Weak Positive Eu 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.

Microbially induced fabrics in the Upper Ediacaran coated grain dolostone, Northeast Sichuan Basin, SW China: Implications for reconstruction of the paleoceanic environment

In the Upper Ediacaran Dengying Formation of the Sichuan Basin, the presence of abundant, well-preserved microbial carbonates provides a unique opportunity to study Precambrian paleoceanography and microbial carbonate origins. Particularly, the underexplored coated grain dolostones of this formation, characterized by distinct microbially induced fabrics and intragranular dissolution, offer crucial insights for understanding late Ediacaran microbial mineralization and diagenetic sequences. Utilizing selected typical samples, we conducted detailed petrographic and geochemical analyses, revealing a dynamic interplay between microbial processes and sedimentary dynamics. We observe the coexistence of constructive micrite envelopes with microbially induced fabrics such as clots and laminations, highlighting microbial biomineralization's key role in fabric formation. The sedimentary dynamics critically determines the formation processes of the coated grains: low-energy settings foster grain agglomeration and consolidation through clot precipitation between grains, while high-energy settings favor smaller grains binding to microbial mats. Geochemically, micrite envelopes play an essential role in preserving distinct rare earth element (REE) signatures. The weak negative Ce anomalies and positive Eu anomalies within these envelopes point to a suboxic to anoxic depositional environment, directly indicative of the microenvironmental conditions conducive to microbial mineralization processes. Furthermore, our study sheds light on the structural evolution of coated grains with hollow nuclei, proposing that their internal pore formations are influenced by both mineral instability and selective dissolution by meteoric freshwater. These findings not only provide fresh insights into complex diagenetic processes in the Dengying Formation but also substantially advance our understanding of early microbial life and environmental adaptations during the Precambrian.

Unravelling the genesis and depositional setting of Neoproterozoic banded iron formation from central Eastern Desert, Egypt

The Neoproterozoic banded iron formations (BIFs) are widely occurred in the Egyptian Eastern Desert. This study integrates field observations, petrographic studies, geochemical data, and lead isotopes to construct the genesis and depositional environment of Wadi El-Mis hama BIF deposits. The iron layers, primarily of oxide facies within a volcano-sedimentary sequence, comprise magnetite-rich beds alternating with jaspilite or silicate laminae. The studied BIFs exhibit a dominant composition of SiO2 and Fe2O3t with relatively low contents of TiO2 and Al2O3. The positive correlation of REEs (La, Sm, Yb) with Zr and low concentrations of HFSEs (Ta, Nb, Th, Hf) indicate a primary formation mechanism of chemical precipitation, maintaining original geochemical signatures. Geochemical patterns show depletion in LREEs, enrichment in HREEs (La/YbPAAS = 0.08–0.12), and positive La anomalies (La/LaPAAS = 1.15–8.57), consistent with seawater influence. Additionally, various geochemical discrimination diagrams supported by elevated super-chondritic Y/Ho values (29.6–38.7), weak positive Eu anomalies, and low contents of transition metals (Cu and Zn), point to the interaction of low-temperature (<200°C) hydrothermal fluids (bearing Fe and Si) with seawater during the deposition of the BIFs. The lack of significant negative Ce anomalies along with low Ni/Co, U/Th, and Cu/Zn ratios, imply that the iron mineralization was precipitated from dysoxic to oxic conditions. The geochemical and Pb isotopic data suggest that the iron deposits formed in an extensional geodynamic setting (intra-oceanic arc basin environment) due to the subduction of the Mozambique Plate, with signatures closely matching other Precambrian Algoma-type BIFs.

Geochemistry and U-Pb zircon geochronology of the Nkout banded iron formations and associated metasiliciclastic rocks, NW Congo craton, southern Cameroon

ABSTRACT The Nkout banded iron formations (BIFs)-hosted iron deposit is located in the Ntem Complex at the northwestern part of the Congo craton, southern Cameroon. We report geochronological and geochemical data of magnetite BIFs and interlayered mica schists from the Nkout iron ore deposit. The BIFs show typical features of chemical precipitates characterized by seawater-like shale-normalized rare earth elements plus yttrium spectra. Weak positive Eu anomalies indicate the contribution of high-temperature hydrothermal fluids to the BIFs, with low detrital input. The absence of true Ce anomalies indicates that the BIFs were likely to be deposited in anoxic seawater. LA-ICP-MS U-Pb detrital zircon data for the BIFs and associated metasiliciclasltic rocks constrained the maximum depositional age at ca. 2.8 Ga, suggesting that the Nkout BIFs are likely the oldest iron formations in the Ntem Complex. Our zircon U-Pb isotope data indicate that the Nkout succession experienced lower-amphibolite facies metamorphism at ca. 2.0 Ga, which correlates with the regional tectono-thermal event linked to the convergence and assembly of terranes between the Congo and São Francisco cratons during the Eburnean-Transamazonian orogeny.

Zircon U-Pb chronology and geochemistry of porphyritic granites: A case study of the Dalancun granites in Zhaoyuan, Shandong Province

The granite in Dalancun, Zhaoyuan, Shandong Province is mainly porphyry granite. In this paper, detailed field geology, petrology, isotopic chronology and whole-rock principal and microgeochemistry have been studied. LA-ICP-MS zircon U-Pb chronology shows that it was formed in the Early Cretaceous (129.7 ± 0.8 Ma), later than Linglong granite (160-150 Ma) and earlier than Laoshan granite (120-114 Ma). It is consistent with the main ore-forming age of Jiaodong large gold deposit and the diagenetic age of Guojialing granite (130-126 Ma). K2O+Na2O=7.41%~9.37%, K2O/Na2O=0.78~1.24; A/CNK=0.89~1.06, MgO=0.63%~1.01%, with high K, Al characteristics, belongs to the quasi-aluminum-peralumin high potassium calc-alkaline series. The strong differentiation of rare earth elements (La/Yb) N=68.24~93.96, weak positive Eu anomaly (δEu=1.26~1.769), enrichment of large ion lithophile elements Ba, Sr, high field strength elements Nb, Ta, Ti deficit, indicating the geochemical characteristics of continental lithosphere. In addition, high Sr (1817×10-6~2017×10-6), low Y (7.55×10-6~9.36×10-6), Sr/Y ratio between 199.89~262.29, Nb/Ta ratio between 14.67~16.73, consistent with continental crust ratio, It shows the characteristics of type I granite and Adakite. Combined with other geochemical indexes and regional geological studies, this paper suggests that the Dellancun porphyritic granite belongs to crust-mantle granite, which may have been formed in the arc environment of continental margin during the transition from compressive to extensional tectonic system in Jiaodong area, and is closely related to the subduction of the paleo-Pacific plate. This study not only helps to understand the Mesozoic tectonic evolution in eastern China, but also provides a basis for revealing the formation mechanism of Jiaodong gold deposit.

Temporal evolution of the Pobeda hydrothermal site (MAR): Utility of proximal sediment cores

Temporal evolution of hydrothermal processes used to be reconstructed based on chemistry and chronology of the sediments around vent fields. However particle-by-particle deposition of metalliferous sediments proximal to sulfide mounds is usually superimposed by mass-wasted events. Quantitative assessment of the relative amount of slumped sulfide debris and plume fallout matter in metalliferous sediments has been done for ∼83 kyr on Pobeda hydrothermal site at the 17°08′ N, Mid Atlantic Ridge.Three sediment cores have been investigated for uranium series isotopes and rare earth elements (REE). Their distribution patterns with an enrichment in heavy REE, a weak positive Eu, and negative Ce anomalies give evidence to the contribution of slumped sulfides from nearby edifices. The 230ThU radiochronology dataset for distal and proximal metalliferous sediments and sulfide deposits from different Atlantic and Pacific hydrothermal sites allowed for the distinction between sediments containing oxidized sulfide debris and those derived predominantly from plume fallout oxyhydroxide particles in terms of 230Th/Fe vs. U/Fe ratios. The calculated amount of plume particulates and the sedimentation rate for each core allowed accurate temporal reconstruction of the hydrothermal activity within the Pobeda site through a novel approach.

Paleoceanographic reconstruction of the Ediacaran Dengying Formation, Sichuan Basin, Southwest China: Implications for the origin of Precambrian microbial carbonates

Abundant microbial carbonates with well-preserved primary textures occur in the upper Ediacaran Dengying Formation in the Sichuan Basin, southwest China, providing an opportunity to study the paleoceanographic conditions and the origin of Precambrian microbial carbonates. However, paleoenvironmental studies of these microbial carbonates are relatively rare, especially with respect to the comparison between the upper and lower parts of the Dengying Formation. In this paper, we present new analyses of carbon and oxygen isotopes and major, trace, and rare earth elements (REEs) for carbonate rocks from the Dengying Formation, in order to reveal the paleoceanographic redox and salinity conditions of the seawater from which these rocks were deposited. These rocks have shale-normalized REE and Y patterns that display weak negative Ce anomalies, weak positive Eu anomalies, and moderate middle REE enrichment, indicating the suboxic seawater for their deposition. From the lower to upper parts of the Dengying Formation, negative Ce anomalies become more prominent, while positive Eu anomalies and middle REE enrichment weaken, suggesting a gradual increase in the seawater oxygen concentration. The heavier oxygen isotopes of the carbonate matrixes and fibrous cements in the second member of the lower part of the Dengying Formation reflect a higher seawater salinity, conforming to the thick evaporates in the second member. The elevated oxygen content and decreased salinity in the seawater promoted the emergence of metazoans that inhibited the growth of microbes, causing the decreasing development of microbial carbonates in the upper part of the Dengying Formation compared to the lower part.

REE and Trace Elements Distribution in the Deep-sea Sediments from the Interoceanmetal (IOM) Polymetallic Nodule Exploration Area in the Clarion-Clipperton Fractures Zone, NE Pacific

Geochemical features of the deep-sea sediments from a high polymetallic nodule area in the eastern part of the Clarion-Clipperton Fractures Zone (CCZ), NE Pacific were studied. Box-corer samples from six stations in individual depth layers 0–3, 3–5, 5–10, 10–20 cm were collected from 4300–4500 m depth. The deep-sea sediments were classified as clayey-siliceous oozes according to the mineral and grain-size analyses. The chemical composition of the studied samples is compatible with that of pelagic sediments. Manganese content varies from 0.16% to 0.70% being the highest in the geochemically active layer (top 7–12 cm) and decreases with depth. The Mn/Fe ratio, Ba, Co, Ni and Cu have the highest values in the first (0–3) and second (3–5) layers and decrease with depth. REE range is 195.84–357.79 ppm showing low variation between the layers. PAAS-normalized REE patterns show significant enrichment of MREE and HREE with strong negative Ce anomaly and weak positive Eu and Y anomalies. The geochemical characteristics of the studied sediments infer polygenic origin. The redox sensitive oxides and barite presense, Eh values and negative Ce anomaly, indicate oxidizing environment of sediment formation.

Origin of the ca. 3.1 Ga Luanjiajie rock assemblage in the northeastern margin of the North China Craton: New constraints on the Mesoarchean geodynamic regime

The North China Craton (NCC) is one of the oldest cratons on Earth and preserves abundant Mesoarchean igneous rocks. However, the petrogenesis of the Mesoarchean magmatism and the associated geodynamic regime in the NCC remain controversial. In this paper, we conducted a systematic study to constrain the petrogenesis of the recently discovered Mesoarchean Luanjiajie rock assemblage (LRA) and infer the regional Mesoarchean tectonic setting. The LRA is composed mainly of tonalite–trondhjemite–granodiorite (TTG) gneiss with minor dioritic gneiss and amphibolite. The amphibolite shows intrusive contact with the surrounding TTG gneiss, whereas the dioritic gneiss displays transitional boundary with the TTG gneiss. Zircon U-Pb dating revealed that these three lithologies were roughly contemporaneous, with crystallization and metamorphic ages of ca. 3.1 and ca. 2.5 Ga, respectively. Geochemical characteristics (e.g., high Sr/Y and LaN/YbN ratios and weak positive Eu anomalies), zircon Hf isotopic features (TDM2 ages of 3822 to 3531 Ma with εHf(t) values of −2.44 to +0.97), and slightly elevated δ18O values (+5.48‰ to +7.24‰) indicate that the ca. 3.1 Ga TTG was derived from the partial melting of ancient hydrated low-K mafic rocks. The Luanjiajie TTG gneiss and other reported Mesoarchean TTGs in the NCC are classified mainly as medium- to low-pressure types. The linear trends between SiO2 content and Fe2O3T, MgO, Y, Yb, Sm, and Gd contents of the ca. 3.1 Ga dioritic gneiss reflect that the dioritic gneiss might be a product related to amphibole fractionation of the coeval TTG magma. The ca. 3.1 Ga amphibolite displays OIB-affinity geochemistry and positive εHf(t) (up to +4.65) and εNd(t) (+0.2 to +7.7) values. The amphibolite was most likely formed through the partial melting of a depleted garnet-peridotite mantle and underwent minor crustal contamination during magma ascent. Hence, the LRA was most likely formed under a plume-related tectonic setting involving basaltic magma underplating and recycling of ancient crust. Combining our new results with existing zircon Hf data, we suggest that the NCC experienced initial continental nuclei formation during the Eo- to Paleoarchean, and followed by episodic crustal growth and recycling during the Meso- to Neoarchean.

Rare earth element geochemistry in soils along arid and semiarid grasslands in northern China

BackgroundRare earth elements (REE) are a group of trace elements that behave geochemically coherently. REE fractionation patterns normalized to reference materials provide a powerful tool for documenting pedogenesis. In-soil processes are particularly difficult to illustrate with respect to contemporary and past climate conditions. In this study, we characterize the rare earth element (REE) contents in bulk soils and respective geochemical fractions (e.g., exchangeable, carbonate-bound, reducible, and oxidizable fractions) and to decipher the relationships between REE geochemistry components and climatic factors across a large-scale northern China transect (NCT).ResultsAcross the NCT, bulk REE concentrations ranged from 55.2 to 241.1 μg g−1 with a main portion in the residual fraction (49–79%), followed by oxidizable fraction (2–40%), reducible fraction (3–22%), carbonate-bound fraction (0.1–16%), and negligible exchangeable fraction. The REE contents of geochemical components (carbonate-bound, reducible, and oxidizable) in topsoils correlated to climate factors (mean annual precipitation, mean annual temperature, potential evaporation, and aridity index (AI)). The normalized abundances to the upper continental crust (UCC) composition show that the middle REE was generally enriched than the light REE and heavy REE in topsoils along the transect. The overall UCC-normalized bulk REE patterns in topsoils and subsoils were similar, characterized by weak negative Ce anomalies and positive Eu anomalies.ConclusionsOur data in topsoils and depth profiles collectively suggest that cycling of REE was primarily regulated by abiotic processes in area with AI < 0.2, while the biological effect on REE circulation in soil played a more effective role in area with AI > 0.3. The similar UCC normalized patterns in topsoils suggest that the REE was originated from a common source with limited influences from other sources (e.g., atmospheric dusts and anthropogenic contributions). Our results to some extent provide evidence for climatic influence REE distribution patterns both in topsoils and subsoils across the continental-scale transect. Our investigation gives insights into future studies on vertical REE mobility and its associated biogeochemical pathways.

New ages of Early Cretaceous magmatic rocks in the Yanbian area (NE China): implications for the subduction and slab rollback of the Paleo-Pacific Plate beneath eastern China during Early Cretaceous

ABSTRACT The precise age of slab rollback of the Paleo-Pacific Plate during Early Cretaceous has been contentious. In this study, we have conducted LA-ICP-MS zircon U-Pb dating, major and trace elements, and Sr-Nd isotopes analyses on the late Early Cretaceous diabase porphyries, andesitic basalts and basanites in the Yanbian area (NE China). Zircon U-Pb dating of basanites, andesitic basalts, and diabase porphyries yielded weighted average ages of 104 ± 1.9 Ma (MSWD = 0.55, n = 5), 106 ± 1.6 Ma (MSWD = 0.68, n = 6), and 111 ± 2 Ma (MSWD = 16, n = 3), respectively. The whole-rock geochemical results indicate that the late Early Cretaceous rocks predominantly belong to the low-K tholeiitic and medium-K calc-alkaline series. The diabase porphyries and andesitic basalts are characterized by large ion lithophile elements (e.g. Ba, K, and Sr) enrichment and high field strength elements (e.g. Nb and Zr) depletion with weak positive Eu anomalies, low initial 87Sr/86Sr ratios (0.70361–0.70394) and high ε Nd (t) values (5.96–7.87). The tectonic discrimination diagrams show that the diabase porphyries and andesitic basalts formed in the continental Island arc environment related to subduction of the Paleo-Pacific Plate in the late Early Cretaceous. In contrast, the basanites are enriched in Ba, Th, and U and depleted in K, Sr, and Ti, with weak negative Eu anomalies. The basanites display high magnesium values (Mg# = 0.79), which are approaching the primary magma. They have low Ti values (TiO2 = 1.45%) and Al values (A12O3 = 9.98–10.15%) and high Co values (78.1–80.2 ppm) and Ni values (549.5–585.5 ppm). The basanites formed in the intraplate extensional tectonic setting and were the products of lithospheric thinning and mantle upwelling during the late Early Cretaceous. The spatial-temporal relationship of late Early Cretaceous magmatic rocks in NE China suggests a slab rollback from high-angle subduction to low-angle subduction of the Paleo-Pacific Plate at ca. 106 Ma.

Neoarchean–early Paleoproterozoic crustal evolution in the Jiapigou terrane in the northeastern part of the North China Craton: Geochemistry, zircon U–Pb dating and Hf isotope constraints from the potassic granitic complex

Neoarchean granitic rocks are important components of Precambrian cratons, and their petrogenesis can provide constraints for understanding the evolution of continental crust. The Neoarchean potassic granites are widely distributed in southern Jilin Province, which is located in the northeastern part of the North China Craton. Newly obtained field geology observations and petrological, geochemical and geochronological data reveal that the Jiapigou potassic granitic complex is mainly composed of biotite granitic gneiss and medium- to coarse-grained granite. All the granitic gneisses and granites are high-K calcium–alkaline to shoshonite, metaluminous to peraluminous (A/CNK = 0.95–1.13; molar Al2O3/(CaO + Na2O + K2O)), enriched in LILEs and LREEs with strongly fractionated REE patterns ((La/Yb)N = 21–179), and depleted in Nb, Ta, Ti and P. The biotite granitic gneisses have low Rb/Sr ratios (∼0.06) and weak positive Eu anomalies (δEu = 1.26–1.51), while the granites have low Th, U and REEs and strong positive Eu anomalies (δEu = 7.27–16.92). The zircons of these granitic rocks generally have core–rim structures based on cathodoluminescence images and show inherited ages of 2729 ± 10 Ma, crystallization ages of 2523 ± 11 Ma to 2526 ± 15 Ma, and crystallization/metamorphism ages of 2480 ± 15 Ma to 2485 ± 9 Ma. The zircon Hf isotope results of the medium-grained monzogranite show that the εHf(t2) values vary from –2.2 to +5.3, and the two-stage model ages (TDM2) are 2.9–3.0 Ga. The magmatic zircons of the early Neoarchean (∼2.73 Ga) and the late Neoarchean (∼2.52 Ga) crystallized in high-temperature magmas (738–890 °C), whereas those of the early Paleoproterozoic (∼2.48 Ga) crystallized in relatively low-temperature (646–702 °C) magmas. These results indicate that the Jiapigou granitic rocks were products of intracrustal recycling during the late Neoarchean and the early Paleoproterozoic. The protoliths of the Jiapigou granitic gneisses were derived from the remelting of the early Neoarchean juvenile crustal rocks in the lower crust during the late Neoarchean. Subsequently, these granitic rocks underwent amphibolite-facies metamorphism and water-fluxed melting in the early Paleoproterozoic, resulting in the association of migmatic granitic gneisses and potassic granites at the shallow crust level. The cratonization in the Jiapigou terrane involved early Neoarchean (∼2.73 Ga) juvenile crust growth and late Neoarchean (∼2.53–2.52 Ga) to the early Paleoproterozoic (∼2.48 Ga) intracontinental reworking.

Tectonic evolution of the Paleoarchean to Mesoarchean Badampahar-Gorumahisani belt, Singhbhum craton, India – Implications for coexisting arc and plume signatures in a granite-greenstone terrain

An association of tonalite-trondhjemite-granodiorite (TTG), other granitoids and greenstones constitute the Paleoarchean-Mesoarchean Singhbhum craton in India. The deformed and metamorphosed greenstone succession of Badampahar-Gorumahisani belt (BGB) in eastern Singhbhum consists of mafic–ultramafic volcanics, intercalated chemical sediments in the lower part and clastic sediments in the upper part. Geology, structure and geochemistry of BGB mafic volcanics, contemporaneous TTG and slightly younger intrusive granite have been integrated to constrain the tectonic setting of Paleoarchean felsic crust generation and coupled deformation of granite-greenstone. Subalkaline, low-K BGB metabasalts show distinct tholeiitic trend of differentiation from komatiitic basalt parent magma. BGB metabasalts with LILE enrichment, negative Nb anomaly, and Ti/V ratio in the range 10–20, are comparable to island arc tholeiites, though the associated komatiites and high-Mg basalts suggest plume related origin. Limited crustal contamination of a komatiitic basalt magma cannot be ruled out altogether. Singhbhum granite batholith with older tonalite-trondhjemite and younger granite-granodiorite plutons were derived from low- to high-K mafic source, together with some TTG reworking for younger granites. With low HREE, higher primordial mantle normalized La/Yb ratio, insignificant Eu anomaly, and coupled depletion of Y and enrichment of Sr, older strongly deformed plutons compare well with the Archean TTG suite. The younger intrusive granites show weak positive Eu anomaly (Eu/Eu* ~ 1.3–1.7) and strongly positive Y anomaly. Geochemical discrimination (e.g. Rb vs Y + Nb) suggests comparison with volcanic arc granites, which is also supported by enrichment of LILE (K, Rb and Ba) and LREE, and relative depletion of HREE and HFSE. However, structural geometry of BGB and adjoining granitoid plutons, association with komatiite, absence of typical signatures of derivation from subduction wedge, and heterogeneity of source for the BGB mafic volcanics, suggest Paleoarchean crustal accretion by mantle plume activity as well as local shallow subduction or subcretion under a transitional tectonic regime.

Contrasting Depositional Environment of Iron Formation at Endengue Area, NW Congo Craton, Southern Cameroon: New Insights from Trace and Rare Earth Elements Geochemistry

The Endengue Banded Iron Formation (BIF) is located in the northwestern edge of the Congo craton in Cameroon. Here, we report geochemical data of trace and rare earth elements (REE) of the Endengue BIF samples from the Archean Ntem complex and investigate their environmental setting. Two types of BIF occur at Endengue area, both containing minimal contamination from terrestrial material. Total REE (ΣREE) contents in the Type 1 BIF are extremely low, ranging from 0.34 to 1.83 ppm, similarly to pure chemical sediments while Type 2 BIF displays ΣREE contents ranging from 2.98 to 24.26 ppm. The PAAS-normalized REE + Y patterns of the two BIF types display LREE enrichment relative to HREE and weak positive Eu anomaly, most likely suggesting that the source of iron and siliceous of the Endengue BIFs is mainly from the contribution of low-temperature hydrothermal alteration of the crust. Type 1 BIF shows very low Nd content (* and positive Ce anomalies which suggests suboxic or anoxic seawater similar to the depositional environment of Elom BIF in Archean Ntem complex. In contrast, Type 2 BIF displays low to moderate Nd contents (1 and 100 ppm with the exception of sample LBR65) with negative correlation between Nd and Ce/Ce* and negative Ce anomalies. These features indicate precipatation of Type 2 BIF from oxic iron-rich solution that changed to oxidized surface by rapid precipitation of the hydrothermal Fe. The Endengue BIFs were deposited in the continental margin ocean in presence of low-temperature hydrothermal fluids mixed with seawater, similar to Paleoproterozoic Kpwa-Atog Boga BIFs within the Nyong group and other Paleoproterozoic Superior-type BIFs worldwide.

Garnet Geochemistry of Reduced Skarn System: Implications for Fluid Evolution and Skarn Formation of the Zhuxiling W (Mo) Deposit, China

A newly discovered tungsten ore district containing more than 300,000 tons of WO3 in southern Anhui Province has attracted great attention. The Zhuxiling W (Mo) deposit in the district is dominated by skarn tungsten mineralization. This paper conducted in suit EPMA and LA-ICPMS spot and mapping analysis of the skarn mineral garnet to reveal the evolution of fluids, metasomatic dynamics, and formation conditions of skarn. Two generations of garnet have been identified for Zhuxiling W (Mo) skarn: 1) Gt-I generation garnet is isotropic, Al-rich grossular without zoning. As a further subdivision, Gt-IB garnet (Adr19-46Grs49-77 (Sps+Pyr+Alm)4-5) contains significantly high content of Ti and Mn compared with Gt-IA garnet (Adr3-42Grs53-96 (Sps+Pyr+Alm)1-5). 2) Gt-II generation garnet is anisotropic, Fe-rich andradite with oscillatory zoning. Gt-II garnet displays compositional changes with a decrease of Fe and an increase of Mn from proximal skarn (Gt-IIA) to distal skarn (Gt-IIB) with the presence of subcalcic garnet for Gt-IIB type (Sps+Pyr+Alm = 56–68). The presence of pyrrhotite associated with subcalcic garnet indicates a relatively reduced skarn system. Gt-I grossular is overall enriched in Cr, Zr, Y, Nb, and Ta compared with the Gt-II andradite, and both W and Sn strongly favor Fe-rich garnet compared with Al-rich garnet. Gt-IA grossular garnet presents a REE trend with an upward-facing parabola peaking at Pr and Nd in contrast to low and flat HREE, and Gt-IB grossular garnet has a distinct REE pattern with enriched HREE. Gt-IIA andradite garnet displays a right-dipping REE pattern (enriched LREE and depleted HREE) with a prominent positive Eu anomaly (Eu/Eu* = 3.6–15.3). In contrast, Gt-IIB andradite garnet shows depleted LREE and enriched HREE with a weak positive Eu anomaly (Eu/Eu* = 0–6.0). The incorporation and fractionation of REE in garnet are collectively controlled by crystal chemistry and extrinsic factors, such as P–T–X conditions of fluids, fluid/rock ratios, and mineral growth kinetics. Major and trace elements of two generations of garnet combined with optical and textural characteristics suggest that Gt-I Al-rich grossular garnets grow slowly through diffusive metasomatism under a closed system, whereas Gt-II Fe-rich andradite represent rapid growth garnet formed by the infiltration metasomatism of magmatic fluids in an open system. The Mn-rich garnet implies active fluid–rock interaction with Mn-rich dolomitic limestone of the Lantian Group in the district.

Provenance, palaeoweathering and depositional environment of the cretaceous deposits from the Babouri-Figuil and Mayo Oulo-Lere basins (North-Cameroon) during the Southern Atlantic opening: Geochemical constraints

The Babouri-Figuil and Mayo Oulo-Lere basins belong to the intra-continental basins which are linked to the opening of Southern Atlantic Ocean. The sediments from these Basins were investigated to infer their provenance and conditions of sedimentation, using major and trace elements geochemistry of the fine facies. The main criteria for samples selection was color and hence twelve representative samples of each color type including black, dark grey, grey, dark reddish green, greenish and brown were selected from each basin. The geochemical compositions have revealed that the studied materials are composed of Fe-shales and Fe-sands. These sediments are characterized by (respectively for the Babouri-Figuil and Mayo Oulo Lere basins): high LREE/HREE ratios (5.11–14.90 and 9.21 to 18.32); weak negative and positive Eu anomaly (Eu/Eu* = 0.82 to 1.32 and 0.93 to 1.38); Al2O3/TiO2 ratios (11.92–31.33 and 16.95 to 35.83); Th/Co ratios (0.35–1.07 and 0.40 to 0.77). These features, together with the Ce vs La/Yb, Zr vs TiO2 and La/Sc vs Th/Co plots, indicate that the sediments are derived from intermediate to felsic rocks with subordinate mafic rocks. The K2O/Na2O ratios (<1), CIA (Chemical Index of Alteration) and PIA (Plagioclase Index of Alteration) (<60) for the two basins suggest that the source rocks have experienced a minor recycling and sorting as well as little weathering. The CIA values and the SiO2 vs Al2O3+K2O + Na2O diagram indicate a cool and arid to semi-arid climate during the deposition. The Sr/Ba values ranging from 0.55 to 6.41, suggest a fluctuating and sometimes high salinity (hypersaline lake) in a lacustrine or swampy environment, and the Ni/Co (<5) and U/Th (0.75) ratio are consistent with oxic conditions. The discriminant function-based multidimensional tectonic diagrams, associated to the ternary diagram (SiO2/20 - K2O + Na2O–Fe2O3 + MgO + TiO2) indicate an arc-collisional related setting, suggesting the origin of the studied materials from Pan-African basement rocks of the Central African Fold Belt (CAFB). This basement was affected by subsidence during the lower Cretaceous, probably the Neocomian, giving the relief that is submitted to the erosion leading to the immature sediments. The present findings reveal a high similarity in geochemical features between the Babouri-Figuil and Mayo Oulo-Lere basins, suggesting that the two basins share the same tectono-sedimentary evolution during the lower Cretaceous.

Neoproterozoic non-glaciogenic iron formation: Insights from Fe isotope and elemental geochemistry of the Shalong iron formation from the Central Tianshan block, southern Altaids

The return of widespread banded iron formation (BIF) deposition during the Neoproterozoic Era after a more than one-billion-year hiatus appears to be a hallmark of the Neoproterozoic, and may be indicative of critical changes in the Earth’s environmental and tectonic conditions. However, consensus on the Fe sources and origin of Neoproterozoic iron formations (NIFs) remains elusive. Here we present new petrographic and geochemical data from the newly discovered Shalong NIF from the Central Tianshan block in order to constrain the iron source and elucidate the origin and depositional conditions of the NIF. The major and trace element composition of the Shalong NIF indicates that it is a relatively pure chemical sediment with a low detrital input. Rare earth elements and Y patterns (REY) exhibit seawater-like signature such as depletion in light REEs (LREEs) and enrichment in heavy REEs (HREEs). The weak positive Eu anomalies (Eu/Eu∗ = 1.11–1.31) and consistent chondritic Y/Ho ratios of 25 to 29 (average 26) suggest a predominant influx of low-temperature hydrothermal fluids. The geochemistry of the Shalong NIF also supports a predominant hydrothermal iron source characterized by low-temperature hydrothermal fluids. The lack of negative Ce anomalies, as well as weak positive Eu anomalies, and positive and variable δ56Fe values (−0.09‰ to +1.28‰) in the Shalong NIF indicates partial Fe2+ oxidation in a relatively large dissolved iron marine basin under oxygen-limited conditions. The Shalong NIF deposition likely took place on the slope of an extensional oceanic basin in a low energy environment. The Shalong NIF has a close association with metamorphized- bimodal volcanic rocks and predominant hydrothermal iron source. Combined with no sign for any glacial influence and depositional age prior to the global Sturtian glaciations, we suggest that the Shalong NIF is a non-glaciogenic Neoproterozoic iron formation in association with volcanic and hydrothermal activity. Our results, in conjunction with previous studies, suggest that glacial events are not essential or necessary for the origin of all the NIFs. Instead, hydrothermal activity seems to be a more important controlling factor for the build-up of ferruginous basins and subsequent iron oxidation events in the Neoproterozoic leading to the sudden recurrence of NIFs.

Rare earth elements in dolostones and limestones from the Mesoproterozoic Gaoyuzhuang Formation, North China: Implications for penecontemporaneous dolomitization

The Mesoproterozoic Gaoyuzhuang Formation (1.54–1.6 Ga), which is up to 1543 m thick, is a dolostone-dominated succession that originated in the Yanshan-liaoxi rift zone. In ascending order, this formation comprises Members 1 to 4. Member 1 is formed of clastic rocks and dolostones, whereas Members 2 and 4 are formed largely of dolostones. Member 3, which is divided in ascending order into three units (Members 3-L, 3-M, 3-T), is a succession of intercalated limestone and dolostone that have total rare earth elements (ΣREE) ranging from 0.5 to 116 ppm and variable Post-Archean Australian Shale (PAAS) normalized profiles. Carbonates from Member 3-M have REE profiles characterized by positive La and Y anomalies, a negative Ce anomaly, and a weak positive Eu anomaly. Relative to Member 3-M, carbonates from Member 2 and Member 3-L have flat REE profiles characterized by enriched light REEs, negative La and Ce anomalies, a positive Eu anomaly, and a lower Y/Ho ratio. Carbonates from Member 3-T and Member 4, relative to those from Member 3-M, are slightly more depleted in light REEs, lack a La anomaly, are enriched in Eu, have less depletion in Ce, and have similar Y/Ho ratios. Correlations between the dolomite content of samples and their [Pr/Sm]PAAS, [La/La*]PAAS, [Ce/Ce*]PAAS, and [Gd/Gd*]PAAS ratios indicate that dolomite-forming processes were probably associated with REE fractionation that reflects mixing between seawater and freshwater. Therefore, the REEs in Mesoproterozoic dolostones have the potential of being used as proxies for constraining the diagenetic environment under which the dolomites formed.

Geochemical, Sr–Nd–Pb and zircon U–Pb–Hf isotopic constraints on the Late Carboniferous back-arc basin basalts from the Chengjisihanshan Formation in West Junggar, NW China

AbstractWest Junggar in the southwestern Central Asian Orogenic Belt is a critical area for the study of the Junggar oceanic basin and may also reveal tectonic evolutionary events before the final closure of the Palaeo-Asian Ocean. The sedimentary formations and paragenetic associations of the Upper Carboniferous Chengjisihanshan Formation in southern West Junggar jointly reveal a back-arc basin setting with zircon U–Pb ages of 313–310 Ma for the basaltic rocks. Geochemically, the basaltic rocks are tholeiitic with low SiO2 (47.76–52.06 wt %) and K2O (0.05–0.74 wt %) but high MgO (6.55–7.68 wt %) contents and Mg no. (52.9–58.9) values. They display slightly flat rare earth element patterns with weak positive Eu anomalies, and show enrichments in large ion lithophile elements relative to high field strength elements with negative Nb and Ta anomalies, exhibiting both N-MORB-like and arc-like signatures, similar to the back-arc basin basalt from the Mariana Trough. The high positive zircon εHf(t) and bulk εNd(t) values as well as high initial Pb isotopes, together with relatively high Sm/Yb and slightly low Th/Ta ratios imply a depleted spinel lherzolitic mantle source metasomatized by slab-derived fluids. The field and geochemical data jointly suggest that the volcanic rocks within the Chengjisihanshan Formation were formed in an intra-oceanic back-arc basin above the northwestward subduction of the Junggar oceanic lithosphere in southern West Junggar. The confirmation of the Late Carboniferous back-arc basin basalts, together with other geological observations, indicate that an arc-basin evolutionary system still existed in southern West Junggar at c. 310 Ma, and the Junggar Ocean closed after Late Carboniferous time.

Geology and geochemistry of Gunung Subang gold deposit, Tanggeung, Cianjur, West Java, Indonesia

The Gunung Subang gold deposit is located in the Sunda arc volcanic belt in West Java, Indonesia. The ore bodies are associated with volcanic rocks from the Miocene Koleberes and Bentang Formation. To constrain the significance of the ore genesis and mineralization, this study was carried out with the aim of examining the deposit using methods such as detailed field observation, petrography, petrochemistry, zircon U-Pb dating, Hf isotope, fluid inclusion analysis, and sulfur and in-situ Pb isotope analyses. The results suggest the following. (1) The volcanic rocks in Gunung Subang belong to the basalt-andesite group, with 48.66–79.17% (average 61.04%) SiO2. The trace element patterns of the volcanic rocks exhibit relatively low HREE content, flat HREE patterns, and weak negative or positive Eu anomalies, which are similar to those of typical arc magmatic rocks. (2) Most zircons selected from the volcanic rocks are magmatic, and only a few are inherited. The magmatic zircons have a weighted mean U-Pb age of 17.0 ± 0.4 Ma, and the inherited zircons are from the Precambrian, dated at 887–2379 Ma. The magmatic zircons have εHf(t) values ranging from −1.24 to 13.59, indicating that the magmatism may be contaminated, predominantly with juvenile crust but also with a small percentage of ancient crust. (3) The sulfur isotopic compositions are fairly uniform, and the δ34S values of pyrite, galena, and sphalerite range from 0.53 to 2.97‰, suggesting a magmatic origin. (4) The Pb isotopic compositions of pyrite and galena are clustered, and some include relatively high amounts of radiogenic lead. The mean ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb are 18.59 ± 0.007, 15.65 ± 0.007 and 38.89 ± 0.017, respectively, indicating that the ore-forming material is highly related to Miocene to Pliocene volcanic rocks. (5) Fluid inclusions in the quartz from the ores are liquid and vapor-liquid phases, with components of the NaCl-H2O system with homogenization temperatures of 240–320 °C and salinities of 0.2–6.2 wt% NaCl. The coexistence of vapor and liquid-rich fluid inclusions suggests that boiling occurred throughout the deposition of the ore. (6) The results of Electron Probe Microanalysis (EPMA) indicate that pyrite is the major Au-bearing mineral. Combined with the observation of scanning electron microscope (SEM), gold is primarily present as Au-Ag-telluride minerals, and there may also be native gold (Au0). Finally, we consider the Gunung Subang is an epithermal gold deposit related to the Mid-Miocene magmatism.