Total REE Contents Research Articles

Geochemistry of the Upper Triassic black mudstones in the Qiangtang Basin, Tibet: Implications for paleoenvironment, provenance, and tectonic setting

The Qiangtang Basin is the largest Mesozoic marine basin in the Tibetan Plateau. The Upper Triassic black mudstones are among the most significant hydrocarbon source rocks in this basin. Here, we present geochemical data for the Upper Triassic black mudstones to determine their paleoenvironment conditions, provenance, and tectonic setting. To achieve these, 30 black mudstones formed in various sedimentary environments were collected from the Zangxiahe, Zana, and Bagong formations. The results show that the total REE concentrations of mudstones from these formations range from 169 to 214 ppm, 204 to 220 ppm, and 141 to 194 ppm, respectively. All samples have chondrite-normalized REE patterns with enrichment of LREE, depletion of HREE and negative Eu and Ce anomalies. Specifically, mudstones from the Bagong Formation exhibit higher negative Eu anomalies and lower REE contents than those from the Zangxiahe and Zana formations.Mudstones from the Zangxiahe and Zana formations with low Sr/Ba and Sr/Cu ratios indicate the humid climate, whereas the high Sr/Ba and Sr/Cu ratios of rocks from the Bagong Formation suggest the arid climate. The low U/Th, (Cu + Mo)/Zn, V/Cr and Ni/Co ratios of rocks from the Zangxiahe, Zana, and Bagong formations are indicators of oxidized conditions. The bivariate diagrams (TiO2 vs. Al2O3, TiO2 vs. Zr, La/Th vs. Hf, and Co/Th vs. La/Sc) reveal that mudstones from the Zangxiahe and Zana formations were potentially derived from intermediate igneous rocks, whereas mudstones from the Bagong Formation were probably sourced from felsic igneous rocks. Their source rocks are mostly deposited in the collisional setting. REE of mudstones from the Zangxiahe, Zana, and Bagong formations were possibly originated from terrigenous detritus, with minor non-terrigenous contributions into the Zana samples. The REE contents of these mudstones are controlled mainly by terrigenous detrital minerals, rather than by the paleoclimate, paleoredox conditions, or organic matter. However, calcite minerals could dilute REE. Therefore, the REE contents of the Bagong Formation mudstones are significant lower than those of the Zangxiahe and Zana formations mudstones.

The spessartine–almandine garnet from Val Codera pegmatite, Central Alps, Italy: a new insight on the crystallochemistry and a 3D image analysis of its inclusions

This paper is a new crystallographic and chemical insight including REE and other trace elements of garnets from the “garnet dike”, located in Val Codera (Central Alps, Italy). The crystals may reach up to 2 cm with trapezohedral shape and show a homogeneous dark red colour; however, there are also present many eye-visible inclusions and cracks. Therefore, synchrotron X-ray computed micro-tomography was carried out on selected samples to visualise the 3D distribution and shape of those inclusions, as well as the distribution of voids and cracks within the crystals. To notice here, that because of those latter features a workability of this mineral could result difficult even though these crystals would otherwise may be interesting as gem. The analysed garnets are intermediate members of the almandine–spessartine solid-solution series, varying in composition from Mn-rich core (up to Sps64 mol%) to Fe-rich rim (up to Alm54 mol%). The samples exhibit a depleted LREE and an enriched HREE patterns with a strong negative Eu anomaly. The increase of total content of REE as well as of Y from core to rim suggests that their incorporation in the [X] site, mainly occupied by Mn and Fe2+, is controlled by the variation of both these cations. In the same way, the cell parameter and the bond lengths, measured both by powder and single crystal X-ray diffraction methods, result between those of pure spessartine and almandine, consistent with the chemistry and related to the variation of the ratio Mn/Fe. The chemical zoning of garnet represents its growth history and makes it a useful indicator of fractional trend of pegmatite-forming melt. Our results suggest that the spessartine-rich garnet crystallisation took place from a Mn-rich melt in presence of volatiles, which prevented the incorporation of Y and REE in the structure favouring the crystallisation of Y and REE-rich accessory minerals.

REE fractionation during the gypsum crystallization in hyperacid sulphate-rich brine: The Poás Volcano crater lake (Costa Rica) exploited as laboratory

The critical role of rare earth elements (Lanthanides plus Yttrium; hereafter REE) in high-tech technologies and consequently their increasing demand from the industry, in addition to the capability of REE to trace water–rock interaction processes, boosted the study of REE in unconventional extreme environments.This study is focused on the geochemical behaviour of REE in the hyperacid sulphate-rich brine of the crater lake of Poás volcano (Costa Rica), where the precipitation of gypsum occurs. This system can hence be considered as a natural laboratory to evaluate the fractionation of REE between the lake water (mother brine) and the precipitating gypsum mineral. Total REE concentrations dissolved in waters range from 1.14 to 2.18mgkg−1. Calculated distribution coefficients (KD) for REE between the gypsum and the mother brine indicate a preferential removal of the light REE (LREE) with respect to the heavy REE (HREE), with KD values mainly decreasing from La to Lu.During the observation period (2007–2009), the distributions of REE concentrations dissolved in lake water normalized to the average local volcanic rock show two different trends: i) LREE depleted patterns, and ii) flat patterns. The identification of the LREE depleted pattern is justified by the KD calculated in this study. We demonstrate that the precipitation of gypsum is able to strongly fractionate the REE in hyperacid sulphate-rich brine, inducing changes in REE concentrations and distributions over time.X-ray computed tomography imaging was performed on gypsum crystal (precipitated from the lake waters) to gain insights on crystal-scale processes possibly controlling the REE geochemistry, i.e. surface processes vs. structural substitution. Accordingly, the heavy metals and possibly the REE seem to be mainly located on the crystal surface rather than inside the crystal, suggesting that a surface process could be the major process controlling REE removal from the water to the crystal.

Petrogenesis of Early Cretaceous granitoids from southwest Zhejiang, NE South China Block and its geodynamic implication

In situ zircon U–Pb ages, whole-rock major and trace elements, and Sr–Nd isotopic compositions of the Sucun, Yunfeng, and Jingning intrusions from southwest Zhejiang, NE South China Block, are presented to trace their petrogenesis and shed light on its lithosphere evolution. LA–ICP–MS U–Pb zircon dating shows that the Sucun quartz monzonite and Jingning monzogranite were emplaced at ~135 Ma, and the Yunfeng quartz monzonite and Jingning granite were emplaced at 104 and 112 Ma, respectively. All these intrusions are metaluminous to weakly peraluminous and lie within high-K calc-alkaline to shoshonite series field (SiO2 = 66–76 wt%, A/CNK = 0.95–1.09, K2O/Na2O = 0.78–1.77). The Yunfeng quartz monzonite clearly have lower K2O and total REE contents, and higher CaO, Na2O, Al2O3, P2O5, MgO, and TiO2 contents, and relatively less enriched Sr–Nd isotopic compositions than those of the Sucun quartz monzonite, indicating that the Yunfeng quartz monzonite were derived from partial melting of a more juvenile lower crust sources compared with the magma source of the Sucun quartz monzonite. The Jingning monzogranite exhibit similar major elements covariations and Nd isotopic compositions, but higher Ba, Sr, and Eu contents and lower Rb, Th, and U contents than those of the Jingning granite. The geochemical features imply that the Jingning monzogranite and granite were fluid-present and fluid-absent anatexis products of the same Paleoproterozoic crustal source, respectively. Whole-rock Sr–Nd isotopic data imply that the estimated amounts of juvenile mantle-derived melts input into the mature crust show southeastward decreasing trend away from the Jiangshan–Shaoxing fault. We propose that roll-back and retreat of the Paleo-Pacific subducting plate might cause extensive asthenosphere mantle upwelling in East China, and the mantle-derived melts tend to rise through the regional main fault zones and preferentially modify the lithosphere nearby these faults.

Geochronology and trace element geochemistry of titanite in the Machangqing Cu-Mo-dominated polymetallic deposit, Yunnan Province, southwest China

The Machangqing Cu-Mo-dominated polymetallic deposit is a porphyry-skarn-epithermal Cu-Mo (-Au) metallogenic system located in the middle part of the Jinshajiang–Ailaoshan alkaline porphyry metallogenic belt. The skarn mineralization of the Machangqing deposit mainly occurs along the contacts between the alkalic porphyry intrusions and the surrounding Lower Ordovician Xiangyang Formation rocks. We present LA-ICP-MS U-Pb ages and trace element data for titanite from the Machangqing deposit in order to investigate the origin of this deposit. Based on mineral textures and assemblages, two types of titanite are recognized in Machangqing: magmatic titanite (Type I) from the granite porphyry and hydrothermal titanite from the mineralized skarn. The coarse-grained magmatic titanite is euhedral and occurs as discrete grains in the interstices of feldspar, quartz and biotite, whereas fine- to medium-grained hydrothermal titanite crystals (Type II) are euhedral to subhedral and occur in association with skarn minerals such as garnet, pyroxene and magnetite. Magmatic titanite has lower FeO, Al2O3, F and Nb/Ta but higher TiO2, Th/U, HFSEs and Lu/Hf than hydrothermal titanite. The magmatic titanite has higher LREE/HREE ratios and total REE contents with stronger negative Eu anomalies than its mineralized skarn counterpart. Trace elemental characteristics of hydrothermal titanite in Machangqing are consistent with relatively low F contents and oxygen fugacities when compared to the neighboring Beiya gold-dominated polymetallic deposit in the same metallogenic belt. The weighted average 206Pb/238U age of 34.3 ± 1.2 Ma of hydrothermal titanite is within error but slightly younger than the age of magmatic titanite (37.5 ± 4.1 Ma), indicating that the skarn mineralization followed the emplacement of the granite porphyry and was broadly coeval with the porphyry mineralization. The porphyry and skarn types of mineralization at Machangqing were formed from the same metallogenic system. Different ore-forming conditions, such as oxygen fugacities and F contents, might be responsible for different metal enrichments at the Machangqing and Beiya deposits.

The braunite (3Mn2O3·MnSiO3)-rich mineralization in the metasedimentary succession from southern Apennines (Italy): Genesis constraints

A massive Mn-bearing mineralization, extensively exploited until the 70 s, locally occurs in the metasedimentary succession of the southern Apennines (at Mormanno site, Calabria-Lucania border, Italy). The mineralization mainly consists of three types distinguished by the relative percentages of the principal mineral phases: braunite, quartz, Mn oxi-hydroxides and Mn-rich epidote (piemontite). Petrographic observations revealed that the ore underwent medium- to high-grade metamorphism that was responsible of the growth of braunite at the expense of “primary” amorphous Mn oxi-hydroxides and quartz. During metamorphism, piemontite also formed where altered phyllosilicates are present in addition to braunite and quartz, suggesting that the aluminosilicates were source of Al necessary to the epidote crystallization. The most important geochemical features of the ore are high Mn contents (up to 71.6 wt% of MnO), low concentrations of transitional elements (Co + Ni+Cu + Zn < 0.1 wt%), high Ba (up to 4455 ppm) and variable total REEs (14.7 < ΣREEs < 329.7 ppm) contents. These are the typical characteristics of the ocean-floor hydrothermal Mn deposits, similarly to what observed in braunite ores occurring in the Alps and northern Apennines. The high LREE/HREE fractionations and the negative Ce and Eu anomalies characterizing the studied ore, suggest the mineralizing fluid was formed by a mixing solution consisting of a Mn-rich hydrothermal fluid and seawater, which attained the ideal condition for the ore precipitation in a distal oceanic area, far from the submarine hydrothermal vent.

Geochemical Characteristics and Radioactive Elements Estimation along Trenches of Um Ara area, South Eastern Desert, Egypt

Um Ara area is located in the extreme south Eastern Desert of Egypt between 581400mE-584900mE and 2503000mN-2504600mN. The area comprises distinct tectono-stratigraphic units beginning with the oldest as follows: Dokhan volcanic and post-collision granitoids (monzogranite, and alkali feldspar granite), and associated with dykes and quartz veins. The granite affected by deuteric K-metasomatism (microclinization) followed by Na-metasomatism (albitization) causing zonation in this part of the pluton. The high radioactivity in the anomalous sites is structurally controlled (strong shearing, fracturing and jointing). The intensity of mineralization is especially significant along joint sets striking NNW-SSE, NNESSW and NE-SW trending. The spacing among these joints ranges from few mm up to about 5 cm. locally, the joints are filled with kaolinite and other alteration products including hematite and Mnoxides. Geochemically, Um Ara granites trenches and core samples show a relative enrichment in K2O compositions due to plagioclase destruction and formation of muscovite/sericitization. Um Ara granites trenches and core samples have variable total REEs contents 117.41 to 308.28 and 256.51 to 397.22 ppm respectively. ΣLREEs in trenches and core samples range from 108.47 to 298.41ppm and from 236.84 to 384.78 ppm respectively, whereas ΣHREEs range from 5.48 to 14.01 ppm and from 12.44 to 19.67 ppm respectively. The lowest concentration of total REEs in core samples is at 100m depth, while the highest values at 145m depth. Um Ara granites trenches, U contents vary from 154 to 506 ppm with an average of 245.7 ppm, and Th contents vary from 47 to 415 ppm with an average of 165.4 ppm. Core samples granite, U contents vary from 128 to 349 ppm with an average of 237.3 ppm, and Th contents vary from 45 to 146 ppm with an average 107.4 ppm. Visible secondary uranium mineralization is represented mainly by uranophane and β-uranophane. Uranium is also contained within a range of U-bearing accessory minerals, mainly fluorite, monazite, xenotime, allanite and zircon. The secondary processes played a main role in uranium enrichment (uranium added to these granitic sites post-magmatically). Um Ara altered granites trenches and core samples rocks are an abnormal radiation and have an extension. So, these anomalies are suitable to be a place for uranium extraction. The result is the continuous of its mining work besides leachability experiments.

Geochemical fingerprinting of Maastrichtian oil shales from the Central Eastern Desert, Egypt: Implications for provenance, tectonic setting, and source area weathering

Upper Cretaceous oil shales accumulated in intracratonic sedimentary basins along the Central Eastern Desert. Comprehensive geochemical analyses have been applied on 3 Maastrichtian oil shale horizons from the Duwi and Dakhla formations (El‐Beida and El‐Nakheil mines). We provide significant clues for chemical weathering, sediment recycling, petrogenesis, and tectonic setting of oil shales' source material. The mineralogical composition of the studied oil shales is generally characterized by carbonates, phyllosilicates, quartz, fluorapatite, and sulphides. The geochemistry is characterized by high Ca, P, Ni, U, and Cr values compared to standard post‐Archean Australian shales (PAAS). Their chondrite‐normalized patterns are distinguished by LREE enrichment, HREE depletion, negative Eu anomaly, and typical shale‐like PAAS‐normalized REE patterns. Furthermore, the total REE content is positively correlated with Si, K, Na, Fe, Al, Ti, and Mn, indicating that the REE of the investigated oil shales originated from terrigenous sources. Diverse proxies (e.g., CIA, CPA, and PIA) and the A–CN–K ternary diagram suppose that the source rock succession of the oil shales experienced moderate to intensive chemical weathering. Various geochemical parameters suggest that the oil shales were predominately derived from intermediate, with a minor contribution from felsic, igneous source rocks without any remarkable recycling signals. The average values of (Gd/Yb)N (1.39), Eu/Eu* (0.76), Th/U (0.29), and discrimination diagrams reveal that the oil shales were derived from Neoproterozoic island arc parent rocks. The geochemical results for oil shale provenance are in accordance with the composition and the palaeotectonic history of the igneous rocks in the Central Eastern Desert of Egypt.

Sources and distribution of yttrium and rare earth elements in surface sediments from Tagus estuary, Portugal

The distribution and sources of yttrium and rare-earth elements (YREE) in surface sediments were studied on 78 samples collected in the Tagus estuary (SW Portugal, SW Europe). Yttrium and total REE contents ranged from 2.4 to 32mg·kg−1 and 18 to 210mg·kg−1, respectively, and exhibited significant correlations with sediment grain-size, Al, Fe, Mg and Mn, suggesting a preferential association to fine-grained material (e.g. aluminosilicates but also Al hydroxides and Fe oxyhydroxides). The PAAS (Post-Archean Australian Shale) normalized patterns display three distinct YREE fractionation pattern groups along the Tagus estuary: a first group, characterized by medium to coarse-grained material, a depleted and almost flat PAAS-normalized pattern, with a positive anomaly of Eu, representing one of the lithogenic components; a second group, characterized mainly by fine-grained sediment, with higher shale-normalized ratios and an enrichment of LREE relative to HREE, associated with waste water treatment plant (WWTP) outfalls, located in the northern margin; and, a third group, of fine-grained material, marked by a significant enrichment of Y, a depletion of Ce and an enrichment of HREE over LREE, located near an inactive chemical-industrial complex (e.g. pyrite roast plant, chemical and phosphorous fertilizer industries), in the southern margin. The data allow the quantification of the YREE contents and its spatial distribution in the surface sediments of the Tagus estuary, identifying the main potential sources and confirming the use of rare earth elements as tracers of anthropogenic activities in highly hydrodynamic estuaries.

Cambrian (~510 Ma) ophiolites of the East Kunlun orogen, China: A case study from the Acite ophiolitic tectonic mélange

ABSTRACTThe Acite ophiolitic mélange represents a remnant of the Proto-Tethys forearc oceanic lithosphere. Two gabbros yield zircon 206Pb/238U ages of 510-512 Ma. The magmatic rocks are divided into three subtypes: (1) Mid-ocean ridge basalt (MORB) -like forearc basalts (FABs), (2) Low-titanium tholeiitic gabbros (LTGs), and (3) Normal calc-alkaline dacites (CADs). The FABs have comparatively higher TiO2 concentrations (1.13–1.42 wt%) and show almost flat REE patterns. In the NMORB normalized trace element patterns, the FABs display flat distributions of high field strength element (HFSE). The εNd(t) values of FABs range from +4.0 to +4.8. These features are similar to the composition of Izu-Bonin-Mariana forearc basalts (FABs). LTGs are characterized by higher contents of MgO (8.85–9.95 wt%) and lower concentration of TiO2(0.29–0.50 wt%). They show LREE-depleted patterns, however having comparatively lower total REE contents than those of FABs. The εNd(t) values of LTGs range from +7.4 to +8.4. These features show that LTG magmas originated from a progressively depleted mantle source. In contrast, the CADs having higher SiO2 contents (63.58–70.92 wt%) have higher total REE contents. In the NMORB normalized trace element patterns, CADs are characterized by enrichment of LILEs and depletion of HFSEs. CADs have negative εNd(t) values ranging from -9.8 to -10.5, which are likely suggestive of a crust-derived source. The rock association of FABs and LTGs, together with tectonic discrimination plots and regional data, suggest a forearc setting above the SSZ for Acite ophiolitic mélange.

LA-ICP-MS titanite U-Pb dating and mineral chemistry of the Luohe magnetite-apatite (MA)-type deposit in the Lu-Zong volcanic basin, Eastern China

The Luohe Fe deposit is the largest magnetite-apatite (MA)-type deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYB) (around 1 Gt Fe ore resource). The main orebody is hosted by volcanic rocks, and widespread titanite mineralization is developed in the footwall of the shallow ore bodies, and in the hanging wall of the deep ore bodies. Trace element chemistry of the well-crystallized hydrothermal titanite from both deep and shallow ore bodies indicates coupled substitution of (Al, Fe)3+ + (F, OH)− for Ti4+ + O2−, and the addition of elements such as Zr, Nb and REEs. Titanite Zr-thermometry yielded a mineralization temperature of ca. 700 °C, suggesting that the Luohe magnetite-apatite deposit was formed at a higher temperature than typical Fe skarn deposits in the metallogenic belt. The high ore-forming temperature may have been a key factor in Ti migration. The distinct LREE enrichment, the relatively high total REE concentrations, and the marked negative Eu anomalies all suggest that the Luohe titanite may have crystallized before apatite and epidote in a high-temperature hydrothermal environment. Variation in the negative Eu anomalies suggests the ore fluid oxygen fugacity increased from deep to shallow levels. Our study suggests that the Th/U ratio is not an efficient igneous vs. hydrothermal titanite discriminator for high-temperature titanite. LA-ICP-MS titanite U-Pb dating of the shallow and deep orebodies yielded ages of 130.0 ± 0.9 Ma and 129.1 ± 0.8 Ma to 129.7 ± 0.8 Ma, consistent with the age of the deep-seated diorite in the area. We propose that the shallow- and deep-level orebodies at Luohe were deposited from the same mineralizing system and mineralization was closely related to the deep-seated diorite.

Genesis of the Permian Kemozibayi sulfide-bearing mafic-ultramafic intrusion in Altay, NW China: Evidence from zircon geochronology, Hf and O isotopes and mineral chemistry

The recently discovered Kemozibayi mafic-ultramafic intrusion and its associated magmatic Cu-Ni sulfide deposits are located at the southern margin of the Chinese Altai Mountain, Central Asian Orogenic Belt in north Xinjiang, NW China. The intrusion is composed of olivine websterite, norite, gabbro and diorite. Disseminated and net-textured Ni-Cu sulfide ores are hosted in the center of the gabbro. In this work, new zircon U-Pb ages, Hf-O isotopic and sulfide S isotopic data, and whole rock and mineral chemical analyses are combined in order to elucidate the characteristics of the mantle source, nature of subduction processes, degree of crustal contamination, geodynamic setting of bimodal magmatism in the region, and the metallogenic potential of economic Cu-Ni sulfide deposit at depth. SIMS zircon U-Pb dating of the gabbro yields Permian ages (278.3±1.9Ma), coeval with the Kalatongke Cu-Ni deposit and with Cu-Ni deposits in the Eastern Tianshan and Beishan areas. Several lines of evidence (positive εHf(t) from +7.1 to +13.3, Al2O3, TiO2 and SiO2 contents in clinopyroxene from olivine websterite, high whole rock TiO2 contents) suggest that the primary magma of the Kemozibayi intrusion was a calc-alkaline basaltic magma derived from depleted mantle, and that the degree of partial melting in the magma source was high. The evolution of the Kemozibayi mafic-ultramafic complex was strongly controlled by fractional crystallization and the crystallization sequence was olivine websterite, norite, and then gabbro. This is evidenced by whole rock Fe2O3 contents that are positively correlated with MgO and negatively correlated with Al2O3, CaO and Na2O, similar LREE enrichment and negative Nb, Ta, Hf anomalies in chondrite and primitive mantle-normalized patterns, and a decrease in total REE and trace elements contents and magnetite content from gabbro through to norite and olivine websterite. Varied and low εHf(t) (+7.1 to +13.3) and high δ18O values (+6.4‰ to +7.2‰) in zircon, high La/Ba, and Rb/Y ratios, and low Nb/La ratios in whole rock samples, suggest 5–10% contamination by subduction related fluid and 10–15% contribution from an upper crustal component. The clinopyroxene TiO2 and Alz values (percentage of tetrahedral sites occupied by Al) in the Kemozibayi intrusion show characteristics of rift cumulate rocks, but minor arc cumulate features, indicating that the mantle source might have experienced a lower degree of subduction metasomatism. A high degree of partial melting of the depleted mantle and subducted oceanic sediments with related fluid metasomatism and crustal contamination in the Kemozibayi mafic-ultramafic intrusion are appropriate for the formation of an economic copper and nickel sulfide deposit. Crustal contamination, crustal S addition and early fractionation of olivine and sulfide induced S saturation and the formation of immiscible sulfide in the Kemozibayi intrusion. With no obvious olivine and Ni-rich sulfide co-crystallization, the Cu grade in the present orebody is higher than the Ni grade, and the proportion of mafic rocks is high in the Kemozibayi mafic-ultramafic complex. Cumulatively, these features suggest that the mafic-ultramafic intrusion underwent early olivine and Ni sulfide segregation, and that deeper or extended portions of the present intrusive body may host Ni mineralization.

A double Fe-Ti oxide and Fe-sulphide liquid immiscibility in the Itsindro Gabbro Complex, Madagascar

The petrology and mineralogy of the Itsindro complex in south-central Madagascar has been investigated through samples obtained from the 320.7 m-deep Lanjanina borehole. The section consists of a 254 m-thick pyroxenite unit with interbedded gabbro layers that overlies a gabbro unit and is itself overlain by a 19 m-thick granite unit. Most of the structures are sub-horizontal. A weak magmatic layering is locally observed but at the scale of the core, the intrusion does not appear to be a layered complex.Pyroxenite and gabbro show a systematic disseminated mineralization consisting of Fe-Ti-P oxides and Fe-(Cu-Ni) sulphides that takes the form of ilmenite-titanomagnetite ± apatite and pyrrhotite ± chalcopyrite ± pentlandite. In the upper zone, from 90 to 72 m, sub-massive centimetre-to decimetre-sized layers of oxides and sulphides comprise a total of 16 m of sub-massive sulphide (the main mineralized zone). In this mineralized zone the oxide/sulphide ratio is close to 1/1. The sulphide is strongly dominated by pyrrhotite, which may locally contain inclusions of molybdenite crystals with the Re sulphide rheniite (ReS2). Oxides are generally euhedral, included in or attached to the Fe-sulphide, and also locally form sub-massive centimetre-sized bands.Apatite as a cumulus phase is ubiquitous. Locally it may account for 30% of the ore-rich samples and some samples consist of apatite-Fe-Ti oxides-Fe-Cu-Ni sulphides with virtually no silicate. Apatite is the main REE carrier but the total REE content remains low (<90 ppm).Mineral compositions and whole rock geochemistry indicate that the rocks are highly differentiated, and in spite of a relatively limited thickness, the differentiation process is observed. Two zones can be distinguished: from the bottom to 162.8 m we see a decrease in the Mg number of olivine and pyroxene, and a drop in TiO2 and Al2O3 for the latter. A reverse trend is then observed within the pyroxenite unit from the 162.8 m level upwards. The main mineralized zone is located in the upper part of this unit, about 70 m above this discontinuity.The highly differentiated Fe-Ti-P facies of the Lanjanina series in the Itsindro Gabbro Complex have been interpreted as corresponding to the Fe-Ti-P rich, Si-poor member remaining after an immiscible segregation of an evolved mafic magma. The granite dykes and the overlying granite unit represent the second, Si-rich member of the immiscibility process.The presence of large amounts of sulphide is attributed to sulphur contamination of the Fe-Ti-rich liquid. Fe-Ti oxides will tend to crystallize on the sulphide droplets and the accumulation of dense Fe-sulphides (liquid) and associated Fe-Ti oxides (solid) will result in this complex and unusual association taking the form of a net texture.

Petrochemistry of Granitoids in Sibolga and its Surrounding Areas, North Sumatra, Indonesia

AbstractPetrochemical studies were conducted on granitoids in Sibolga and surrounding areas in North Sumatra, Indonesia. The granitoids presented the characteristics of A‐ and I‐type ilmenite series. The syenogranites from Sarudik, the syenogranite, quartz syenite, and quartz alkali feldspar syenite from Sibuluhan Sihaporas, and the monzogranites from Sibolga Julu are A‐type granitoids that have high SiO2, CaO, Al2O3, Na2O + K2O, and Rb content, a high FeO/MgO ratio, and low Ba, Sr, and Zr content. The alkali feldspar syenite from Sarudik, the quartz alkali feldspar syenite from Tukka, and the quartz syenite from Adian Koting show I‐type characteristics. I‐type granitoids have lower SiO2, Na2O + K2O, Rb, Yb, and total REE (ΣREE) content and low FeO/MgO ratio. However, quartz syenite from the Sibolga Julu, which exhibits I‐type characteristics, has high ΣREE content but a low FeO/MgO ratio.The granitoids consisted mainly of K‐feldspar megacrysts, quartz, plagioclase, and biotite with hornblende in some cases. Accessory minerals were zircon, apatite, allanite, and titanite. Cordierite and corundum occurred as xenocrysts and inclusions in K‐feldspar in syenogranite (Sarudik) and quartz syenite (Adian Koting). SiO2 and ΣREE content and the Rb/Sr ratio are positively correlated to each other.The REE are enriched in quartz alkali feldspar syenites from Sibolga Julu and Sarudik, quartz syenite from Tarutung, and quartz alkali feldspar syenite, syenogranite, and quartz syenite from Sibuluhan Sihaporas. These highly‐differentiated granitoids were formed within the plate settings. In contrast, the ΣREE content of hornblende‐bearing granitoids that were formed in volcanic arc settings is low. The coexistence of cordierite and corundum with biotite and hornblende indicates that the metaluminous magma was contaminated by metasedimentary basement rocks.

Rare earth elements in tributaries of Lake Baikal (Siberia, Russia)

REE dissolved in waters from sixty five sampled tributaries of Lake Baikal show diversity in abundances (∑REE = 0.02–2.16 μg/L) and distribution patterns ([La/Yb]N = 0.20–5.58; [La/Tb]N = 0.30–2.59). Total REE contents vary seasonally and yearly, while REE patterns remain generally similar. The REE patterns in stream waters correlate with those in the drained rocks and hence, seem to be predominantly controlled by watershed lithology. Rivers having small watersheds around Lake Baikal belong to six lithological provinces with different stream-water REE patterns: the Olkha plateau and the ranges of Primorsky, Baikalsky, Barguzin, Ulan-Burgassy, and Khamar-Daban. The Khamar-Daban province, in turn, is lithologically heterogeneous and comprises four subprovinces with different respective patterns of dissolved REE. Rock-water interaction in subsurface aquifers that feed the Baikal inlets provides much stronger lithological control of REE than the weak interaction of short and rapid streams with rocks on the surface. REE fractionation in stream waters show negative Ce anomalies in almost all samples (Ceanom = 0.12–0.97), except for those of large rivers (Selenga, Turka, Barguzin, Upper Angara, and Kichera). The reason is that the great length, relatively slow current, and the presence of streambed fine alluvium make the water chemistry more uniform and the REE patterns less dependent on local variations in watershed lithology. The sampled tributaries represent a range of physico-chemical and geomorphological conditions which can be used as reference for calibrations in future studies of sediment sources and, possibly, as a valuable tool for mineral exploration.

The Central Tianshan Block: A microcontinent with a Neoarchean-Paleoproterozoic basement in the southwestern Central Asian Orogenic Belt

The Central Tianshan Block (CTB) is one of the major continental constituents of the western Central Asian Orogenic Belt (CAOB). Its nature and origin is essential for understanding the accretionary processes and the reconstruction of the architectural framework of the CAOB. The Wuwamen Metamorphic Complex (WMC), which is believed to represent the basement of the CTB, was incorporated into the Wuwamen ophiolitic mélange during the Paleozoic evolution of the South Tianshan Ocean. The WMC is mainly composed of amphibolite-facies volcano-sedimentary rocks, including migmatites, orthogneisses, amphibolites, schists and locally marbles. Zircon U–Pb geochronology suggests that the protoliths of the interlayered biotite-two feldspar and biotite-plagioclase-hornblende gneisses in the Yangjuan area were formed between 2529 and 2513Ma and overprinted by amphibolite-facies metamorphism between 1815 and 1809Ma. Subsequently, the gneisses were intruded by ∼1724Ma meta-mafic dykes and ∼787Ma leucogranite dykes/veins. Geochemical data reveal that the biotite-two feldspar gneisses have high-silica adakite affinities with high SiO2 and Na2O contents as well as high Sr/Y ratios (61–117) and slightly evolved zircon Hf isotopic compositions. Consequently, they are interpreted to be the products of partial melting of relatively old subducted oceanic crust. In contrast, the interlayered biotite-plagioclase-hornblende gneisses have high MgO, Cr, Co and Ni contents and prominent negative Nb–Ta–Ti anomalies, indicating derivation from partial melting of an ancient sub-continental lithospheric mantle in a continental arc setting. The ∼1.72Ga meta-mafic dykes have geochemical similarities of typical Fe-Ti basalts and are characterized by incompatible element enrichments and primitive Sr–Nd–Hf isotopic signatures ((87Sr/86Sr)i=0.703446−0.703535); ɛNd(t)=+6.49 to +6.90; zircon ɛHf(t)=+0.57 to +10.6), which is consistent with partial melting of a relatively fertile, isotopically depleted asthenospheric mantle in a post-collisional extensional setting. In addition, the ∼787Ma leucogranites are trondhjemitic in composition and display extremely low total REE contents and highly evolved Nd–Hf isotopic signatures (ɛNd(t)=−4.71 to −4.53; zircon ɛHf(t)=−19.4 to −13.0). They are thought to have no petrogenetic relationship with the host orthogneisses and to be products of a foreign melt injection. Our new data provide robust evidence for a Neoarchean–Paleoproterozoic basement of the CTB. Comparison of the tectono-thermal-magmatic events reveals that the CTB and the Tarim Craton have a partly overlapping Precambrian crustal evolution, and thus the CTB could have originated from the Tarim Craton. However, a major difference between both is the presence of juvenile Mesoproterozoic magmatism in the CTB and its absence in the Tarim Craton and, therefore, further geological and geochronological research is required to determine the precise origin of the CTB and its tectonic affinity with adjacent terranes in the western CAOB.

Fluid inclusion and geochemistry studies of calcite veins in Shizhu synclinorium, central China: Record of origin of fluids and diagenetic conditions

Calcite veins in carbonate fracture have been investigated by petrographic, fluid inclusion, geochemical analyses and coupled with basin modeling techniques to provide useful insights into fluid activity and deformation conditions of the Cambrian to Triassic Shizhu synclinorium from the western region of Mid-Yangtze, central China. The results of the fluid inclusion microthermometry show a wide range of homogenization temperatures (78.6-215.5 °C) and salinities (0.18-23.11 wt.% NaCl equivalent), indicating the formation under diverse fluid conditions. All the calcite veins have negative Ce anomalies, which are the typical characteristic of marine carbonate sediments; it is therefore plausible that calcite veins were precipitated from the marine basin fluid. The stable carbon iso-topic compositions of calcites (δ 13CV-PDB=-2.5‰-4.26‰) and host limestones (δ 13CV-PDB=-3.56‰-5.80‰) are very similar with a correlation coefficient of 0.86, however, four calcites from the Lower Permian and Lower Triassic show lower δ 13C values relative to the host limestones, and they are depleted in total REE concentrations (ΣREE ratio varies from 0.74 to 2.06), suggesting the derivation of dissolved carbon from marine carbonates hosting the calcite veins and, less commonly, from the degradation of organic matter. Calculated δ 18O of the fluids-precipitating calcites (δ 18OV-SMOW=-0.41‰-14.42‰), 87Sr/86Sr ratios varying in the range of coeval seawater and the distinct REE pattern simultaneously suggest calcite-forming fluids in each stratigraphic unit could have formed from the involvement of fluids that originated from coeval seawater and evolved through different degrees of water rock interaction. However, the presence of more radiogenic 87Sr/86Sr ratios than coeval seawater and pronounced positive Eu anomalies in calcites of Lower to Middle Ordovician rocks indicate that terrestrial input from upper strata mudstone and siliciclastic rocks could be involved in the precipitation of the Ordovician calcite. Fluid-inclusion data combined with burial and thermal history modeling indicate there was large-scale flow of evolved basinal fluids through the carbonate formation fractures spanning a time frame from 135 to 50 Ma (Early Cretaceous-Eocene). Therefore, the geochemical characteristics of calcite veins can provide the basis for deformation events in Late Yanshanian and Early Himalayan orogeny.

Mechanism of mineralization in the Changjiang uranium ore field, South China: Evidence from fluid inclusions, hydrothermal alteration, and H–O isotopes

The Changjiang uranium ore field, which contains >10,000tonnes of recoverable U with a grade of 0.1–0.5%, is hosted by Triassic two-mica and Jurassic biotite granites, and is one of the most important uranium ore fields in South China. The minerals associated with alteration and mineralization can be divided into two stages, namely syn-ore and post-ore. The syn-ore minerals are primarily quartz, pitchblende, hematite, hydromica, chlorite, fluorite, and pyrite; the post-ore minerals include quartz, calcite, fluorite, pyrite, and hematite. The fluid inclusions of the early syn-ore stage characteristically contain O2, and those of the late syn-ore and post-ore stage contain H2 and CH4. The fluid inclusions in quartz of the syn-ore stage include H2O, H2O–CO2, and CO2 types, and they occur in clusters or along trails. Homogenization temperatures (Th) for the H2O–CO2 and two-phase H2O inclusions range from 106°C to >350°C and cluster in two distinct groups for each type; salinities are lower than 10wt% NaCl equiv. The ore-forming fluids underwent CO2 effervescence or phase separation at ∼250°C under a pressure of 1000–1100bar. The U/Th values of the altered granites are lowest close to the ore, increase outwards, but subsequently decrease close to unaltered granites. From the unaltered granites to the ore, the lowest Fe2O3/FeO values become lower and the highest values higher. The REE patterns of the altered granites and the ores are similar to each other. The U contents of the ores show a positive correlation with total REE contents but a negative correlation with LREE/HREE ratios, indicating the pitchblende is REE-bearing and selectively HREE-rich. The δEu values of the ore show a positive correlation with U contents, indicating the early syn-ore fluids were oxidizing. The δCe values show a negative correlation, indicating the later mineralization environment became reducing. The water–rock interactions of the early syn-ore stage resulted in oxidization of altered granites and reduction of the ore-forming fluids, and it was this reduction that led to the uranium mineralization. During alteration in the early syn-ore stage, the oxidizing fluids leached uranium from granites close to faults, and Fe2O3/FeO ratios increased in the alteration zones. The late syn-ore and post-ore alteration decreased the Fe2O3/FeO ratios in the alteration zones. The δ18OW–SMOW values of the ore-forming fluids range from −1.8‰ to 5.4‰, and the δDW–SMOW values range from −104.4‰ to −51.6‰, suggesting meteoric water. The meteoric water underwent at least two stages of water–rock interaction: the first caused the fluids to become uranium-bearing, and the second stage, which was primarily associated with ore-bearing faults, led to uranium deposition as pitchblende, accompanied by CO2 effervescence.

Mineral chemistry and genesis of the Permian Cihai and Cinan magnetite deposits, Beishan, NW China

Cihai and Cinan are Permian magnetite deposits related to mafic-ultramafic intrusions in the Beishan region, Xinjiang, NW China. The Cihai mafic intrusion is dominantly composed of dolerite, gabbro and fine-grained massive magnetite ore, while gabbro, pyrrhotite+pyrite-bearing clinopyroxenite and magnetite ore comprise the major units in Cinan. Clinopyroxene occurs in both deposits as 0.1–2mm in diameter subhedral to anhedral grains in dolerite, gabbro and clinopyroxenite. High FeO contents (11.7–28.9wt%), low SiO2 (43.6–54.3wt%) and Al2O3 contents (0.15–6.08wt%), and low total REE and trace element contents of clinopyroxene in the Cinan clinopyroxenite imply crystallization early, at high pressure. This clinopyroxene is FeO-rich and Si and Ti-poor, consistent with the clinopyroxene component of large-scale Cu-Ni sulfide deposits in the Eastern Tianshan and Panxi ares, as well as Tarim mafic intrusion and basalt, implying the Cinan mafic intrusion and sulfide is related to tectonic activity in the Tarim LIP. The similar mineral chemistry of clinopyroxene, apatite and magnetite in the Cihai and Cinan gabbros (e.g., depleted LREE, negative Zr, Hf, Nb and Ta anomalies in clinopyroxene, lack of Eu anomaly in apatite and similarity of oxygen fugacity as indicated by V in magnetite), indicate similar parental magmatic characteristics. Mineral compositions suggest a crystallization sequence of clinopyroxenite/with a small amount of sulfide – gabbro – magnetite ore in the Cinan deposit, and magnetite ore – gabbro – dolerite in Cihai. The basaltic magma was emplaced at depth, with magnetite segregation (and formation of the Cinan magnetite ores) occurring in relatively low fO2 conditions, after clinopyroxenite and gabbro fractional crystallization. The evolved Fe-rich basaltic magma rapidly rose to intermediate or shallow depths, forming an immiscible Fe-Ti oxide magma as fO2 increased and leaving a Fe-poor residual magma in the chamber. The residual magmas was emplaced at different levels in the crust, forming the Cihai gabbro and dolerite, respectively. Finally, the immiscible Fe-Ti oxide magma was emplaced into the earlier formed dolerite because of late magma pulse uplift, resulting in a distinct boundary between the magnetite ores and dolerite.

Migmatization and low-pressure overprinting metamorphism as record of two pre-Cretaceous tectonic episodes in the Santander Massif of the Andean basement in northern Colombia (NW South America)

The core of the Santander Massif in the northern Andes of Colombia is dominated by migmatitic gneisses with a <1.71Ga protolith and was affected by continuous interactions of oceanic plates to the west and the northwestern corner of the South American continental plate. The exposed metamorphic core of the massif offers a unique opportunity to understand the tectonic evolution of northwestern South America. We present new metamorphic petrology and geochemistry data from the Bucaramanga Gneiss in the Santander Massif to document part of this tectonic evolution from late Proterozoic to Jurassic times.Metapelitic migmatite gneiss, quartz-feldspathic gneiss, and amphibolite from the Bucaramanga Gneiss recorded metamorphic peak conditions in the range of 660–850°C at pressures of >7.5kbar. Lithologies are overprinted by low-pressure metamorphism, related to extensive Jurassic intrusions and linked with growth of cordierite and equilibration of low-pressure mineral assemblages, recorded metamorphic conditions are <750°C and <6.5kbar.Observed leucosomes display significant compositional variations and can be grouped in three groups: i) Group One leucosomes with high total REE content, high LREE/HREE, and negative Eu anomaly, ii) Group Two leucosomes with low total REE, low LREE/HREE, and positive Eu anomalies, and iii) Group Three leucosomes with relatively low LREE/HREE and strong positive Eu anomaly. Geochemical data support the interpretation that Group Two leucosomes crystallized from melts originated in a partial melting event affecting mostly pelitic and quartz-feldspathic lithologies with fluid-present melting reactions. The evaluation of mesosomes (amphibolite, pelitic and quartz-feldspathic rocks) as potential protoliths or restites indicates that at least two pelitic samples of the analyzed lithologies have characteristics consistent with the occurrence of fluid-present melting reactions involving quartz and feldspar. The leucosomes produced by crystallization of modified partial melts contrast with several other leucosomes that were injected; however, in some cases the melts crystallized as injected leucosomes show consistent geochemistry with partial melting of lithologies geochemically similar to the ones observed in the unit.The migmatization and the low pressure metamorphic overprint are related here to two main tectonic events: an early Paleozoic tectonic pulse produced by subduction of the oceanic crust of the Iapetus Ocean beneath northwestern Gondwana, and an Upper Triassic to Lower Jurassic tectonic pulse produced by subduction of oceanic crust of the proto-Pacific ocean beneath western Pangaea.