Main Mineral Assemblages Research Articles

Fe–Cu mineralization of Tangal-e-Sefid; a magnetite rich massive sulfide deposit from Kuh-e-Sarhangi district, Central Iran

The Tangal-e-Sefid Fe–Cu mineralization forms syngenetic stratiform deposit in the Late Neoproterozoic volcano sedimentary sequence from the Central Iran. Early Cambrian metamorphic rocks are associated ore-bearing geologic units. The mineralization includes early oxide phase as a magnetite-rich bodies that are overprinted by a pyrite-chalcopyrite-rich sulfide phase. The most current alteration zone includes propylitic-carbonate, chlorite, sericite and silicic with a well-developed distribution of chloritization, especially in the layered part. Magnetite, pyrite and chalcopyrite comprise the primary main mineral assemblage, which is accompanied by malachite, covellite, hematite and goethite as the secondary minerals associated with epidote, chlorite, quartz and calcite minerals. Magnetite chemistry reveals the hydrothermal evolution of mineralization, and all the examined magnetite fall within fields of magnetite from VMS deposits. Fluid inclusion analysis of quartz and calcite coexisting with magnetite represent homogenization temperature range of 198 °C and 357 °C with a cooling trend from the massive toward the layered parts. The measured fluid salinity identifies two distinct medium-salinity fluids with mean values corresponding to 15 and 22 wt% NaCl. There is no significant difference in terms of temperature and salinity measured in calcite and quartz minerals. However, the average measured temperature values of fluids trapped in calcite (189–336 °C) are slightly lower than quartz (227–357 °C). Since the ore deposit distribution is spatially associated with actinolite schist, thermometric data of actinolite show temperature fluctuations of 310–315 °C and mineral formation pressures of 2.5–3 Kbar, which are correlated with the low-grade metamorphism.Primary hydrothermal fluids derived from submarine magmatism in an extensional system of seafloor were enriched in Fe and Cu (± Zn and possibly Pb) and it is the responsible for the first stage of magnetite formation and following the overprinting pyrite and chalcopyrite mineralization. The ore deposit geometries associated with magnetite mineralization and sulfide replacement styles; reveals that in the second stage of mineralization, hydrothermal fluid is mixed with oceanic water and eventually metal sulfides are deposited. The mineralizaton zone associated with the volcano-sedimentary sequence is affected by low-grade regional metamorphism related to Pan-African orogeny and represent the green schist territory as the VMS deposits related to Archean.

Adsorption of aqueous perfluorooctane sulfonate by raw and oleylamine-modified Iranian diatomite and zeolite: Material and application insight

Perfluorooctanesulfonate (PFOS) is a hazardous chemical, and its presence in surface and groundwater poses a risk to environmental quality and human health. Containment is often applied to immobilize PFOS to stop or minimize the exposure. Mineral-based materials became promising adsorbents. However, there is scope to develop adsorbents using locally available minerals and understand their adsorption mechanisms. Here, we developed oleylamine-modified composites using naturally occurring Iranian zeolite and diatomaceous earth (DE). The mineralogy and surface properties of materials were fully characterized, and the adsorption of PFOS from simulated wastewater was linked to it. Clinoptilolite in zeolite sample, calcite and kaolinite in DE were main mineral assemblages. The raw samples also contained silica as a main constituent of them. Thermogravimetric analysis suggested that the materials were successfully modified with the oleylamine molecules in the material's structure and surfaces. These were further supported by scanning electronic microscopy (SEM), Fourier transmission infrared spectroscopy (FTIR), and surface and pore size analysis. After adsorption at various pHs, the isotherm of adsorption was also performed at ambient temperature. Modified DE and zeolite tend to adsorb PFOS (14.1 and 25.5 mg/g, respectively) more than their raw counterparts (4.72 and 0.39 mg/g, respectively). Adsorption models suggest monolayer and, in rare cases multilayer binding capacities and affinities toward PFOS. We analyzed the post-adsorption materials and discovered that electrostatic and hydrophobic interaction was likely the main cause of PFOS adsorbed to the material. This research helps to improve our knowledge of how PFOS adheres to untreated and surfactant-altered zeolite and DE in aquatic environments.

The Clay Mineralogy and Geochemistry of Sediments in the Beibu Gulf, South China Sea: A Record of the Holocene Sedimentary Environmental Change

In this study, we analyzed the clay mineralogy and geochemistry of surface and drill core samples from the northeastern Beibu Gulf in order to unravel the sediment provenance of, and factors controlling, the sedimentary environment. The main clay mineral assemblage in the surface sediment samples included kaolinite (27–72%), smectite (4–51%), illite (7–20%), and chlorite (8–17%). The study area comprises three major clay distribution zones (from the northeastern coastal area to central Beibu Gulf basin), i.e., the kaolinite-dominated, kaolinite–smectite, and smectite-dominated zones. The zoning of the clay mineralogy and major and trace elements indicated the mixing of coarse terrigenous sediments with distal fine sediments. Early Holocene sea level rise was documented in core B15-1, which had three sedimentary units (divided into Units 1–3 from top to bottom), as revealed by the changes in the dominant clay minerals and geochemical/oxide ratios (SiO2/Al2O3, Rb/Sr, Sr/Ba, and Ti/Ca) in Unit 2. Unit 1 and Unit 3 were likely deposited in the continental and marine environments, respectively. The low sedimentation rate and hydrodynamic disturbance may have influenced the deposition process more than climatic fluctuations.

Geochemistry, petrogenesis, and tectonic setting of the Cúcamo mafic and intermediate volcanic rocks from the Ahualulco Volcanic Complex, San Luis Potosí, Mexico

The Ahualulco Volcanic Complex (AVC) is situated in the north-central part of the San Luis Potosí Volcanic Field (SLPVF) that is found in the southern portion of the Mesa Central (MC). The Cúcamo, AVC is mainly composed of mafic and intermediate volcanic rocks. The present study focuses on understanding the evolution, origin, and magmatic processes and petrogenesis of mafic and intermediate rocks in the Cúcamo, AVC. The Quaternary mafic rocks have porphyritic textures with the mineral assemblage of olivine, and clinopyroxene. These volcanic rocks display high K calc-alkaline basaltic compositions with enrichment in light rare earth elements (LREEs) and incompatible elements. Geochemical modeling reveals that mafic magmas were derived through a partial melting process of a spinel lherzolite source at low degrees of melting (~2 to 15 %) in an extensional regime. The intermediate volcanic rocks show porphyritic and glomeroporphyritic textures with matrix formed by randomly oriented microlites. The main mineral assemblage consists of plagioclase, K-feldspar, and clinopyroxene. These volcanic rocks are characterized by calc-alkaline basaltic andesitic and andesite compositions with enrichment in light rare earth elements and incompatible elements. Geochemical modeling suggests that intermediate rocks were derived from high ratios of assimilation and fractional crystallization processes between mafic melts and continental crust in an extensional environment.

Petrogenesis of juvenile pelletal lapilli in ultramafic lamprophyres

The Chadobets alkaline-carbonatite complex (Siberian craton) is a natural laboratory for all varieties of ultramafic lamprophyres, including damtjernites formed by fluid-explosion mechanisms, which contain a large number of pelletal lapilli. Data obtained from comprehensive mineralogical, structural and chemical studies of these pelletal lapilli show strong similarity with the main magmatic mineral assemblage of damtjernites, suggesting a juvenile composition for them. The composition of phlogopite, carbonate and fluorapatite in pelletal lapilli is mostly constrained toward primitive compositions (beginning) of mineral crystallization trends in ultramafic lamprophyres. According to the petrographic and mineralogical features found in pelletal lapilli from damtjernites, these can be divided into three types based on the conditions and depth of formation. Estimated late magmatic temperatures of pelletal lapilli mica and fluorapatite formation vary from 815 to 990 °C.

Mineralogy, Geochemistry and Fluid Inclusion Study of the Stibnite Vein-Type Mineralization at Rizana, Northern Greece

The stibnite mineralization at Rizana (Kilkis ore district; Serbo-Macedonian metallogenic province; northern Greece) occurs along a NE–SW-trending brittle shear-zone, which transects a two-mica and an augen-gneiss of the Vertiskos Unit. Barren Triassic A-type granites and satellite pegmatites and aplites, as well as Oligocene-Miocene plutonic, subvolcanic and volcanic rocks that are variably hydrothermally altered and mineralized, outcrop in the broader region. The mineralization appears as veins, discordant lodes and disseminations. Veins and discordant lodes exhibit massive and brecciated textures. Historic underground mining (1930s–1950s) produced 9000 t of stibnite ore, grading 40% Sb on average. The main ore mineral assemblage includes stibnite + berthierite + sphalerite + pyrite + chalcopyrite + native antimony and traces of wolframite, galena, tetrahedrite, marcasite, pyrrhotite, arsenopyrite, realgar, native arsenic and native gold. Quartz, minor barite and ankerite are the gangue minerals. Sericitization and silicification developed along the shear-zone, forming hydrothermal halos of moderate intensity in the two-mica gneiss. Locally, valentinite, goethite and claudetite are present due to the supergene oxidation of the stibnite mineralization. Bulk ore geochemistry shows enrichments in specific elements including As, Au, Cd, Se, Tl and W. Fluid inclusion microthermometry showed that the mineralization was formed under a limited range of temperatures and salinities. The fluids had low to slightly moderate salinities (6.6–8.1 wt% equiv. NaCl) with low homogenization temperatures (217–254 °C, with a maximum at 220 °C).

A spectral approach on mineralogy and geochemistry of garnet skarns in Arc-Type granitoids

Garnets that common constituent of skarn type iron deposits are wide ranges of chemical compositions, and they are also important as a semi-gemstone mineral. This study has been investigated garnets and inclusions of its formed in contacts of Pertek granitoid and Keban marble by using a combination of multiple techniques including Raman spectrum, electron microprobe, petrography and LA-ICP-MS. The main mineral assemblage observed in the skarn formation is diopside, garnet, quartz, magnetite, calcite and pyrite. The garnets are in size between 1 and 7 cm and have reddish, greenish and light–dark brown colour. The compositions of the garnets are mainly grossular-andradite, andradite-grossular, less grossular and andradite. Increased porosity and horizontal flow of hydrothermal fluids during metasomatism resulted in the formation of garnets of different sizes. Raman spectroscopy studies on garnets show to be that the idiomorphic and semi-idiomorphic garnets are predominantly grosular, andradite and uvarovite in the core. Raman spectroscopy studies on garnets showed to have predominantly grossular, andradite and uvarovite composition in the core of the idiomorphic and semi-idiomorphic garnets. As for the rare earth element (REE) analysis and distribution model, the chondrite-normalized REE patterns of all garnets in the study area have similar trends and generally in the right-slopping shape depleted in HREE and enriched in LREE, mostly controlled by adsorption. High Fe2O3, CaO and MgO contents may cause to high mobility during skarn formation associated with contact metamorphism and cation exchange via hydrothermal fluids (eg uvarovite in the core). All garnets have generally positive Ce and positive Eu anomalies. These results suggest that the garnets in the studya area formed in conditions showing increase in pH and high oxygen fugacity or decrease in temperature in the source of the hydrothermal fluids.

Pre-eruptive excess volatiles and their relationship to effusive and explosive eruption styles in semi-plugged volcanoes

Understanding the role of various factors influencing eruption style is challenging, but it can aid in adapting different hazard mitigations and crisis responses for explosive or effusive events. Here, we focus on the role of magma storage conditions in controlling eruption styles at basaltic andesite volcanoes, and how they can be related to monitoring data. We study the cycle of explosive (1990, sub-Plinian) → effusive (2007, dome) → explosive (2014, sub-Plinian) eruptions from Kelud (Kelut) volcano, Indonesia. We conducted petrological analyses of the eruption products and phase equilibria experiments using pumice and explored a range of temperatures, pressures, oxygen fugacity, and volatile contents. We show that we can reproduce the main mineral assemblage (plagioclase ± pyroxenes ± magnetite ± amphibole ± olivine) and phenocryst content (30–50 wt%) of the magmas from the three eruptions at T = 975 ± 39°C, p = 175 ± 25 MPa, fO2 = nickel–nickel oxide buffer, and about 4–6 wt% water in the melt (ca. 3 to 5 wt% and ca. 4 to 7 wt% for the 1990 and 2014 eruptions, respectively). However, geothermobarometric results also indicate that some crystals of amphibole were sourced from higher pressures. We infer from a synthesis of our data and historical observations that the high phenocryst content of the 2007 dome (∼70 wt%) likely resulted from slow magma ascent toward the surface alongside progressive degassing and re-equilibration at a lower volatile content (∼1 wt% water in the melt). Mass balance calculations on the sulfur budget of the 1990, 2007, and 2014 magmas show that the explosive events contained an excess fluid phase at pre-eruptive conditions, and we propose that this led to their higher explosivity compared to the 2007 dome. The accumulation of excess fluids during decadal-long repose depends on how plugged the volcanic system is, or its ability to passively release magmatic fluids, prior to eruption. Such condition could be inferred from monitoring records, including changes in gravity of the plumbing system over time, and thus contribute to better anticipate eruptive style.

Identifying and Mapping Alteration Minerals Using HySpex Airborne Hyperspectral Data and Random Forest Algorithm

Airborne hyperspectral remote sensing data provide rapid, non-destructive, and near laboratory quality reflectance spectra for mineral mapping and lithological discrimination, thereby ushering an innovative era of remote sensing. In this study, NEO HySpex cameras, which comprise 504 spectral channels in the spectral ranges of 0.4–1.0 μm and 1.0–2.5 μm, were mounted on a delta wing XT-912 aircraft. The designed flexibility and modular nature of the HySpex aircraft hyperspectral imaging system made it relatively easy to test, transport, install, and remove the system multiple times before the acquisition flights. According to the design fight plan, including the route distance, length, height, and flight speed, we acquired high spectral and spatial resolutions airborne hyperspectral images of Yudai porphyry Cu (Au, Mo) mineralization in Kalatag District, Eastern Tianshan terrane, Northwest China. By comparing the features of the HySpex hyperspectral data and standard spectra data from the United States Geological Survey database, endmember pixels of spectral signatures for most alteration mineral assemblages (goethite, hematite, jarosite, kaolinite, calcite, epidote, and chlorite) were extracted. After a HySpex data processing workflow, the distribution of alteration mineral assemblages (iron oxide/hydroxide, clay, and propylitic alterations) was mapped using the random forest (RF) algorithm. The experiments demonstrated that the workflow for processing data and RF algorithm is feasible and active, and show a good performance in classification accuracy. The overall classification accuracy and Kappa classification of alteration mineral identification were 73.08 and 65.73%, respectively. The main alteration mineral assemblages were primarily distributed around pits and grooves, consistent with field-measured data. Our results confirm that HySpex airborne hyperspectral data have potential application in basic geology survey and mineral exploration, which provide a viable alternative for mineral mapping and identifying lithological units at a high spatial resolution for large areas and inaccessible terrains.

Petrology and geochemistry of the Palaeo‐Mesoarchean Banded Iron formations (BIFs) from the central Bundelkhand greenstone belts, Bundelkhand Craton, India: Source characteristics and depositional environment

Palaeo‐Mesoarchean Banded Iron formations (BIFs) of central Bundelkhand greenstone belts occur from Babina (in the west) to Mahoba (in the east) via Mauranipur along with the E–W trending of the Bundelkhand Tectonic Zone (BTZ). In the present study, we report petrological, geochemical, and mineralogical data of BIFs from the Babina, Mauranipur, and Mahoba greenstone belts of central Bundelkhand Greenstone Complex to address their source characteristics and deposition environment. The lithological association of BIFs in the Babina and Mauranipur greenstone belts consists of pillowed basic–ultrabasic volcanics and metasediments, whereas in the Mahoba greenstone belt, BIFs and quartzite occur within basic volcanics. These lithological associations collectively suggest that the entire succession was deposited in a marine environment. Based on geological setting, they are classified as Algoma‐type. The main mineral assemblages of the Babina and Mauranipur BIFs are composed of magnetite, quartz, ±garnet, hornblende, chlorite, and apatite, whereas the Mahoba BIFs along with magnetite and quartz, poised of two distinct mineral assemblages; the first one is Opx–Cpx–Grt and the other one is Grt–Hbl. The preliminary P–T study of these BIFs implies that they had gone through amphibolite‐ to granulite‐facies metamorphism. The geochemical characteristics show that BIFs from the Babina and Mauranipur greenstone belts are dominated by higher quartz and magnetite (total content = 90.63–99.95 wt.%), whereas the Mahoba greenstone belt is characterized by lesser quartz and magnetite components (total content = 86.68–94.58 wt.%). The PAAS (Post Archean Average of Australian Sediments)‐normalized REE patterns display significant positive Eu anomaly (Eu/Eu* up to 2.50) with well‐represented negative Ce anomalies (Ce/Ce* = 0.27–1.20). The REE and trace element ratios of BIFs from the Babina and Mauranipur greenstone belts suggest that they were formed by high‐T hydrothermal fluid activity without or very less continental input. Moreover, the Mahoba greenstone belt BIFs are formed by low‐T hydrothermal fluid activity with substantial continental input. Their chemical variation suggests the shallowing of the proto‐basin from the Babina in the west to Mahoba in the east. Tectonic discrimination plots imply the BIFs of the Babina, Mauranipur, and Mahoba greenstone belts of the central Bundelkhand Greenstone Complex are formed in a back‐arc tectonic setting.

Textures and chemical compositions of the Narm iron oxide-apatite deposit in Kuh-e-Sarhangi District (Central Iran): Insights into the magmatic-hydrothermal mineralization

The Narm deposit is located in the Kuh-e-Sarhangi district which is a main part of the most significant Iranian iron mineralization belt, the Kashmar-Kerman Tectonic Zone (KKTZ) in Central Iran. The Narm deposit comprises an estimated total of ∼ 135000 tons of iron ore with an average grade of ∼ 55% Fe and is hosted in Early Cambrian volcano sedimentary rocks of the Rizu formation. Ore occurrences in this deposit consist of lens-shaped magnetite ore bodies, magnetite-apatite-actinolite veins and locally rare brecciated dolomite with magnetite clasts. Magnetite, pyrite, chalcopyrite and specularite associated with apatite, actinolite, biotite and carbonate minerals form the primary main mineral assemblage which is accompanied by hematite and goethite as the secondary minerals. Magnetite as the most current mineral of Narm deposit reveal the magmatic to hydrothermal evolution of mineralization. Magmatic magnetite minerals (Mag I) are dark-gray inclusion-rich magnetite spatially correlated with high temperature Ca-Fe alteration. The brighter inclusion-free hydrothermal magnetite groups (Mag II and Mag III) form during the temperature decreasing of the mineralizing fluid. According to the magnetite chemistry examination, most magnetite fall into the field for magnetite from iron-oxide apatite (IOA) deposits. Apatite minerals with F/Cl > 2, belong geochemically to the fluorapatite type. In addition to the primary dolomite, there are some hydrothermal Fe-rich dolomites and Mn bearing ones, indicating the hydrothermal fluid playing the important role for Fe-rich mineralization. In respect to fluid evolution, fluid inclusion analysis of calcite and apatite minerals form the magnetite paragenesis assemblage represent homogenization temperature range for fluid between 325 and 557℃. The salinity of fluid varied from 7.7 to 11.6 wt% NaCl equivalent and a cooling trend with the dominant chlorine complex as an agent for deposition of the Fe-rich ores. The geochemical characteristics of the δ18Ofluid values of magnetite (from + 6.1 to + 10.4‰) and δ18Ofluid values of actinolite (from + 7.7 to + 12.5‰) represent the magmatic-hydrothermal (δ18Ofluid > + 0.9 ‰) formation process. The iron rich Al-clinochlore composition from the alteration zone indicates a temperature range between 250 and 330℃ which points to a temperature reduction of hydrothermal fluids in this mineralizing zone. The integrated geochemical data from this investigation, including mineral chemistry, microthermometry of fluid inclusions and oxygen isotope data all reveal a magmatic-hydrothermal genesis for this deposit.

Stable isotope (δD, δ18O) data on the structural water of two sericite samples from the Cu-Au high-sulphidation deposit Chelopech, Bulgaria – a tentative study

The epithermal high-sulphidation Cu-Au Chelopech deposit is characterized by a well-developed and well-traceable hydrothermal footprint manifested in the volcanic host rocks. The economic ore mineralization is embedded in the strong silicification, included among the advanced argillic zone of alteration, smoothly transitioning to quartz-sericite alteration that evolves into widespread propylitics. The quartz-sericite alteration zone is accessible for exploration only in underground mining galleries and exploration drillings. The main mineral assemblage in this zone is quartz, sericite, pyrite, minor rutile/anatase and relics of apatite and feldspar. According to XRD data from the studied samples, sericite was defined as illite and muscovite/sericite 2M1 polytype. The abundance of heavy stable isotopes (D, 18O) in the structural water of two sericite samples is the object of this study. A special attention was paid to the separation of extraneous waters from the structural one by thermal fractionation. The extracted structural water was converted to hydrogen and carbon dioxide before the isotopic measurements. The obtained results, put into a δD vs. δ18O plot, indicate that sericite structural water is “heavier” than meteoric water, within uncertainty limits.

Intrusion-hosted gold deposits of the southeastern East Sayan (northern Central Asian Orogenic Belt, Russia)

Our paper reviews a series of relatively low-grade gold deposits and prospects hosted by granitoid intrusions within the southeastern East Sayan in the Altaid Belt (northern Central Asian Orogenic Belt, Russia). The studied deposits/prospects are characterized by the presence of Te-, Sb- and Bi-bearing minerals. However, they differ in their main gold-associated mineral assemblages, which permits to distinguishing four mineralization assemblages: 1) gold-telluride; 2) gold-tetradymite; 3) gold-stibnite; and 4) gold-bismuth sulfosalt. The gold-telluride mineralization (1) is represented by the Tainskoye deposit, and the Khoringolskoye and Sagangolskoye prospects which are defined by paragenesis of native gold with Au-Ag-Bi-Pb-Ni tellurides, such as hessite, petzite, altaite, wehrlite, calaverite, melonite, altaite, and tellurobismuthite. The gold-tetradymite mineralization (2) which is represented by the Konevinskoye deposit, is hosted by the Saylag granitoid pluton, and contains tetradymite as one of the principal minerals associated with native gold. The gold-stibnite mineralization (3), represented by the Tumannoye prospect, is also hosted in a granitoid complex consisting of granodiorite and leucogranite. The gold-bismuth-sulfosalt mineralization (4) is documented at the Pogranichnoye prospect, where native gold is associated with the bismuth-bearing minerals: bismuthinite, galeno-bismuthite, lillianite, and native bismuth. Estimated δ18O values of ore-forming fluids from intrusion-hosted deposits range from 5.7 to 7.4‰, indicative of a magmatic origin; δ34S values of sulfides vary from −4.2 to 4.5‰, which corresponds to magmatic sulfur. The geological setting, element geochemistry of host granitoids, stable isotope ratios, and mineralogy of the studied deposits are consistent with a magmatic origin of the mineralizing fluids.The studied deposits are spatially associated with either Neoproterozoic or Early Paleozoic granitoids, belonging to the two main orogenic stages of the East Sayan geodynamic evolution. At ∼850 Ma, during the Neoproterozoic stage, deposits with dominant gold-telluride assemblages, formed in association with granitoids characterized by geochemical features of island arc granites. During 458–439 Ma, in the Early Paleozoic stage, gold-tetradymite, gold-stibnite, gold-telluride and gold-bismuth-sulfosalt assemblages were formed that are spatially associated with orogenic granites with different geochemical compositions. Most gold-bearing mineral assemblages are intersected by post-mineral Late Paleozoic dykes. The origin of these different gold-sulfide-telluride assemblages is explained with their genetic association with granitoid intrusions of different ages and compositions.

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.

Mineralogy of the Vorontsovskoe gold deposit (Northern Urals). Part 1: history of study, mineral assemblages, list of minerals

This work is the frst paper in a series of publications dedicated to mineralogy of the Vorontsovskoe gold deposit in the Northern Urals. The deposit is unique for both Russia and world with regard to the diversity and originality of Tl–Hg–Mn–As–Sb–S mineralization. Based on available literature and our data, we compiled an exhaustive list of 209 mineral species of the deposit, including eight new minerals found by the authors: vorontsovite, ferrovorontsovite, tsygankoite, gladkovskyite, luborzakite, pokhodyashinite, gungerite and auerbakhite. In addition, 40 and 89 minerals are found for the frst time in Russia and the deposit, respectively. Systematic studies of ores revealed nine main mineral assemblages, seven of which are confned to carbonate breccias. They contain more than 70 rare sulfdes, tellurides and sulfosalts including 30, 12 and 9 minerals, where Tl, Hg and Mn are species-defning elements, respectively. In this paper, we characterize the history of study of the deposit, its geological position and structure, ore types, mineral assemblages, and analytical methods.

Middle to late Eocene chemical weathering history in the southeastern Tibetan Plateau and its response to global cooling

Continental weathering is a crucial factor to maintain the balance of the carbon cycle and thus the livability of the Earth. The Tibetan Plateau (TP), known as the Third Pole of the Earth, has undergone strong uplift since the Cenozoic, thus making it an ideal area to study continental weathering processes and associated driving factors. However, early Cenozoic weathering records are rare in this region. Here, we report high quality mineralogical and geochemical records of clay fraction (< 2 μm) of a middle to late Eocene sequence retrieved from the Qujing Basin, southeastern TP, to track the early Cenozoic chemical weathering history in this area. The results show that the main clay mineral assemblages contain illite-smectite mixed layers, illite and palygorskite. The chemical weathering intensity indicators (e.g., clay mineral assemblages and the chemical index of alteration (CIA)) collectively indicate a clear long-term modulation of global cooling on regional chemical weathering processes during middle to late Eocene, as indicated by high correlation between global temperatures and chemical weathering indicators and the similarity in their periodicity characteristics. Moreover, a sudden decrease of the chemical weathering intensity with large fluctuations was identified at 42–39.9 Ma, which may be linked to rapid transport of clay minerals caused by tectonic activity and/or an increased precipitation in the southeastern TP. This study indicates a control of global cooling on chemical weathering of the southeastern TP area during middle to late Eocene, while tectonic activity or other factors might exert catalytic influences at some certain periods.

Magnetite compositions from the Baba Ali iron deposit in the Sanandaj-Sirjan zone, western Iran: Implications for ore genesis

The economically significant Baba Ali iron ore deposit, is located in the Sanandaj-Sirjan zone, western Iran, is associated with diorite. The mineralization trends in a ENE-WSW direction with thickness that varies from 30 m up to 50 m and a length of about 250 m. The main mineral assemblages are massive magnetite, epidote, actinolite, tourmaline, quartz, calcite, and pyrite. Using different magnetite compositional discrimination diagrams, the low concentrations of Ti (0.11 wt%), Al (0.06 wt%), Cr (19.62 ppm), and Co (20.26) and high Ni/Cr ratios (6.34) are consistent with a hydrothermal origin, similar to typical skarn mineralization. The different elemental compositions of various magnetite generations might be due to an ore-forming fluid that initially a high-T magmatic hydrothermal character, then evolved to a lower temperture meteoric water dominated system. It seems that Fe-bearing saline fluids infiltrated the carbonate host rocks, causing high temperature, acid metalliferous fluids to become neutralized during reaction with carbonates, causing a decrease in iron solubility and saturating as a magnetite with increasing pH and decreasing the temperature. The close spatial and temporal association of the mineralization with the fracture systems suggests that a pressure decrease also had a significant impact on the iron solubility, triggering ore precipitation and replacement at relatively low temperatures (~300 °C), under reducing conditions.

Rare earth element geochemistry of altered pyroclastic rocks in the Hashtjin area of north-west Iran

AbstractThis study evaluates the rare earth element (REE) geochemistry in altered trachyandesitic ignimbrites, tuff and lava flows in the Hashtjin area by assessing chondrite-normalized REE patterns and Y/Ho geochemical ratios. Modifications in the REE patterns took place along altered fault zones that were affected by hypogene and supergene alterations. The precursor volcanic and pyroclastic rocks contain phenocrysts of plagioclase accompanied by augite, zircon, apatite and pyrite. Based on X-ray diffraction analysis, the main mineral assemblages of the altered units consist of kaolinite as the main clay mineral, SiO2 polymorphs (quartz and cristobalite) and anatase as a minor constituent. The chondrite-normalized REE patterns of argillic samples reveal fractionation of light REEs (LREEs) compared to heavy REEs (HREEs), together with a marked Eu anomaly and a weak W-type tetrad effect related to the weak non-charge radius control (CHARAC) behaviour of REEs and slightly higher Y/Ho and Zr/Hf ratios. The relationship between the Y/Ho and Zr/Hf ratios and recognizable T3 and T4 effects (tetrad effect) suggests that an increasing degree of water–rock interaction occurred during hypogene alteration processes by acidic hydrothermal fluids that were overprinted by supergene alteration. Water–rock interaction and adsorption by Mn-oxides and clay minerals are considered to have played important roles in determining the close to non-CHARAC behaviour of REEs during the argillic alteration of the pyroclastic rocks in the Hashtjin area.

The Lukoganskoe Au–Cu Skarn Deposit (Eastern Transbaikalia): Mineral Composition, Age, and Formation Conditions

Abstract —The Lugokanskoe deposit is located in southeastern Transbaikalia and has been studied for a long time by many researchers. However, the type of its formation is still debatable. In this paper we study the mineral composition of ores by modern methods, recognize and describe the main gold mineral assemblages, and present detailed data on the chemical composition of native gold and sulfide minerals and their isotope composition. We have established that gold–pyrite–chalcopyrite–arsenopyrite and gold–bismuth parageneses localized in skarn deposits are the main productive assemblages. Study of the sulfur isotope composition of sulfide minerals has shown an endogenous source of sulfur of the ore minerals. The carbon and oxygen isotope compositions of carbonates of ore-bearing veins indicate the participation of a magmatic fluid. The established age of the gold mineralization and igneous rocks of the Shakhtama complex, together with direct geological observations, points to their spatial, temporal, and genetic relationships. According to their petrochemical and geochemical characteristics, the igneous rocks of the Shakhtama complex are I-type ilmenite (reduced) granitoids. Study of fluid inclusions by heating and cooling and Raman spectroscopy has shown that the mineral formation was accompanied by a gradual decrease in the content of salts in the ore-forming fluids and by a decrease in their homogenization temperatures. Optical observations demonstrate that the fluid was heterogeneous at the early stages of the mineral formation. The evolution of the ore system was accompanied by a change in the gas phase composition of fluid inclusions from predominantly nitrogen–carbon dioxide to essentially aqueous, with carbon dioxide impurity (H2O + CO2 ± N2 → H2O ± CO2). The research data testify to the magmatic nature of fluids and the participation of meteoric waters at the late stages of the ore-forming process. The data obtained have led to the conclusion that the Lugokanskoe gold deposit is related to reduced intrusions formed at a shallow depth.

Lithogeochemistry and 3D geological modeling of the apatite-bearing Mesquita Sampaio beforsite, Jacupiranga alkaline complex, Brazil

The Mesquita Sampaio beforsite is a mineral exploration area with apatite mineralization occurring near the northern portion of the Morro da Mina apatite deposit (Cajati Mine), in the Jacupiranga Alkaline Complex. This study presents the mineralogical, geochemical and 3D geological modeling data of this target. Four lithotypes are presented for Mesquita Sampaio: the fine-grained beforsite (FGB), the coarse-grained beforsite (CGB), the xenolith zone (XEN), and the jacupirangite (JAC). The 3D geological model shows the Mesquita Sampaio beforsite as a vertical body with an NNW direction and approximately 350 meters in length, 150 meters in width, and 400 meters of depth, with open extensions. The main mineral assemblage of the beforsite is dolomite, apatite, phlogopite, serpentine, calcite, and magnetite, in order of abundance. The distribution of major elements for both types of beforsites (FGB and CGB) indicates the same composition with only a small variation in the mineral proportions. Although no direct contact relation between Mesquita Sampaio and Morro da Mina is known, both may be the same carbonatite body with a gradual compositional variation, or the Mesquita Sampaio beforsite could be a distinct late or even earlier carbonatite pipe.