Mineral Chemistry Studies Research Articles

From melt- to crystal-rich magmatic systems during rift localization: Insights from mineral chemistry in Central Afar (Ethiopia)

Magmatism plays a key role in accommodating and localizing extension during continental breakup. However, how the crustal magmatic systems evolve at the continental-ocean transition is poorly understood. We address these questions by studying the evolution of the magmatic system in the rift of Central Afar (Ethiopia), currently marking the transition from continental rifting to oceanic spreading. We focus on the voluminous and widespread Upper Stratoid Series (2.6–1.1 Ma) and the following Central Afar Gulf Series (1.1–0.6 Ma), the latter corresponding to localization of volcanism in narrow magmatic segments. We carried out the first systematic study of major and trace element mineral chemistry for these two Series and integrated it with geothermobarometry estimates and geochemical modeling, to reconstruct the evolution of the magmatic system architecture during rift localization. The Upper Stratoid magmas evolved by fractional crystallization in a melt-rich, moderately zoned, middle-lower crustal (10–18 km) magmatic system, from where they rose directly to the surface. Polybaric plagioclase convection and dissolution of a plagioclase-rich crystal mush is recorded in the phenocryst texture and chemistry. The Central Afar Gulf magmas evolved at similar depth in a more complex and dynamic storage system, with magma rising and mixing through multiple, relatively small, crystal-rich and interconnected reservoirs. Our study documents the transition during the continental breakup, from an overall stable and melt-rich magmatic system feeding the voluminous and homogeneous Upper Stratoid eruptions to a more dynamic, interconnected and crystal-rich situation feeding small-volume eruption while the rift localizes.

Enhanced classification of pyrite generations based on mineral chemistry using uniform manifold approximation and projection (UMAP)

Trace element signatures are tracked to unravel the genetic history of ore deposits in several mineral systems. This is particularly true for pyrite, a ubiquitous component of many ore-forming systems, including orogenic gold deposits. Here, we critically compare the efficacy of utilizing Uniform Manifold Approximation and Projection (UMAP) versus Principal Component Analysis (PCA) as dimensionality reduction tools applied to a 31 element dataset collected using Laser Ablation Inductively Coupled Mass Spectrometry of pyrite grains from the Kibali gold district (Democratic Republic of Congo). Because of the non-linearity inherent to mineral chemistry and because of its superior preservation of local (distances within clusters) and global (separation between clusters) data relationships, the UMAP approach outperforms dimensionality reduction by PCA. We further present a workflow in which UMAP dimensionality reduction is followed by k-means clustering to guide the classification of pyrite generations in the Kibali case study. Validating this approach with trace elements and UMAP + k-means on a seed-by-seed petrography basis shows that the workflow significantly enhances the original pyrite classification, previously based on texture. This study thus emphasizes the utility of employing advanced statistical analysis methods to capture the intricate nature of pyrite formation. These findings will shape best practices for handling large multi-element datasets in pyrite mineral chemistry studies and are extrapolatable to other mineral systems in which trace element signatures are used to infer the conditions of ore deposit genesis.

Petrography, mineral chemistry and FTIR spectral study of lateritic iron oxide concretions from the Damodar River basin, India

Petrography, mineral chemistry and FTIR spectral study of lateritic iron oxide concretions from the Damodar River basin, India

Petrological and geochemical insights into the magma plumbing system of the Daliuchong dacite eruption, Tengchong Volcanic Field, SW China

The formation of highly evolved, dacitic magmas has been attributed to various processes, including crystal fractionation, partial melting of overlying crust, and/or assimilation of crustal material into an evolving magma chamber. These processes are undoubtedly primary processes involved in the formation of dacites, but they may not be the only mechanism involved in the formation of high-silica dacites. For instance, mafic magma replenishment has been proposed as an additional mechanism but has not been assessed, and thus, its role has not been well-constrained. The Daliuchong volcano is the result of one of the largest eruptive events within the Tengchong Volcanic Field (TVF) in southwest China during the Early-Middle Pleistocene. Here, we conducted detailed mineral textures, mineral chemistry, and geochemical studies on Daliuchong pyroclastic rocks to explore the pre-eruptive storage conditions and evolution processes of the magma. The Daliuchong pyroclastic rocks are dacitic in composition. The samples show porphyritic textures characterized by phenocrysts of plagioclase, amphibole, clinopyroxene, orthopyroxene, and Fe-Ti oxides. Additionally, two distinct types of glomerocryst are identified: a gabbroic glomerocryst containing plagioclase + clinopyroxene + orthopyroxene ± Fe-Ti oxides assemblage and a dioritic glomerocryst containing plagioclase + amphibole ± pyroxene ± Fe-Ti oxides assemblage. Both phenocryst and glomerocryst show rich micro-textures. The Daliuchong dacite exhibits bulk compositional heterogeneity. Analysis of bulk-rock data suggests that this heterogeneity may arise from both the differentiation of the dacite itself and the injection of mafic magma. The compositional similarity between the Daliuchong dacite and experimentally produced partial melts of metamorphic basalt supports that the Daliuchong dacite was predominantly formed through the partial melting of the mafic lower crust. Thermobarometry estimation indicates that clinopyroxenes with high Mg# crystallized at 560–870 MPa, whereas amphibole and clinopyroxenes with low Mg# crystallized at 185–300 MPa. Based on the observed phase relations and the calculated crystallization conditions, we propose that during the differentiation of the Daliuchong dacite, heterogeneous dacitic magma formed by partial melting accumulated in a deep magma reservoir (21–32 km) before subsequently ascending toward shallower depths. Crystallization of plagioclase, amphibole, Fe-Ti oxides, and small amounts of pyroxene and apatite occurred at a shallower depth (7–10 km). The presence of coarse-sieve texture, fine-sieve texture, and oscillatory zoning with high amplitude in plagioclase suggests intermittent injection of mafic magma into the shallow magma reservoir, with the eruption of dacitic magma occurring after the final mafic magma replenishment. The petrological evidence above advocates that primitive magma replenishment could have been involved in the formation and triggered the eruption of dacite in the Daliuchong volcano.

Discrimination of ivory from extant and extinct elephant species using Raman spectroscopy: A potential non-destructive technique for combating illegal wildlife trade.

The use of elephant ivory as a commodity is a factor in declining elephant populations. Despite recent worldwide elephant ivory trade bans, mammoth ivory trade remains unregulated. This complicates law enforcement efforts, as distinguishing between ivory from extant and extinct species requires costly, destructive and time consuming methods. Elephant and mammoth ivory mainly consists of dentine, a mineralized connective tissue that contains an organic collagenous component and an inorganic component of calcium phosphate minerals, similar in structure to hydroxyapatite crystals. Raman spectroscopy is a non-invasive laser-based technique that has previously been used for the study of bone and mineral chemistry. Ivory and bone have similar biochemical properties, making Raman spectroscopy a promising method for species identification based on ivory. This study aimed to test the hypothesis that it is possible to identify differences in the chemistry of mammoth and elephant ivory using Raman spectroscopy. Mammoth and elephant tusks were obtained from the Natural History Museum in London, UK. Included in this study were eight samples of ivory from Mammuthus primigenius, two samples of carved ivory bangles from Africa (Loxodonta species), and one cross section of a tusk from Elephas maximus. The ivory was scanned using an inVia Raman micro spectrometer equipped with a x50 objective lens and a 785nm laser. Spectra were acquired using line maps and individual spectral points were acquired randomly or at points of interest on all samples. The data was then analysed using principal component analysis (PCA) with use of an in-house MATLAB script. Univariate analysis of peak intensity ratios of phosphate to amide I and III peaks, and carbonate to phosphate peaks showed statistical differences (p<0.0001) in the average peak intensity ratios between Mammuthus primigenius, Loxodonta spp. and Elephas maximus. Full width at half maximum hight (FWHM)analysis of the phosphate peak demonstrated higher crystal maturity of Mammuthus primigenius compared to living elephant species. The results of the study have established that spectra acquired by Raman spectroscopy can be separated into distinct classes through PCA. In conclusion, this study has shown that well-preserved mammoth and elephant ivory has the potential to be characterized using Raman spectroscopy, providing a promising method for species identification. The results of this study will be valuable in developing quick and non-destructive methods for the identification of ivory, which will have direct applications in archaeology and the regulation of international trade.

Evolution of Ore-Forming Fluids at Azegour Mo-Cu-W Skarn Deposit, Western High Atlas, Morocco: Evidence from Mineral Chemistry and Fluid Inclusions

The Azegour Mo-Cu-W skarn deposit, located on the northern side of the Western High Atlas, occurs in lower Cambrian volcanic and sedimentary rocks. The mineralizations are linked to the hydrothermal alterations that affected carbonated layers of the lower Cambrian age during the intrusion of the calc-alkaline hyperaluminous Azegour granite. Four stages of the skarn and ore mineral deposition have been identified as follows. Firstly, (i) the early prograde stage and (ii) the late prograde stage. These prograde stages are characterized by anhydrous minerals (wollastonite, garnets, and pyroxenes) associated with scheelite mineralization. Based on mineral chemistry studies, the early prograde stage is dominated by andradite (Ad72.81–97.07) and diopside (Di61.80–50.08) indicating an oxidized skarn; on the other hand, the late prograde stage is characterized by a high portion of grossular (Gr66.88–93.72) and hedenbergite (Hd50.49–86.73) with a small ratio of almandine (Alm2.84–34.99), indicating “strongly reduced” or “moderately reduced” conditions with low f(O2). The next two stages are (iii) the early retrograde stage and (iv) the late retrograde stage, which contain hydrous minerals (vesuvianite, epidote, chlorite, muscovite, and amphibole) associated with sulfide. Fluid inclusions from pyroxene and quartz (prograde skarn stage) display high homogenization temperatures and high to low salinities (468.3 to &gt;600 °C; 2.1 to &gt;73.9 wt% NaCl equiv.). The boiling process formed major scheelite mineralization during prograde skarn development from dominated hydrothermal magmatic fluid solutions. By contrast, fluid inclusions associated with calcite–quartz–sulfide (retrograde skarn stage) record lower homogenization temperatures and low salinities (160 to 358 °C; 2.0 to 11.9 wt% NaCl equiv.). The distribution of the major inclusions types from the two paragenetic stages are along the trend line of fluids mixing in the salinity–homogenization temperature (magmatic water), illustrating the genesis of ore-forming fluid by mixing with fluids of low temperatures and salinities (metamorphic and meteoric waters).

Origin and Tectonic Implications of I-type Granites, North Delhi Fold Belt, NW, India: Insights from Whole Rock Geochemistry and Mineral Compositions

Abstract A comprehensive whole-rock geochemical and mineral chemistry study was carried out of granites from the Narnaul and surrounding isolated hills of North Delhi Mobile Belt, NW India to constrain their genesis, and tectonic environment. The distinctive features of granites are meta to peraluminous nature and decreasing trend of P2O5 with high SiO2 (&amp;gt;71%) ascribed to its metaigneous origin. In detail the chemical diversity of the studied samples is reflected by their affinity to Calc-alkaline to shoshonite series with high K2O/Na2O (mostly&amp;gt;1), low MgO (&amp;lt; 0.11%) and variation in Mg# (upto 0.5) and maficity with increasing SiO2. The positive Eu anomalies, high LILE, highly enriched LREE (La/Sm = 4.13–19.33) pattern and comparatively elevated concentrations of Cs, La, Ta, Dy, Zr, Pb and Yb suggest different episodes of magmatic differentiation. Our data indicate that melting of older subduction-related igneous rocks and the underwent fractional crystallization in hydrous conditions is the possible mechanism for the generation of I-type granites in collisional tectonic setting.

New evidence for the presence of the Inner Tauride Ocean: Lithological, geochronological and P-T correlations with the Tavşanlı and Afyon zones of Central Anatolia (Türkiye)

The Tavşanlı and Afyon zones, which are well defined in western Türkiye, represent high pressure-low temperature (HP-LT) metamorphic rocks formed in a subduction zone. The Korumaz and Hınzır mountains (northeast of Kayseri) between the Central Anatolian Crystalline Complex and the Eastern Tauride Belt are represented by a HP-LT metamorphic sequence, which is thought to have formed between the Carboniferous and the Cretaceous. Here we present 40Ar/39Ar data from white micas within the Lower-Middle Triassic calc-phyllites from Korumaz Mountain, which yielded an age of 87 ± 2 Ma. Therefore, based on new mineralogical and geochronological data we suggest that this low-grade (HP-LT) metasedimentary sequence correlates with the Afyon Zone. Tectonically emplaced on-top of the HP-LT metasedimentary units of the Korumaz and Hınzır mountains is a meta-accretionary complex consisting of serpentinite, listvenite, marble, phyllite, calc-phyllite, metabasite, and meta-plagiogranite. Petrographic and mineral chemical studies show that the meta-accretionary complex was subjected to metamorphism under the HP-LT conditions that are similar to those of the Tavşanlı Zone in western Türkiye. Zircon Hf isotopic compositions from the meta-plagiogranite in the Korumaz and Hınzır mountains exhibit a depleted mantle signature with positive epsilon Hf(t) values of +5.6 to +13.1, and yielded a U-Pb crystallization age of 101.14 ± 0.84 Ma, which is the oldest age determined in oceanic crustal rocks of the ophiolites from the Tauride Belt. Geochemically, the metabasites and meta-plagiogranites from the meta-accretionary complex show subduction-related characteristics. White micas from phyllites and listvenite, which dominate the matrix of the meta-accretionary complex, yielded 40Ar/39Ar metamorphic ages ranging from 78 to 60 Ma. Our new geological and geochronological data suggest that metamorphic units of the Korumaz and Hınzır mountains are the eastern continuation of the Afyon and Tavşanlı zones. This indicates that the Inner Tauride Ocean, the existence of which has been controversial in the literature, was indeed present between the Central Anatolian Crystalline Complex and the Eastern Tauride Belt.

Late orogenic high-temperature overprint in (ultra)high-pressure ([U]HP) metamorphic rocks of the North Qinling Orogen, Central China: Insights into the geodynamics of the exhumation of (U)HP metamorphic rocks

Exhumed (ultra)high-pressure ([U]HP) metamorphic rocks are markers of paleo-subduction zones and can provide crucial implications for the geodynamic evolution of subduction-exhumation. A variety of (U)HP metamorphic rocks from many collisional orogens have commonly documented a low-pressure high-temperature overprint, which appears to indicate a pervasive geodynamic process behind (U)HP metamorphic rock exhumation. A good case study of garnet amphibolite and associated garnet-biotite gneiss with an unambiguous low-pressure high-temperature overprint in the Xixia-Xiaguan area of the North Qinling Orogen, central China, provides a significant opportunity to uncover this geodynamic process. A detailed study of petrography, mineral chemistry, and phase equilibria modeling suggests that the investigated garnet amphibolite and garnet-biotite gneiss have jointly experienced multistage exhumation processes with near-isothermal decompression from the (U)HP eclogite facies (26.8−29.0 kbar/740−775 °C) to the medium-pressure (MP) amphibolite facies (8.7−10.0 kbar/740−775 °C), subsequent remarkable near-isobaric heating to the low-pressure (LP) granulite facies (6.8−8.2 kbar/832−835 °C), followed by final decompression and cooling to the LP amphibolite facies (5.8−6.8 kbar/740−757 °C). U-Pb age dating of zircons from the garnet amphibolite and garnet-biotite gneiss yielded metamorphic ages of ca. 500 Ma, 450 Ma, and 400 Ma, which we consider to represent (U)HP eclogite-facies, LP granulite-facies, and late LP amphibolite-facies metamorphism, respectively. According to the regional geology and the metamorphic history we have determined, this multistage exhumation process of two-step exhumation stages delimited by a phase of LP near-isobaric heating was associated with slab rollback. Accordingly, we propose a tectonic model of exhumation driven by slab rollback for the geodynamic evolution of (U)HP metamorphic rock exhumation in the North Qinling Orogen. Additionally, a combination of previous studies and our new results suggests that exhumation driven by slab rollback appears to be a fundamental mechanism for the (U)HP metamorphic rock exhumation within collisional orogens.

The propylitic alteration in the Ponce Enriquez Gold Mining district, Azuay province, Ecuador: genetic constraints from a mineral chemistry and fluid inclusions study

Wide areas of the Southern sector of Ecuador are characterized by Cretaceous mafic volcanic rocks, pervasively altered by a propylitic mineralogical assemblage with epidote, chlorite, and quartz with minor titanite, illite and prehnite. These propylitically altered rocks host significant gold mineralization in veins, deeply exploited in the last decades. Porphyry Au-Cu deposits also occur in the area. This work focuses on the study of this propylitic alteration to assess the genetic conditions and the relation with the distance from the porphyry system, through the chemical composition of the secondary minerals (particularly trace elements in epidote and chlorite) and fluid inclusion data. The aim is to improve knowledge of the trace element geochemistry of secondary minerals in propylitic alteration and their relationship to the causative porphyry systems. The results of this work indicate that alteration affecting the rocks of the Pallatanga Fm. can be related to the propylitic hydrothermal alteration halo of porphyry copper deposits. Fluid inclusion investigation in quartz veins associated with epidote and chlorite evidenced at least two pulses of hydrothermal fluids characterized by temperature around 345°C for the first one and around 305°C for the second one. The salinity of the fluids is always low (around 1.5 wt% NaClequiv.) suggesting a dominantly meteoric component. Evidence of boiling processes have not been observed. We hypothesize that the hydrothermally altered “green rocks” of the Ponce Enriquez Mining District could be envisaged as the propylitic halo of the Miocene hornblende-bearing Au-Cu porphyry deposits of Gaby-Papa Grande, and discuss the epidote and chlorite trace element composition in this scenario. The results help improving the general knowledge of the propylitically altered rocks in southern Ecuador and add new data for the use of trace element chemistry of epidote and chlorite in mineral exploration elsewhere.

Clastic-volcaniclastic Rocks in Papaghni Sub-basin: Implications for Syn-sedimentary Felsic Volcanism during the Deposition of Paleoproterozoic Vempalle Carbonate Sequence, Cuddapah Basin, India

Abstract Based on detailed field, petrographic, mineral chemistry and geochemical studies, succinct details on the clastic - volcaniclastic sequence recorded within the dolomite horizon in Paleoproterozoic Vempalle Formation, Cuddapah basin, India are presented. The clastic rocks represented by rudaceous-arenaceous sequence and the volcaniclastic rock represented by an intervening tuffaceous horizon are noticed in the Vempalle dolomite near Tummalapalle area, in southwestern part of the Cuddapah basin. The conglomerate is matrix supported with dominantly chert clasts and the arenaceous unit is represented by fine grained quartz arenite. The tuffaceous unit is finely laminated exhibiting typical; clast matrix texture characteristic of fine grained pyroclastic rocks. Zircon, apatite, rutile and monazite are the accessory phases in the tuff. Geochemically, the felsic tuff is rhyolitic, peraluminous and shows relative enrichment of LREE. The chemical composition of the feldspar falls close to the K-feldspar end member in the Or-Ab-An plot. The clastic - volcaniclastic sequence in the Vempalle carbonate sequence indicate a localised break in carbonate precipitation accompanied by syn-sedimentary felsic volcanism during the evolution of Paleoproterozoic segment in the Cuddapah basin, India.

Petrography, mineral chemistry, metamorphism and Raman spectroscopic studies of Mahadevpur fall meteorite, India

The Mahadevpur (27°39′48″ N, 95°51′32″ E; Arunachal Pradesh, India) meteorite is a fall‐type, coarse‐grained rock consisting of a heterogeneous mixture of chondrules of varying types occurring in a matrix comprising mostly chondrule fragments. It was earlier classified as poorly shocked (S1, unshocked) ordinary chondrite (H4/5). The minerals observed in the Mahadevpur meteorite include olivine, orthopyroxene, clinopyroxene, albitic‐plagioclase, Fe‐Ni alloy and troilite with minor amounts of chromite and phosphate. Olivine and orthopyroxene are almost equilibrated. The textures and compositional features of minerals and chondrule margins indicate a limited degree of recrystallization. The equilibration temperature (700–800°C) for the Mahadevpur chondrite is consistent with the computed temperature (752–902°C) derived from the coexisting orthopyroxene‐clinopyroxene pairs. In the light of new evidences observed during the present study, such as olivine exhibiting undulose extinction and planar to sub‐planar fractures, olivine and orthopyroxene showing strong mosaicism, the shock‐melted plagioclase glasses, the presence of high‐pressure mineral phases and silicate darkening, the earlier proposed shock stage pronouncement has been modified to S4–S5 with supporting petrographic, sub‐microscopic and Raman spectroscopic data. This is the first comprehensive account of the Mahadevpur chondrite with an upgraded shock stage classification.

Mineralogical study of electrum grain size, shape and mineral chemistry in process streams from the Krumovgrad mine, Bulgaria

The gold ore mined at the Krumovgrad mine, Bulgaria is processed by flotation following grinding and regrinding stages securing the targeted liberation size of the electrum which is the principal precious metal-bearing mineral. The findings from a SEM-based automated mineralogy study aiming to quantify electrum occurrences in the processing scheme are presented and the implications of these characteristics on precious metals recovery are discussed. It has been found that Au/Ag grade ratios in the electrum tend to increase in the concentrate streams and decrease in the tailings. The mineralogical analysis indicated that in comparison to the precious metals grains met in the reground rougher concentrate, those detected in the cleaner concentrate are characterized by about 10 % higher gold content. Moreover, particles possessing zones largely enriched in gold (zones where electrum is detected as having at least 90 % gold grade) are more frequently observed in the cleaner concentrate. Grains morphology suggests that the stirred media detritor (SMD) used in the regrinding circuit does not significantly modify particle shape. Likewise, opting for a staged flotation reactors (SFRs) as the main flotation units seem to be a judicious choice by maintaining conditions favouring the recovery of the ultrafine gold.

Mineralogy and Mineral Chemistry of the Au-Ag-Te-(Bi-Se) San Luis Alta Deposit, Mid-South Peru

A mineralogical and mineral chemistry study was carried out in the San Luis Alta telluride-rich gold deposit, mid-south Peru, to contribute towards determining its formation and improving the ore processing. The San Luis mineralization is considered an intrusion-related gold deposit located in the Arequipa segment of the Coastal Batholith. The mineralization occurs in quartz veins hosted in diorites and granodiorites from the Tiabaya Super-Unit. These veins are sulfide-rich in the deep areas and contain abundant iron oxides. Sulfides are mainly pyrite with minor chalcopyrite and galena. Native gold and telluride minerals are abundant. Mineral chemistry was determined using an electron microprobe. The mineralogy of veins was classified into four stages. Gold occurs in the three last stages either in large grains, visible to the naked eye, or, more frequently, in grains of less than 10 µm. Gold appears as grains encapsulated in pyrite, Fe oxides, quartz and filling fractures. The first stage is characterized by the deposition of quartz and massive pyrite, which does not contain gold. During the second stage, hessite, calaverite, petzite and altaite are formed. Additionally, Bi-tellurides, mainly volynskite, rucklidgeite, kochkarite and tellurobusmuthine, are formed. Some of these minerals occur as blebs encapsulated in pyrite, suggesting that a Bi-Te-rich melt was formed from the ore-forming hydrothermal fluid and transported the Au and Ag elements. This stage was followed by a fracturing event and tellurobismuthite, tetradymite and montbrayite precipitated. In the last stage, a supergene replacement formed covellite, bornite and goethite. Te-Bi minerals do not appear in this stage, but selenium minerals occur in minor amounts. Chlorargyrite and iodargyrite occur and are associated with gold.

The role of uranyl complex decomposition and redox conditions in the precipitation of hydrothermal uranium deposits: Insights from chlorite mineralogy and geochemistry in the Shazhou uranium deposit, Xiangshan, SE China

Uranium precipitation involves the decomposition of uranyl complexes and the reduction of hexavalent uranium, which can occur sequentially or simultaneously within one redox reaction. The redox condition of hydrothermal fluid plays a vital role in controlling the migration and precipitation of uranium in hydrothermal uranium deposits. However, little attention has been paid to the role of uranyl complex decomposition in uranium precipitation. In this study, chlorite mineralogy and geochemistry were examined to clarify the process of uranium precipitation in the Shazhou deposit, Xiangshan uranium orefield, Southeast China. Based on comprehensive petrographic and mineral chemistry studies of chlorites obtained from altered granite porphyries and uranium ores, five types of chlorites were identified: (1) chlorite present in the form of pseudomorphous biotite, which was produced by the hydrothermal alteration of biotite in rocks that underwent hematitization and chloritization (Chl-I); (2) chlorite filling the cleavage cracks in biotite in rocks that underwent hematitization and chloritization (Chl-II); (3) chlorite occurring in pyrite veins (Chl-III); (4) chlorite intergrown with pitchblende in ore veins (Chl-IV); and (5) chlorite occurring in calcite veins (Chl-V). Chlorite geothermometry revealed that the formation temperatures of the five types of chlorites ranged from 219 °C to 254 °C. Mineral chemistry analyses revealed that the five types of chlorites formed in a reductive fluid environment, where the oxygen fugacity at different stages is similar, with log fO2 values ranging from −41.6 to −39.1. Uranium precipitation started only in stage Chl-IV. The examination of the mineral assemblage revealed that the ore-forming fluid was rich in F−, HPO32−, and CO32−. Comprehensive investigation of chemistry and physicochemical conditions of the ore-forming fluid revealed that oxidized uranium (UO22+) could be complexed with HPO42− and F−, and uranyl phosphate and the uranyl fluoride complexes were the main uranium species when uranium precipitation and the decomposition occurred at stage Chl-IV. However, the assessment of oxygen fugacity of the solution equilibria between the UO2(s) and the uranyl phosphate and uranyl fluoride complexes revealed that the reducibility of the fluid favoring the reduction of uranyl ions (UO22+) to U4+ was insufficient to reduce the uranyl phosphate and uranyl fluoride complexes. This indicates that the breakup of uranyl phosphate and the uranyl fluoride complexes to release uranyl ions should occur first at stage Chl-IV, reducing uranyl ions to U4+, and leading to uranium precipitation. Hence, the decomposition of uranyl complexes played an important role in uranium precipitation. With the increase in pH, uranyl phosphate and the uranyl fluoride complexes gradually decomposed and became reducible. Moreover, the decrease in ligand concentration was conducive to the decomposition of uranyl phosphate and the uranyl fluoride complexes. During the formation of the Shazhou deposit, the fluid boiling process induced the loss of volatiles, such as CO2, CH4, HF, and H2S, leading to an increase in pH and decrease in HPO42−, F−, and CO32− concentrations in the fluid. These factors also led to the decomposition of uranyl fluoride and the uranyl phosphate complexes and the precipitation of fluorapatite and calcite. Uranium was then reduced by the action of Fe2+ and S− (pyrite) from a hexavalent to a tetravalent, and finally, uranium was precipitated to form uranium ores.

New insights into chemical and spectroscopic characterization of beryl mineralization related to leucogranites in the west Wadi El Gemal area, southern Eastern Desert of Egypt

Our combined mineral chemistry and spectroscopic studies enabled us to envisage details about composition and genesis of beryl from the Eastern Desert of Egypt. Two types of beryl were investigated including emerald from ancient workings in the schist-related type whereas the other type is non-gem and granite-related. On the basis of FeOt-Cr2O3-MgO ternary classification, the Wadi El Gemal emeralds (alkali-type, sodic) are Mg-bearing while the Mikpid-Akarem beryls (normal-type) are Fe2+-bearing. Intense green colour of emerald results in a 323 cm−1 Raman shift due to ∼0.77 wt% Cr22O3 derived from serpentinite. In the Mikpid-Akarem district (belonging to the granite-related type), beryls are either faint blue or white and lack Cr, Mg and AlIV. On the other hand, they are relatively enriched in Fe and AlVI compared with the Wadi El Gemal emeralds (up to 0.13 & 1.97 apfu and 0.04 & 1.68 apfu, respectively), which is a function of the peraluminous composition of the granite host. Al3+ is substituted in the Y-site by di- to tetravalent cations (Mg2++Fe2++Cr3++Ti4+) and this is paired with a vacancy substitution by Na+. The Wadi El Gemal emeralds have H2O & K+ in the 2a-site and H2O & Na+ in the 2b-site. The latter site is vacant in the Mikpid-Akarem beryls.The FTIR spectra of our beryls and emeralds show a series of stretching and bending vibrations due to the presence of type I H2O (OH-Na band at 3657 cm−1) and type II H2O (stretching at 3591–3657 cm−1 & bending at 1631 cm−1). Lacking of Li+ leads to the absence of OH-Li band that is indicative of type III H2O. Hypothetical H2O in the Wadi El Gemal emerald (green beryl) amounts 2.24–2.92 wt% whereas it amounts 1.21–1.25 wt% in the Mikpid-Akarem non-green beryl. The asymmetric stretching vibration at 2921–2923 cm−1 is related to CH due to the presence of CH4, and possibly CO2, in the fluid inclusions trapped in the analyzed crystals. Some beryls, e.g. the Um Kabu emerald, has a Raman shift at 672 cm−1, which indicates maximal cation substitutions in the octahedral sites and minimal H2O in the channel sites. An uncommon shift at 147 cm−1 can be attributed to defect structure due to presence of radioactive zircon and monazite inclusions. Fe2+ substitution in the octahedral sites is appreciable and results in an UV–Vis-NIR band at 836 nm.

Unravelling magmatic-hydrothermal processes at Salobo and GT-46 IOCG deposits, Carajás mineral province, Brazil: Constraints from whole-rock geochemistry and trace elements in apatite

The iron oxide-copper-gold (IOCG) deposits of the northern portion of the Carajás Province, Amazonian Craton, Brazil, were primarily formed at the 2.5 Ga IOCG event. These deposits commonly exhibit wide Fe–K hydrothermal alteration halos (i.e., almandine-grunerite-biotite-magnetite) and display a temporal and spatial association with the 2.5 Ga granitic bodies, particularly in the GT-46 and in the world-class Salobo deposit. However, this association still lacks investigation concerning the metallogenic influence of the 2.5 Ga granitic bodies in the IOCG formation. Within these deposits, ca. 2.5 hydrothermally altered granites are recognized and include the Old Salobo, at the Salobo deposit, and G1 and G2, at the GT-46 deposit. Whole-rock geochemistry combined with petrography has pointed out that the G1 and Old Salobo are represented by granites to monzogranites, ferroan, metaluminous to weakly peraluminous with negative Eu anomaly. On the contrary, the G2 granite seems to represent a distinct body with granite composition, ferroan, metaluminous to weakly peraluminous with positive Eu anomaly. This granite exhibit pegmatitic textures, high Sr/Y ratios and listric-shaped normalized REE patterns, which suggest crystallization under high water contents. These granites primarily plots in the field of the volcanic arc granites, though some shifts towards to syn-collisional granites are observed. Effects of hydrothermal alteration are commonly seen in these granites both in textural aspects and geochemistry. Apatite mineral chemistry studies shows that magmatic signature in apatite is preserved in the G2 granite (G2core apatite), Old Salobo granite (OSA) and in the GT-46 ore zone (OZGT-A apatite), represented by enrichment in LREE and negative Eu anomaly. Hydrothermally altered apatite is recognized in the Old Salobo (OSB) and G2 granite (G2rim) and in the ore zone of GT-46 deposit and (OZGT-B) with depletion in LREE, which suggest that the same fluid might have affected both deposits. Truly hydrothermal apatite crystals from the Salobo and GT-46 ore zones shows high (Eu/Eu*)N values, indicative of high oxygen fugacity and alkaline fluids. Strong HREE depletion of the apatite grains from the central zone of the deposits suggest coeval almandine crystallization in the Fe–K alteration halos and evolved hydrothermal fluids. These results indicate that magmatic and hydrothermal process coexist in the IOCG deposits from the northern sector of Carajás Province. This might suggest that the 2.5 Ga IOCG deposits from the Carajás Province can have formed during the development of wide magmatic-hydrothermal systems.

Mineral chemistry and oxygen isotope studies on Sn (±W) mineralization from Pedra Branca Granite Massif, Central Brazil

Mineral chemistry and oxygen isotope studies on Sn (±W) mineralization from Pedra Branca Granite Massif, Central Brazil

Geothermobarometery–mineralogical and mineral chemistry studies of Dacite and trachydacite of Masahim volcano south and east of Shahr-e-Babak north

Geothermobarometery–mineralogical and mineral chemistry studies of Dacite and trachydacite of Masahim volcano south and east of Shahr-e-Babak north

The petrogenesis of chemically zoned, phonolitic, Plinian and sub-Plinian eruptions of Somma-Vesuvius, Italy: Role of accessory phase removal, independently filled magma reservoirs with time, and transition from slightly to highly silica undersaturated magmatic series in an ultrapotassic stratovolcano

A mineralogical, major, LA-ICP-MS trace element mineral chemistry and bulk-rock geochemical study of juvenile samples of the Mercato, Avellino, Pompeii and Pollena eruptions, collected in stratigraphically and volcanologically well-characterized sections of the Somma-Vesuvius stratovolcano (Roman Magmatic Province), along with reference data on the 1944 CE and the Pomici di Base eruptions, highlights the compositional variability of bulk-rock and glass from leucite phonotephrite to garnet-bearing phonolite . The latter products have extreme fractionation of trace elements (e.g., La/Yb n = 126, Zr/Y = 89, Zr/Hf =78, Nb/Ta = 40; Th/U = 2.3), very low Sc, V, Y, HREE and very high As, Tl, Cs, Pb, Th and U. The Pomici di Base products, older than the eruptions described above, range from leucite-bearing shoshonites to trachytes , are devoid of garnet and belong to an independent liquid-line-of-descent, having also different fractionation between trace elements (e.g., La/Yb n = 15; Zr/Y = 12.4, Zr/Hf = 50, Nb/Ta = 15.6; Th/U = 3 in the Pomici di Base trachytes). A marked chemical variability of the observed phases is found. The geochemistry of garnet, amphibole , clinopyroxene and other phases shows wide variations of concentrations and elemental ratios (e.g., La/Yb n up to 520 in the sadanagaite coexisting with garnet). The magmatic evolution is dominated by low-pressure, oxidized, nearly closed-system fractional crystallization of clinopyroxene, plagioclase , leucite, ±magnetite, ±biotite, ±olivine and apatite in the transition tephrite-phonotephrite, and of potassic sanidine (±hyalophane), Fe-clinopyroxene, melanite garnet±Fe-amphibole in the transition tephriphonolite-phonolite. Mineralogic and geochemical evidence and modelling points out the existence of independent, zoned magma batches throughout the activity of the stratovolcano, which possibly started to crystallize at similar depths. The Somma-Vesuvius magmas thus evolved in shallow independent reservoirs with respect to those of the neighbouring volcanic complexes (Campi Flegrei, Ischia) as shown, for instance, by the contrasting compositional trends of clinopyroxene and amphibole, and have very limited evidence of crustal contamination (and/or carbonate assimilation). • Pollena, Pompeii, Avellino and Mercato are zoned eruptions with phonolites • The magmas which formed the various eruptions evolved independently • Different garnets dominate HREE, Y and HFSE; amphibole is strongly HREE-poor • The phonolites and their mineral compositions have no tendency to become peralkaline • F, Cl and H 2 O dominate the volatile budget