Mineral Chemical Analyses Research Articles

Multistage gedrite in gedrite–hypersthene-bearing high-grade granulites from Daltonganj, Chhotanagpur granite–gneissic complex, Jharkhand, as evident from TEM and textural relations

The studied rock consists of mineral phases garnet–cordierite–gedrite–hypersthene–biotite–quartz and lies nearly 14 km southwest of Daltonganj in the western part of Chhotanagpur granite–gneissic complex. Textural relations of mineral phases, mineral chemistry and transmission electron microscopy (TEM) analyses of the rock samples suggest the early (gedrite1), middle (gedrite2) and late stages (gedrite3) formation of gedrite. Hypersthene appears through the metamorphic reaction gedrite $$+$$ quartz $$=$$ orthopyroxene $$+$$ garnet $$+$$ cordierite $$+ \,\hbox {H}_{2}$$ O due to the breakdown of gedrite3 at the thermal peak of 869 $$^{^{\circ }}$$ C/7.79 kbar. The bundles, prismatic and fibrous forms of three gedrites have been observed in the TEM images. TEM images and selected area electron diffraction patterns show the distribution of metallic element position at the different lattice site. The P–T estimates of the rock from garnet–orthopyroxene, garnet–cordierite and garnet–biotite exchange geothermometers and garnet–sillimanite–cordierite–quartz geobarometers vary from 775 $$^{\circ }$$ to 869 $$^{\circ }$$ C, 642 $$^{\circ }$$ to 703 $$^{\circ }$$ C, 480 $$^{\circ }$$ to 617 $$^{\circ }$$ C and 6.76 to 7.79 kbar, respectively.

In-situ mineral chemistry and chronology analyses of the pitchblende in the Shazijiang uranium deposit and their implications for mineralization

In-situ mineral chemistry and chronology analyses of the pitchblende in the Shazijiang uranium deposit and their implications for mineralization

Mineralogical Study of the Advanced Argillic Alteration Zone at the Konos Hill Mo–Cu–Re–Au Porphyry Prospect, NE Greece

The Konos Hill prospect in NE Greece represents a telescoped Mo–Cu–Re–Au porphyry occurrence overprinted by deep-level high-sulfidation mineralization. Porphyry-style mineralization is exposed in the deeper parts of the system and comprises quartz stockwork veins hosted in subvolcanic intrusions of granodioritic composition. Ore minerals include pyrite, molybdenite, chalcopyrite, and rheniite. In the upper part of the system, intense hydrothermal alteration resulted in the formation of a silicified zone and the development of various advanced argillic alteration assemblages, which are spatially related to N–S, NNW–SSE, and E–W trending faults. More distal and downwards, advanced argillic alteration gradually evolves into phyllic assemblages dominated by quartz and sericite. Zunyite, along with various amounts of quartz, alunite, aluminum phosphate–sulfate minerals (APS), diaspore, kaolinite, and minor pyrophyllite, are the main minerals in the advanced argillic alteration. Mineral-chemical analyses reveal significant variance in the SiO2, F, and Cl content of zunyite. Alunite supergroup minerals display a wide compositional range corresponding to members of the alunite, beudantite, and plumbogummite subgroups. Diaspore displays an almost stoichiometric composition. Mineralization in the lithocap consists of pyrite, enargite, tetrahedrite/tennantite, and colusite. Bulk ore analyses of mineralized samples show a relative enrichment in elements such as Se, Mo, and Bi, which supports a genetic link between the studied lithocap and the underlying Konos Hill porphyry-style mineralization. The occurrence of advanced argillic alteration assemblages along the N–S, NNW–SSE, and E–W trending faults suggests that highly acidic hydrothermal fluids were ascending into the lithocap environment. Zunyite, along with diaspore, pyrophyllite, and Sr- and Rare Earth Elements-bearing APS minerals, mark the proximity of the hypogene advanced argillic alteration zone to the porphyry environment.

A Geometallurgical Approach to Tailings Management: An Example from the Savage River Fe-Ore Mine, Western Tasmania

At the Old Tailings Dam (OTD), Savage River, Western Tasmania, 38 Mt of pyritic tailings were deposited (1967 to 1982) and have since been generating acid and metalliferous drainage (AMD). Mineral chemistry analysis confirmed high concentrations of refractory cobalt in pyrite (up to 3 wt %). This study sought to determine, through a series of bench scale tests, if Co could be liberated using biohydrometallurgical techniques. Four bulk tailings samples were collected across the OTD, from up to 1.5 m depth, targeting three sulphide-bearing facies. The study was conducted in four stages: (1) bacterial adaption using BIOX® bacteria; (2) biooxidation optimization with pH, temperature and Fe medium parameters tested; (3) flotation test work to produce a sulphide concentrate followed by biooxidation; and (4) Fe and Co precipitation tests. The BIOX® culture adapted to the bulk composite (containing 7 wt % pyrite) in ~10 days, with biooxidation occurring most efficiently at pH 1.5–1.6 and 40 °C whilst the Fe medium concentration was identified as a less-controlling parameter. Flotation produced a 71% pyrite concentrate with total oxidation occurring after 14 days of biooxidation with 99% of Co leached. At pH 3, Co was effectively separated from Fe, however Ni and Cu were also present in the pregnant liquor solution and therefore required refining before production of cobalt hydroxide, the intermediate saleable product. This study shows that adopting a geometallurgical approach to tailings characterisation can identify if mine waste has commodity potential and how best to extract it therefore unlocking the potential for unconventional rehabilitation of AMD affected sites.

Petrology and geochemistry of the 2014–2015 Holuhraun eruption, central Iceland: compositional and mineralogical characteristics, temporal variability and magma storage

The 2014–2015 Holuhraun fissure eruption provided a rare opportunity to study in detail the magmatic processes and magma plumbing system dynamics during a 6-month-long, moderate- to large-volume basaltic fissure eruption. In this contribution, we present a comprehensive dataset, including major and trace elements of whole-rock and glassy tephra samples, mineral chemistry, and radiogenic and oxygen isotope analyses from an extensive set of samples (n = 62) that were collected systematically in several field campaigns throughout the entire eruptive period. We also present the first detailed chemical and isotopic characterization of magmatic sulfides from Iceland. In conjunction with a unique set of geophysical data, our approach provides a detailed temporal and spatial resolution of magmatic processes before and during this eruption. The 2014–2015 Holuhraun magma is compositionally indistinguishable from recent basalts erupted from the Barðarbunga volcanic system, consistent with seismic observations for magma ascent close to the Barðarbunga central volcano, followed by dyke propagation to the Holuhraun eruption site. Whole-rock elemental and isotopic compositions are remarkably constant throughout the eruption. Moreover, the inferred depth of the magma reservoir tapped during the eruption is consistently 8 ± 5 km, in agreement with geodetic observations and melt inclusion entrapment pressures, but inconsistent with vertically extensive multi-tiered magma storage prior to eruption. The near constancy in the chemical and isotopic composition of the lava is consistent with the efficient homogenization of mantle-derived compositional variability. In contrast, occurrence of different mineral populations, including sulfide globules, which display significant compositional variability, requires a more complex earlier magmatic history. This may include sampling of heterogeneous mantle melts that mixed, crystallized and finally homogenized at mid- to lower-crustal conditions.

Phase Equilibrium Modeling of Retrograded Eclogite at the Kekesu Valley, Eastern Segment of SW Tianshan Orogen and Tectonic Implications

In this study, we report newly found strongly retrograded eclogite blocks in the Kekesu Valley, eastern segment of southwestern Tianshan, northwestern China. Based on detailed petrographical studies and mineral chemistry analyses, we constrain P-T evolution of two representative samples via phase equilibrium modeling method using THERMOCALC software. The peak stage is evidenced by the primary inclusions of omphacite+glaucophane+lawsonite (inferred)+chlorite+white mica in garnet. Using grossular and pyrope isopleth thermobarometry in garnet, the P peak conditions for this stage were constrained to be ~23 kbar, 480–500 °C and the T peak conditions were constrained to be 19–20 kbar, 500–520 °C. Subsequent exhumation led to the transformation of lawsonite to epidote. Using Si-inphengite barometry, the P was constrained to be 16–20 kbar at T of 500–520 °C for this stage. Further retrograde stage was evidenced by the mineral assemblages of glaucophane+garnet+epidote+paragonite+ chlorite+white mica+quartz. Later stage retrogression is evidenced by glaucophane+epidote+chlorite+ quartz+white mica+albite+actinolite with P of 5–9 kbar when T<490 °C, indicating the eclogite may have been exhumed to the middle crust level and overprinted by blueschist-greenschist facies metamorphism. The results show that eclogite at the eastern segment of SW Tianshan shares similar P-T path to the HP rocks from the western segment, implying a similar P-T evolution for these rocks.

The Acoculco Caldera Complex magmas: Genesis, evolution and relation with the Acoculco geothermal system

The Acoculco Caldera Complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt, México. The ACC was formed 2.7 Ma, and since then, volcanic activity has persisted until 0.06 Ma, through the emission of domes, cinder cones, fissure lava flows and two ignimbrite eruptions dated at 1.2 and 0.65 Ma. The pervasive hydrothermal alteration in the central part of the ACC has motivated considerable geothermal exploration, however, none of it assessed the relation between the magmatic heat supply and the evolution of magmas, including their reservoir depth.Magmatic processes were investigated with petrography, major oxides-trace-element geochemistry, mineral chemistry and isotopic analysis. In addition, we performed a series of hydrothermal experiments in order to constrain the storage depth for the magma tapped 1.2 Ma during a reactivation of the caldera. We investigated the origin of magmas tapped during the eruptive history of the ACC, and the magmatic processes (magma mixing, assimilation and crystallization) that modified these magmas.After the caldera collapse 2.7 Ma, the local stress field was probably modified and allowed the ascent of peralkaline magmas through new plumbing systems. Such magmas mixed with calc-alkaline magmas and formed the post-caldera volcanism. Influx of new magma is considered as the probable heat supply that maintains the active magmatic system, whereas assimilation has been negligible, but yields insights about the depth at which processes have occurred. Our work suggests that there are either shallow storage zones (200–500 bars) where magmas crystallize and either erupt or stall and cool, or there are intrusive magmas unrelated to the ACC suite that have accumulated nearby and heat the reservoir magmas.

Diamond exploration and regional prospectivity of Western Australia

Pre-1.6 Ga rocks comprise around 45% of the onshore area of Western Australia (WA), constituting the West Australian Craton (WAC) (including the Archean Yilgarn and Pilbara Cratons) and the western part of the North Australian Craton (NAC). These areas provide the conditions suitable for diamond formation at depth, and numerous diamondiferous lamproite and kimberlite fields are known. As emplacement ages span close to 2500 Ma, there are significant opportunities for diamond-affinity rocks being present near-surface in much of the State, including amongst Phanerozoic rocks. WA’s size, terrain, infrastructure and climate, mean that many areas remain underexplored. However, continuous diamond exploration since the 1970s has resulted in abundant data. In order to advance future exploration, a comprehensive database of results of diamond exploration sampling (Geological Survey of Western Australia 2018) has been assessed. The Yilgarn and Pilbara Cratons have spinel indicators almost exclusively dominated by chromite (>90% of grains), whereas (Mg,Fe,Ti)-bearing Al-chromites account for more of the indicator spinels in the NAC, up to 50% of grains at the Northern Territory (NT) border. Increasing dominance of Al in chromites is interpreted as a sign of weathering or a shallower source than Al-depleted Mg-chromites. Garnet compositions across the State also correlate with geological subdivisions, with lherzolitic garnets showing more prospective compositions (Ca-depleted) in WAC samples compared to the NAC. WAC samples also show a much broader scatter into strongly diamond-prospective G10 and G10D compositions. Ilmenites from the NAC show Mg-enriched compositions (consistent with kimberlites), over and above those present in NT data. However, ilmenites from the WAC again show the most diamond-prospective trends. Numerous indicator mineral concentrations throughout the State have unknown sources. Due in part to the presence of diamondiferous lamproites, it is cautioned that some accepted indicator mineral criteria do not apply in parts of WA. For example Ca-depleted garnets, Mg-depleted ilmenites and Cr-depleted and Al-absent clinopyroxenes are all sometimes associated with strongly diamondiferous localities. Quantitative prospectivity analysis has also been carried out based on the extent and results of sampling, age of surface rocks relative to ages of diamond-prospective rocks, and the underlying mantle structure. Results show that locations within the NAC and with proximity to WA’s diamond mines score well. However, results point to parts of the WAC being more prospective, consistent with mineral chemical data. Most notable are the Hamersley Basin, Eastern Goldfields Superterrane and the Goodin Inlier of the Yilgarn Craton. Despite prolific diamond exploration, WA is considerably underexplored and the ageing Argyle mine and recent closure of operations at Ellendale warrant a re-evaluation of diamond potential. Results of mineral chemistry and prospectivity analysis make a compelling case for renewed exploration.

The Variscan evolution in the basement cobbles of the Permian Ponteranica Formation by microstructural and petrologic analysis

This work reconstructs the metamorphic evolution recorded in the cobbles of the Ponteranica Formation to be compared with the evolutions of the basements of the central Southern Alps and of the clasts of Aga and Vedello and Dosso dei Galli conglomerates. The Ponteranica Formation is part of the lower volcano-sedimentary cycle of the Permian sequences of the western Orobic basin. These sequences cap the Variscan basement in which different tectonometamorphic units were recognized and contoured. We sampled all the crystalline rock types preserved as clasts such as different types of metapelites, meta-intrusives, granitoids, and tourmalinites and reconstructed the relative chronology of the superposed equilibrium assemblages. Only for metapelitic rocks the PT paths were reconstructed on the ground of mineral chemical analyses and geothermobarometry. The cobbles of the Pontereanica Formation record two types of metamorphic evolution. In Type 1 cobbles, the M1 metamorphic assemblage is characterised by white mica, biotite, garnet, staurolite, ± kyanite and is interpreted as the result of the record of the Variscan collision. This assemblage is followed by M2 assemblage characterised by white mica, chlorite, ± biotite that represents the Variscan exhumation. In type 2 cobbles, relict andalusite and possible cordierite characterise the pre-M2 metamorphic re-equilibration that developed under a high thermal regime. The PT paths recorded in cobbles are consistent with the tectono-metamorphic evolutions recorded in the basement units suggesting that during the deposition of conglomerates, the type of tectono-metamorphic units exposed to erosion were equivalent to that presently exposed in the Orobic Alps.The evolution of type 1 cobbles is similar to the evolution recorded in the clasts of Aga and Vedello conglomerates, whereas the clasts of Dosso dei Galli Conglomerate recorded only the effects of the Variscan tectonic burial. The occurrence of tourmalinite cobbles may imply an age up to Upper Permian / Lower Triassic for the Ponteranica Formation. In this view, the age of the Permian conglomerates of central Southalpine decreases westward with the increase of the thermal maturity of the metamorphic imprints recorded in clasts. These observations are consistent with a westward migration of the extensional tectonics responsible for the basin opening.

The 3.5 Ga São Tomé layered mafic-ultramafic intrusion, NE Brazil: Insights into a Paleoarchean Fe-Ti-V oxide mineralization and its reworking during West Gondwana assembly

The São Tomé mafic-ultramafic intrusion, located in the Borborema Province, NE Brazil, consists of a group of metamorphosed dismembered intrusive rocks, composed of ol-websterite and clinopyroxenite, which host stratiform meter-sized massive ilmenite-magnetite lenses with up to 1.98 wt% V2O3 in magnetite. The ol-websterite rocks display dark discontinuous olivine bands intercalated with cumulate augite, whereas the clinopyroxenite rocks are composed mainly of anhedral augite. The mineral chemistry analysis of the mafic phases resulted in contents up to Fo87.28 and 4605 ppm of Ni in the olivine crystals and high-MgO augite (22.59 wt% MgO), suggesting that these rocks formed from the differentiation of a primitive, fertile peridotite melt. The retention of high nickel contents in the olivine crystals, despite the crustal contaminated signature revealed by Sm-Nd isotopes, suggests that there was no early sulfur saturation in the magma; additionally, the magnetite crystallization did not trigger sulfur saturation, inhibiting Ni-Cu sulfide precipitation or even the formation of individual platinum group metals (PGM) grains. Metamorphism at amphibolite-facies conditions resulted in the development of penetrative foliation and mineral assemblages, wherein tschermakite is dominant. Ultimately, the metasomatic imprint included the following stages: (1) a K-P-H2O stage, represented by newly formed phlogopite, apatite, chlorite and hornblende; (2) a calcic stage, with epidote-group minerals, calcite and minor pyrite, and (3) the least potassic alteration forming diablastic phlogopite and microcline. LA-ICP-MS U-Pb zircon data reveal a concordant age of 3506 ± 29 Ma for the São Tomé intrusion, interpreted as the igneous crystallization age, and thus, it represents the oldest record of continental crust generation in South America. 40Ar/39Ar phlogopite dating, on the other hand, revealed plateau ages of 597 ± 5 Ma and 517 ± 6 Ma. These Ediacaran-to-Cambrian ages represent the emplacement of local granites, which bear heavily on the source of magmatic fluids.

Química mineral e condições de cristalização do Plúton Granítico Barcelona, extremo nordeste da Província Borborema, Nordeste do Brasil

O Plúton Granítico Barcelona (PGB) localiza-se na porção extremo nordeste da Província Borborema, oeste do Domínio São José do Campestre, com idade Ediacarana presumida. O PGB é formado por três fácies petrográficas/texturais distintas, a fácies granito porfirítico dominante no PGB, a partir da análise de micropetrografia associada às análises de química mineral. A fácies granito porfirítico (biotita monzogranito) possui plagioclásio (oligoclásio com An25-20%), K-feldspato (microclina pertítica) e quartzo compondo sempre (&gt; 70 % modal). O anfibólio é a hastingsita com moderadas razões Mg/(Mg+Fe), pressões de cristalização entre 5,0-6,0 kbar e temperatura de 700°C, finalizando com 3,0-5,0 kbar e 500°C no eutético ternário granítico. O conteúdo de AlT do anfibólio, os valores de pistacita Ps = 28 no epídoto e o processo de martitização da magnetita é indicativa de ambiente com ƒO2 moderada a elevada. As biotitas possuem trend com leve tendência para o pólo da annita, denotando reequilíbrio em estágio subsolidus/tardi-magmático. Em diagramas discriminantes de séries magmáticas as biotitas se comportam como às de afinidade geoquímica subalcalina, coerentes com a cálcio-alcalina médio a alto K dos granitos porfiríticos. Zonação em cristais de plagioclásio, K-feldspato e allanita, é indicativo da importância do processo de cristalização fracionada na evolução/cristalização da fácies porfirítica deste plúton. A atuação de fluidos ricos em H2 O e CO2 propicia a transformação tardia de biotita e feldspatos, além de geração de mirmequitas. A presença de agregados de pequenos cristais de plagioclásio mirmequíticos exibindo contatos com ângulos de 120° indica cristalização em equilíbrio eutético.

New archaeometrical results related to Neolithic blueschist stone tools from Borsod-Abaúj-Zemplén County, Hungary

26 blueschist Neolithic polished stone tools coming from archaeological sites in Borsod-Abaúj-Zemplén County (NE Hungary) were archaeometrically analysed. All items were described and documented macroscopically and magnetic susceptibility was measured on each tool. Non-destructive XRD analyses were made on all implements to separate blueschists from other metamorphic rocks, because this technique can make a difference between the diagnostic sodic amphiboles and other amphiboles, which was confirmed by EDS/SEM in each case. Electron microprobe analyses were carried out on polished sections and selected intact axes by using “original-surface” method. Non-destructive Prompt Gamma Activation Analysis (PGAA) was performed on selected pieces. Blueschist polished stone tools were classified into five groups based on their mineral components and metamorphic evolution. Mineral chemistry analyses suggest that blueschists suffered a polyphase metamorphism. Our data were compared to the nearest blueschist outcrop in the Meliata Unit (Slovakia); it is supposed that the source area of the blueschist implements is the Meliata Unit and/or the blueschist pebbles from Cretaceous conglomerate in the Pieniny Klippen Belt.

Internal Characteristics of Loose Solid Source of Debris Flow in Zhouqu

In this paper, with debris flow in Zhouqu as the research object, combined with experiments such as cation exchange capacity (CEC), mineral chemical composition and water quality analysis, relation between water and salt in solid source forming debris flow was studied via soil column leaching test and soluble salt analysis, and internal characteristics of debris flow was accordingly showed. It was found that, the soil was loose, and the content of gravel and sand was high, and the content of fine particle was low. The soluble contents at the slope of the accumulation body were described as, collapsed accumulation body > landslide accumulation body, slope toe > slope top, gentle slope > steep slope, also related to length of the slope. The results indicated that accumulations released a large number of base ion after intense weathering, which migrated with water, concentrated and enriched at the slope toe. Saline soil with high salt content collapsed when encountering water and then formed mudflow, thus becoming the internal power to trigger and initiate debris flow to some extent.

Age and petrogenesis of the Lundy granite: Paleocene intraplate peraluminous magmatism in the Bristol Channel, UK

The Lundy granite forms part of the Lundy Igneous Complex, which is the southernmost substantive expression of magmatism within both the British Cenozoic Igneous Province and the wider North Atlantic Igneous Province. Its Qz + Pl + Kfs + Bt ± Grt ± Tpz mineralogy and peraluminous character contrast with other British Cenozoic Igneous Province granites farther north but are similar to the granites of the adjacent Early Permian Cornubian Batholith. We present the results of mapping, petrographical and mineral chemical analysis, and the first U–Pb zircon ages for the granite (59.8 ± 0.4 – 58.4 ± 0.4 Ma) and cross-cutting basic dykes (57.2 ± 0.5 Ma), which confirm a Paleocene age for magmatism. Zircon inheritance is limited but two cores imply the presence of Early Palaeozoic igneous rocks in the unexposed basement of SW England. The anomalous southerly location of the Lundy Igneous Complex is a consequence of mantle melting arising from the superimposition of localized lithospheric extension, related to intraplate strike-slip tectonics, with the distal ancestral Icelandic plume. Granite generation primarily reflects crustal partial melting during the emplacement of mantle-derived melts. The change in geochemical character between the Lundy granite (peraluminous) and other British Cenozoic Igneous Province granites (metaluminous or subalkaline) indicates a fundamental crustal source control between contrasting peri-Gondwanan and Laurentian basement provinces. Supplementary material: Sample locations, petrographic summaries, mineral chemistry data and secondary ionization mass spectrometry U–Pb analytical data are available at https://doi.org/10.6084/m9.figshare.c.3886783

Indium Mineralization in the Yejiwei Sn‐Polymetallic Deposit of the Shizhuyuan Orefield, Southern Hunan, China

AbstractThe Yejiwei deposit, which is located in the southern Hunan W–Sn–Pb–Zn Metallogenic Belt in south China, is a large‐scale porphyry–skarn–veinlet‐type deposit containing 806 t In. Mineralization occurs as porphyry‐type Sn (stockworks), skarn‐type Sn–Cu, marble‐hosted‐type Sn–Cu (veinlet), and vein‐type Pb–Zn ores. Thirty‐five ore samples were collected from the Yejiwei deposit for bulk and mineral chemical composition, microscopic observation and electron microprobe analyses. The porphyry‐type Sn ores contain variable amounts of In (2.3–76 ppm; mean of 17.4 ppm) with local In enrichment (226 ppm) and 1000 × In/Zn values are 3.8–52.4. The skarn‐type Cu–Sn ore is richest in In (12.3–214 ppm; mean of 114 ppm), and 1000 × In/Zn values are 2.4–117. In contrast, the In content of the marble‐hosted‐type Cu–Sn ores is relatively low (7.4–34.9 ppm; mean of 20.3 ppm), and 1000 × In/Zn is in the range of 0.61–5.5, and the vein‐type Pb–Zn ores in the external zone contain the lowest In contents (7.2–17.0 ppm; mean of 12.1 ppm) with 1000 × In/Zn values of 0.07–0.09. The ore minerals in the deposit include pyrite, pyrrhotite, cassiterite, and In‐bearing minerals of sphalerite, chalcopyrite, and stannite. Although only trace amounts of sphalerite are hosted in the porphyry ores, the sphalerite contains the highest In content (0.27–10.1 wt.% In) in the deposit. We observed the highest In contents of all In‐bearing sphalerite reported in China. The In contents of sphalerite in the skarn‐type ore range from 0.15 to 0.56 wt.%, whereas the marble‐hosted‐, and vein‐type ores have lower In contents (0.00–0.04, and 0.03–0.06 wt.%, respectively). The In resources of the Yejiwei deposit are mainly hosted in skarn ores of the No. 31 and No. 32 orebodies. The genesis of In in the Yejiwei deposit was closely related to the shallow intrusive environment of related igneous rocks. As W–Sn–Mo–Bi–Cu–Pb–Zn–Ag mineralization is widespread in south Hunan, this study would suggest a focus on skarn‐type Sn–Zn deposits for the future prospecting of In resources.

Mineral chemistry and P-T conditions of the adakitic rocks from Torud–Ahmad Abad magmatic belt, S-SE Shahrood, NE Iran

Torud-Ahmad Abad magmatic belt is located 175km east and southeast of Shahrood in the northern part of the Central Iran Structural Zone and includes a thick sequence of Paleocene to middle Eocene volcanic and volcanosedimentary rocks. This magmatic belt was formed by numerous hypabyssal igneous adakitic domes constituting basaltic andesite, andesite, trachyandesite, dacite, trachydacite, and dacite. The investigated rocks are mainly composed of pyroxene, amphibole, and plagioclase, with minor biotite and opaque minerals. Mineral chemical analysis reveals that plagioclase composition varies from albite to labradorite, clinopyroxene varies from diopside to augite, and amphibole varies from Mg-hastingsite to Mg-hornblende.Amphibole geothermobarometry suggests crystallization temperatures of 850–1050°C, at 2–6kbar and the temperature of 920–970°C, at a pressure of 3–4.5kbar, which are conditions in agreement with andesite and dacite formation. Clinopyroxene crystallized at temperatures of 1020–1170°C, at 2–10kbar, indicating crystallization at crustal depths of maximum 30km for the studied intrusive rocks in the Torud-Ahmad Abad magmatic belt.

Genesis of the Permian Kemozibayi sulfide-bearing mafic-ultramafic intrusion in Altay, NW China: Evidence from zircon geochronology, Hf and O isotopes and mineral chemistry

The recently discovered Kemozibayi mafic-ultramafic intrusion and its associated magmatic Cu-Ni sulfide deposits are located at the southern margin of the Chinese Altai Mountain, Central Asian Orogenic Belt in north Xinjiang, NW China. The intrusion is composed of olivine websterite, norite, gabbro and diorite. Disseminated and net-textured Ni-Cu sulfide ores are hosted in the center of the gabbro. In this work, new zircon U-Pb ages, Hf-O isotopic and sulfide S isotopic data, and whole rock and mineral chemical analyses are combined in order to elucidate the characteristics of the mantle source, nature of subduction processes, degree of crustal contamination, geodynamic setting of bimodal magmatism in the region, and the metallogenic potential of economic Cu-Ni sulfide deposit at depth. SIMS zircon U-Pb dating of the gabbro yields Permian ages (278.3±1.9Ma), coeval with the Kalatongke Cu-Ni deposit and with Cu-Ni deposits in the Eastern Tianshan and Beishan areas. Several lines of evidence (positive εHf(t) from +7.1 to +13.3, Al2O3, TiO2 and SiO2 contents in clinopyroxene from olivine websterite, high whole rock TiO2 contents) suggest that the primary magma of the Kemozibayi intrusion was a calc-alkaline basaltic magma derived from depleted mantle, and that the degree of partial melting in the magma source was high. The evolution of the Kemozibayi mafic-ultramafic complex was strongly controlled by fractional crystallization and the crystallization sequence was olivine websterite, norite, and then gabbro. This is evidenced by whole rock Fe2O3 contents that are positively correlated with MgO and negatively correlated with Al2O3, CaO and Na2O, similar LREE enrichment and negative Nb, Ta, Hf anomalies in chondrite and primitive mantle-normalized patterns, and a decrease in total REE and trace elements contents and magnetite content from gabbro through to norite and olivine websterite. Varied and low εHf(t) (+7.1 to +13.3) and high δ18O values (+6.4‰ to +7.2‰) in zircon, high La/Ba, and Rb/Y ratios, and low Nb/La ratios in whole rock samples, suggest 5–10% contamination by subduction related fluid and 10–15% contribution from an upper crustal component. The clinopyroxene TiO2 and Alz values (percentage of tetrahedral sites occupied by Al) in the Kemozibayi intrusion show characteristics of rift cumulate rocks, but minor arc cumulate features, indicating that the mantle source might have experienced a lower degree of subduction metasomatism. A high degree of partial melting of the depleted mantle and subducted oceanic sediments with related fluid metasomatism and crustal contamination in the Kemozibayi mafic-ultramafic intrusion are appropriate for the formation of an economic copper and nickel sulfide deposit. Crustal contamination, crustal S addition and early fractionation of olivine and sulfide induced S saturation and the formation of immiscible sulfide in the Kemozibayi intrusion. With no obvious olivine and Ni-rich sulfide co-crystallization, the Cu grade in the present orebody is higher than the Ni grade, and the proportion of mafic rocks is high in the Kemozibayi mafic-ultramafic complex. Cumulatively, these features suggest that the mafic-ultramafic intrusion underwent early olivine and Ni sulfide segregation, and that deeper or extended portions of the present intrusive body may host Ni mineralization.

Effect of organic matter removal on U-series signal in clay minerals

Clay minerals hold some key information on time and rate of weathering processes that could be deciphered by U-series disequilibria. However a removal of organic matter is generally required prior to clay mineral chemical analyses, especially in soils. The impact of two protocols of organic matter removal on the U-series disequilibria in three different clay minerals was therefore investigated. Natural or commercial kaolinite, illite and montmorillonite were treated with hydrogen peroxide and by calcination. Both investigated treatments impact the U-series signature of clay minerals. Yet the extent of this impact strongly depends on the clay-type: while kaolinite remains almost insensitive to the treatments, illite shows some equivocal shifts since the activity ratios measured after calcination could describe more accurately the pure clay signal than initial activity ratios, and montmorillonite displays the larger reactivity. Overall, the H2O2 treatment seems to induce more disturbance than the calcination process, mostly on U isotopes in illite and montmorillonite. Due to the oxidizing power of H2O2, the redox state of U held in the clays, i.e.: U(IV) or U(VI), strongly influences the extent of U leaching. In addition, a preferential leaching of 234U relative to 238U was observed in montmorillonite sample only, resulting to a U isotopic ratio fractionation. Conversely, during calcination, the most pronounced effect arises for Th isotopes. The congruent feature of Th loss, i.e.: Th loss≈mass loss, affecting the three clay minerals suggests a loss of Th through volatilization during this treatment. U or Ra are also affected, but to a lesser extent, by this volatilization. Overall, calcination causes a two- or three-fold reduced change of the (230Th/234U) and (234U/238U) activity ratios relative to H2O2 treatment. The (226Ra/230Th) ratio was clearly the less sensitive to any treatment. It was only slightly affected in kaolinite after calcination, and the observed change suggests an improvement of the measured value due to a removal of a Th-bearing external matter. Albeit not perfect, calcination should therefore be preferred to H2O2 treatment for organic matter removal before U-series analyses.

Magnetic properties of serpentinized peridotites from the Dongbo ophiolite, SW Tibet: Implications for suture‐zone magnetic anomalies

AbstractMagnetic properties of a suite of variably serpentinized peridotites from the Dongbo ophiolite, SW Tibet (China), have been investigated to determine the magnetic signatures of suture zones. The degree of serpentinization (S) for these peridotites is mainly in the range of S &lt; 60%. Petrography, mineral chemistry, and thermomagnetic analyses reveal that magnetite occurring in the interior of various serpentine veins is the predominant magnetic phase. Magnetic hysteresis and first‐order reversal curve diagrams suggest that the magnetite is mixture of interacting single‐domain, and pseudo‐single‐domain and/or multidomain particles. Superparamagnetic magnetite occurs in the S = 40–60% serpentinized but weakly magnetic dunites. Overall, the magnetite content and magnetic susceptibility increase consistently from ~0 to 20% of serpentinization and then decrease from S = 30 to 60%. Peak magnetite abundance is at S~25% (density, ~3.1 ± 0.05 g cm–3) and may suggest localized enrichment of fluids. The low magnetite abundance in dunite results from low‐temperature serpentinization. Finally, strong intensities of magnetization (1–10 Am–1) reside in the S &gt; 60% and S = 20–30% serpentinized peridotites, indicating that peridotites with such degrees of serpentinization contribute to the aeromagnetic anomalies within the Yarlung‐Zangbo suture zone in south Tibet.

Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado–Penalva do Castelo Shear Zone (Central Iberian Zone)

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D 3 : (a) D 3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D 3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T &gt; 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D 3 events. Supplementary material: The petrology data, mineral chemistry analyses of the calc-silicate units and EPMA analytical conditions, and the P–T modelling methodology can be found at https://doi.org/10.6084/m9.figshare.c.3785648