Chondrite-normalized REE Patterns Research Articles

Genesis of W mineralization in the Yangla Cu-polymetallic deposit (NW Yunnan, China): Constraints from scheelite microstructure, trace element, U-Pb dating and Sr isotope geochemistry

The Yangla Cu-polymetallic deposit is the largest skarn Cu deposit in the Jinshajiang suture zone. Recently, an new W orebody was identified in the depths of the Cu orebodies. However, the genetic relationship between W and Cu orebodies, along with the tungsten ore-forming processes, remains unclear. This study delves into the careful investigation of three types of scheelites focusing on microstructure, trace element compositions and Sr isotope geochemistry. The aim is to elucidate the source and evolution of ore-forming fluids and ore genesis in the multi-episodic metallogenic systems. Primary scheelite (Sch-I) generally displays oscillatory zoning in CL images with the lowest REE (3.8–142 ppm) and highest Sr (604–1480 ppm) contents. Secondary scheelite (Sch-III), commonly precipitated as overprinting rims on primary crystal, has the highest REE (145–1071 ppm) and lowest Sr (376–793 ppm) contents. In contrast, Sch-II represents a transitional stage from primary to secondary, with REE and Sr contents at intermediate levels. Sch-I is characterized by LREE enrichment and a positive Eu anomaly in the chondrite-normalized REE patterns, whereas Sch-III shows MREE-rich patterns with a weak positive Eu anomaly. Scheelite U-Pb dating (30.3 ± 1.5 Ma) indicates that the W mineralization is not genetically related to Triassic granitoids in the Yangla district. Instead, it is likely associated with magmatic activities and tectono-thermal events in the Jinshajiang fault zone during the Cenozoic. The high initial 87Sr/86Sr ratios (0.71771–0.72229) in the scheelite, significantly higher than those of Triassic granites and ore-hosting marble, suggest that the geochemical features of Sch-I are inherited from the parental ore-forming fluids. These fluids are likely derived from the mixed hydrothermal fluids involved by the Neoproterozoic strata and/or concealed magmatism.

Vermilion monazite: A new Archean reference material for U-Pb and Sm-Nd microanalysis

Monazite (Vermilion) from the ∼2.7 Ga Vermilion Granitic Complex (Minnesota, USA) was investigated as a potential reference material for U-Pb geochronology, using electron microprobe, high-precision ID-TIMS and high-spatial resolution techniques (LA-(Q/SF/MC)-ICP-MS and SHRIMP). Vermilion monazite is relatively homogeneous in BSE images. It is characterised as a monazite-(Ce) with uniform, relatively LREE-enriched chondrite-normalised REE patterns (Eu/Eu* = 0.074–0.085). ID-TIMS U-Pb analyses yielded concordant to slightly (−0.6% to 1.8%) discordant isotope data with weighted mean 207Pb*/206Pb* dates of 2668.6 ± 1.8 Ma (2σ, MSWD = 1.3, n = 3) and 2666.8 ± 1.8 Ma (2σ, MSWD = 4.0, n = 4). High-spatial resolution data from SHRIMP and LA-(Q/SF/MC)-ICP-MS agree with the ID-TIMS age. Using Vermilion monazite as primary reference material, LA-SF-ICP-MS analyses reproduced the age of several other reference monazites within −1.13% to +1.64% accuracy. Nd isotopes obtained via ID-TIMS and solution MC-ICP-MS returned consistent 143Nd/144Nd ratios of 0.510694 ± 0.000040 (2σ) and 0.510726 ± 0.000014 (2σ), respectively. The LA-MC-ICP-MS 143Nd/144Nd ratios of 0.51071 ± 0.00001 (2σ) and 0.51065 ± 0.00001 (2σ) are also in agreement within uncertainties. Results of Sm-Nd isotope analyses by LA-MC-ICP-MS from two laboratories yielded mean 147Sm/144Nd ratios of 0.0869 ± 0.0002 (2.5% RSD, 2σ, n = 58) and 0.0835 ± 0.0001 (2.2% RSD, 2σ, n = 50), reflecting small variations in individual fragments. The initial epsilon Nd (εNdi) varies from −0.8 ± 0.5 to 0.6 ± 0.4 (2σ). The reproducibility of our results shows that Vermilion monazite can be used as a primary reference material for U-Pb geochronology and is suitable as a secondary reference material for Sm-Nd isotopic tracing. The Archean age of Vermilion monazite, the oldest available as a reference material, will facilitate the investigation of Archean crustal evolution processes and broaden the prospect of studying ancient metamorphic events by combining matrix-matched U-Pb geochronology and Sm-Nd isotopic tracing.

Post-collisional calc alkaline-adakites from a metasomatized subcontinental lithospheric mantle: Sr-Nd isotopes and bulk-rock geochemistry of Miocene volcanic rocks from the Varzaghan area, NW Iran

ABSTRACT The NW Iran is located at the western termination of the Alborz Mountain range and continuation of the Sanandaj-Sirjan magmatic-metamorphic zone at the NW. The volcanic rocks in the Varzaghan area are distributed over different compositional ranges from high-K basalt, andesite, dacite, and trachyandesite, dacite, to rhyolite with adakitic signatures. Chondrite-normalized REE diagrams show flat MREE and HREE patterns for calc-alkaline volcanic rocks and enrichment in LREE, indicating a garnet-free source. Adakitic rock sample chondrite-normalized REE patterns are relatively steep with significant enrichment in LREE and depletion in HREE, indicating a garnet-bearing source or garnet fractionation process. The arc trace element signatures, narrow range of initial87Sr/86Sr(i) = 0.70425–0.70487, different initial143Nd/144Nd(i) = 0.51264–0.51286 and the presence of adakitic rocks reveal that the volcanic rock sources were not the same and can be related to the partial melting of metasomatized mantle and lower continental crust sources. Slab break-off and upwelling of hot asthenosphere in a post-collisional tectonic regime resulted in the melting of the subcontinental lithosphere and lower continental crust which produced calc-alkaline-adakitic volcanic rocks in the Varzaghan area. The main petrological processes involved in the volcanic rocks petrogenesis were fractional crystallization and crustal assimilation.

Allanite in Mantle Eclogite Xenoliths

Abstract Rare-Earth Elements (REE) are key geochemical tracers of crust–mantle differentiation, but there are few direct data on REE-rich minerals in mantle rocks. Here, we report the combined petrography and comprehensive chemical and isotopic characterization of three coesite- and kyanite-bearing eclogite xenoliths from the Udachnaya kimberlite pipe (Siberian craton), which are unusual in that two xenoliths (one with diamond and graphite) contain discrete, idiomorphic crystals of allanite at the grain boundaries of garnet and omphacite. Another xenolith contains allanite as part of a complex aggregate of calcite, apatite, barite, and celestine hosted by serpentine, which is a low-temperature secondary minerals likely result from metasomatic reaction at shallower depths during the transport of eclogite by the erupting kimberlite melt. The bulk rock composition reconstructed from the trace element composition of garnet and omphacite show marked depletion in LREE, precluding equilibration with kimberlite melt, whereas the measured bulk compositions show chondrite-normalized REE patterns with conspicuous depletions of Ce–Pr–Nd relative to La and Sm. The presence of 0.005 to 0.008 wt % of allanite, texturally and chemically out of equilibrium with the rock-forming minerals, allows balancing the LREE and Sm–Nd budget of the rock, whereas Th and U require additional hosts. This not only highlights the utility of measuring bulk eclogite xenoliths in bringing this unusual component to light, but also demonstrates that the long-known incompatible element enrichment in bulk eclogites reflects the deposition of discrete phases rather than merely bulk kimberlite melt addition. Although allanite is stable in metabasalts at the pressure–temperature conditions of 1025°C to 1080°C and 3.6 to 4.8 GPa recorded by the eclogite xenoliths, its association with Ba-Sr minerals suggests its formation via reaction of the host eclogites with kimberlite melt. This is supported by the similarity in 143Nd/144Nd ratios between bulk eclogite (0.51227–0.51249) and the host kimberlite at eruption, whereas clinopyroxene in part retains unradiogenic Sr (87Sr/86Sr = 0.70205 ± 0.00011) related to ancient depletion. The discovery of allanite in the Udachnaya eclogites demonstrates that this REE mineral can form when omphacite and grossular-rich garnet in eclogite breakdown in contact with REE- and alkali-rich carbonatite/kimberlite melt, and may be more common than hitherto recognized. Crystallization of allanite in the cratonic mantle eclogite reservoir may also help explain the difference in LREE abundances between the more strongly enriched carbonatite/kimberlite at depth and the final erupted product. It is likely that allanite is overlooked at eclogites xenoliths, while it is common accessory mineral, hosting REE in orogenic UHP/HP eclogites. Further studies are required to deciphered the peculiarities in metamorphic history recorded in eclogites xenoliths and orogenic eclogites, as well as the differences ancient (Archean/Proterozoic) and Phanerozoic subduction processes.

Petrogenesis of cassiterite-rich greisens from Guengue hill, Mayo Darlé, Northwestern Cameroon

The studied cassiterite-rich greisens of Mayo Darlé are hosted in granites located on the NE slope of the Guengue hill. They have three various structural forms including massive, vein, and stockwork. This study aims to describe all the different structural forms of greisen and also explains the main processes of greisen deposition related to the tin mineralization. The petrographic study shows that the massive form presents two facies: the massive greisens are rich and barren in cassiterite respectively; the stockworks and veins are all mineralized. The mineralogical composition recorded for each of the forms are Quartz + Muscovite + Cassiterite + Zinnwaldite + Plagioclase for the cassiterite-bearing greisen; Quartz + Muscovite + Tourmaline + Chlorite for the barren greisen and Quartz + Muscovite + Cassiterite + Tourmaline + Chlorite for the veins and stockworks. This mineralogy could indicate a varied chemical nature of the hydrothermal/magmatic fluids and the host granite. Also, the presence of minerals such as tourmaline, zinnwaldite and chlorite can equally indicate that there were three main stages in the granitic process: (i) tourmalinization, (ii) lepidolitization, and (iii) chloritization. The greisens have a chemical composition rich in SiO2 (over 71 %) and Al2O3 (5.23–43 %). The chondrite-normalized REE pattern shows a negative Eu anomaly (0.0005–0.15) resulting from the disappearance of feldspars and biotite from the geochemical system, which is gradually transformed to quartz and chlorite respectively during leaching. The ratios of Rb/Sr, Zr/Hf, and Nb/Ta indicates an M-type tetrad effect, which also characterizes the variable values of the Eu anomaly. From a mining point of view, the areas most suitable for further prospection would be those hosting the massive, cassiterite-rich greisens. The significant occurrence of tourmaline and zinnwaldite in the studied greisens could indicate a possible Sn–W–Li polymetallic deposit.

Genesis and tectonic setting of an Early Palaeozoic skarn in the East Kunlun Orogenic Belt, China: constraints from adakitic rocks and garnet megacrysts

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) in the northern Tibet Plateau has undergone a long multi-stage history of drifting, accretion, and collision of the Proto- and Palaeo-Tethyan oceans. It has been addressed a trench-arc-back-arc basin system of the Proto-Tethys in the E-KOB, and the tectonic history of this major ocean has been well constrained by previous studies. However, the evolution of the back-arc basin still remains unclear. In this study, the Yugusayi monzodiorite, quartz-bearing syenite, and gradually contacted skarn rocks characterized by garnet megacrysts, in the North Qimantagh Belt were investigated. To reveal the tectonic setting and skarnization mechanism, geochemical and geochronological studies were conducted on the intrusions and garnet skarn, and the elemental compositions of the garnet megacrysts were also analysed. These monzodioritic rocks have high MgO (6.87–8.31 wt.%), Sr and La contents, low Y and Yb contents, and relatively high La/Yb (31–55) and Sr/Y (41–51) ratios, resembling to high-Mg adakites. They display right-sloping chondrite-normalized REE patterns, enriched in LILEs, and depleted in Ti with significant Nb-Ta troughs, which are similar to those of continental arc rocks. Zircons from monzodiorite and skarn samples have contemporaneous U-Pb ages of 459 ± 3 Ma and 454 ± 4 Ma with εHf(t) values of − 8.53 to − 4.64 and − 7.7 to + 12.8, respectively. In addition, the chemical zoning garnet megacrysts are composed mainly of grossular and andraditic grossular, and are characterized by spoon-shaped REE patterns. Based on our new data and the regional geology, it can be concluded that the Yugusayi intrusive rocks formed in a subduction-related tectonic setting and resulted in the skarnization. These data not only provide key constraints on the initial subduction of the Qimantagh back-arc basin’s oceanic crust, but also offer a unique opportunity to explore the Caledonian skarnization in the Qimantagh Belt.

Geochemical dichotomy of Tethyan oceanic mantle and the supercontinent connection: Insights from Os isotopic signatures of mantle peridotites from Naga Hills ophiolite

Naga Hills Ophiolite (NHO) represents a thrusted section of relict Neo-Tethyan oceanic lithosphere comprising distinct crustal and upper mantle lithologies that preserve imprints of different stages of evolution of ancient ocean basins and subsequent accretion onto continental margins during late Cretaceous-Eocene collision involving Indian and Eurasian Plates. This study presents bulk geochemistry and Re-Os isotopic compositions for the mantle peridotites of NHO to address their petrogenetic aspects, implications for Neo-Tethyan upper mantle heterogeneity and thermo-tectonic evolution of Neo-Tethyan oceanic lithosphere through subduction-accretion-collision processes. Geochemical characteristics of the mantle peridotites, as reflected from their chondrite normalized REE patterns, binary relationship between fractionation depletion indices, low Re/Os, suggest that these are mantle residues resulting from ∼ 5–20 % of melt extraction from a spinel peridotite source that experienced decompression melting in a fore-arc extensional setting in response to subduction initiation. Isotopically, these mantle peridotites are subchondritic with 187Os/188Os (0.1218–0.1266) and γOs values of −3.73 to −0.09 comparable with the depleted MORB source. LILE-LREE enrichment, HFSE depletion, and U-shaped chondrite-normalized REE patterns demonstrated by the studied samples with Re addition collectively corroborate multistage petrogenetic processes in compliance with geodynamic transition from extensional to compressional regime in a suprasubduction zone environment and partial melting of a chemically heterogeneous source mantle. This compositional heterogeneity can be equated with mantle depletion by melt extraction and refertilization by influx of slab-dehydrated fluids and percolation of boninitic melts. The non-radiogenic Os isotope signatures, negative γOs values and wide spectrum of melt extraction ages for these mantle peridotites correspond to (i) multiple melt production and melt extraction events prior to the opening of Neo-Tethyan seaway (ii) detachment and incorporation of ancient, depleted SCLM fragments into oceanic mantle concurrent with subduction driven ocean basin closure-congregation of Gondwana Supercontinent and emergence of Neo-Tethyan embryonic ocean in response to disintegration of Gondwana Supercontinent.

A late Neoarchean continental arc system in the northeastern North China Craton: constrain from the Guanghua mélange in the Tonghua area, southern Jilin Province

ABSTRACT The late Neoarchean tonalite – trondhjemite – granodiorite (TTG) and mafic rocks record essential information regarding the early tectonic evolution of the North China Craton, which is crucial for understanding the onset and development of the plate tectonic regime. In this study, we present a comprehensive analysis of petrography, geochronology, and geochemistry of the Guanghua Mélange from the Tonghua area in NE China. The mélange comprises basalt, gabbro, pyroxene diorite, and trondhjemite, and is classified into three groups: basaltic – gabbroic rocks (Group #1), dioritic rocks (Group #2), and trondhjemites (Group #3). Zircon U-Pb analyses indicate that they formed at 2596–2550 Ma, 2531–2515 Ma, and 2502 Ma, respectively, with metamorphism occurring during ~ 2500–2450 Ma. Meanwhile, geochemical data indicate that the calc – alkaline basaltic rocks (Group #1) exhibit slight depletion of Nb or Ta with weakly negative Eu anomalies, suggesting derivation from the lithospheric mantle and formation within a continental arc environment. The pyroxene diorites (Group #2) display similar geochemical characteristics to Group #1, likely formed by partial melting of depleted mantle that underwent sgignificant metasomatism due to variable amounts of slab-derived fluids or melts. The tholeiitic granitoids (Group #3) exhibit strongly fractionated chondrite-normalized REE patterns with high (La/Yb)N and Sr contents, low Y contents, and depletion of Nb, Ta or Ti, resembling adakites generated in arc setting, indicating likely formation through partial melting of subducted oceanic crust. This process was controlled by the initial plate tectonics under a higher geothermal gradient. Consequently, when combined with previous findings, we conclude that an initial subduction of oceanic plate operated in the northeastern North China Craton during the late Neoarchean (>2.55 Ga). Subsequently, it transformed into modern-type plate tectonics during ~ 2.55–2.53 Ga, eventually followed by the continent-continent collision at ~ 2.50–2.45 Ga, resulting in the formation of the Ji – Liao – Ji (Jilin – Liaoning – Hebei) Collision Orogenic Belt.

Geochemistry of Rocks at the Neskevara Rare-Metal Deposit of the Vuoriyarvi Alkaline–Ultramafic Complex, Kola Peninsula

The Vuoriyarvi Paleozoic alkaline–ultramafic complex with carbonatites is made up of a great diversity of rocks with various ore mineralization. The paper presents data on the geochemistry of pyroxenites, phoscorites, and carbonatites from the Neskevara deposit of rare metals. The pyroxenites of the rare-metal deposit are significantly enriched in Nb, Ta, and Th relative to the primitive mantle and the primary alkaline–ultramafic melt composition calculated for the Kola alkaline province and are characterized by high Nb/Ta, Zr/Hf, and Th/U ratios of 29, 35, and 14, respectively. HFSE are maximally enriched in the phoscorites and carbonatites of stages II and III, with the highest concentrations of Nb (16 000 ppm), Th (2800 ppm), and Zr (4000 ppm) found in the calcite–tetraferriphlogopite phoscorites, in which pyrochlore crystallization on the liquidus was identified. The rocks of the carbonatite series are strongly enriched in LREE relative to carbonaceous chondrite. The calcite–dolomite carbonatites of the late magmatic–carbothermal stage show REE enrichment up to 25 800 ppm. The chondrite-normalized REE patterns and (La/Yb)N ratio indicate that REE were systematically more strongly fractionated in the sequence pyroxenite (70)—phoscorite (90)—calcite (540) and dolomite (3790) carbonatites The crystallization sequence of minerals in the rare-metal phoscorites and carbonatites of intermediate stages indicates that magnetite and pyrochlore crystallized nearly simultaneously. The crystallization temperatures of such associations are, according to data of the magnetite–ilmenite thermometer, lower than 500–600°C, at ∆NNO = –0.3 and + 1.5 and corresponded to the temperature at which the rare-metal ore mineralization of the main stage was formed.

In situ U-Pb dating of rutile and titanite from the Chaihulanzi lode gold deposit, northern North China Craton and its geological significance

The Chifeng-Chaoyang region, located along the northern margin of the North China Craton, is an important gold producer in China, with a proven gold reserve of about 500 tons. Most gold deposits are fault-controlled and formed in the Mesozoic, but the absolute ages of gold mineralization in this region have not been well constrained due to the lack of suitable dating minerals. This study presents in situ U-Pb ages of hydrothermal rutile and titanite for the Chaihulanzi gold deposit and obtains a distinctive age. The Chaihulanzi deposit (∼27 t Au) is located in the western portion of the Chifeng-Chaoyang region and mainly hosted in the carbonaceous mica schist of the Neoarchean-Paleoproterozoic Jianping Group along NW to NWW striking faults. It contains auriferous quartz veins and disseminated ores in hydrothermal alteration zones, with pyrite and pyrrhotite as the predominant ore minerals. Based on field and petrographic observations, four stages of mineralization process can be identified, involving stage I quartz-pyrite, stage II quartz-pyrite-pyrrhotite-chalcopyrite, stage III quartz-polymetallic sulfides, and stage IV quartz-calcite. Stage II represents the main gold stage and contains abundant hydrothermal rutile and titanite, which are intergrown with pyrite, pyrrhotite and native gold. Rutile grains have high contents of V, Nb, Fe, Cr, W and low contents of Zr and yield a Tera-Wasserburg lower intercept age of 285.4 ± 8.5 Ma (n = 37, MSWD = 3.9). Titanite grains show low Th/U and La/Yb ratios (0.03 to 0.66 and 0.26 to 3.98, respectively) and flat chondrite-normalized REE patterns (LREE/HREE = 0.88–5.72, Eu/Eu* = 0.71–1.71), with a Tera-Wasserburg lower intercept age of 282.3 ± 3.4 Ma (n = 20, MSWD = 2.1). Combined with the emplacement age of a post-ore mafic dyke (ca. 275.5 Ma), the timing of the Chaihulanzi deposit can be constrained to Early Permian, representing a new gold mineralization epoch in the Chifeng-Chaoyang region. On the basis of available geological and geochronological evidence, the Chaihulanzi deposit can be classified as an orogenic gold deposit formed during the southward subduction of the paleo-Asian Ocean. These new findings likely have important implications for future regional gold exploration.

Early Paleozoic granitic magmatism and mineralization in the Debao Sn-Cu deposit, South China: Constraints from zircon and cassiterite U-Pb geochronology and whole-rock geochemistry

Numerous W-Sn deposits are developed in South China. While the majority of these deposits are related to the Yanshanian granites, a few of them are associated with the Early Paleozoic granites, and their geological significance is poorly understood. The Debao deposit is a typical skarn Sn-Cu polymetallic deposit that is spatially related to the Early Paleozoic Qinjia granites. Zircon U-Pb dating of three lithofacies (coarse-grained biotite granite, medium-grained porphyritic biotite granite and fine-grained porphyritic biotite granite) of the Qinjia granites yielded emplacement ages of 437.8 ± 3.0 Ma, 440.2 ± 2.0 Ma and 441.0 ± 3.0 Ma, respectively, consistent with cassiterite U-Pb ages of skarn ore (439.6 ± 2.7 Ma), indicating a close genetic relationship between the granitic magmatism and Sn mineralization. Biotite from the Qinjia granites belong to the ferro-biotite series with relatively high F contents (up to 0.70 wt. %). The Biotite compositions plot near the Ni-NiO (NNO) buffer on a Fe3+-Fe2+-Mg2+ diagram, suggesting formation under relatively low oxygen fugacity. The Qinjia granites are characterized by highly silicious (70.37–72.88 wt. %), high alkali (Na2O + K2O = 6.98–8.88 wt. %) and metaluminous to strongly peraluminous with the A/CNK ratios of 0.92–1.21, belonging to the I-S transform-type granites. They are enriched in U, Th, Zr, Nd and Hf, and depleted in Ba, Sr, Nb, Ta and Ti. They display right-declining chondrite-normalized REE patterns with significant negative Eu anomalies (EuN/Eu* = 0.33–0.66), and have low Nb/Ta and K/Rb ratios, indicating that the Qinjia granites experienced a high degree of fractional crystallization. The zircon εHf(t) values (−2.7 to +1.4), together with TDMC of 1.59 to 1.33 Ga, indicate that the granites formed via partial melting of Mesoproterozoic crustal with minor addition of mantle materials. The reduced (fO2 near NNO and Ce4+/Ce3+ ratios of zircon < 100), fractionated and volatile-rich magma facilitated the enrichment of Sn in the residual melt and hydrothermal fluids. Combined with previous studies on the Early Paleozoic magmatic rocks and tectonic activities in South China, we suggest that the Qinjia granites and associated Sn-Cu mineralization were likely formed in the post-collisional extensional environment during the Wuyi-Yunkai orogeny.

Monazite record of assimilation and differentiation processes in the petrogenesis of Himalayan leucogranites

The Himalayan leucogranites have received particular attention as they record the continental crust-building processes of a collisional orogenic system. Deciphering the magmatic evolution processes and potential sources is essential for understanding the origin of the Himalayan leucogranites. In-situ monazite Th-Pb ages, trace element geochemistry, and Nd isotope ratios have been determined with whole-rock geochemical data for the leucogranites and their country rocks in the Yadong area, southern Tibet. Monazites in most of the studied leucogranites are marked by multiple age populations with variable REE ratios and wide ranges of Nd isotope ratios, reflecting a complex history of magmatic processes. Abundant inherited monazites are identified in the Yadong leucogranites, and most of them are excluded from the source regions due to their distinguished Nd isotope compositions from the magmatic monazites. The inherited monazites share close affinities in geochronological dates and geochemical compositions with those in the country rocks, strongly suggesting a major origin of the country-rock contamination. This highlights the contamination-assimilation processes in the genesis of the Yadong leucogranites. It is worth noting that xenocrystal monazites from the metasedimentary country rock are less preserved in the leucogranites than those from the granitoid country rock, which probably reflects the different assimilation reactions for different types of country rock. Assimilation of the metapelitic rocks released low content of xenocrystic monazite and high volume of partial melt into the leucogranites, whereas assimilation of the granitoid rocks released high content of xenocrystic monazite. In addition to the contamination-assimilation processes, the uniform Nd isotope data with large variations in REE ratios for magmatic monazites in some leucogranite samples suggest a subsequent closed-system fractional crystallization process during the magma evolution. Besides, some magmatic monazites in the muscovite granites show a significant tetrad effect in the Chondrite-normalized REE patterns, indicating that the magma system has evolved into an aqueous fluid-available environment. This supports an important role of the magmatic differentiation process in the formation of the Yadong leucogranites. Taken together, a petrogenic model involving AFC (assimilation and fractional crystallization) processes is proposed, and the geochemical variations of the Yadong leucogranites are mainly attributed to the post-source processes. It is suggested that the rock geochemistry of the Himalayan leucogranites often dose not directly reflect the nature and the petrogenetic processes in the source regions.

Origin, dynamics, and chemical evolution of garnet-bearing leucogranitic magma, Eastern Desert of Egypt: Controls on the rare-metal enrichment in the A-type magmatism

Egypt hosts numerous rare-metal granites, i.e., highly evolved granites enriched in rare metals (Ta, Nb, Be, Sn, Zr, Th, and REE). However, the processes involved in the rare-metal enrichment are not fully understood. We present new data on the textural characteristics and chemical composition of rare-metal mineralization associated with microgranite dikes in the Ras Abdah area of the Egyptian Eastern Desert. These dikes are garnet-bearing leucogranites (GLG) composed of perthitic alkali-feldspars and quartz. When compared to other Egyptian A-type granites, microgranite dikes are alkaline rocks with particularly higher HREE contents. Zircon, huttonite, fergusonite (Y), and Fe-Ti-Zn oxides (magnetite, Zn-bearing ilmenite and pyrophanite) are largely associated with the altered domains, which are also enriched in Nb, Zr, Y, Ta, Th, and REE. However, similarities between the chondrite-normalized REE patterns of the altered and unaltered domains of the GLG dikes favor the hypothesis of a unique magmatic signature. Moreover, the chemical and textural features of rare-metal minerals indicate that the alteration of primary minerals was caused by deuteric fluids or aqueous residual melt exsolved from the parental granitic magma (autometasomatism). Garnet compositions are rich in the spessartine component (up to 84 %), which is typical of garnet in highly evolved pegmatitic rocks. Furthermore, garnet exhibits no major element zoning but shows chemical fluctuations in trace element concentrations, suggesting correspondingly abrupt changes in melt composition due to sequential magma pulsing. This magma emplacement may cause crystal nucleation and oscillatory crystallization followed by magmatic segregation. Overall, parental magma type, dike injection, and magmatic-hydrothermal processes all play a role in the unusual enrichments of rare metals.

Xenocrystic zircon in Central New York State Kimberlites record Neoproterozoic disturbance of Grenville Basement rocks

Zircon grains from two central New York State (USA) Mesozoic kimberlite dikes intruding Middle and Upper Devonian sedimentary rocks were analyzed for UPb and LuHf isotopes, and trace elements, and were derived from different parts of the continental crust and/or upper mantle. Zircon from the Six Mile Creek (SMC) kimberlite has variable physical properties and “felsic” chondrite normalized REE patterns, and yields dates (321–1732 Ma) compatible with entrainment of sedimentary xenoliths during accent through the upper crust. In contrast, zircon from the Taughannock Creek (TAG) kimberlite have near concordant UPb dates that are distributed between intercept ages of 572 ± 25 and 960 ± 20 (2σ) Ma (MSWD = 0.61) with homogeneous LuHf systematics and 176Lu/177Hf ratios <0.0001. Examination in CL reveal TAG zircon grains vary in internal zoning and structures, but they are geochemically and isotopically similar. Their evolution includes early Neoproterozoic (ca. 950 Ma) granulite-facies metamorphism in equilibrium with garnet growth (flat to negative sloping HREE patterns and Ti-in-zircon temperatures >900 °C) during the final assemblage of the supercontinent Rodinia, followed by subsequent Pb-loss, resulting in a discordant array and decoupling of the UPb and LuHf isotopic systems prior to entrainment. Concordant zircon UPb dates, anchoring the lower intercept, yield an age of 596 ± 7 (2σ) Ma for the timing of Pb-loss. This most likely occurred in a zone of rifting and mafic magmatism beneath Central New York and Pennsylvania during the opening of the Iapetian Ocean.

Carbonatitic Magma Fractionation and Contamination Generate Rare Earth Element Enrichment and Mineralization in the Maoniuping Giant REE Deposit, SW China

Abstract Carbonatite intrusions host the world’s most important light rare earth element (LREE) deposits, and their formation generally requires extraordinary fertile sources, magmatic evolution, and hydrothermal events. However, carbonatitic magma evolution, particularly the role of fractional crystallization and contamination from silicate rocks in REE enrichment, remains enigmatic. The Maoniuping world-class REE deposit in southwestern China, is an ideal target to decipher magmatic evolution and related REE enrichment as it shows continuous textual evolution from medium- to coarse-grained calcite carbonatite (carbonatite I) at depth, to progressively pegmatoidal calcite carbonatite (carbonatite II) at shallow levels. In both types of calcite carbonatites, four generations of calcite can be classified according to petrographic and geochemical characteristics. Early-crystalizing calcite (Cal-I and Cal-II) are found in carbonatite I and exhibit equigranular and a polygonal mosaic textures, while late calcites (Cal-III and Cal-IV) in carbonatite II are large-size oikocrysts (&amp;gt;0.5 mm in length) with strain-induced undulatory extinction and bent twinning lamellae. All these generations of calcite yield similar, near-chondritic, Y/Ho ratios (26.6–28.1) and are inferred to be of magmatic origin. Remarkably, gradual enrichment of MgO, FeO and MnO from Cal-I to Cal-IV is coupled with a significant increase in REE contents (~800 to 2000 ppm), with LREE-rich and gentle-to-steep chondrite-normalized REE patterns ((La/Yb)N = 3.1–26.8 and (La/Sm)N = 0.9–3.9, respectively). Such significant REE enrichment is ascribed to protracted magma fractional crystallization with initial low degree of fractional crystallization (fraction of melt remining (F) = ~0.95) evolving to late stage (F = 0.5–0.6) by formation of abundant calcite cumulates. Differential LREE and HREE behavior during magma evolution largely depend on separation of phlogopite, amphibole, and clinopyroxene from the carbonatitic melt, which is indicated by progressively elevated (La/Yb)N ratios ranging from 3.1 to 26.8. The four generations of calcite have significantly different C and Sr isotopic compositions with δ13CV-PDB decreasing from −3.28 to −9.97‰ and 87Sr/86Sr increasing from 0.70613 to 0.70670. According to spatial relations and petrographic observations, the relative enrichment of δ13C and depletion in 87Sr/86Sr ratios of Cal-I and Cal-II show primary isotopic characteristics inherited from initial carbonatitic magma. By contrast, the variable Sr and C isotopic compositions of Cal-III and Cal-IV are interpreted as the results of contamination by components derived from silicate wall rocks and loss of CO2 by decarbonation reactions. To model such contamination processes, Raleigh volatilization and Monte Carlo simulation have been invoked and the model results reveal that carbonatitic melt-wall rock interaction requires 40% radiogenic Sr contamination from silicate rocks and 35% CO2 degassing from carbonatitic melt. Moreover, positive correlations between decreasing δ13C values and increasing REE contents, together with bastnäsite-(Ce) precipitation, indicate further REE accumulation during the contamination processes. In summary, alongside REE-rich magma sources, the extent of fractional crystallization and contamination during carbonatitic magma evolution are inferred to be important mechanisms in terms of REE enrichment and mineralization in carbonatite-related REE deposits worldwide.

Geochemistry and petrogenesis of ophiolitic rocks from the Indus Suture Zone (ISZ), Ladakh Himalaya: insights for depleted mantle beneath an intra-oceanic island arc complex

ABSTRACT The Indus Suture Zone preserves the ophiolitic remnants of the eastern part of the Neo-Tethys Ocean. Parts of these ophiolitic remnants are exposed along the Khangral-Chiktan and Dras-Kargil road sections of western Ladakh. The observed rock types include ultramafic-mafic cumulates, gabbros, and volcanics emplaced as faulted blocks over the Mesozoic Dras arc complex. Geochemically, these rocks show sub-alkaline tholeiitic characteristics with basalt to basaltic-andesite compositional variation. Based on modal mineralogy, the ultramafic cumulates classify as olivine-websterite, the mafic cumulates as olivine-norite, and the gabbros as norite to hornblende-gabbro. The ultramafic cumulates depict depleted chondrite normalized REE patterns [(La/Yb)N = 0.6–1.1] while the mafic cumulates display depleted to enriched REE patterns [(La/Yb)N = 0.6–3.2]. Fractionated patterns are observed in gabbros [(La/Yb)N = 1.6–4.1], while flat and enriched chondrite normalized REE-patterns in volcanics [(La/Yb)N = 1.0–12.3] similar to NMORB, and EMORB. The multi-element patterns depict subduction-related geochemical characteristics such as enriched LILE and depleted HFSE compared to the primitive mantle. Presence of Mg-rich olivines, orthopyroxenes, and clinopyroxenes while Ti-poor clinopyroxenes in ultramafic-mafic cumulates reflect their derivation from previously depleted mantle sources at high-pressure and temperature comparable to the base of modern intra-oceanic island arc tholeiitic sequences. The studied rocks exhibit close similarity to an intra-oceanic subduction system contemporaneous to the Dras-Shergol-Suru-Thasgam ophiolitic slices of western Ladakh.

Geochemistry and origin of hydrothermal apatite in Carlin-type Au deposits, southwestern China (Gaolong deposit)

Apatite can record the evolution of magmatic–hydrothermal ore systems, but it is unclear whether it is suitable for investigating low-temperature, sediment-hosted hydrothermal ore systems. To fill the knowledge gap. we investigated the Gaolong Carlin-type Au deposit in the Guangxi Zhuang Autonomous Region, southwestern China (Youjiang Basin). Results show that the hydrothermal apatite occurs in quartz–sulfide ores, has an intact crystal form and homogeneous texture, and is F-rich (2.77–3.67 wt%) and Cl-poor (<0.01 wt%). The apatite is enriched in rare earth elements (REEs; ΣREE = 739–2481 ppm) and has middle-REE-enriched chondrite-normalized REE patterns with positive Eu anomalies (δEu = 1.35–2.00). These REE features were inherited from a reducing hydrothermal system in which anion complexation was dominant. The apatite has high Sr contents (up to 0. 9 wt%) and relatively uniform initial 87Sr/86Sr ratios (0.709055–0.709208), suggesting that Sr was leached or derived from the deep continental crust. In situ U–Pb dating of 156.8 ± 8.3 Ma for apatite shows that the Late Jurassic–Early Cretaceous was an important time for the formation of Carlin-type Au deposits in the Youjiang Basin. The Au mineralization was associated with far-field tectonism and deeply sourced hydrothermal ore-forming fluids. Hydrothermal apatite can be used to fingerprint low-temperature, sediment-hosted hydrothermal ore systems.

Metallogeny and exploration strategy for volcanic rocks hosting world class Be-U-F mineralization at Spor Mountain, Utah, U.S.A.

We report new geochemical data (major-elements, trace-elements, and Nd-Pb isotopes) for the Spor Mountain Formation (SMF), a volcanic sequence that hosts the largest deposit of beryllium in the world. Understanding the genesis of the deposit is critical for improved recognition of regional exploration targets containing additional volcanic-hosted beryllium resources. The deposit occurs in an area of extensive alkalic rhyolitic ash flow tuffs and calderas underlain by Paleozoic and older limestone, dolomite, shale, and quartzite. The lower fluorite and bertrandite-rich tuff and upper topaz-rich rhyolite of the SMF have overlapping zircon crystallization ages (25.59 + 0.29/−0.45 Ma). Enrichment of Be, U, and F is a key attribute of the SMF inherited from melting heterogeneous source materials and magma hybridization in the continental lithosphere. Two mineralization stages are implicated: magmatic, and hydrothermal/metasomatic. The magmatic stage occurred early, as indicated by the high beryllium content in early-crystallizing zircon. The post-magmatic/hydrothermal stage involved F-rich fluids that reacted with glass and carbonate nodules in the tuff to form bertrandite (and U-rich fluorite and opal) and high-grade mineralization. The rhyolite best displays the magmatic stage of enrichment. The SMF is alkaline. Chondrite-normalized REE patterns for tuff and rhyolite are regular, stacked subparallel, and imply a shared source derivation and evolution. The tuff and rhyolite plot along a sub-vertical array for Pb isotopes and suggest they are cogenetic; both have negative values of εNd. Pb isotopic ratios for SMF do not match the slopes, initial isotopic values, or estimated basement ages of precious and base metal mining districts in Utah; thus, they do not share a source region or evolution. Crustal ages from Nd isotopes (average TDM ~ 2.29 Ga) are older than basement ages obtained by Pb isotopes. The deposit occurs in a transitional area of thinned crust near the boundary between two basement domains, the eastern Great Basin and Colorado Plateau, which have contrasting ages. Many geochemical attributes are shared between SMF and the Ermakovka deposit in Russia: they represent the world's major commercial Be deposits. Rare-metal indices of granite-related mineralization show that SMF resembles other global beryllium-bearing granitic deposits. Despite this coincidence, none of these examples matches the mineralogy, geologic setting, or type of rare metal mineralization at Spor Mountain.

Geochemical constraints on the multistage evolution of the Yilashan ophiolite, central Tibet

The Yilashan ophiolite in the southern part of the central Bangong-Nujiang suture zone comprises variably deformed harzburgite, dunite, and less abundant gabbro and pyroxenite pods and crosscutting veins. Cr-spinels in podiform chromitite bodies are characterized by high Cr# (64.3–74.3), high Mg# (58.2–67.5), and consistently low contents of TiO2 (≤0.12 wt%). Primary melt calculations designate parental boninitic magmas with Al2O3(liquid) contents of 11.67–13.49 wt%, and (FeO/MgO)liquid values of 0.92–1.12.Whole-rock geochemical data of fresh pyroxenite samples are characterized by negative K, Ta, Nb and Zr anomalies, no Eu anomaly, and chondrite-normalized REE patterns comparable to the N-MORB melts. The gabbro samples, in contrast, are high-K calc-alkaline, with elevated contents of LILE (Rb, K, Th) and depleted HFSE (i.e., Nb, Ta, P, Ti), consistent with subduction-related magmas. The fractionated chondrite-normalized REE patterns, high LREE/HREE values, and negative Eu anomalies of these gabbroic rocks are similar to the island arc basalts. Pyroxenite shows high PPGE/IPGE values, variable Pd/Ir (55.7–316.7), and left-leaning primitive mantle-normalized PGE pattern, whereas massive chromitite is enriched in IPGE relative to PPGE. The PGE patterns of pyroxenite and chromitite, though distinct, depict significant fractionation and partial melting in their formation. The elevated contents of PPGEs in pyroxenite suggest derivation from sulfur-saturated magmas as chromitite crystallization scavenged the compatible IPGEs from the parental melt. This assumption is supported by clinopyroxene in pyroxenite with variable Al2O3 contents and variable Cr2O3 contents (0.32 to 0.73 wt%). For the fractionated gabbro unit, zircons yielding a concordia U-Pb age of 162.8 ± 3.0 Ma and widely variable εHf(t) values (−9.2 to +12.0) suggest formation from heterogenous or reworked source materials during a Middle Jurassic subduction increment in the Neo-Tethyan Ocean.

Heavy rare earth element and crustal-derived silicon enrichment in Huayangchuan carbonatites, Qinling orogenic belt

The Huayangchuan carbonatite is found in the North Qinling orogenic belt. It is distinguished from most carbonatites worldwide by its abundance of quartz and high heavy rare earth element (HREE) content. The origin of Si supersaturation and HREE enrichment in carbonatite is still unknown. Here, we conducted a detailed mineralogical investigation and in situ geochemical analyses on a suite of drill-core samples obtained from Huayangchuan carbonatite. In situ titanite and monazite UPb dating yielded ages of 212.3 ± 6.4 Ma and 211.8 ± 5.0 Ma, respectively, corresponding to the post-collision stage of the Qinling orogeny. Clinopyroxene and amphibole in the carbonatite are commonly intergrowth with quartz grains and trapped by quartz as mineral inclusions. The mineralogical textures and compositions indicate that quartz crystallized simultaneously with the mafic minerals from a Si supersaturation magma. The high Si activity during clinopyroxene formation may be related to Si assimilation from the wall-rocks. In situ Nd isotopic compositions of apatite and titanite show consistent εNd(t) ranging from −4.8 to −9.0, similar to the values of the whole-rocks and calcites. This indicates that the REE were derived from carbonatite magma without an external source. Two types of calcites were identified based on their cathodoluminescence characteristics, and they have variable chondrite-normalized REE patterns (La/Yb(CN) ranging from <0.01 to 19) but similar HREE contents. This implies that the carbonatite underwent strong crystal fractionation of early LREE-rich minerals, which may be essential for HREE enrichment. Therefore, we proposed that Si supersaturation in the carbonatite was probably caused by wall-rock assimilation, and fractional crystallization yielded primary HREE enrichment in the residual melts. Moreover, HREE enrichment in highly evolved carbonatites is further aided by high Si and alkaline activities.