounded with U-Pb ages ranging from 3.63 to 3.17 Ga. All these features suggest that the Simlipal basin formed during the Mesoarchean Era on a slowly subsiding continental platform undergoing extension, which received reworked and recycled sediments from a diverse but granitoid-dominated, intensely-weathered, mostly Paleoarchean source. The mafic volcanic rocks are low-Ti, tholeiitic basalts showing low CaO/Al2O3 and TiO2/Yb values, and flat HREE pattern indicating derivation from a spinel lherzolite mantle at shallow depth (<80 km). The presence of xenoliths of crustal rocks, quartz-normative nature, wide range of magnesium number (39–80), primitive mantle-normalized negative Nb, Ta and Ti anomalies, and high U/Nb and Th/Yb ratios reflect that the magma underwent significant fractional crystallization and crustal assimilation. The Simlipal volcanic rocks are consanguineous with the spatially close and coeval gabbro-anorthosite bodies, layered mafic–ultramafic rocks, and A-type Mayurbhanj Granite of the Singhbhum Craton, together forming an intraplate bimodal volcanic-plutonic association. We construe that the widespread Mesoarchean magmatism and sedimentation was linked to mantle upwelling possibly related to an event of lithospheric delamination and crustal extension.

Intermediate sulfidation epithermal Pb - Zn (± Ag ± Cu ± In) and low sulfidation Au (± Pb ± Ag ± Zn) mineralization styles in the Gonzalito polymetallic mining district, North Patagonian Massif

The Gonzalito polymetallic mining district is located within the northeast tip of the 600 km-long NE-trending polymetallic belt of the North Patagonian Massif, in Patagonia, Argentina. It comprises Paleozoic igneous-metamorphic rocks of the Mina Gonzalito complex, which are intruded by Middle Triassic trachytic-andesitic dykes of the Monasa Fm. through a complex system of high-angle fractures. Rhyolitic dykes crosscut the pre-Jurassic rocks. Numerous semi-parallel vein-like mineral deposits, whose classification and age have been subject of debate, sharply crosscut the igneous-metamorphic, trachytic-andesitic and rhyolitic units. Mineralization shows a strong structural control through a complex system of semi-parallel NW-N-NE oriented fracture-hosted mineral deposits. Extensional jogs and open dilational fractures allow multiband vein infills, vein-like hydrothermal and fault-tectonic breccias, stockwork veins and veinlets with primary growth open-space and boiling-derived textures, indicating a brittle geological setting that is proper of the epithermal environment. Based on the structural setting, sulfide-gangue-alteration mineral assemblages and textures, sulfide mineral chemistry and whole-rock geochemical analyses, two different styles of hydrothermal deposits can be distinguished: massive-sulfide intermediate sulfidation epithermal Pb – Zn ± Ag ± Cu ± In and disseminated-sulfide low sulfidation Au ± Pb ± Ag ± Zn. The first has a main-stage pyrite – In-rich sphalerite – chalcopyrite – galena ore assemblage in scarce quartz - carbonates gangue. The second shows disseminated pyrite ± sphalerite ± chalcopyrite ± galena with economic grades of Au in abundant quartz – adularia ± fluorite gangue. SHRIMP U–Pb zircon data of rhyolites reveal Early Jurassic ages of 193 ± 2 and 191 ± 2 Ma and, along with their bulk-rock geochemical features, they can be included within the Chon Aike magmatic province, locally known as Marifil volcanic complex. Crosscutting relationships between the Lower Jurassic rhyolites and the epithermal veins in the Gonzalito district, and the presence of Oligocene basalts partially covering the epithermal deposits towards the western area, constrain the mineralization event between the Lower Jurassic and the Oligocene. However, the crosscutting relationships of this district along with the already proven genetic, temporal and spatial links between epithermal ore and fluorite-rich deposits and the Jurassic magmatism-rifting of Patagonia suggest that the studied epithermal deposits might be yielded closer or during the Lower Jurassic.

Rare element minerals\u2019 assemblage in El Quemado pegmatites (Argentina): insights for pegmatite melt evolution from gahnite, columbite-group minerals and tourmaline chemistry and implications for minerogenesis

The pegmatite district of El Quemado (NW Pampean Ranges, NW Argentina) hosts several Ordovician pegmatite bodies of the LCT (Li, Cs, Ta) type. We present paragenetic assemblages for a set of samples from two of the El Quemado pegmatite groups, Santa Elena and Tres Tetas, and mineral chemistry analyses for gahnite, columbite-group minerals, tourmaline, micas, albite, microcline, and discuss the relation between their major element composition and the degree of evolution of pegmatite melts. The chemical composition of rare element minerals allows recognizing an evolutive trend reaching highly differentiated compositions, with complex paragenetic assemblages including Li-, Zr-, U-, Zn-, P-, Mn- and Ta-bearing minerals. The temperature of crystallization during the magmatic phase was below 400 °C. Non-pervasive hydrothermal alteration, testified by a moderate presence of phyllosilicates, affected the pegmatite bodies. Chlorite geothermometry indicates that the circulation of post-magmatic hydrothermal fluids occurred at a temperature ranging between 200 °C and 250 °C. The mineralogical features recognized in the El Quemado pegmatite rocks have implications for the metallogenesis of the region, suggesting that the pegmatites potentially contributed to the genesis of Ta-Nb oxide placer mineralizations.

Evolution of the gold (copper) mineralization in the porphyry stock and the related skarn zones and epithermal veins in the Astarghan area, NW Iran: Evidence from fluid inclusion, mineral chemistry and sulfur isotope analyses

The Astarghan area in northwest Iran (~50 km north of Tabriz) represents an excellent example of coexisting and associated skarn, porphyry and epithermal mineralizations of copper and gold. This contribution aims to study the geology, alteration and mineralization in these deposits, as well as the occurrence of gold and its host minerals, and present the physico-chemical characteristics of ore-forming fluids and the sulfur origin, in order to get a better understanding of development and evolution of these mineralizations. The Astarghan porphyry stock (Oligo–Miocene) intruded the upper Cretaceous flysch-type sedimentary sequence (calcareous sandstone and limestone with intercalations of siltstone, shale and marl) and brought about metamorphism and metasomatism along the contact. The Cu–Au mineralization in the Astarghan area occurred mainly in three forms: (1) skarn patches along the contact (Zone A), (2) stockwork-type and disseminated porphyry mineralization within the potassic (at Kaghdara and Boiokwari areas) and phyllic (at Sildirimdara area) alteration zones (Zone B), and (3) native gold-bearing low-sulfidation epithermal veins within the argillic alteration zone at Nowrozkala area (Zone C). The micro-thermometric studies on the porphyry-type Kaghdara potassic alteration zone showed that homogenization temperatures for liquid-rich and vapor-rich 2-phase, as well as halite-bearing multiphase inclusions range about 160–420 °C, 220–460 °C, and 320–580 °C, respectively. Salinities of the 2-phase and multiphase inclusions have ranges of 5–25 and 40–70 wt% NaCleq., respectively. However, fluid inclusions present in the epithermal veins of Nowrozkala are mainly liquid-rich 2-phase, and their homogenization temperature and salinity values lie within the ranges of 118–325 °C and 0.6–7.4 wt% NaCleq., respectively. The approximate estimated depth of the sulfide and gold mineralization in the porphyry system ranges from 1.6 to 2.5 km, and in the epithermal system is < 0.9 km. The sulfur isotopic analysis of pyrite from the porphyry and epithermal mineralizations in the potassic alteration zone at Kaghdara and argillic alteration zone at Nowrozkala indicate that the δ34S values (–0.1 to –0.3 and –0.5 to –1.1‰, respectively) are close to the range of magmatic sources. Finally, electron probe micro-analyses indicate that besides independent particles of 50–200 μm in the epithermal veins, gold is mainly hosted by tetrahedrite within the porphyry-related phyllic alteration zone, but especially stibnite within the epithermal veins; these minerals, as well as epithermal polyhedral pyrites contain native gold inclusions.

Monazite Microstructures and Their Interpretation in Petrochronology

The phosphate mineral monazite (LREE,Y,Th,Ca,Si)PO4 occurs as an accessory phase in peraluminous granites and Ca-poor meta-psammopelites. Due to negligible common Pb and very low Pb diffusion rates at high temperatures, monazite has received increasing attention in geochronology. As the monazite grain sizes are mostly below 100 μm in upper greenschist to amphibolite facies meta-psammopelites, and rarely exceed 250 μm in granulite facies gneisses and in migmatites, microstructural observation and mineral chemical analysis need the investigation by scanning electron microscope and electron probe microanalyzer, with related routines of automated mineralogy. Not only the microstructural positions, sizes and contours of the grains, but also their internal structures in backscattered electron imaging gray tones, mainly controlled by the Th contents, can be assessed by this approach. Monazite crystallizes mostly euhedral to anhedral with more or less rounded crystal corners. There are transitions from elliptical over amoeboid to strongly emarginated grain shapes. The internal structures of the grains range from single to complex concentric over systematic oszillatory zonations to turbulent and cloudy, all with low to high contrast in backscattered electron imaging gray tones. Fluid-mediated partial alteration and coupled dissolution-reprecipitation can lead to Th-poor and Th-rich rim zones with sharp concave boundaries extending to the interior. Of particular interest is the corona structure with monazite surrounded by apatite and allanite, which is interpreted to result from a replacement during retrogression. The satellite structure with an atoll-like arrangement of small monazites may indicate re-heating after retrogression. Cluster structures with numerous small monazite grains, various aggregation structures and coating suggest nucleation and growth along heating or/and enhanced fluid activity. Microstructures of monazite fluid-mediated alteration, decomposition and replacement are strongly sutured grain boundaries and sponge-like porosity and intergrowth with apatite. Garnet-bearing assemblages allow an independent reconstruction of the pressure-temperature evolution in monazite-bearing meta-psammopelites. This provides additional potential for evaluation of the monazite microstructures, mineral chemistry and Th-U-Pb ages in terms of clockwise and counterclockwise pressure-temperature-time-deformation paths of anatectic melting, metamorphism and polymetamorphism. That way, monazite microstructures serve as unique indicators of tectonic and geodynamic scenarios.

An investigation of the 27 July 2018 bolide and meteorite fall over Benenitra, southwestern Madagascar

Several dozen stones of an ordinary chondrite meteorite fell in and around the town of Benenitra in southwestern Madagascar during the early evening of 27 July 2018, minutes after a widely observed meteor fireball (bolide) transit and detonation. The event was confirmed by low-frequency infrasound recordings received at ~17h15 UTC (Coordinated Universal Time; 19h15 local time) at the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) infrasound station I33MG near Antananarivo, 542 km north-northeast of Benenitra. An energy release equivalent to 2.038 kt of TNT was calculated from the infrasound signals. Seismograph readings at the SKRH station 77 km north-northwest of Benenitra recorded a twostage signal consistent with the arrivals of an initial air-coupled ground wave at 16h48:08 UTC and a stronger pulse at 16h49:22 UTC linked directly to the atmospheric pressure wave. The infrasound and seismic signal arrival times suggest that the bolide entry and detonation occurred at approximately 18h46 local time (16h46 UTC), entry was from the northwest, and the detonation hypocentre was located within ~20 km of Benenitra. Despite meteorite debris being found among buildings within Benenitra, there was no damage to structures or injuries reported. Eyewitness accounts and photographic records indicate that approximately 75 mostly intact stones were collected; however, the remoteness of the area, the rugged nature of the terrain and sales of fragments to meteorite collectors have limited scientific analysis of the fall and the extent of the strewn field. The total mass of recovered stones is estimated at between 20 kg and 30 kg, with one fragment of 11.2 kg and several of ~1 kg. Petrographic and mineral chemical analyses indicate that the stones belong to the L6 class of ordinary chondrites. Cosmogenic radionuclide analysis confirms that the fall is linked to the bolide event. The name Benenitra has been officially accepted by the Meteoritical Bulletin Database.Significance:&#x0D; &#x0D; Eyewitness reports, CTBTO infrasound records, seismograph records and cosmogenic radionuclide analysis confirm a meteorite fall over Benenitra on 27 July 2018.&#x0D; Petrographic and geochemical analyses confirm that the meteorite is an L6 ordinary chondrite.&#x0D; Recovery of meteorite falls is rare; this is Madagascar’s second known meteorite fall and the first that can be linked to a bolide.&#x0D; Regional and global science monitoring networks can be interrogated to improve the understanding of bolide events.&#x0D; Interaction of scientists with local communities is important to dispel misunderstandings around scientific phenomena, and can improve collection of data.&#x0D;

Andesitic (SiO2: ~60 wt%) monogenetic volcanism in the northern Colombian Andes: Crystallisation history of three Quaternary volcanoes

The Quaternary Villamaría-Termales Monogenetic Volcanic Field (VTMVF) is one of the monogenetic fields located within the subduction related, volcanic chain in the northern Colombian Andes. The dominantly andesitic composition of the products from this field provides an opportunity to study the crystallisation history of monogenetic volcanic products, associated with long term crustal stagnation before eruption. This work focuses on the Victoria, Gallinazo and Tesorito monogenetic effusive volcanoes, using a combination of petrography, mineral chemistry and whole-rock chemical analysis, in addition to geothermobarometers, to estimate the crystallisation conditions of the main mineral phases and infer the depths at which such phases crystallised. We also present two K/Ar radiometric ages for Victoria and Gallinazo volcanoes in order to add to the known age (1.2 Ma) of the Tesorito volcano. This helps us to timeframe this volcanism within the ~140 km volcanic chain where it is located. The rocks are porphyritic with a glassy and microcrystalline groundmass, and display glomeroporphyritic and disequilibrium textures such as textures sieve in plagioclase and reaction rims in pyroxene. The main mineral phases forming the rocks are plagioclase (An22–67), orthopyroxene (Wo02 En65–80 Fs18–33), clinopyroxene (Wo39–45 En39–49 Fs07–17), amphibole (magnesiohastingsite, magnesihornblende, edenite and tschermakite), and Fe-Ti oxides Usp37–23 Mag63–77 and Ilm79–62 Hem21–39. Olivine (Fo71–87) only appears in the Victoria volcano. The volcanoes are subalkaline andesites with calc-alkaline affinity. Estimation of the crystallisation conditions (i.e. temperature and pressure) for in-equilibrium mineral phases, indicate the depths at which they crystallised. Olivine crystallised at 1198–1191 °C and ~1.4 GPa, orthopyroxene-clinopyroxene coexisted at 963–880 °C and 0.78–0.47 GPa, plagioclase formed at 928–908 °C and 0.76–0.17 GPa, amphibole crystallised at 998–877 °C and 0.79–0.16 GPa, and Fe-Ti oxides crystallised at 937–904 °C. This suggests that crystallisation starts at depths of ~53 km and it continues until the surface. The obtained radiometric ages for the Gallinazo (1.92 ± 0.06 Ma) and Victoria (1.97 ± 0.06 Ma) volcanoes indicate that they are ~0.7 Ma older than the nearby Tesorito volcano.

Integration of remote-sensing, structural, and geochemical data for characterizing granitoid rocks in Um Naggat pluton, Central Eastern Desert, Egypt

Integration of remotely-sensed data, field work in addition to geochemical and mineral chemistry analyses, indicates that the Um Naggat granite pluton (UNP) has distinctive lithologic and structural characteristics. Image transformation techniques of using Landsat-8 Operational Land Imager (OLI) data successfully discriminated lithologic characteristics and highlighted hydrothermally altered areas. Shuttle Radar Topography Mission (SRTM) and OLI data allowed highlighting the structural contacts and revealed that the dominant structural trends are NW-SE, NE-SW, and N-S. Petrographic examination, field, and structural survey results revealed that the (UNP) pluton consists of biotite alkali feldspar granite (BAFG) and alkali granite (AG) in addition to a hybrid granite zone (HGZ). The BAFG phase is dominated by an alkaline suite with composition ranging from granite to alkali feldspar granite and pertains to post-orogenic granite (POG) and within-plate granites (WPG). The AG represents the youngest alkali granite phase of magmatism in the studied pluton. Various enclaves from diorites and microgranites have been recorded within the HGZ. Field relationships confirmed with quantitative analysis of magmatic fabric measurements indicate that the AG granitic phase was emplaced as subparallel-elongated sheets or dike-like masses trending E-W and associated with an extensional regime overprinted by the right-lateral strike-slip regime. This phase of “within plate” hypersolvus granite has excessive of large-ion lithophile (LIL) and high field strength (HFS) elements such as Zr, Nb, and Ga that belongs to A-type granites, being depicted by elevated alkali composition and high Fe2O3t/(Fe2O3t +MgO) ratios, and excessive amount of Zr, Y, Nb, and Ga. Microprobe analysis reveals that the compositions of amphibole mineral in the HGZ extent from magnesio-hornblende to actinolite and the temperature of the plagioclase range from 900 to 1050 °C, while the plagioclase of the BAFG is extremely pure albite in composition.

The gabbro to amphibolite transition along a hydration front

AbstractIn most metamorphic terrains the transition to the preserved mineral assemblage can be established to have occurred during progressive metamorphism through lower‐grade facies involving fluid saturated conditions. However, in some circumstances metamorphism can occur in a non‐progressive way as a distinct overprint of either igneous protoliths such as gabbro, or of earlier higher‐grade metamorphic rocks such as granulites. In this contribution we use thermodynamic modelling, together with petrographic and mineral chemistry analysis, to study a suite of amphibolite facies metamafic rocks that were variably affected by metamorphism, with the objective of explaining the textures and mineral assemblages observed. The study samples form a continuum in terms of textural relationships and mineral compositions, but they can be divided into three main types: metagabbro, lineated metagabbro, and amphibolite. Metagabbro samples preserve both relict igneous and metamorphic textures and a combination of igneous and metamorphic mineralogy. They preserve a variety of reaction textures, such as coronas of garnet around ilmenite, double coronas of low‐Al (actinolitic) and high‐Al (pargasitic) hornblende around clinopyroxene and limited degrees of recrystallization of igneous plagioclase. Lineated metagabbro samples preserve similar reaction textures as the metagabbro but show a strong lineation and have higher proportions of pargasitic hornblende and polygonal plagioclase combined with lower proportions of clinopyroxene, igneous plagioclase and actinolitic hornblende. Amphibolite samples are composed of pargasitic hornblende and polygonal plagioclase, presenting little igneous plagioclase and practically no garnet and clinopyroxene, and they preserve none of the reaction textures of the other two types. These samples are interpreted to be near chemically equilibrated on a hand sample scale, although with some mineral zoning preserved. and pseudosections show that the main mineral assemblage differences between the amphibolites and the other two types are primarily controlled by the amount of H2O that equilibrated with the rocks. While all rocks experienced an influx of fluid, only the amphibolites represent thermodynamically fluid‐saturated conditions, the metagabbro and lineated metagabbro having equilibrated under H2O undersaturated conditions, allowing the presence of garnet. In parallel with the variation in fluid influx, the variation in microstructure appears to be the result of varying length scales of diffusion. The complex corona structures in the metagabbros are interpreted to reflect the pre‐existing igneous texture and the apparent low mobility of Al during metamorphism. Here, igneous clinopyroxene is largely pseudomorphed by the actinolitic hornblende whereas the higher‐Al pargasitic hornblende and garnet occur adjacent to plagioclase or replacing it. Zoning patterns in amphibole crystals in the amphibolite are similar to those in metagabbro samples, which is consistent with earlier stages of the evolution of the amphibolite actually being similar to those preserved in the metagabbro.

A Textural and Mineralogical Study of the Shanzhuang Banded Iron Formation, Southeastern Margin of the North China Craton: Implications for the Overprint History of Hydrothermal Alteration and Supergenesis after Mineralization

AbstractThe newly discovered Shanzhuang BIF is hosted in the Shancaoyu Formation of the Taishan Group within the Eastern Block, southeastern margin of the North China Craton. The ores can be subdivided into three types in terms of mineral assemblages, corresponding to three types (I, II, III). The element concentration of the type I magnetite is similar to that of the type II magnetite, while the type III magnetite is similar to that of the schist. In general, magnetite and hematite grains from the ores show high concentrations of Mn (1317, 1162 ppm), Co (787, 1023 ppm), Al (2224, 2435 ppm) and Ti (540, 300 ppm), Whereas magnetite is depleted in Si (420 ppm) and hematite enriched in Si (1690 ppm). Detailed petrographic and mineral chemical analysis of magnetite, hematite, amphibole/hornblende and pyroxene, reveals that almost all the minerals occur as subhedral‐anhedral grains with pits and fractures, and the BIF is recrystallized to metamorphic assemblages of high amphibolite facies. Hornblende is highly enriched in Fe, Mg and Ca, but depleted in K and Na, mostly belonging to magnesiohornblende. In addition, the ratios of Mg/(Mg+Fe2+), Fe3+/(Fe3++Fe2+), Si/(Si+Ti+Al) and Al/Si are 0.48–0.64, 0.17–0.36, 0.79–0.88 and 0.14–0.27, respectively. It is suggested that hornblende is neither a typical magmatic origin nor a typical metamorphic. Pyroxene has the characteristics of high Ca and Fe, but low Ti and Al, with end‐member components En, Wo and Fs in the ranges of 25.22–28.64 wt%, 43.71–46.40 wt% and 24.51–27.62 wt%, respectively, belonging to clinopyroxene, and mostly diopside, might be formed during the prograde metamorphism in the absence of H2O. The carbonate such as dolomite‐ankerite series is probably a precursor mineral of the BIF deposit. Mass mineral chemical and structural characteristics indicate that the Shanzhuang iron deposit has been subjected to varying degrees of oxidized hydrothermal superimposed reformation, metamorphism, and supergenesis after mineralization, during which some elements have been migrated in some degree.

Mineral Chemistry–thermobarometry and Petrography of Metamorphic Sole Rocks of Kömürhan Ophiolite (SE Turkey): Constraints to Evolution and Emplacement

This paper presents the generation of metamorphic sole rocks through the detailed geochemical and petrographical analysis of field work carried out on the Kömürhan ophiolite. The metamorphic sole rocks of Kömürhan ophiolite are defined as amphibo-lite (Pl+Mg–Hbl+Ttn±Ap) plagioclase–amphibole schist (Pl+Mg–Hbl+Cpx+Ttn±Zrn±Ap), plagioclase–clinopyroxene–amphibole schist (Pl+Di+Mg–Hbl+Ttn±Ap), and epidote–plagioclase amphibole schist (Ep+Pl+Mg–Hbl+Ttn±Ap±Qtz±Zrn). This research mainly reports comprehensive petrography and mineral chemistry analyses of metamorphic sole rocks of Kömürhan ophiolite of SAOB (Southeast Anatolian Orogenic Belt) together with a goal of presenting geothermobarometric examination and unravelling the mineral sys-tematics. The metamorphic sole rocks have been observed as a thin slice and these rocks are seen at the base of the tectonites, metamorphosed in amphibolites facies throughout the intra–oceanic supra-subduction geodynamic environment. The Kömürhan ophiolite includes from the top to bottom volcanics, sheeted dike complex, isotropic gabbros cumulates, and tectonites and shows a complete oceanic lithospheric fragments. Analyses of mineral chemistry and petrography of metamorphic sole rocks have been used to exhibit the metamorphic processes of these rocks. Mineral chemistry analyses of pyroxene phenocrysts in the metamorphic sole rocks of Kömürhan ophiolite present similarities island arc tholeiite (IAT), proposing that protolith of the sole rocks was related to the supra-subduction geodynamic environment. The amphibolites were occurred by metamorphism of island arc tholeiite–type volcanics that separated from the front of the obducted ophiolite (Kömürhan ophiolite) and after that underplated.

Metamorphic evolution and SIMS U–Pb geochronology of orthopyroxene‐bearing high‐P semipelitic granulite in the Fuping area, middle Trans‐North China Orogen

AbstractThe Fuping area lies in the middle part of the Trans‐North China Orogen, which is a critical region for understanding the metamorphic–tectonic evolution of this orogenic belt in the Precambrian. The newly discovered orthopyroxene‐bearing semipelitic granulite in this area is a coherent stratigraphic unit, among which three generations of metamorphic mineral assemblages (peak, post‐peak and cooling retrograde) have been identified based on petrographic observation and mineral chemical analysis. The peak metamorphic mineral assemblage mainly consists of porphyroblastic garnet plus matrix minerals including biotite, plagioclase, orthopyroxene, quartz as well as accessory minerals including pyrite, monazite, zircon and apatite. Geothermobarometry calculations and phase equilibria modelling confirm that the representative samples record different P–T conditions and metamorphic stages due to retrogressive metamorphism, with the highest P–T conditions reaching high‐P granulite facies. The retrieved clockwise metamorphic P–T paths pass from 12.5 to 13.5 kbar/855–880℃ (peak stage) through 8.5–11 kbar/850–886℃ (post‐peak stage) and to a speculative cooling phase (retrograde stage), reflecting near isothermal decompression (ITD) followed by near isobaric cooling (IBC). This P–T path is interpreted to reflect a subduction and/or collision event followed by a rapid exhumation. Secondary ion mass spectrometry (SIMS) U–Pb dating on metamorphic monazite and zircon yielded metamorphic ages ranging from 1,825 to 1,815 Ma, possibly constraining a retrograde metamorphic age. Therefore, the semipelitic granulite in the Fuping area records the subduction/collision event between the Eastern and Western Blocks of the North China Craton during the Late Palaeoproterozoic.

Metamorphic Differentiation via Enhanced Dissolution along High Permeability Zones

AbstractMetamorphic differentiation, resulting in segregated mineral bands, is commonly recorded in metamorphic rocks. Despite the ubiquitous nature of compositionally layered metamorphic rocks, the processes that are responsible for metamorphic differentiation receive very little attention. Here, detailed petrography, quantitative mineral chemistry and bulk rock analyses are applied to investigate compositional variations and assemblage microstructure. Furthermore, thermodynamic modelling is applied to provide additional constraints on the P–T–XH2O conditions of assemblage formation and mass transfer. The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone lithology, defined by the interlayering of amphibolite with leucocratic domains. Within the granulite, quartz-lined fractures surrounded by amphibolite-facies alteration haloes represent relics of initial fluid infiltration associated with brittle failure. The fracture assemblage (quartz + plagioclase + zoisite + kyanite ± muscovite ± biotite) is identical to that occurring within leucocratic domains of the shear zone. Consequently, the compositional layering of the shear zone lithology is linked to fluid infiltration along localized zones of high permeability that result from fracturing. Mass-balance calculations indicate that quartz-lined fractures and compositional differentiation of the shear zone resulted from mass redistribution internal to the shear zone rather than partial melting or precipitation of minerals from externally derived fluid. The process of internal fractionation within the shear zone is driven by enhanced dissolution along highly permeable fracture planes resulting in the loss of MgO, Fetot and K2O from the leucocratic domains. Elements dissolved in the fluid are then transported and ultimately either precipitated in comparatively impermeable amphibolite domains or removed from the system resulting in an overall mass loss. The mass transfer causing metamorphic differentiation of the shear zone is the result of coupled reaction and diffusion under differential stress. The mechanisms of mass redistribution observed within this shear zone provides further insight into the processes that facilitate mass transfer in the Earth’s crust.

Episodic Melt Percolating and Chromite Mineralization in Yushigou Peridotite, North Qilian Suture Zone

AbstractHarzburgite‐dunite associations are the dominant type of ultramafic heterogeneities in the exhumed peridotites. Although the dunite network are usually small in volume of exposed mantle rocks, their mineral structures and chemical variations are widely observed in various tectonic environments and record complex mantle evolution. The Yushigou peridotite complex displays a 25 km2 mantle section belonging to a partly dismembered ophiolite in North Qilian suture zone, northwest China. It is mostly composed of depleted spinel harzburgite re‐equilibrated at temperatures of 900 °C, contain dunite veins and minor podiform chromite at its Moho level.Here we report the petrological observations and mineral chemical analyses of harzburgite‐dunite associations in the Yushigou peridotite complex. Based on the data we classified high Cr # (51–74) and low Cr# (19–34) dunite veins in this Yushigou complex. The podiform chromite are mainly high Cr# affinity and are production of melt‐harzburgite interaction. There are three main microstructures recorded the episodic melt‐rock interaction process: (1) complex spinel mineral inclusions both in harzburgite and two kind of dunite record widespread melt percolating. The observed primary inclusions (not related to later low temperature fluid activities) including hydrous and anhydrous silicates and minor sulphides, such as PGE and Fe‐Cu‐Ni related sulfidies, olivine, orthopyroxene, clinopyroxene, amphibole and phlogopite. (2) In all dunite samples, clinopyroxene dominated sieve‐textured rims around spinel are more abundant and more thicken than the harzburgite; implying stained/pulsated melt supply after the chromite crystallization. (3) Millimeter scale veined reaction patches in some dunite samples consist of clinopyroxene, amphibole, phlogopite, Fe‐Ni sulfidies and carbonates. Such metasomatic minerals assemblages may response for the later fluid infiltration after chromite mineralization. All these petrological observations and mineral chemical compositions (major and trace element contents) thus provide a wealth of information on the pathways of migrating melts in the mantle wedge, and provide constrain on nature and evolution of the uprising magma. Strong enrichment of incompatible trace elements in dunite clinopyroxene (LREE/HREE &gt;10) possible reflect a slab origin of the percolating melt which is not observed elsewhere participating in chromite forming.

Geochemical and metamorphic record of the amphibolites from the Tuting–Tidding Suture Zone ophiolites, Eastern Himalaya, India: implications for the presence of a dismembered metamorphic sole

AbstractThe Tuting–Tidding Suture Zone (TTSZ), exposed along Dibang and Lohit river valleys in Arunachal Himalaya, NE India, is the easternmost continuation of the Indus–Tsangpo Suture Zone (ITSZ) and consists of ophiolites associated with metabasics and carbonates. Amphibolites, existing at the base of the ophiolite complex, were studied using whole-rock, mineral chemical analyses and pressure–temperature (P-T) pseudosection modelling to understand their metamorphic and petrogenetic history, and interpret the tectonic environment of their formation. They exhibit two-stage deformation, where D1 is depicted by polymineralic inclusion trails in former melt pools and the main foliation represents D2. Sub-alkaline tholeiitic character, high-field-strength element (HFSE) ratios and mid-oceanic ridge basalt (MORB) -like rare earth element (REE) patterns with negative Eu anomaly indicate that the protolith of these amphibolites originated in a spreading regime by extensive partial melting of a depleted mantle source at shallow depth. Petrography, mineral chemistry and P-T modelling indicate a three-stage metamorphic history for them. M1 is the prograde (c. 2.1 GPa, c. 450°C) defined by garnet centre compositions corresponding to the D1 event. The existence of former melts in the samples demarcates the M2 stage (1.4–1.8 GPa, c. 600°C). The rocks later underwent retrogression (M3: 0.8–1.0 GPa, 480–520°C), which corresponds to the D2 event. These observations suggest that the protolith of the TTSZ amphibolites originated in a mid-oceanic ridge setting, which accreted below a subduction zone where it underwent M1 metamorphism followed by M2 metamorphism, corresponding to partial melting of the rocks. Finally, the M3 event occurred during the obduction phase of the ophiolite complex, where the amphibolites were obducted as the metamorphic sole of the TTSZ ophiolites.