Chondrite-normalized REE Distribution Patterns Research Articles

Carbonatite-related delicate REE mineralization processes revealed by fluorocarbonates and monazite: Insights from the giant Bayan Obo REE-Nb-Fe deposit, China

Fluorocarbonates and monazite are common economic REE minerals in carbonatite-related REE deposits, such as the largest known REE deposit at Bayan Obo, China. The geochemical evolution of REE minerals is a sensitive and effective indicator of delicate REE mineralization processes. In this study, the fluorocarbonates and monazites were classified into three generations, corresponding to three REE mineralizing stages. The first generation (Gen-1) of bastnäsite and monazite are generally characterized by elevated La/Ce, La/Nd and La/Lu ratios, with right-declining chondrite normalized REE distribution patterns. They have low Th and radiogenic Sr content, with forming age of about 1.3 Ga and εNd(t) around 0, indicating a primary origin and orthomagmatic crystallization from carbonatite melts. The second generation (Gen-2) of REE minerals, formed at 1.25–1.2 Ga, normally constitute banded aggregates with associated hydrothermal gangue minerals (fluorite, aegirine, sodium amphibole etc.). The Gen-2 REE minerals are featured by moderate-low La/Ce, La/Nd, La/Lu ratios but slightly elevated 87Sr/86Sr0 ratios, indicating a Mesoproterozoic hydrothermal origin related to carbonatitic fluids. Gen-2 bastnäsite and monazite precipitated earlier in low-fluid/rock-ratio environment with right-declining LREE distribution patterns, while the subsequent Gen-2 parisite and huanghoite display convex LREE patterns and higher OH/F ratios, indicative of relative La-depletion and increased ambient fluid/rock ratios during progressive hydrothermal REE mineralization. The third generation (Gen-3) of REE minerals are generally megacrysts in the vein-type ores formed in the early Paleozoic REE mineralization stage (470–420 Ma). The Gen-3 REE minerals are typified by significantly variable REE, Sr and Nd isotopic components, indicating extensive hydrothermal metasomatism and intense contamination of crustal material. Induced by activities of early Paleozoic subduction-related fluids, Gen-3 REE minerals were formed through local REE re-enrichment within the ore-hosting dolomite, rather than by introduction of REE from external sources. In this case study, a comprehensive model of the delicate REE mineralization processes of the giant Bayan Obo REE-Nb-Fe deposit has been established. This study highlights the great potential of fluorocarbonates and monazites widely distributed in carbonatite-related REE deposits as universal geochemical indicators of late-stage evolution of carbonatite and delicate magmatic-hydrothermal REE mineralization processes.

Monazite geochronology and geochemistry constraints on the formation of the giant Zhengchong Li-Rb-Cs deposit in South China

The giant Zhengchong Li-Rb-Cs deposit (ore resource: 270 k tonnes (t) Li2O @ 0.557 wt%, 110 kt Rb2O @ 0.225 wt%, and 6 kt Cs2O @ 0.013 wt%) in South China is associated with quartz-zinnwaldite-topaz rock, which is composed mainly of quartz, zinnwaldite and topaz with a porphyritic texture. However, the genetic link between these rocks and their Jiuyishan granite batholith host is unclear, and the magmatic-hydrothermal process involved in the rare-metal mineralization remains enigmatic. In this study, we analyzed the in-situ U-Pb age and mineral chemistry of monazite, a ubiquitous accessory phase at Zhengchong, to reconstruct the precise geochronological framework of this highly-fractionated felsic magmatic system, and the origin of the Li-Rb-Cs-rich quartz-zinnwaldite-topaz rock. Based on petrographic observations and back-scattered electron (BSE) imaging, monazite crystals in different rocks at Zhengchong have the following characteristics: (1) Mnz-I is mainly BSE-zoned monazite disseminated in granite porphyry, coexisting with zircon or included in zinnwaldite, (2) Mnz-II consists of relatively fine-grained BSE-zoned or homogeneous monazite distributed in feldspar phenocryst-bearing quartz-zinnwaldite-topaz rock, (3) Mnz-III is BSE-zoned monazite with embayed texture in the core, distributed in coarse-grained quartz-zinnwaldite-topaz rock, and (4) Mnz-IV is homogenous, BSE-gray monazite from fine-grained porphyritic quartz-zinnwaldite-topaz rock (as the main orebodies). All monazite crystals are characterized by high Th content, indicating magmatic origin. The LA-ICP-MS monazite U-Pb ages indicate granitic magmatism and mineralization cluster around 150 Ma, implying a close genetic link between the granite porphyry host and the quartz-zinnwaldite-topaz rock. We compared the chondrite-normalized REE distribution patterns and principal component analysis and partial least squares-discriminant analysis of the monazite analyzed, and suggested that the BSE-zoned Mnz-I, II and III are likely of the same type, significantly different from the homogenous Mnz-IV. The gray rims (under BSE) of Mnz-I, II and III are similar to Mnz-IV, and differ markedly from the bright core (under BSE) in their lower HREE and ThO2, but higher Eu and Eu/Eu* contents. When combined with the whole-rock composition data, the monazite compositional variation cannot be attributed to rapid changes in magma composition, depressurization, and increased water content, but rather to the increasing oxygen fugacity and the decreasing temperature and Si activity, accompanied by the elevated fractionation of granitic magma. Thus, the variation in monazite composition reflects the formation of Zhengchong Li-Rb-Cs deposit, which may be the result of high magmatic differentiation of fluorine-rich A-type magmas. Although multistage rare metal metallogenic events have been identified in the Nanling region, it is suggested that the Late Jurassic Zhengchong Li-Rb-Cs mineralization occurred within a short duration (<1 million years). Overall, the monazite chemical and geochronological signatures are good pathfinders for magmatic evolution and Li-Rb-Cs mineralization in the Jiuyishan complex, which could be used as a potential prospecting tool for highly-evolved granite-related rare metal deposits.

Zirconium-REE mineralization characteristic in Sakju and Ryongpho intrusions of Sakju Complex distributed in the western area, DPR Korea

Alkali intrusions of Sakju Complex distributed in the western area, DPR Korea were formed in mid-Proterozoic era. The Zirconium-REE mineralization occurs in Sakju and Ryongpho intrusions of Sakju Complex. Zr and Hf are the typical lithophile elements and always exist together under the closed relation to alkali and acid igneous rock. In our country, Zr and Hf mineral resources are exploited in the coast placer deposits and some in the alluvial placers. In addition, deposits related to alkali syenite, carbonatite and pegmatite deposites are found. The contents of Zr, Hf and REEs in intrusions of the Sakju complex are much higher than the average contents of the earth's crust and those in alkali rocks. The REE values in the constituent rocks of Ryongpho intrusion, from earlier differentiation phase to later phase, gradually decrease in the order of alkali pyroxenite to alkali gabbro to alkali amphibole syenite to alkali feldspar syenite. The constituent rocks have the similar chondrite-normalized REE distribution patterns, i.e., rich in LREE and deficient in HREE, and no or weak negative anomaly of Eu. REEs are concentrated in the fluorite zone experienced Ca–F metasomatism in alkali pyroxenite and alkali gabbro to form a medium-high grade REE ore body. The carbonatite in Sakju intrusion and alkali metasomatic belts heavily experienced the albitization in Ryongpho intrusion are unique useful mineral resources capable of recovering simultaneously Hf, Sr, REEs as well as Zr.

Evidence for Mesoarchean subduction in Southern Bundelkhand Craton, India: Geochemical fingerprints from metavolcanics of Kurrat - Girar - Badwar Greenstone Belt

The metavolcanic suites from southern Bundelkhand Craton, India provides a comprehensive record of the little-explored evolutionary history of Southern Bundelkhand Craton. The present work is a detailed petrological and geochemical account of the metavolcanics to (i) constrain the mantle processes and tectonic realm involved in its genesis and (ii) evaluate the Archean geodynamics of Bundelkhand Craton. The metavolcanics of Kurrat, Badwar and Girar are compositionally slightly evolved tholeiitic basalts. Chondrite normalized REE distribution pattern reflects a slightly super-chondritic LREE distribution pattern (Kurrat metavolcanics, KMV: (La/Yb)N = 1.9–2.9, Badwar-Girar metavolcanics, BGMV: (La/Yb)N = 0.89–2.86) and negative Nb and Ti anomaly on a primitive mantle normalized trace element variation diagram. ƐNd(t) of the samples vary from −1.0 to 6.3 and ƐNd(t) versus 87Sr/86Sr diagram imply the absence of crustal contamination. These attributes are consistent with the formation of the metavolcanics from a slightly enriched heterogeneous mantle source at shallower depths where the original depleted MORB underwent later episode(s) of enrichment via fluid addition. The metavolcanics originated in an intra-oceanic subduction setting, essentially exhibiting back-arc basin basalts (BABB) characteristics. TDM age (3.07 Ga) calculated for the KMV is coherent with the 2.98 Ga age of the BGMV from the adjacent Badwar-Girar volcano-sedimentary sequence. Collectively, all the features implicate the formation of metavolcanics in a modern style Mesoarchean extension-subduction geodynamic regime.

Sandstone Geochemistry of Dalbuing Formation of Yinkiong Group, Arunachal Pradesh, NE India: Implications for Provenance, Paleoweathering and Tectonic Settings

This study presents the results of geochemical investigations of early to middle Eocene sandstones belonging to the Dalbuing Formation of Yinkiong Group, occurring in and around Yamne valley of Arunachal Pradesh. Geochemically, these sandstones are wacke and lithic arenite, with the former being dominant than the other. The various indices of weathering (CIA, CIW, PIA) suggest intense chemical weathering of the source rock. Moreover, the average Th/U ratio (mean = 6.97) also indicates intense chemical weathering in the source areas and derivation of the sediments from the upper continental crust. However, the plot of chemical index of alteration (CIA) vs. index of compositional variability (ICV) shows that most of the sandstones are geochemically mature and were derived from intensively weathered source rock. The sandstones were deposited dominantly in semi-arid condition. Tectonically, these sandstones are found to have affinities towards both the passive margin (PM) and the active continental margin (ACM) settings, which further implies existence of multiple tectonic activities during their deposition. Moreover, the elemental ratios of Eu/Eu*, La/Sc, Th/Sc, La/Co, Th/Co, Th/Cr and chondrite normalized REE distribution pattern suggest their derivation from sources of felsic composition. The high ratio of Zr/Sc indicates the enrichment of heavy mineral zircon supplemented by sediment recycling.

Provenance, tectonic setting and source area-paleoweathering of sandstones of the Bahariya Formation in the Bahariya Oasis, Egypt: An implication to paleoclimate and paleogeography of the southern Neo-Tethys region during Early Cenomanian

The Lower Cenomanian Bahariya Formation forms the most prolific hydrocarbon reservoir in the Western Desert of Egypt, and represents a good example to illustrate the paleoclimatic conditions and paleogeographical evolution in the southern Neo-Tethys region during Early Cenomanian. It is also particularly important for its diverse vertebrate fauna including dipnoans, chelonians, crocodyliforms, squamates, plesiosaurs, decapod crustaceans and dinosaurs, which reflect sustained regional biostratigraphical and paleoenvironmental significance. In this study, a multidisciplinary approach of fieldwork, petrography and bulk-rock geochemistry of sandstones of the Bahariya Formation has been performed to constrain their provenance, tectonic setting, source area-paleoweathering and paleoclimatic conditions within the Neo-Tethyan realm. Petrographically, the studied sandstones are mainly of quartz-arenite type with subordinate sub-arkose and sublithic ones. Quartz types, trace and rare earth element concentrations and ratios indicate the sandstones were sourced from felsic granitic rocks with a rare contribution of felsic gneisses. The modal analysis reflects that the sandstones are mainly of cratonic interior and quartzose recycled tectonic provenance. The chondrite-normalized REE distribution pattern of the Bahariya sandstones are characterized by enriched LREE and flat HREE similar to those of UCC with negative Eu/Eu* anomalies. Major and trace elements of the sandstones suggest their deposition on a Neo-Tethys passive continental margin related to the Early Cretaceous rifting. The weathering indices (CIA, CIW, PIA and ICV) and the Al2O3-(CaO + Na2O)-K2O diagram suggest moderate to severe chemical weathering in the source area under semi-humid to humid climatic conditions. In conclusions, the Bahariya sandstones could be sourced by a fluvial reworking of moderately- to intensively-weathered Precambrian granites and gneisses of the surrounding Gebel Uweinat Massive (at the southwest of the study area), and were deposited in a low-lying basin (Bahariya area) of the Neo-Tethys passive continental margin. In a broad interest, results of this study could contribute in a reconstruction of the paleoclimatic conditions and paleogeographical evolution of the southern Neo-Tethys region during Early Cenomanian.

Agates from Paleoproterozoic Volcanic Rocks of the Onega Structure, Central Karelia

Agate mineralization in Central Karelia, Northwest Russia is related to the Paleoproterozoic volcanic rocks of the Ludicovian Superhorizon (2.05–1.95 Ga) within the Onega structure. Agates and parental volcanic rocks were studied with optical and electron microscopy, electron microprobe, X-ray diffraction, Raman spectroscopy, and ICP-MS. It is shown that fine-grained quartz, and fibrous and fine-flake chalcedony are the major minerals in the agate structure. Inclusions of coarse-crystalline calcite, and microinclusions of chlorite, iron oxides and hydroxides, hydroxylapatite, epidote, mica (phengite), apatite, pyrite, chalcopyrite, titanite, and leucoxene are present in agates. The presence of contrast rhythms within agate amygdules is marked by compositional variation in impurity mineral phases and different microtextures of silica layers represented by different-grained aggregates of quartz, fine-flake and fibrous chalcedony, and quartzine. This indicates stage-by-stage mineral crystallization at variable temperatures and pressures during agate formation, which may also reflect the heterogeneity of the initial hydrothermal fluid. High concentrations of Ti, Cr, Mn, Ni, and Cu (10–120 ppm) and the low concentrations of Li, Co, Ga, Zn, Sr, Zr, Mo, and Sn (0.5–10 ppm) are characteristic of quartz–chalcedony agates. Calcite in agates is characterized by high concentrations of Mn (1253–6675 ppm), Sс, Ti, Ni, Sr, Y, La, Ce, and Nd (5–56 ppm). The chondrite-normalized REE distribution pattern in agates shows a decay profile from La to Lu and a negative Eu anomaly in some samples. Low contents of rare metals and REE in agates compared to parental volcanic rocks indicate a gradual chemical depletion of circulating fluids during agate formation.

Decoding the oxygen fugacity of ore-forming fluids from garnet chemistry, the Longgen skarn Pb-Zn deposit, Tibet

The relationship between the compositional zonation of garnet in skarn deposits and the evolution of physicochemical conditions of ore-forming fluids is not clear and needs further investigation. This study presents the characteristics of trace elements of garnet from the Longgen skarn Pb-Zn deposit, Tibet with an aim to reveal the evolution trend of the physical and chemical conditions (fO2 and pH) of ore-forming fluids and to explore the fingerprint of garnet trace elements in different types of skarn deposits.Based on robust alteration halo, mineral assemblages and petrographic observation, the garnets of the Longgen skarn Pb-Zn deposit are divided into two types: (1) distal exoskarn garnet near limestone with ore-bearing and fine oscillatory zoning (Adr39.9-75.0; Grt1, Adr represents andradite) and (2) proximal exoskarn garnet (core: Adr32.9-70.7; rim: Adr83.3-99.5) with Pb-Zn-rich mineral and coarse oscillatory zoning (Grt2). According to the total rare earth element (ΣREE) content in the garnet compositional zone, Grt1 is further divided into three component zones: Grt1-1 (heavy rare earth element (HREE)-enriched chondrite-normalized REE distribution patterns and positive Eu-anomaly (δEu > 1, δEu = 2*(Eusample/Euchondrite)/(Smsample/Smchondrite + Gdsample/Gdchondrite)), Grt1-2 (HREE-enriched REE patterns with negative Eu-anomaly (δEu < 1)), and Grt1-3 (light rare earth element (LREE)-enriched REE patterns with δEu > 1). Similarly, five component zones were identified in Grt2: Grt2-1 (REE-rich REE patterns with δEu < 1), Grt2-2 (LREE-depleted REE patterns with δEu < 1), Grt2-3 (steady REE patterns with δEu < 1), Grt2-4 (LREE-enriched REE patterns with δEu > 1), and Grt2-5 (LREE-enriched REE patterns with δEu < 1) from the core to rim, respectively.The fractionation of LREE with HREE and the behavior of Eu indicate that the physical–chemical conditions during the formation of Grt1-1-Grt1-2 and Grt2-1-Grt2-3 were near neutral (pH = 6–7), and changed to mildly acidic in Grt1-3 and Grt2-4-Grt2-5 (pH < 6–7). In addition, the characteristics of low U (<1 ppm) and the linear relationship between Sn and Eu indicate that the fO2 of the ore-forming fluid was relatively oxidized in comparison to the garnets found in other skarn deposits, and Grt1-1-Grt1-3 and Grt2-1-Grt2-5 have a gradual increasing oxidative ability. The linear relationship between ΣREE and Y/Ca indicates that Grt1 and Grt2 grew in a near-closed system. The incorporation of REE3+ into the garnet was mainly controlled by a “yttrogarnet” (YAG)-type substitution, Ca site vacancy, fluid chemistry, and physicochemical conditions of precipitation. However, the compositional zone at the transition zone between Al-rich and Fe-rich segments in Grt2-4 may be formed in an open system, as suggested by the exchange of LREE and Cl−. In conclusion, garnet crystal growth was affected by the crystal chemistry and physicochemical conditions of the skarn deposit. This study demonstrates that factor analysis and discrimination diagrams (log Sn vs. log U, log Ce vs. log U, and log (Ce + Hf) vs. log U) are effective methods for distinguishing different types of skarn deposits by their contents of redox-sensitive trace elements in garnet.

Geochemical studies of monazites from coastal sands of Kandivalasa–Dibbalapalem coast, Andhra Pradesh

The present study deals with the geochemical and geochronological studies of monazite from Kandivalasa–Dibbalapalem coast, north Andhra Pradesh. The average concentration of ThO2 and UO2 from beach sands is 11.38 and 0.40 wt.% and from dune sands is 11.94 and 0.63 wt.%. The average REE from beach and dune sands is 58.64 and 57.94 wt.% and they are rich in La, Ce, and Nd. The most common characteristic feature to all monazite shows that the total REE content exceeds that of the actinides (Thru). Monazite that formed after the garnet break down contains significantly higher amounts of Y and HREE in fine sediments. The chondrite-normalized REE distribution patterns of monazite show uniformly enriched light rare earth element (∑LREE) which could be due to the preferential incorporation of lighter lanthanides and positive europium anomalies indicate that monazite was formed from magma/anatectic melt with high oxygen fugacity. Back-scattered electron (BSE) images of monazite showing compositional zoning and the bright outermost rim which is attributed to the influence by variations of LREE, HREE, U, Th, Pb and Y concentrations. The primary controlling factor, radiation damaged monazite yields in most cases significantly, with a strong dependence on the degree of metamictization based on the BSE intensity. The geochemical dating (U–Th–Pb) of monazite ranging age from 487 to 977 Ma from grain interior, can signify the age of Grenvillean evolution. This interpretation changes the existing tectonothermal history of the northern Eastern Ghats Belt. Our data show that the two adjacent crustal domains of the Eastern Ghats Belt show distinctly contrasting Neoproterozoic histories.

Geochemistry, geochronology and Hf isotope of granitoids in the Chinese Altai: Implications for Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The Chinese Altai in northwestern Xinjiang has numerous outcrops of granitoids which provide critical information on accretionary orogenic processes and crustal growth of the Central Asian Orogenic Belt. Zircon U–Pb ages, Hf-isotopic compositions and whole-rock geochemistry of monzogranite and granodiorites in the Qinghe County are employed to elucidate Paleozoic tectonics of the Chinese Altai. Granodiorites have crystallization ages of 424.6 ± 3.1 Ma (MSWD = 0.23) and 404.0 ± 3.4 Ma (MSWD = 0.18); monzogranite was emplaced in the early Permian with a crystallization age of 293.7 ± 4.6 Ma (MSWD = 1.06). Both granodiorites and monzogranite are I-type granites with A/CNK ratios of 0.92–0.97 and 1.03–1.06, respectively. They also show similar geochemical features of high HREE and Y contents, low Sr contents and Sr/Y ratios, as well as enrichment of Cs, Rb, Th and U, and depletion of Nb, Ta, P and Ti. These geochemical features indicate that the monzogranite and granodiorites were formed in an arc setting related to subduction. The gneissic monzogranites display high SiO2 and K2O contents, and belong to the high-K calc-alkaline series. In the chondrite normalized REE distribution pattern, the monzogranite samples exhibit enrichment of LREE with strong negative Eu anomalies (δEu = 0.44–0.53), zircon εHf(t) values from +7.24 to +12.63 and two-stage Hf model ages of 463–740 Ma. This suggests that the monzogranite was generated from the mixing of pelitic and mantle material. The granodiorite samples are calc-alkaline granites with lower contents of SiO2 and Na2O + K2O, higher contents of TiO2, Fe2O3t, MgO and CaO compared to the monzogranite samples. They also show enrichment of LREE and moderate negative Eu anomalies (δEu = 0.54–0.81), as well as slightly higher differentiation of LREE than that of HREE. The 425 Ma granodiorite has zircon εHf(t) values from −0.51 to +1.98 and two-stage Hf model ages of 1133–1240 Ma, whereas the 404 Ma granodiorite displays those of +2.52 to +7.50 and 816–1071 Ma. Geochemistry and zircon Hf isotopic compositions indicate that granodiorites were formed by partial melting of juvenile lower crust. Together with regional geology and previous data, the geochemical and geochronological data of the monzogranite and granodiorites from this study suggest long-lived subduction and accretion along the Altai Orogen during ca. 425–294 Ma.

The tetrad-effect in rare earth elements distribution patterns of titanium-rich bauxites: Evidence from the Kanigorgeh deposit, NW Iran

The titanium-rich bauxite horizon of the Kanigorgeh bauxite (northeast of Bukan, NW Iran) is a part of the globally significant known Irano-Himalaya karst bauxite belt. This bauxite horizon was developed as discontinuous stratified layers and/or lenses within carbonate rocks of the Ruteh Formation of Upper Permian age. Studies of whole rock geochemistry along a selected profile across the horizon revealed that Ce anomaly values of the bauxite ore samples vary from 0.22 (negative) to 7.34 (strongly positive). The remarkable variations in ratios of some isovalent elements such as Y/Ho, Nb/Ta, and Zr/Hf along the profile illustrate non-CHARAC behavior which can be attributed to so-called tetrad-effect phenomenon in this residual horizon. The chondrite-normalized REE distribution patterns represent the irregular shapes known as M- and W-shape tetrad-effects. The illustration of diagrams of fourth tetrad-effects versus Ce anomaly values and ratios of Y/Ho, Nb/Ta, and Zr/Hf indicated that the studied residual horizon comprises two distinct zones with different geochemical behaviors. The upper zone, from top surface down to ~7.3m, was deposited under oxidizing conditions represented by the bauxitic composition. The lower zone, starting from ~7.3m down to the bottom of the horizon, was precipitated under reducing conditions manifested by the kaolinitic composition. The obtained results led us to deduce that the tetrad-effects can be used as a geochemical indicator to evaluate the depositional conditions of bauxite deposits. It has been further concluded that certain mechanisms such as mineral crystallization, REE-complexation, penetration of meteoric acidic surface waters, and lateritization intensity had striking influence upon occurrence of the tetrad-effects in the studied titanium-rich bauxite ores in the Kanigorgeh district.

The 1.0 Ga S–type granite in the East Kunlun Orogen, Northern Tibetan Plateau: Implications for the Meso– to Neoproterozoic tectonic evolution

The East Kunlun Orogen (EKO) is characterized by widely distributed granitoids with different ages, which are keys to understanding the tectonic evolution of the Central China Orogenic Belt. Zircon U–Pb ages and Hf isotopic compositions, as well as the whole rock geochemistry of the gneissic granite from the basement rock, are carried out to elucidate the Meso– to Neoproterozoic tectonics of the EKO. The Al–rich minerals, including muscovite and tourmaline, and the A/CNK ratios (1.07–1.18) indicate S–type affinity of the granite. The granite displays high SiO2 and K2O contents, and slightly enrichment in LREE in chondrite normalized REE distribution pattern with strong negative Eu anomalies (δEu=0.10–0.15). The samples exhibit positive anomalies of Rb, Th, U and Pb, and depletion of Ba, Nb, Ta, Sr and Ti. Meanwhile, the high Rb/Ba and Rb/Sr ratios and low (CaO+FeO+MgO+TiO2) contents indicate that they were derived from pelitic material. Together with the negative εHf(t) values ranging from −5.97 to −2.34 and two–stage Hf model ages varying from 1968 to 1786Ma, the gneissic granite is suggested being originally derived from partial melting of the metasedimentary rocks of the Paleoproterozoic Jinshuikou Group in the central EKO due to the crust thickening. Most zircon grains from the gneissic granite show typical magmatic zircon morphology, and yield an U–Pb upper intercept age of 1006±20Ma (MSWD=1.5), representing the crystallization age. Integrated with the regional geology, our results suggest that the EKO has been probably involved into a Meso– to Neoproterozoic plate collisional event related to the assembly of the Rodinia supercontinent.

Heavy mineral concentrations in the sandstones of Amij Formation with particular emphasis on the mineral chemistry and petrographic characteristics of monazite, western desert of Iraq

The heavy minerals in the clastic unit of the Lower Jurassic Amij Formation exposed in the western desert of Iraq were studied. The uppermost part of the clastic unit contains thin, placer-like black sandstone horizons that are radioactive and abnormally rich in heavy minerals (0.6–56%), dominated by opaque (65%) and transparent (35%) heavy minerals. The minerals, in the order of decreasing abundance are pseudorutile, goethite, zircon, hematite, magnetite, monazite, rutile, leucoxene, tourmaline, ilmenite, chromite, and few others. Electron probe microanalysis (EPMA), microscopic and autoradiographic observations and analysis showed that the monazite is monazite-(Ce) type with an average composition of (Ce0.39Nd0.16La0.19Pr0.04Sm0.02Gd0.02Eu0.01Y0·04Th0·06U0·01Ca0·05Fe0.01)(P0·98Si0.03)O4.Monazite consists predominantly of REE-oxides (57.93%) and P2O5 (29.31%), with minor amounts of ThO2 (6.60%), Y2O3 (1.92%), UO2 (0.76%), CaO (1.14%), SiO2 (0.69%), and FeOt (0.17%). The dominant compositional substitution operating between REE and P were a mixture of the complex cheralite type substitution ([REE]−2 [Th][Ca]) and the coupled huttonite type substitution ([REE]−1 [P]−1 [Th][Si]). The chondrite-normalized REE distribution patterns of monazite show enrichment in LREE with positive Eu- and Pr-anomalies of 1.46 and 9.13, respectively. The median values of (La/Sm)CN and (La/Nd)CN ratios are 4.35 and 1.97, respectively. Zircon which is the dominant transparent mineral is Hf-rich that is composed of 30.61% SiO2, 57.58% ZrO2, 7.03% HfO2, 2.04% Y2O3, 0.56% ThO2, 0.19% UO2, and 0.19% Al2O3 corresponding to a formula (Zr0.909Hf0.065Th0·004U0·001Y0.031)Σ1.011(Si3·966Al0.028)Σ0.999O4. Rutile and tourmaline form 7% and 4% of the heavy minerals. Ilmenite which is one of the predominant heavy minerals forms 2.5% of the opaques because it is pervasively altered to Ti-Fe oxides. In addition of zircon and monazite, the chemical compositions of most of the other heavy minerals are also given in this study. The expected dominant source of heavy minerals and their host sandstones are most probably the felsic igneous and metamorphic complexes of the Arabian Shield, currently located ∼600 kms to the south of the studied area. The heavy minerals were carried from the source area by northward moving rivers and sorted out by ocean waves as black sand concentrations at the delta mouth along the southern beaches of the Neo-Tethys Ocean under passive margin tectonic setting. This border was apparently bordering the current western part of Iraq during Upper Jurassic period.

Origin and geochemistry of agates in Permian volcanic rocks of the Sub-Erzgebirge basin, Saxony (Germany)

Mineralogical and geochemical investigations of agates from Permian volcanic rocks of the Sub-Erzgebirge basin (Saxony, Germany) were made to constrain the genesis and characteristics of these spectacular forms of silica in acidic volcanic host rocks. Samples from the main agate occurrences of Chemnitz, Hohenstein-Ernstthal, St. Egidien and Zwickau were selected for detailed analyses.The results of the study show that agate formation can be related to volcanic activity (Rochlitz ignimbrite) and the subsequent alteration of the volcanic rocks. Most agates originate from the infill of silica into cavities of lithophysae (high-temperature crystallization domains), which formed during cooling of welded ignimbrite. Agate formation temperatures of probably >150°C were calculated from fluid inclusion and oxygen isotope studies, which indicate that the mobilization and accumulation of silica started already during a late phase of or soon after the volcanic activity.The trace-element composition of chalcedony and macrocrystalline quartz in agates is different from that of quartz from magmatic or metamorphic rocks and pegmatites. Elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) were released during the alteration processes and accumulated in the SiO2 matrix of the agates. Extraordinary high contents of Ge (>90ppm), B (46ppm) and U (>18ppm) were also detected, which can exceed the Clark concentration and sometimes the element concentration in the surrounding host rocks. In addition, chondrite-normalized REE distribution patterns of the agates show strong negative Eu-anomalies, enriched HREE as well as tetrad effects. These patterns are different from the REE patterns of the volcanic host rocks and point to fractionation processes during agate formation. The specific geochemical features indicate interactions of the host rocks with magmatic volatile fluids and heated meteoric water, and transport of SiO2 and other elements both in aqueous solution and via stable fluorine (and chlorine) compounds such as SiF4, BF3, GeF4, and UO2F2.The results of cathodoluminescence (CL) microscopy and spectroscopy revealed a microstructure of the agates that is similar for all occurrences. Characteristic features are irregular internal textures and sector zoning in quartz as well as luminescence colours and spectra, respectively, which are caused by a typical high defect density (oxygen vacancies, silanol groups). According to these results, the formation of agates can be explained by crystallization via an amorphous silica precursor under non-equilibrium conditions.

Determination of the source rocks for the diatexites from the Edough Massif, Annaba, NE Algeria

The crystalline Edough Massif is located in the oriental part of the Algerian coastline. It consists of two tectonically superposed units of gneisses, augen-gneisses and migmatitic gneisses in the lower unit and micaschists in the upper unit. The crystalline rocks underwent a low to moderate degree of metamorphism; the gneisses suffered partial melting. They display migmatitic features such as nebulitic structures with contorted leucosome layers and K-feldspar porphyroblasts and thus can be classified as diatexites. The mineralogical composition of these rocks is very homogenous and consists of K-feldspar, micas and quartz. The feldspar-rich, arkosic nature of the outcrop implies a granitic source rock. High K2O/Na2O ratios and high A/CNK>1.1 indicate an S-type granite source and a peraluminous composition of the protolith respectively. Chondrite normalized REE distribution patterns of the Edough diatexites show gently inclined patterns with a minor negative Eu anomaly (Eu/Eu*=0.36–0.49), which points to a very slightly differentiated granitic source. The REE pattern and trace element data of the diatexites are similar to those of average Proterozoic upper continental crust, which suggests that they are derived mainly from upper continental crust and were deposited in continental margins.

Ore mineralization in naphtides and problems of its genesis

The paper presents several examples of metalliferous naphtide mineralization hosted in sedimentary and magmatic rocks and discusses different models of their origin. Geological, geochemical, experimental, and thermodynamic data were used to support a hypothesis assuming that metalliferous naphtide deposits were formed during cold and hot (with melt generation) mantle degassing with subsequent condensation of heavy hydrocarbons and organometallic compounds (OMC) in fault zones. Important criteria for discrimination between metalliferous naphtide mineralization of different origin include endo- and biomarkers, Nd and Sr isotopic data, and chondrite-normalized REE distribution patterns. The formation of endogenic metalliferous naphtide deposits does not rule out the possible assimilation of organic components from the host rock. Juvenile water and inorganic gases were found in the hydrothermal solutions contributing to the formation of these deposits.

Rare earth element geochemistry of hydrothermal deposits from Southwest Indian Ridge

The REE compositions of hydrothermal deposits and basalt samples from the Southwest Indian Ridge (SWIR) were determined with ICP-MS. The results show that there are significant differences between different types of samples although all samples show relative LREE enrichment. The contents of REE in hydrothermal sulfides and alterated rocks samples are lower (from 7.036×10−6 to 23.660×10−6), while those in the white chimney deposits are relatively higher (ranging from 84.496×10−6 to 103.511×10−6). Both of them are lower than basalts. Chondrite-normalized REE distribution patterns show that sulfides and alterated rocks samples are characterized by significant positive Eu anomalies. On the contrary, white chimney deposits have obvious negative Eu anomalies, which may be caused by abundant calcite existing in the white chimney samples. Both the content and distribution pattern of REE in sulfides suggest that REE most possibly is originally derived from hydrothermal fluids, but influenced by the submarine reducing ore-forming environment, seawater convection, mineral compositions as well as the constraint of mineral crystallizations.

Trace element geochemistry of nyerereite and gregoryite phenocrysts from natrocarbonatite lava, Oldoinyo Lengai, Tanzania: Implications for magma mixing

The abundances of Li, P, Cl, V, Mn, Rb, Sr, Y, Cs, Ba, Pb, Th, U and REE, within and between, phenocrysts of nyerereite and gregoryite occurring in natrocarbonatite lavas erupted from the active volcano Oldoinyo Lengai (Tanzania) have been determined by electron microprobe, LA-ICP-MS and SIMS. These data show that, in general, nyerereite is enriched in Rb (71–137ppm), Sr (14,485–23,240ppm), Y (2.0–8.9ppm), Cs (1.6–5.3ppm), Ba (4000–11,510ppm), but poorer in Li (21–91ppm), P (820–1900ppm) and V (5.1–47ppm) relative to gregoryite (Rb=43–106; Sr=4255–7275; Y=0.3–4.0; Cs=0.6–5.1; Ba=1125–7052; Li 84–489; P=6790–15,860; V=33–155ppm). Nyerereite is highly enriched in REE (La=236–973; Ce=395–1044ppm) relative to gregoryite (La=59–309; Ce=59–301ppm). Chondrite normalized REE distribution patterns for nyerereite and gregoryite are parallel and linear with no Eu anomalies. They show extreme enrichment in light REE and depletion in heavy REE (nyerereite La/YbCN=1759–7079; gregoryite La/YbCN=1051–10,247). Significant differences exist in the abundances of trace elements within and between coexisting crystals occurring in diverse natrocarbonatite flows, although there do not appear to be any significant secular variations in phenocryst compositions in lavas erupted from a given vent. It is concluded that both major, minor and trace element compositional data for nyerereite and gregoryite phenocrysts occurring in natrocarbonatite lavas are derived by the crystallization of several different batches of magma in a continuously replenished fractionating magma chamber. Natrocarbonatite lavas are considered to be hybrids formed by the mixing of both crystals and melts formed from several batches of natrocarbonatite magma; thus bulk rock compositions cannot represent the compositions of the primary magma composition before the onset of fractionation. Differentiation of natrocarbonatite melts leads to enrichment of residua in Ba and Mg.

Tetrad effects in the rare earth element patterns of granitoid rocks as an indicator of fluoride-silicate liquid immiscibility in magmatic systems

This paper focuses on reasons for the appearance of tetrad effects in chondrite-normalized REE distribution patterns of granitoids (Li-F granites, peralklaine granites, ongonites, fluorine-rich rhyolites, and granitic pegmatites). The analysis of published data showed that the alteration of such rocks by high- and/or low-temperature metasomatic processes does not result in most cases in the appearance or enhancement of M-type tetrad effects in REE patterns. These processes are accompanied by the removal or addition of lanthanides, a W-type sag appears between Gd and Ho, and negative or positive Ce anomalies develop sometimes in REE patterns. The formation conditions of peculiar rocks enriched in Ca and F from the Ary Bulak ongonite massif (eastern Transbaikalia) and the character of REE distribution in these rocks and melt inclusion glasses were discussed. Based on the obtained data and the analysis of numerous publications, it was concluded that REE tetrad effects in rare-metal granitoids are caused by fluoride-silicate liquid immiscibility and extensive melt differentiation in the accumulation chambers of fluorine-rich magmas. A considerable increase in fluorine content in a homogeneous granitoid melt can cause its heterogenization (liquation) and formation of fluoride melts of various compositions. The redistribution of lanthanides between the immiscible liquid phases of granitoid magma will result in the formation of M-type tetrad effects in the silicate melts, because the REE patterns of fluoride melts exhibit pronounced W-type tetrad effects. The maximum M-type tetrad effect between La and Nd, which is observed in many rare-metal granitoids, is related to the character of REE partitioning between fluoride and silicate melts and F- and Cl-rich magmatic fluids. The low non-chondritic Y/Ho ratio (<15) of many rare-metal granitoids may be indicative of a contribution of fluoride melts to the differentiation of F-rich silicic magmas, from which these rocks were formed. The influence of high-temperature F-Cl-bearing fluids on melts and/or granitoid rocks results in an increase in Y/Ho ratio owing to the elevated solubility of Ho in such fluids.

LREE-rich hibonite in ultrapotassic rocks in southern India

We describe a rare terrestrial occurrence of hibonite (CaAl 12TiO 19) from the ultrapotassic rocks which occur as veinlets in graphite-bearing spinellite in the Achankovil Shear Zone (ACSZ), southern India. The ultrapotassic rocks are characterized by an unusual mineral assemblage, kalsilite + leucite + corundum + spinel + perovskite + apatite + hibonite ± phlogopite, along with mantle-derived graphite and Fe-sulfides. Three types of hibonite were identified in terms of chemical zoning as observed in back scattered images: (i) unzoned hibonite, (ii) zoned hibonite with brighter core and dark rim, and (iii) zoned hibonite with brighter core, a less bright intermediate zone and a dark outer rim corresponding to the variations in the distribution of LREEs (light rare earth elements), Ca, Al and Ti in the hibonite structure. The core regions of hibonite are highly enriched in LREEs and Sr, and depleted in Ca, Ti and Al, compared to the rim areas. Chondrite-normalized REE distribution patterns for hibonite exhibit extreme LREE enrichment (La N/Yb N = 763 to 374,494) with a negative Eu anomaly. Apatite displays MREE-enriched patterns. Partitioning of LREEs in hibonite and apatite crystallized from the melts is chiefly determined by their preference for hibonite > apatite. We propose a model that considers the formation of the LREE-rich Ti-depleted hibonite from ultrapotassic silica-undersaturated melts. We envisage that the melts were generated by low degree partial melting of a phlogopite-bearing peridotite that was fertilized in LREEs. The ultrapotassic rock that occurs as veins along the ACSZ represents a melt influx associated with deep-seated extension after the final amalgamation of the Gondwana supercontinent.