Quartz Muscovite Research Articles

The Formation of K-Cymrite in Subduction Zones and Its Potential for Transport of Potassium, Water, and Nitrogen into the Mantle

The conditions of the formation of K-cymrite in volatile-rich pelite and partially devolatilized mica quartz–muscovite–chlorite schist were experimentally investigated at pressures of 5.5, 6.3, and 7.8 GPa and temperatures ranging from 900 to 1090°C corresponding to hot subduction geotherm. Experimental samples at these P–T conditions formed assemblage of solid phases (Grt + Coe + Phe + Cpx + Ky, with accessory Po + Ru + Zrn ± Mnz) and water-enriched supercritical fluid–melt. Analysis of the obtained data indicates that the stability of phengite and its potential replacement by K-cymrite depends on the P–T conditions and the amount of volatiles in the metasediment. In samples of volatile-rich pelite and mica schist at 5.5 GPa and 900°C, as well as at 6.3 GPa and 1000°C, phengite remains stable in equilibrium with 3–13 wt % of the fluid–melt. With increasing pressure up to 7.8 GPa and temperature up to 1090°C, the fraction of supercritical fluid–melt in pelite reaches 20 wt %, while phengite disappears. Only 5 wt % supercritical fluid–melt are formed in the schist at 7.8 GPa and 1070°C, while most part of phengite is preserved. For the first time, phase assemblage with phengite and K-cymrite (±kokchetavite) was obtained in the pelite and schist samples at 7.8 GPa and 1070°C. The assemblage was identified using Raman mapping. At stepwise devolatilization (with removal of fluid–melt portion forming in equilibrium with volatile-bearing minerals that are stable at P–T conditions of experiments), phengite has been preserved up to 7.8 GPa and 1090°C, but K-cymrite is not formed in the absence of fluid–melt. It was concluded that the most effective transport of volatiles (first of all, water) in the metasediment to depths over 240 km may occur during its partial and early (before the formation of supercritical fluid–melt) devolatilization. In this case, almost all phengite may reach depths of 240 km during metasediment subduction and then transform into water-bearing K-cymrite, or, in the presence of nitrogen in the metasediment, into nitrogen-bearing K-cymrite, thus facilitating the further transport of LILE (large-ion lithophile elements), water, and nitrogen. However, the formation of a significant portion of supercritical fluid–melt leads to the complete dissolution of phengite with increasing P–T conditions, making further transport of LILE, water, and nitrogen impossible. During deep multi-stage devolatilization, phengite remains stable up to depths of 240 km; however, during further subduction, it likely transforms into an anhydrous K-hollandite (KAlSi3O8).

Characterization of a Metamorphosed Volcanic Stratigraphy and VMS Alteration Halos Using Rock Chip Petrography and Lithogeochemistry: A Case Study from King North, Yilgarn Craton, Western Australia

Despite countless advances in recent years, exploration for volcanogenic massive sulfide (VMS) deposits remains challenging. This is particularly the case in the Yilgarn Craton of Western Australia, where outcrop is limited, weathering is deep and extensive, and metamorphism is variable. At Erayinia in the southern Kurnalpi terrane, intercepts of VMS-style mineralization occur along ~35 km strike length of stratigraphy, and a small Zn (-Cu) deposit has been defined at King (2.15 Mt at 3.47% Zn). An extensive aircore and reverse circulation drilling campaign on the regional stratigraphy identified additional VMS targets, including the King North prospect. Through a combination of detailed rock chip logging, petrography (inc. SEM imaging), and lithogeochemistry, we have reconstructed the volcanic stratigraphy and alteration halos associated with the King North prospect. Hydrothermal alteration assemblages and geochemical characteristics at King North (Mg-Si-K enrichment, Na depletion, and high Sb, Tl, Eu/Eu*, alteration index, CCPI, and normative corundum abundance values) are consistent with an overturned VMS system. The overturned footwall stratigraphy at King North is dominated by metamorphosed volcanic rocks, namely the following: garnet amphibolite (tholeiitic, basaltic), biotite amphibolite (andesitic, calc-alkaline), chlorite–quartz schist (dacitic), and narrow horizons of muscovite–quartz schist (dacitic to rhyolitic, HFSE-enriched). The hanging-wall to the Zn-bearing sequence is characterized by quartz–albite schists (metasedimentary rocks) and thick sequences of amphibolite (calc-alkaline, basaltic andesite). An iron-rich unit (>25% Fe2O3) of chlorite–actinolite–quartz schist, interpreted as a meta-exhalite, is associated with significant Cu-Au mineralization, adjacent to a likely syn-volcanic fault. Extensive Mg metasomatism of the immediate felsic footwall is represented by muscovite–chlorite schist. Diamond drilling into the deep hanging-wall stratigraphy at both King North and King has also revealed the potential for additional, stacked VMS prospective horizons in the greenstone belt stratigraphy. The discovery of HFSE-enriched rhyolites, zones of muscovite–chlorite schist, presence of abundant sulfide-rich argillaceous metasedimentary rocks, and a second upper meta-exhalite horizon further expand the exploration potential of the King–King North region. Our combined petrographic and lithogeochemical approach demonstrates that complex volcanic lithologies and VMS alteration signatures can be established across variably metamorphosed greenstone belts. This has wider implications for more cost-effective exploration across the Yilgarn Craton, utilizing RC drilling to reconstruct the local geology and identify proximal halos, and limiting more costly diamond drilling to key areas of complex geology and deeper EM targets.

Empirical Method for Engineering Geological Evaluation of Pelosika Dam Diversion Tunnel Design, Southeast Sulawesi, Indonesia

This paper explains a Pelosika Dam Diversion Tunnel in metamorphic rocks (quartz muscovite phyllite) and aims to find out excavation methods, stand-up time, and support systems for tunnel construction. The purpose of this research is that the development of science in tunnel construction with the characteristic of metamorphic rock will increase. Tunnel location in poort rock mass quality based on previous geology investigations. Based on current geology and engineering geological mapping results, diversion tunnel through poor rock mass of metamorphic rocks (phyllite) which have slightly-highly weathered. We classified rock mass quality by GSI, RMR, and Q-System methods which produce the almost same results (poor rock mass quality). The GSI Chart and Pettifer-Fookes Chart which excavation method has the results of the digging and ripping. RMR value determines the stand-up time last 42-240 minutes after excavation. RMR and Q-System values generate the support system which used rock bolts (length= 1.8 m), shotcrete (thick=10-15 cm), and steel sets (spacing=1,5m) if required. A purpose of combination from the different excavation methods and support systems to complement each other and improve the safety of the tunnel without compromising the cost-effectiveness of construction.

EXCASS system and Rock Mass Rating (RMR) classification for determining excavation method and tunnel support at Pelosika Dam Spillway Tunnel

Pelosika dam spillway tunnel was designed to drainage flood discharge and prevent overtopping. In recent investigations, the rock mass classification has not been done specifically in the spillway tunnel. This research aims to obtain technical geological data and information to define excavation methods and tunnel support. For more comprehensive research, engineering geological investigations were carried out regarding the Geological Strength Index (GSI), selecting excavation methods using EXCASS system and Rock Mass Rating (RMR) for tunnel support. The geological mapping showed that the study site was composed of 2 lithological units: quartz muscovite phyllite and pebblely sand deposits. The GSI value in the tunnel is between 14-25, indicating poor and very poor quality. Adopting the EXCASS system, the optimum method of excavation (EPIopt) in the Pelosika dam spillway tunnel are ripper easy and digger. While RMR values of 9 – 25 are classified as poor quality. Consequently, the appropriate method for the excavation sequence is using the top heading and bench and multiple drifts. And the recommended support system is a systematic rock bolt with wire mesh and shotcrete in the crown and walls with medium to light steel rib.

Early Cretaceous terminal convergence between the North China‐Mongolia and Siberia continents: Imprints from the Chagan Obo shear zone in central Inner Mongolia, North China

The ENE‐striking Chagan Obo shear zone in central Inner Mongolia is critical to constrain the termination of convergence between the North China‐Mongolia and Siberia continents. We recognized three ENE‐striking zones with strong deformation in the Permian biotite monzogranite within the shear zone and investigated the structural and kinematic characteristics. The strong deformation zones developed dense ENE‐striking and NNW‐dipping foliations. The widespread shear sense indicators such as S‐C foliations, mica fish, σ‐type, structures, and typical tight‐to‐isoclinal asymmetric folds show top‐to‐the‐SSE thrusting through NNW–SSE compression. Two 40Ar‐39Ar ages of muscovites from the muscovite quartz schists in this shear zone show 144.1 ± 1.2 Ma and 142.1 ± 0.9 Ma, respectively, providing the peak compression to be Early Cretaceous (144–142 Ma). Combined with the regional syn‐/post‐collisional magmatic rocks (166–155 Ma) related to the closure of the Mongol‐Okhotsk Ocean, this shear zone's ages (144–142 Ma) suggest that the continental convergence between the North China‐Mongolia and Siberia continents lasted from Late Jurassic to Early Cretaceous. The Early Cretaceous shoshonitic volcanism (137–133 Ma) and rift basins in this region indicate the onset of orogenic collapse in the eastern Central Asian Orogenic Belt.

Nitrogen fractionation in mica metapelite under hot subduction conditions: Implications for nitrogen ingassing to the mantle

Nitrogen partitioning between its main hosts is investigated using partly devolatilized natural quartz–muscovite–chlorite schist (mica schist), with NH4+-rich muscovite, under conditions corresponding to hot subduction: 3.0–7.8 GPa, 750–1090 °C, and oxygen fugacity (fO2) about the NNO (Ni-NiO) buffer. At these conditions, mica schist transforms into an eclogitic assemblage, while muscovite (C2/c) recrystallizes successively into two high-pressure white mica phases. The resulting phases are phengite (P3112) and a phase intermediate between the dioctahedral and trioctahedral mica series (C2/m). During dehydration and decarbonation reactions at run P-T conditions mica schist releases an H2O-CO2 fluid containing 0.3–14 rel.% N2 and < 0.1–3.7 rel.% NH3. The contents of NH4+ determined by Fourier transform infrared spectroscopy (FTIR) decrease from 1600 to 2000 ppm in primary muscovite to ∼1500 ppm in phengitic muscovite at 6.3 GPa and 1000 °С and to 610–990 ppm in the intermediate between the dioctahedral and trioctahedral mica phase at 7.8 GPa and 1070-1090 °C. Ammonium in the analyzed metapelite shows incompatible behavior (DNH4Mica−Fluid=0.05–0.15) at ≥ 5.5 GPa and temperatures and fO2 common to hot oxidizing slabs, which is expected to cause its efficient outgassing from subducting metasediments depleted in volatiles. Nitrogen partitioning under hot subduction conditions at sub-arc depths is generally controlled by devolatilization of measediments, phase transitions in N-bearing micas, and changes of fluid composition near the second critical point.

Granite Pluton at the Panasqueira Tungsten Deposit, Portugal: Genetic Implications as Revealed from New Geochemical Data

Core samples recovered from exploration boreholes and granite/greisen outcrops at the Panasqueira world-class tungsten deposit in central Portugal were subjected to chemical analyses and petrographic studies. We present a geochemical dataset and the trace element compositions of quartz and micas from a large part of the unexposed Panasqueira granitic pluton. Our data suggest that the hidden granite body is more complicated than previously believed. It consists of a flat cupola of porphyritic granite with only traces of mineralization at Rio and a steep stock of greisenized leucogranite surrounded by a swarm of flat quartz–muscovite veins rich in wolframite between Barroca Grande and Panasqueira. The contents of W (Sn, Nb, Ta) in muscovite markedly drop at a transition from the unmineralized greisen body to quartz veins. The W deposit was formed in three principal stages: (1) intrusion of porphyritic two-mica granite accompanied with local near-contact greisenization and uncommon quartz–wolframite veinlets; (2) intrusion of a more strongly fractionated leucogranite and formation of the cupola and apophyses; (3) circulation of hydrothermal fluids from deeper parts of the granite body into the cupola, greisenization, hydraulic fracturing and opening of flat structures in and outside the cupola and formation of ore veins.

Evolution of the magmatic–hydrothermal system and formation of the giant Zhuxi W–Cu deposit in South China

The Zhuxi deposit is a recently discovered W–Cu deposit located in the Jiangnan porphyry–skarn W belt in South China. The deposit has a resource of 3.44 million tonnes of WO3, making it the largest on Earth, however its origin and the evolution of its magmatic–hydrothermal system remain unclear, largely because alteration–mineralization types in this giant deposit have been less well-studied, apart from a study of the calcic skarn orebodies. The different types of mineralization can be classified into magnesian skarn, calcic skarn, and scheelite–quartz–muscovite (SQM) vein types. Field investigations and mineralogical analyses show that the magnesian skarn hosted by dolomitic limestone is characterized by garnet of the grossular–pyralspite (pyrope, almandine, and spessartine) series, diopside, serpentine, and Mg-rich chlorite. The calcic skarn hosted by limestone is characterized by garnet of the grossular–andradite series, hedenbergite, wollastonite, epidote, and Fe-rich chlorite. The SQM veins host high-grade W–Cu mineralization and have overprinted the magnesian and calcic skarn orebodies. Scheelite is intergrown with hydrous silicates in the retrograde skarn, or occurs with quartz, chalcopyrite, sulfide minerals, fluorite, and muscovite in the SQM veins.Fluid inclusion investigations of the gangue and ore minerals revealed the evolution of the ore-forming fluids, which involved: (1) melt and coexisting high–moderate-salinity, high-temperature, high-pressure (>450 °C and >1.68 kbar), methane-bearing aqueous fluids that were trapped in prograde skarn minerals; (2) moderate–low-salinity, moderate-temperature, moderate-pressure (~210–300 °C and ~0.64 kbar), methane-rich aqueous fluids that formed the retrograde skarn-type W orebodies; (3) low-salinity, moderate–low-temperature, moderate-pressure (~150–240 °C and ~0.56 kbar), methane-rich aqueous fluids that formed the quartz–sulfide Cu(–W) orebodies in skarn; (4) moderate–low-salinity, moderate-temperature, low-pressure (~150–250 °C and ~0.34 kbar) alkanes-dominated aqueous fluids in the SQM vein stage, which led to the formation of high-grade W–Cu orebodies. The S–Pb isotopic compositions of the sulfides suggest that the ore-forming materials were mainly derived from magma generated by crustal anatexis, with minor addition of a mantle component. The H–O isotopic compositions of quartz and scheelite indicate that the ore-forming fluids originated mainly from magmatic water with later addition of meteoric water. The C–O isotopic compositions of calcite indicate that the ore-forming fluid was originally derived from granitic magma, and then mixed with reduced fluid exsolved from local carbonate strata. Depressurization and resultant fluid boiling were key to precipitation of W in the retrograde skarn stage. Mixing of residual fluid with meteoric water led to a decrease in fluid salinity and Cu(–W) mineralization in the quartz–sulfide stage in skarn. The high-grade W–Cu mineralization in the SQM veins formed by multiple mechanisms, including fracturing, and fluid immiscibility, boiling, and mixing.

Ore Genesis of the Takatori Tungsten–Quartz Vein Deposit, Japan: Chemical and Isotopic Evidence

The Takatori hypothermal tin–tungsten vein deposit is composed of wolframite-bearing quartz veins with minor cassiterite, chalcopyrite, pyrite, and lithium-bearing muscovite and sericite. Several wolframite rims show replacement textures, which are assumed to form by iron replacement with manganese postdating the wolframite precipitation. Lithium isotope ratios (δ7Li) of Li-bearing muscovite from the Takatori veins range from −3.1‰ to −2.1‰, and such Li-bearing muscovites are proven to occur at the early stage of mineralization. Fine-grained sericite with lower Li content shows relatively higher δ7Li values, and might have precipitated after the main ore forming event. The maximum oxygen isotope equilibrium temperature of quartz–muscovite pairs is 460 °C, and it is inferred that the fluids might be in equilibrium with ilmenite series granitic rocks. Oxygen isotope ratios (δ18O) of the Takatori ore-forming fluid range from +10‰ to +8‰. The δ18O values of the fluid decreased with decreasing temperature probably because the fluid was mixed with surrounding pore water and meteoric water. The formation pressure for the Takatori deposit is calculated to be 160 MPa on the basis of the difference between the pressure-independent oxygen isotope equilibrium temperature and pressure-dependent homogenization fluid inclusions temperature. The ore-formation depth is calculated to be around 6 km. These lines of evidence suggest that a granitic magma beneath the deposit played a crucial role in the Takatori deposit formation.

Petrographic, geochemical and structural characteristics of gold-bearing metasedimentary rocks from the Atacora structural unit, Northwestern Bénin Republic

The metasedimentary rocks of the Atacora structural unit around Natitingou in Benin are composed of quartzites, quartz muscovite schist and mica schist. Being auriferous, the petrographic, geochemical and structural features of these rocks were investigated to ascertain the source of gold and its relation to structural deformations. Quartz, muscovite and opaque minerals constitute the dominant minerals in the quartz muscovite schist. The quartzites and mica schists are predominantly composed of quartz, plagioclase, microcline, muscovite and chlorite in varying proportions, with biotite, garnet and epidote in mica schist. The rocks have quartzose sedimentary composition and plot within the passive margin setting on major element discrimination diagrams. The Pan-African ductile and brittle deformation phases produced predominantly folds, strike-slip faults, dextral shear plane and fractures within the area. Gold-bearing veins in the host rocks occur within and near the N-S to NE-SW trending microfolds and the major brittle E-W and NE-SW faults. Gold mineralization is hosted within the NE-trending shear zones and NE-SW trending asymmetric folds which are attributed to transpressional D2 phase of the Pan-African orogeny. Pathfinder elements for gold mineralization in the Atacora are W–Cu–As–Mo ± Sn. This may suggest interaction between host rocks and mineralizing fluids, probably from deformation and metamorphic processes during the late stages of the Pan-African Orogenic event. However, fluid inclusion and stable isotopic studies are required to constrain the origin of ore fluids.

Preservation of erniettomorph fossils in clay-rich siliciclastic deposits from the Ediacaran Wood Canyon Formation, Nevada: Ediacaran preservation in sandstone beds

Author(s): Hall, JG; Smith, EF; Tamura, N; Fakra, SC; Bosak, T | Abstract: Three-dimensionally preserved Ediacaran fossils occur globally within sandstone beds. Sandy siliciclastic deposits of the Ediacaran Wood Canyon Formation (WCF) in the Montgomery Mountains, Nevada, contain two fossil morphologies with similar shapes and sizes: one exhibits mm-scale ridges and a distinct lower boundary and the other is devoid of these diagnostic features. We interpret these as taphomorphs of erniettomorphs, soft-bodied organisms with uncertain taxonomic affinities. We explore the cast-and-mould preservation of both taphomorphs by petrography, Raman spectroscopy, X-ray fluorescence microprobe and X-ray diffraction. All fossils and the surrounding sedimentary matrix contain quartz grains, iron-rich chlorite and muscovite. The ridged fossils contain about 70% larger quartz grains compared to the ridgeless taphomorph, indicating a lower abundance of clay minerals in the ridged fossil. Chlorite and muscovite likely originated from smectite and kaolinite precursors that underwent lower greenschist facies metamorphism. Kaolinite and smectite are inferred to have been abundant in sediments around the ridged fossil, which enabled the preservation of a continuous, distinct, clay- and kerogen-rich bottom boundary. The prevalence of quartz in the ridged fossils of the WCF and in erniettomorphs from other localities also suggests a role for this mineral in three-dimensional preservation of erniettomorphs in sandstone and siltstone deposits.

Fluid inclusion and stable isotope constraints on the source and evolution of ore-forming fluids in the Bailongshan pegmatitic Li-Rb deposit, Xinjiang, western China

The newly discovered Bailongshan pegmatitic lithium–rubidium (Li–Rb) deposit in West Kunlun–Karakorum (western China) is a world-class Li deposit. In this paper, we present detailed field observations and fluid inclusion (FI) data for four major mineral zones at Bailongshan; i.e., the fine albite (FAZ), blocky feldspar (BFZ), quartz–muscovite (QMZ), and spodumene–quartz (SQZ) zones. FIs are abundant in quartz and spodumene, and include four types: (1) two-phase L-type, (2) two-phase V-type, (3) three-phase S-type, and (4) three-phase (CO2–H2O–NaCl) or two-phase (CO2-rich) C-type. Quartz contains all four FI types, whereas spodumene contains mainly C-type FIs. Microthermometric measurements show that the L-type FIs in the FAZ, BFZ, and SQZ homogenised at 283–338, 156–224, and 223–421 °C, respectively, with corresponding salinities of 15.4–26.5, 4.4–10.4, and 10.5–30.6 wt% NaCl equivalent. The S-type FIs in the FAZ and SQZ homogenised at 183–285 and 214–298 °C, respectively. The C-type FIs in spodumene and quartz homogenised at 256–321 and 234–286 °C, with corresponding salinities of 4.4–18.1 and 4.4–12.6 wt% NaCl equivalent, respectively. The ore-forming fluids were of medium–low-temperature and medium–low-salinity, and no systematic fluid compositional variations were identified in each zone. The average CO2 densities and trapping pressures were estimated at 0.72–0.91 g/cm3 and 3.00–3.75 kb, respectively, corresponding to mineralisation depths of 9–11 km. Supercritical fluids likely contributed to the concentration of ore-forming elements in the fluids, and fluid boiling may have led to CO2 degassing and Li ore deposition. H–O isotope data show that quartz grains from all four zones have δ18OV-SMOW values of 8.6‰–10.2‰ and δDV-SMOW values of −38‰ to 107‰. The ore-forming fluids were likely sourced from pegmatitic magmas.

Early-Late Devonian Post-Collision Extension Due to Slab Breakoff Regime along the Northern Margin of North China Craton: Implications from Muscovite 40Ar-39Ar Dating

Ordovician-Silurian subduction, Early Devonian arc-contient collision and followed post-collision extension are recorded in the north of the North China Craton. Most previous research has focused on the first two processes. Discussion on the post-collisional extension and its tetonic regime is still limited. In this study, 40Ar-39Ar muscovite ages obtained from the central part of the northern North China Craton were analyzed to shed light on the timing of post-collision extension. Garnet muscovite schist and muscovite quartz schist in the Jianshan Formation yielded 40Ar-39Ar muscovite plateau ages of 412 ± 3 Ma and 391 ± 3 Ma, respectively. Two other 40Ar-39Ar muscovite plateau ages (389 ± 2 Ma and 397 ± 2Ma) were obtained for two Mesoproterozoic monzogranites intruding into the Jianshan Formation. Based on previous research, the northern North China Craton underwent a collision event with Bainaimiao arc at c. 415 Ma, followed by post-collision extension in early Late Paleozoic. Therefore, combined with the newly acquired muscovite 40Ar-39Ar dating results, the Jianshan Formation might go through regional metamorphism at c. 412 Ma during the collision process. Subsequently, the Jianshan Formation and monzogranites intruding into it went through rapid exhumation along with metamorphism at c. 397–389 Ma in a post-collision extensional setting. The muscovite 40Ar-39Ar ages provide new markers for the exhumation history and the post-collisional extension setting during Early-Late Devonian in the study area. Furthermore, slab breakoff as the cause for this extensional setting is argued by the emplacement of the Early-Late Devonian alkaline rocks.

Geology and origin of the Zhunuo porphyry copper deposit, Gangdese belt, southern Tibet

The Zhunuo porphyry copper deposit in the Gangdese belt of southern Tibet contains 2.37 Mt at 0.57% Cu. In this deposit, Eocene rhyolite (51.6 ± 1.0 Ma) and quartz porphyry (49.1 ± 0.6 Ma) were intruded by monzogranite (14.7 ± 0.3 Ma) and monzogranite porphyry (14.5 ± 0.2 Ma) which commonly host mafic microgranular enclaves (14.9 ± 0.3 Ma). Late-mineralization diorite porphyry (14.2 ± 0.2 Ma) and post-mineralization lamprophyre dikes (12.2 ± 0.1 Ma) and granite porphyry (12.0 ± 0.2 Ma) crosscut the Miocene intrusions. Two quartz–molybdenittie veins yielded molybdenite Re-Os model ages of 14.8 ± 0.1 Ma and 14.4 ± 0.1 Ma, and another two quartz–muscovite–llite–pyrite–molybdenite veins yielded molybdenite Re-Os model ages of 14.2 ± 0.1 Ma and 13.5 ± 0.1 Ma. Zhunuo is characterized by early potassic alteration and associated quartz A veins centered on the monzogranite porphyry and adjacent monzogranite, distal propylitic alteration in the rhyolite, and late-stage phyllic alteration and associated quartz B veins, followed by D veins that affected quartz porphyry and potassic-altered rocks. Copper ores are mainly distributed in the potassic alteration zone that was overprinted by phyllic alteration. Copper mineralization likely occurred during potassic alteration stage and was locally remobilized and redeposited during phyllic alteration stage, or occurred during phyllic alteration stage. The post-mineralization granite porphyry has lower whole-rock Sr/Y ( 40; eNd(t) = − 6.9 to − 6.1; U/Yb = 1 to 10). Limited input of juvenile melts could explain the low potential for copper mineralization in the post-mineralization granite porphyry at Zhunuo.

Magmatic-hydrothermal processes recorded by muscovite andcolumbite-group minerals from the Bailongshan rare-element pegmatites in the West Kunlun-Karakorum orogenic belt, NW China

The mineralization of rare-element pegmatites is attributed to complex magmatic-hydrothermal processes such as magma fractionation, fluid exsolution and fluid/melt metasomatism. However, it is still unclear how these processes affect the evolution of pegmatites and the fractionation of rare elements. Here we use the textures and major and trace element compositions of muscovite and columbite-group minerals (CGM) from the newly discovered Bailongshan Li-Cs-Ta (LCT)-type pegmatites in the West Kunlun-Karakorum orogenic belt, NW China, to decipher the magmatic-hydrothermal processes related to enrichment of rare elements. Four major regional zones are recognized in the Bailongshan area; quartz-albite-tourmaline (QAT) zone, quartz-muscovite (QM) zone, quartz-albite-spodumene (QAS) zone and quartz-spodumene (QS) zone. The QAT and QM zones are barren, whereas the QAS and QS zones host six major LiRb ore segments (I to VI) along the NWW-SEE strike. From the QM zone through the QAS to the QS zone, the K/Rb ratios of muscovite decrease gradually from 26.0 to 7.8 and CGM has Mn#s increasing from 0.28 to 0.56, consistent with the trend of progressive evolution of the pegmatites. The Ta# profiles across individual CGM grains reveal a three-stage growth process, i.e., from a Nb-rich CGM magmatic stage to a Ta-rich CGM magmatic stage to a Nb-rich CGM hydrothermal stage. High-resolution elemental mapping of primary muscovite from the QAS and QS zones shows that melt metasomatism induced local enrichment of F, Li, Cs and Ta and the precipitation of secondary lepidolite along the rims or cleavages of primary muscovite. In addition, late-stage, F-poor fluid metasomatism caused the release of Fe, Mn, B, Rb and Cs from primary muscovite, and induced complex zoning patterns and crystallization of hydroxylapatite in cleavage planes of primary muscovite. Secondary muscovite has higher Li, Be, B, Rb and Cs and lower Nb than primary muscovite, indicating that these elements may have been partially dissolved and remobilized from primary spodumene, muscovite and CGM into late-stage hydrothermal fluids, from which the secondary muscovite and Nb-rich CGM crystallized. Therefore, the complex zoning patterns and compositional variations of both muscovite and CGM from the Bailongshan pegmatites record a progressive magmatic-hydrothermal process related to the rare-metal mineralization.

Mineralogical and Geochemical Evidence for Granite Metasomatism in Dikes in the North of the Berezovskoe Ore Field (Middle Urals)

Abstract —We present results of petrographic, geochemical, and mineralogical studies of apogranite metasomatites associated with sulfide–quartz gold ore veins. The studies show a predominance of muscovite and quartz–muscovite metasomatites. Formation of muscovite metasomatites was accompanied by the accumulation of W, Sc, Zr, Hf, Ga, REE, U, Th, Ta, and Nb and the genesis of new accessory minerals: monazite-(Ce), apatite, zircon, scheelite, W-containing rutile, uraninite, thorianite, cassiterite, etc. Compared with the primary granites, quartz–muscovite metasomatites are richer in Pb, Bi, As, Sb, Co, Ni, Ba, In, Cd, Mo, Te, Ag, and Au (elements of the gold ore assemblage). The high contents of these trace elements are due to abundant galena, fahlores, chalcopyrite, and pyrite among the accessory minerals. Metasomatism of granites was followed by the removal of SiO2, which was then spent for the formation of quartz veins. We have revealed that the distribution of metasomatites of different types within a dike body affects directly the distribution of sulfide–quartz veins and thus determines the ore content of the dike body fragments.

First-principles calculations of equilibrium silicon isotope fractionation in metamorphic silicate minerals

In this study, silicon isotope fractionation factors for metamorphic silicate including muscovite, epidote, and kyanite are calculated using first-principles methods based on density-functional theory. The reduced partition function ratio (103lnβ) of 30Si/28Si decreases in the order of muscovite > epidote ≈ kyanite. Combining with previous calculations, our results show that the equilibrium silicon isotope fractionation factors are correlated with average Si–O bond length. These results predict that measurable Si isotope fractionation can occur during metamorphic fluid activities. At 1000 K, the calculated 103lnα values for quartz–epidote, quartz–kyanite, and quartz–muscovite are 0.51‰, 0.49‰, and 0.08‰, respectively. The calculation results are generally consistent with the observed Si isotopic data in two metamorphic eclogite-vein systems, showing that our work can provide useful information in investigating the Si isotope fractionation in metamorphic fluid evolution.

40Ar/39Ar Age of Gold Ore Metasomatites of the Albyn Deposit, Mongol–Okhotsk Fold Belt

The paper reports the results of 40Ar/39Ar geochronological studies of the muscovite–quartz–albite metasomatites of the ore bodies and host rocks of the Albyn deposit located in the Selemdzha–Kerbi structural zone of the eastern part of the Mongol–Okhotsk Fold Belt. To a first approximation, the age of the hydrothermal ore process responsible for the formation of the Albyn deposit can be estimated within an interval of 131–130 Ma. At the same time, we cannot exclude the existence of an earlier stage of this process at ~135 Ma, although this assumption requires further substantiation. A close age estimate (~134–130 Ma) was obtained earlier for the ore metasomatites of the Malomyr deposit located in the same structural zone. There are no data on the magmatic manifestations of close age within the considered region, which makes it impossible to relate the ore mineralization of the Albyn deposit with magmatic processes. At the same time, the age of the thermal event superimposed on the host rocks of the Afanasiev Formation is 131 ± 3 Ma, which likely indicates the regional nature of this event. We suggest that a significant role in the mobilization, redistribution of ore matter, and formation of the Albyn deposit was played by large-scale deformations accompanied by hydrothermal activity.

Mineralogical constraints on the magmatic–hydrothermal evolution of rare-elements deposits in the Bailongshan granitic pegmatites, Xinjiang, NW China

The Bailongshan pegmatite district in south Xinjiang Province, NW China, is a newly discovered area of rare-element pegmatites. It is a superlarge Li–Rb reserve that includes an estimated 3.45 million tons of Li2O and 176,000 tons of Rb2O. On the basis of a detailed characterization of textures and minerals in the Bailongshan 2# and 5# pegmatites, six zones are identified: a saccharoidal albite zone (zone I), a graphic pegmatite zone (zone II), a blocky microcline zone (zone III), a muscovite–quartz zone (zone IV), a cleavelandite–spodumene zone (zone V), and a quartz–spodumene zone (zone VI). The detailed mineralogical characteristic is performed by EPMA for illustrating both the existent state of rare-elements and the magmatic-hydrothermal evolution of the Bailongshan pegmatite. Lithium, Be, Rb, Nb, Ta, and Sn are major economic rare-elements, which are concentrated in zones V and VI. The rare-elements are hosted by minerals including spodumene, montebrasite, eucryptite, lepidolite, beryl, Rb-enriched K-feldspar, columbite-group minerals, cassiterite, and tourmaline, which formed from the fractional crystallization of pegmatitic magma. The Bailongshan pegmatites show lower degrees of fractional crystallization than many other Li-Cs-Ta (LCT-family) pegmatites worldwide, as evidenced by the relatively low Nb-Ta, Zr-Hf, and K-Rb fractional degree in the carrier minerals. In zones V and VI, the primary minerals record intense interaction with hydrothermal fluids, which caused the alteration of spodumene and mica, the development of Cs- or Na-enriched veinlets in beryl, and the formation of Mn- and Ta-enriched veinlets coupled with micropores within columbite-group minerals. The hydrothermal fluids were composed predominantly of Li, F, Cs, and Ta, which conformed to the evolved trend of pegmatitic melt. These late hydrothermal fluids exsolved from the magma as it differentiated, and may have the benefit with rare-elements metallogeny.

Epigenetic Graphitization in the Basement of the Siberian Craton as Evidence of the Migration of Hydrocarbon-enriched Fluids in the Paleoproterozoic

This paper reports on diagnostic and structural studies that were first carried out for carbonaceous material of quartz–muscovite dynamoschists from the schistosity zone in biotite migmatites, pegmatites, and diabases of the southern part of the Baikal ledge of the Siberian craton. The carbonaceous material is represented by phanerocrystalline and microcrystalline graphite with residual hydrocarbon radicals. Native Ni, Sn, zincous Cu, Fe–Ni compounds, sulfides of Cu, rutile, monazite, and zircon were revealed in the intergrowths with carbonaceous material. The carbon isotopic composition ranges from –29.19‰ to –31.58‰, except for carbonaceous material from the schistocity diabase, where δ13C = –24.93‰. 40Ar–39Ar dating of muscovite gave an age of 1947 ± 7.8 Ma pointing to the relation between dynamic metamorphism with accretion of the Akitkan fold system and the ancient complexes of the craton. It was concluded that the deposition of native carbon and metals was caused by migration of essentially hydrocarbonate fluid in the formations of the upper crust (Fig. 4, Table 1).