Light Enrichment Research Articles

Distribution, speciation and mobility of metals in sediments of the Tianxiu hydrothermal field, Carlsberg Ridge, Northwest Indian Ocean

Tianxiu is the only ultramafic-hosted hydrothermal field that has been discovered on the slow-spreading Carlsberg Ridge (CR), northwest Indian Ocean. Sequential extraction has been performed on the surface sediments from the near-vent and far-field of Tianxiu to characterize the compositions of metalliferous sediments, the speciation of selected metals and the potential risks of metal mobility. The near-vent sediments of Tianxiu are mainly derived from hydrothermal inputs and are enriched in Fe, Zn, P, Pb, Mo and especially Cu, Mg, Co and Ni with significantly higher contents compared to metalliferous sediments formed in basalt-hosted hydrothermal systems. Most of the ore-forming elements (Fe, Cu, Zn, Mn, Co and Ni) and rare-earth elements (REEs) exist in the residual fraction and Fe Mn oxide fraction, followed by the carbonate fraction and organic fraction. The chondrite-normalized REE pattern in each phase is characterized by relative enrichment in light (L)REEs and positive Eu anomalies, consistent with the REE patterns of hydrothermal fluids. The far-field sediments of Tianxiu are mainly derived from background pelagic sedimentation while the sediments at some sites are influenced by a small amount of hydrothermal plume fallout. The investigated elements mainly exist in the Fe Mn oxide fraction, residual fraction and carbonate fraction. In particular, ore-forming elements Cu, Pb, Mn, Co and Ni are mostly enriched in the Fe Mn oxide fraction, which is attributed to their adsorption by hydrothermal particulates in the plumes. The potential mobility of each metal is evaluated in terms of its total content in the unstable phases, which reveals that most of the heavy metals (e.g., Cu, Zn, Mn and Pb) in the metalliferous sediments have high potential mobility (> 50%). • Heavy metals are particularly enriched in the near-vent sediments. • Heavy metals are mainly present in the residual fraction and Fe Mn oxide fraction. • Most of the heavy metals in the near-vent sediments have high potential mobility.

Late Neoarchean crust–mantle interaction and tectonic implications in western Shandong Province, North China Craton: Evidence from a granodiorite pluton and associated magmatic enclaves

This study presents original data on the microstructure, mineralogy, whole–rock geochemistry, and isotopes of a Late Neoarchean granodiorite pluton and associated mafic microgranular enclaves (MMEs) in western Shandong Province of the eastern part of the North China Craton (NCC). The MMEs hosted in the granitic pluton are more mafic, and amphibole and biotite are the predominant mafic minerals. The sharp contacts and fine–grained textures of the MMEs indicated a rapid quenching process during a magma mingling event. Zircon and titanite UPb dating indicated that the host granodiorite pluton formed at circa 2546 Ma, and the MMEs formed at approximately 2538 Ma. The dating results indicated a crust–mantle interaction. Geochemically, the host pluton has high silica contents (SiO2 = 67.68–68.51 wt%), Mg# values (51–53) and low Aluminum Saturation Index ratios (A/CNK = 0.92–0.95), which are indicative of I–type granitic affinity. Moreover, they show arc–type trace element features characterized by enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) (e.g., Cs, Rb, K, and Pb), but depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, P, and Ti). In contrast, the MMEs have lower silica contents (SiO2 = 52.07–56.03 wt%), but higher Mg# values (56–68) and Cr, Ni and Cu contents of 454–633, 159–225 and 98.3–207 ppm, respectively, corresponding to a mantle component. Similar to the host pluton, the MMEs show enrichment of LREEs and LILEs, but depletion of HFSEs, reflecting an arc–type magma origin. Isotopically, the host granodiorite and MMEs both have relatively depleted NdHf isotopic compositions, with εNd(t) values of −0.67–1.22 and − 0.31–1.89, and εHf(t) values of 5.63–2.03 and − 0.18–4.48, respectively. Zircon oxygen isotopes reveal that the host granodiorite and MMEs have slightly higher oxygen isotopes than the normal mantle zircon (5.3‰), with mean δ18O values of 6.35‰ and 6.81‰, respectively, indicating assimilation of minor supracrustal materials in the magma source. However, a combination of Nd–Hf–O isotopes suggests that the MMEs haveslightly enriched isotopic compositions than the host granodiorite pluton, suggesting a “reversed isotope” feature. This important finding also enlightens that “reversed isotope” phenomenon represented magma mingling between isotopically relatively enriched mafic and relatively depleted felsic magmas. Furthermore, the Ce4+/Ce3+ ratios in zircon from the host granodiorite and MMEs indicated a low oxygen fugacity, with mean values of 24.25 and 39.69, respectively. Although the host granodiorite and MMEs have the similar oxygen fugacity ranges, the MMEs have high Cu concentrations of 98.3–207 ppm (mean value = 151 ppm), indicating a Cu–rich magma source. Integrating the petrogenesis of the Late Neoarchean granodiorite pluton and associated MMEs, it is inferred that the host granodiorite pluton was derived from partial melting of mafic lower crust with minor insignificant ancient materials, and the MMEs were derived from partial melting of depleted mantle metasomatized by sulfur–rich sediment melts/fluids (chalcophile element). Tectonically, our samples reveal an affinity with an arc in a syn–collisional setting. Together with regional rock assemblages, we suggest that the onset of plate tectonics (subduction) was to occur by the end of Neoarchean in the NCC.

Geochronological, mineralogical and geochemical studies of sulfide mineralization in the Yueyawan mafic intrusion in the East Tianshan orogenic Belt, NW China

The Yueyawan Cu-Ni sulfide deposit, located in the northern part of Dananhu island arc, is a new case of Cu-Ni mineralization related to Early Permian mafic magmatism in the East Tianshan orogenic belt, NW China. Olivine gabbro, hornblende gabbro, and gabbro are the major rock types of this complex. Disseminated sulfide mineralization with a pentlandite + pyrrhotite + chalcopyrite assemblage occurs in olivine gabbro. Zircon crystals from olivine gabbro yielded a U-Pb age of 275 ± 2 Ma, almost contemporaneous with the postcollisional extension-related Cu-Ni deposits in the East Tianshan nickel belt (e.g., Tulaergen, Xiangshan, Huangshan, Tudun, and Baixintan deposits). The Yueyawan olivine gabbro is characterized by light rare earth element enrichment, pronounced negative Nb-Ta anomalies, low initial 87Sr/86Sr ratios from 0.703282 to 0.703420, positive εNd(t) values from 3.4 to 7.2, zircon εHf(t) values from + 15.5 to + 18.5, indicating that the parental magma of the Yueyawan intrusion was derived from a metasomatized mantle source, similar to other contemporaneous mafic–ultramafic intrusions in the region. The δ34S values of sulfide of the Yueyawan deposit are similar and vary from 0.60 to 1.85 ‰, which are within the range of typical mantle values as well as that of the country rocks, providing no direct evidence for addition of external sulfur. However, the negative correlation between Se/S (54.01 to 77.56 × 10−6) ratios and sulfur contents in the disseminated sulfides indicate crustal contamination played a critical role for sulfide segregation of the magma. The olivine Fo and Ni contents from sulphide-bearing mafic rocks show a negative correlation. Olivine chemistry and Sr-Nd-Hf-S isotope data indicate that fractional crystallization and crustal contamination played a role in triggering sulfide saturation in the Yueyawan mafic magma.

Late Carboniferous bimodal volcanic rocks of the West Junggar Terrane, NW China: Implications for the postcollisional tectonic setting of the southwestern CAOB

ABSTRACT The West Junggar terrain (WJT), as a crucial part of the Central Asian Orogenic Belt (CAOB), is distributed with numerous igneous rocks, which provide critical information for crustal growth. However, the closure of the Junggar Ocean (JO) and the beginning of the postcollisional tectonic stage of the WJT have been controversial. This study delimited the regional lithologic units based on remote sensing geological mapping and recognized a series of bimodal volcanic rocks (BVR) in Hala’alate Mountain of the WJT. LA-ICP‒MS zircon U‒Pb geochronology and geochemistry were used to discuss the petrogenesis of the BVR and determine the stage of regional tectonic evolution. Geochronological results yield crystallization ages of 302 ± 4 Ma, 298 ± 2 Ma, 304 ± 1 Ma, and 303 Ma± 2 Ma for the basaltic andesite, basalt, and two rhyolitic samples, respectively. Basalts and basaltic andesites are calc-alkaline, and display enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Notably, basaltic andesites in this area were once misjudged as sanukitoids owing to their low contents of Mg#, Ni, Cr, and other characteristics that are inconsistent with the typical definition of sanukitoids. The rhyolites are A2-type granitoids with high SiO2 contents and are depleted in Nb, Ta, P, Ti, and Sr, showing enriched LREE patterns with negative Eu anomalies. These features indicate that the magma of the mafic end-member of the BVR is derived from the partial melting of the depleted lithospheric mantle, whereas the felsic end-member magma can be associated with the remelting of the lower crust due to the upwelling and underplating of mafic magma. In combination with previous studies of simultaneous basic dikes and felsic rocks, a postcollisional tectonic stage was proposed for the WTJ during the late Carboniferous, suggesting that the JO was closed.

Triassic volcanism on the North margin of the North China Craton: Insights for lithospheric modification during closure of Paleo-Asian Ocean

As a key tectonic region linking the northern margin of the North China craton (NCC) and eastern Central Asian Orogenic Belt (CAOB), western Liaoning, with extensively distributed magmatic activity, provides natural samples to investigate the evolutionary history of the Paleo-Asian Ocean plate subduction to closure. The Middle Triassic (ca. 241 Ma) trachyandesites and Late Triassic (ca. 223 Ma) basaltic andesites were first identified from the Tiaojishan Formation, western Liaoning. They have variable SiO 2 contents (52.48–60.31 wt%) and alkaline or high-K calc-alkaline to shoshonite compositions, and belong to the metaluminous series. Middle Triassic trachyandesites are enriched in large ion lithophile elements (LILEs), high field strength elements (HFSEs), and light rare earth elements (LREEs) with high Nb/La ratios (0.63–0.76). They have homogeneous whole-rock Sr Nd ( 87 Sr/ 86 Sr i = 0.704684–0.704808, ε Nd (t) = −4.6 to −4.0) and variable zircon Hf isotopic signatures (ε Hf (t) = −10.3 to 8.2), indicating that they probably originated from the partial melting of the subcontinental lithospheric mantle with the addition of asthenosphere materials. In contrast, the Late Triassic basaltic andesites show arc-related geochemical affinities, such as LILEs and LREEs enrichment and HFSEs depletion. They have more enriched whole-rock 87 Sr/ 86 Sr i (0.704895–0.705394), ε Nd (t) (−16.1 to −14.0), and zircon ε Hf (t) (−13.1 to −8.8) than the Middle Triassic trachyandesites, suggesting that they probably represent partial melts of the subduction-related fluids-modified lithospheric mantle. The modification of the northern margin of the NCC lithospheric mantle is closely related to the final subduction of the Palaeo-Asian oceanic plate and the subsequent post-orogenic extension. • Triassic volcanic rocks are founded in Tiaojishan Formation, Western Liaoning. • Study area was in a post-orogenic extensional setting in the Middle-Late Triassic. • Ocean Subduction modified the North China Craton lithosphere in the Triassic.

Geochemical, mineralogical, and petrological analyses for the interpretation of the sedimentary environment of the Middle-Late Ordovician Majiagou Formation (northern China) as a tool for more effective gas exploration

Core samples from the deeply buried Ordovician Majiagou Formation below the Huainan Coalfield (E China) have been investigated for their carbonate types, major and trace elements (including rare earth elements) and C and O isotopes. The objective was to get a better insight into the possible occurrences of gas (and possibly oil) derived from Carboniferous coals. It was found that the carbonates are dolomites with strongly varying amounts of CaO and MgO. The low concentrations of SiO 2 , Al 2 O 3 , and Fe 2 O 3 indicate deposition in a normal marine environment with little terrigenous input. The Na 2 O/K 2 O, Fe/Mn and Sr/Ba ratios, as well as the Ga values indicate mainly a marine salinity and a hot and humid climate. The slight depletion of Ce and Eu, the depletion of heavy REE and the enrichment of light REE indicate deposition in a reducing environment. It thus appears that the Majiagou Formation below the Huainan Coalfield closely resembles that in the eastern part of the Ordos Basin, where several gas reservoirs are present, so that the Majiagou Formation under the Huainan Coalfield represents a promising target for hydrocarbon exploration.

Coexistence of Carboniferous oceanic island basalts with Permian supra‐subduction zone ophiolites in the Changning–Menglian accretionary wedge: Implication for tectonic reconstruction

This paper reports two types of basalt that have different ages (Carboniferous and Permian) but occur next to each other in the northern part of the Changning–Menglian accretionary wedge, southwestern China. We use the geochronology and geochemical data to evaluate the tectonic evolution of the Palaeo‐Tethys during this period. Zircon grains from the mafic‐ultramafic rocks associated with the Permian basalts in the Xiaomengtai area yield a U–Pb age of 281 Ma and εHf(t) values from +9.2 to +12.8. The Permian mafic‐ultramafic rocks and the associated basalts are all characterized by normal mid‐ocean ridge basalt (N‐MORB)‐like chondrite‐normalized rare earth elements patterns, moderately negative Nb‐Ta anomalies in the mantle‐normalized immobile incompatible trace element patterns, and positive εNd(t) values from +4.2 to +6.5, which are consistent with the geochemical characteristics of mafic‐ultramafic rocks in supra‐subduction zone (SSZ)‐type ophiolite. On the contrary, the Carboniferous basalts, which are associated with marine carbonates, are characterized by light REE enrichments, slightly positive Nb‐Ta anomalies, and εNd(t) values from +2.8 to +4.0. These features are similar to those of typical oceanic island basalts (OIBs) worldwide. Modelling results using REEs show that the parental magmas for the Carboniferous OIBs and the Permian basalts were likely derived from mantle peridotites at the depths of garnet and spinel stability, respectively, consistent with the formation depth of these two different types of basalt globally. The occurrence of these two different types of mafic‐ultramafic rocks with significantly different ages in the same area supports the view that they are the remnants of the accreted Palaeo‐Tethys oceanic crust. The Carboniferous OIBs are considered to be parts of an OIB‐carbonate seamount chain that originally formed in the southern part of the Palaeo‐Tethys. The Permian mafic‐ultramafic rocks are regarded as fragments of SSZ‐type ophiolites that were present in the northern part of the Palaeo‐Tethys. These different pieces of oceanic crust were accreted to the Simao–Indochina continental Block by subduction between the Late Permian and the Triassic.

Origin and distribution of rare earth elements (REEs) in the soils of Meizhou City, southern China with high abundance of regolith-hosted REEs

Currently, regolith-hosted rare earth element (REE) deposits in South China are the major source of global REEs, particularly heavy REEs (HREEs). This type of REE resource contains abundant movable, bioavailable, and ion-exchangeable REEs and may act as an environmental hazard; however, little is known regarding REEs in soils from areas with abundant deposits. Here, we investigated the REEs in the soil, surface water and crops, collected from Meizhou City, Guangdong Province, South China, as a representative region with abundant regolith-hosted REEs. Mean REE concentration (ΣREE) in the soil (318 μg g−1) exceeded the local soil background (169 μg g−1). The LREE/HREE ratio ranged from 2.04 to 69.4 in soil samples, suggesting the enrichment of light REEs (LREEs) compared with HREEs. Most soil samples exhibited positive Ce anomalies and negative Eu anomalies. The highest ΣREE (1162 μg g−1) was detected in forest land, followed by garden land (793 μg g−1) and paddy field (519 μg g−1). According to the correlation analysis, pH as well as soil organic matter (SOM), Fe, and Mn content contributed to REE enrichment, while SOM and Mn content affected REE fractionation. REE enrichment in soil may be related to natural sources (e.g., parent materials) and anthropogenic activities (e.g., fertilizer application and coal combustion). The REE contamination of surface water was not serious. However, ΣREE in crops (7.08 μg g−1·dw) indicated that these elements migrate from soils to plants, producing potential adverse effects on crops and humans. These findings shed new light on the geochemical behavior of REEs in the environments with abundant regolith-hosted REEs and can serve as a reference to prevent and control REE contamination.

Garnet geochemistry of the giant Beiya gold–polymetallic deposit in SW China: Insights into fluid evolution during skarn formation

The Beiya deposit in the central Jinshajiang–Ailaoshan metallogenic belt of SW China is the country’s largest gold skarn and third largest gold deposit. In this study of the Beiya deposit, we use garnet mineralogy and geochemistry to delineate three skarn alteration zones (i.e., contact, proximal and distal skarn). We investigate the possible controls on the rare earth element (REE) incorporation into skarn-type garnet, the ore-fluid evolution, and metasomatic dynamics. Garnet in the Beiya deposit belongs to the grossular–andradite solid solution series, and comprises three types: (1) Grt1 andradite (Adr67–88Grs9–32) in the contact skarn, exhibiting light REE (LREE) enrichments, heavy REE (HREE) depletions, and positive Eu anomalies; (2) Oscillatory-zoned Grt2 andradite (Adr55–98Grs1–44) in proximal skarn, which has moderate to low LREE/HREE enrichments and varying Eu anomalies; (3) Grt3 andradite (Adr29–76Grs22–69) in distal skarn with low LREE/HREE enrichments and minor negative Eu anomalies. Grt1, Grt2 and Grt3 yielded lower intercept U–Pb ages of 36.4 ± 1.8 Ma (MSWD = 0.87, n = 67), 34.8 ± 0.7 Ma (MSWD = 1.8, n = 31) and 33.4 ± 1.0 Ma (MSWD = 1.2, n = 25). Garnet shows compositional changes from Grt1 to Grt3 due to the fluid-wallrock reactions. With decreasing andradite and increasing grossular contents, the (La/Yb)N ratio decreases. Ore-forming fluids changed from diffusive metasomatism to infiltration metasomatism, and then again to diffusive metasomatism. Meanwhile, the ore-fluid physicochemical conditions changed from acidic to near-neutral (both oxidised), whilst the water/rock ratio first increased and then decreased. The proximal skarn recorded high water/rock ratio with intermittent boiling events, leading to fluctuating oxygen fugacity, which is conducive to ore deposition. Therefore, the skarn-type garnet composition from the various alteration zones can reveal the fluid activity and physicochemical fluid evolution path, preserving rich information about the mineralisation process.

Essential processes involving REE-enrichment in biogenic apatite in deep-sea sediment decoded via multivariate statistical analyses

Deep-sea sediments highly enriched in rare-earth elements (REE), termed “REE-rich mud,” have recently attracted attention as a potential mineral resource for industrially critical metals. Biogenic apatite or fish teeth and bone debris comprises the mineral phase which primarily hosts REE in deep-sea sediments; thus, biogenic apatite is a key material to decipher the formation process of REE-rich mud. We investigated the key process(es) controlling REE-enrichment in biogenic apatite through a novel data-driven approach. We analyzed the in situ chemical composition of biogenic apatite grains collected from REE-rich mud in the western North Pacific Ocean. Subsequently, the dataset of 685 analytical points × 35 elements was statistically analyzed via independent component analysis. Our new approach extracted four independent components (ICs) as geochemically meaningful and statistically independent signatures: substitution of elements into apatite lattice (IC1), the relatively rapid adsorption process that occurred at the sediment-water interface (IC2), elemental transfer from volcanic materials during early diagenesis (IC3), and metalliferous or detrital impurities physically adhering to or infiltrating apatite grains (IC4). Data distributions within the real and IC spaces provide insights into physicochemical processes involving elemental incorporation into biogenic apatite. We argue that after settlement on the seafloor, the fish bones and dentine in teeth adsorb REE from the overlying seawater and REE-enriched porewater at the sediment-water interface, resulting in the preferential enrichment of light rare-earth elements into apatite from these solutions. Further, that REE adsorbed on the apatite crystal surface was incorporated into the crystal lattice via substitution. The IC2 signal was intensified in the layers of “extremely REE-rich mud”, suggesting that the REE adsorption process was facilitated during the formation of layers with remarkably high bulk-sediment REE contents. This may reflect an increased supply of REE-carrier substances such as organic matter, which degraded on the seafloor and released REE at the sediment-water interface, which is associated with changes in paleoceanographic conditions including ocean surface bioproductivity.

Major, trace element, and Nd isotopic compositions of banded iron formation and shales from the Sirsi shelf, Dharwar Craton, India: Implications for paleo-seawater chemistry, post-depositional alteration, and provenance

We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb)SN (0.37–0.53) and (Pr/Yb)SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*)SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb)SN (1.73–18.48) and (Pr/Yb)SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*)SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial εNd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(Xelement/XTi)sample/(Xelement/XTi)reference-1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-depositional fluid overprint. Geochemical compositions of the Sirsi clastic rocks suggest a felsic provenance, while Nd isotopic compositions of selected shale samples indicate that these sediments were most likely derived from granitoids in the Goa-Dharwar Sector.

Alkaline rocks of the Bobaomby volcanic field point to a petrogenetic link between Comoros and northern Madagascar lithosphere

The Bobaomby volcanic field (10–11 Ma) is the northernmost volcanic area of Madagascar, and is a monogenetic volcanic field comprising outcrops of lava flows, dykes, scoria cones, tuff rings and plugs, widely scattered over an area of roughly 500 km2. The volcanic rocks range in composition from nephelinite, basanite and tephrite, through tephritic phonolite, to F- and Cl-rich peralkaline phonolite (MgO from 13 to 0.01 wt%), and the serial affinity varies from sodic to potassic. A few mica-amphibole-rich lamprophyric dykes have tephritic composition and ultrapotassic affinity. The mafic lavas host intrusive xenoliths with evident cumulate features (wehrlites, composite olivine gabbros s.l., amphibole clinopyroxenites and “kaersutitites”), as well as various types of mantle-derived xenoliths and xenocrysts in the most primitive rocks. The very wide compositional variations of the observed phases (olivine, clinopyroxene, amphibole, oxides, feldspars, feldspathoids, apatite, titanite, aenigmatite and other accessories) in lavas, dykes and cognate xenoliths are fully consistent with the variable degree of differentiation of the host lavas/dykes, and pointing out to limited open-system or polybaric crystallization. The mafic lavas have marked enrichment in incompatible elements and light rare-earth element (LREE) (e.g., Lan/Ybn = 19–27), whereas concave REE patterns are found in the peralkaline phonolites, as a result of removal of accessory titanite starting from tephritic phonolite magmas. The gabbroic/ultramafic xenoliths are interpreted as crustal cumulates of basanitic and tephritic magmas. Several liquid lines of descent in the basanites and tephrites are evident from the trace-element distribution, and from the differing geochemistry of the evolved rocks. The isotopic compositions reach extreme values (e.g., 206Pb/204Pb = 20.065 in the ultrapotassic lamprophyre) when compared to the rest of the Cenozoic/Recent Madagascan volcanic rocks, but similar to those of the Comoros archipelago, suggesting analogies of mantle sources and enrichment processes in the lithosphere of this volcanic archipelago. The origin of the Bobaomby mafic rocks is compatible from a derivation from low degree partial melting of an incompatible element-enriched peridotite source (possibly located in the lowermost lithospheric mantle) rich in volatile-rich phases (pargasite, locally also phlogopite and possibly carbonates), matching the sources of other Cenozoic volcanic areas throughout Madagascar, and perhaps Comoros.

Petrogenesis of the Alubaogeshan intrusion in the Maodeng–Xiaogushan area, southern great xing'an range, NE China: Implications for magma evolution and tin–polymetallic mineralization

• The Alubaogeshan intrusion formed at ca. 134 Ma and belongs to transitional I–A 2 -type granite . • The Alubaogeshan intrusion was formed by partial melting of a Neoproterozoic juvenile crust. • The syenogranite porphyry evolved from the porphyritic monzogranite. • Fluid–melt interaction is crucial for W mineralization. • The magma evolutionary degree influences the size of the Sn–polymetallic deposits in the SGXR. The Alubaogeshan intrusion, consisting of porphyritic monzogranite in the central part and syenogranite porphyry in the edge, is genetically related to the Maodeng–Xiaogushan Sn–polymetallic deposit. This paper aims to investigate the genetic link between these two granite phases and the relationship between magma evolution and Sn mineralization. LA-ICP-MS zircon U–Pb dating of the porphyritic monzogranite and syenogranite porphyry yielded ages of 134 ± 1.0 Ma and 133 ± 0.5 Ma, respectively. The Alubaogeshan intrusion has high SiO 2 and total alkali (Na 2 O + K 2 O) contents, with low Al 2 O 3 , Fe 2 O 3 , FeO, and MgO contents, belonging to high-K calc-alkaline metaluminous–weakly peraluminous granites. Chondrite-normalized rare earth element (REE) patterns are characterized by light REE enrichment and heavy REE depletion, with extremely negative Eu anomalies. In addition, the intrusion was also enriched in Rb, Th, U, K, Zr, Hf, and light REE and strongly depleted in Ba, Sr, P, and Ti. Mineralogical and geochemical characteristics, together with high zircon saturation temperature, indicate that the Alubaogeshan intrusion belongs to transitional I–A 2 -type granites. Compared with the porphyritic monzogranite, the syenogranite porphyry reveals higher SiO 2 content and lower TiO 2 , Al 2 O 3 , Fe 2 O 3 , FeO, CaO, MgO, and P 2 O 5 contents, with more obvious negative anomalies of Eu, Ba, Sr, P, and Ti. The temporal and spatial relationship and geochemical characteristics of the two rocks indicate that the syenogranite porphyry evolved from the porphyritic monzogranite. The modeling results show that the fractional crystallization of amphibole, plagioclase, K-feldspar, biotite, apatite, monazite, and allanite occurred during the magma evolution. The granitic magma differentiation plays a key role in the formation of the Maodeng–Xiaogushan Sn–polymetallic deposit. The ε Hf ( t ) values of zircon range from 2.58 to 7.08, and the two-stage Hf model ages range from 619 Ma to 850 Ma, implying that the Alubaogeshan intrusion was predominantly derived from partial melting of a Neoproterozoic juvenile crust. The TE 1,3 values of the ore-related granites from W–(Sn) deposits are generally higher than 1.1, suggesting that fluid–melt interaction is crucial for W mineralization. The degree of magmatic evolution is one of the controlling factors for the size of Sn–polymetallic deposits in the southern Great Xing′an Range.

Crust–mantle interactions in Zijinshan ore field identified by zircon SIMS U–Pb age and zircon Hf–O isotopes

AbstractThis study presents zircon U–Pb geochronology and Hf and O isotope data for granitic rocks in the Zijinshan ore field, southwestern Fujian Province, China. The intrusive rocks comprise monzogranite, granite and granodiorite. The magmatic zircon U–Pb ages from these granitic rocks can be divided into four episodes: episodes 1 (157.9–159.9 Ma) and 2 (141 Ma) in the Late Jurassic and episodes 3 (108.1–103.2 Ma) and 4 (97.5–99.7 Ma) in the Middle and Late Cretaceous, respectively. Patterns of rare-earth elements (REE) show enrichment in light REE and obvious negative Eu anomalies. These rocks are also enriched in Rb, Th, U, La, Ce, Nd and Hf, and depleted in Ba, Nb, Sr, P and Ti. The increasing La/Yb ratio and decreasing heavy REE content with decreasing age may imply an increasing contribution of mantle-derived materials from the Late Jurassic to Cretaceous. The zircon ϵHf(t) and δ18O values, ranging from −37.7 to −2.8 and 12.0 ‰ to 6.3 ‰, respectively, indicate that the lower crust is an important source of granitic rocks. There was a significant increase in ϵHf(t) values and a decrease in δ18O values in the younger magmatic episodes (3 and 4), which ranged from −11.4 to −0.6 and 10.7 to 6.3 ‰, respectively. This suggests an increasing contribution of mantle-derived magma to the crustal melts from the Late Jurassic to late Early Cretaceous in response to the changing regional tectonic setting from compression to extension and an increasing interaction between the crust and mantle.

Mineralogy and Geochemistry of the Upper Ordovician and Lower Silurian Wufeng-Longmaxi Shale on the Yangtze Platform, South China: Implications for Provenance Analysis and Shale Gas Sweet-Spot Interval

The sediment provenance influences the formation of the shale gas sweet-spot interval of the Upper Ordovician–Lower Silurian Wufeng–Longmaxi shale from the Yangtze Platform, South China. To identify the provenance, the mineralogy and geochemistry of the shale were investigated. The methods included optical microscopy analysis, X-ray diffraction testing, field-emission scanning electron imaging, and major and trace element analysis. The Wufeng–Longmaxi shale is mainly composed of quartz (avg. 39.94%), calcite (avg. 12.29%), dolomite (avg. 11.75%), and clay minerals (avg. 28.31%). The LM1 interval is the shale gas sweet-spot and has the highest contents of total quartz (avg. 62.1%, among which microcrystalline quartz accounts for 52.8% on average) and total organic carbon (avg. 4.6%). The relatively narrow range of TiO2–Zr variation and the close correlation between Th/Sc and Zr/Sc signify no obvious sorting and recycling of the sediment source rocks. Sedimentary sorting has a limited impact on the geochemical features of the shale. The relatively high value of ICV (index of compositional variability) (1.03–3.86) and the low value of CIA (chemical index of alteration values) (50.62–74.48) indicate immature sediment source rocks, probably undergoing weak to moderate chemical weathering. All samples have patterns of moderately enriched light rare-earth elements and flat heavy rare-earth elements with negative Eu anomalies (Eu/Eu* = 0.35–0.92) in chondrite-normalized diagrams. According to Th/Sc, Zr/Sc, La/Th, Zr/Al2O3, TiO2/Zr, Co/Th, SiO2/Al2O3, K2O/Na2O, and La/Sc, it can be inferred that the major sediment source rocks were acidic igneous rocks derived from the active continental margin and continental island arc. A limited terrigenous supply caused by the inactive tectonic setting is an alternative interpretation of the formation of the sweet-spot interval.

Origin of microbial–hydrothermal bedded dolomites in the Permian Lucaogou Formation lacustrine shales, Junggar Basin, NW China

Dolomite is widely developed in the shale sequences of saline lakes and has significance in paleoenvironmental, paleoclimatic, and petroleum geology research. In this study, a new end-member type of dolomite is discovered from the Permian Lucaogou Formation in the Junggar Basin, northwest China, along with its geological significance. The microbial–hydrothermal origin of the bedded dolomite is evident from the development of hydrothermal minerals (including barite and cubic pyrite) and methanogenic micro-organisms. In terms of geochemistry, the bedded dolomite has high (Ba/La)N ratios (~2.8) and Sr contents (~594.9 ppm) and positive Sr anomalies (Sr/Sr* ~3.8). Chondrite-normalized rare earth element (REE) patterns exhibit light REE enrichment, heavy REE depletion, slightly positive Ce anomalies, and negative Eu anomalies. The bedded dolomite has relatively low 87Sr/86Sr ratios (~0.705283), high δ13C values (~6.63‰ ), and low δ18O values (~−9.92‰ ). All these indicate that the dolomitizing fluid is a mixed source of hydrothermal fluids and lake waters, and the hydrothermal fluids and microbial methanogenesis provided the Mg2+ and C, respectively. The combination of microbial and hydrothermal activity overcame the limitation of dolomitization hydration shells around Mg2+. In terms of geological significance, the studied Lucaogou Formation was deposited in an extensional tectonic setting under hot–arid climatic conditions, which controlled the high-frequency interbedding and meter-scale cyclicity of sediment deposition of dolomites and shales. The combined effects of tectonism and paleoclimate may be important factors for the formation of bedded dolomite, enrichment of organic matter and hydrocarbons in saline lacustrine shales.

Nature and evolution of diagenetic fluids in the deeply buried Cambrian Xiaoerbulake Formation, Tarim Basin, China

Significant amounts of petroleum are found in the deeply buried Cambrian Xiaoerbulake Formation in the Tarim Basin. However, limited understanding of origin of diagenetic fluids hampers petroleum exploration. In this study, petrological and high-resolution geochemical methods have been performed to understand the origin and evolution of diagenetic fluids, and their effects on the formation of dolostone reservoir. The Xiaoerbulake Formation is characterised by microbialites. The matrix is characterised by modern seawater-like rare earth element and yttrium (REY) patterns and δ18O values of −6.02 ± 2.11‰ Vienna Pee Dee Belemnite (VPDB). Fine-crystalline dolomite cements have similar δ13C values and heavy rare earth element enrichment patterns to that of matrix dolomite, yet with lower δ18O values of −7.47 ± 1.2‰ VPDB. In the deep-burial regimes, medium- to coarse-crystalline dolomite cements and saddle dolomites were precipitated at temperatures of 90–120 and 130–190 °C, respectively, lower δ18O values, distinct middle rare earth element enrichment, and high Fe and Mn contents compared with matrix and fine-crystalline dolomite cements. These imply thermochemical reduction of Fe- or Mn-rich oxides and/or hydrothermal fluids. Compared with other types of dolomite cements, calcite cements are characterised by low δ13C values (as low as −6‰ VPDB), light rare earth elements enrichment and significantly elevated Y/Ho ratios (86 ± 59.3), likely related to the oxidation of organic matter during thermochemical sulfate reduction (TSR). Consequently, diagenetic fluid may have evolved from: Cambrian seawater, meteoric water, shallow-buried seawater-dominated pore water, medium-deep-buried ferrous-rich pore water, lately high-salinity hydrothermal fluid-dominated pore water and finally to acidic fluids related to TSR dissolution. Reservoir quality is enhanced by hydrothermal pulses and TSR in deep-burial settings. This study is significant for those concerned with the formation of deep-buried carbonate reservoirs.

Discovery of antiskarn-hosted strategic metal mineralization in the Upper Cretaceous Twihinate carbonatite intrusion (West African Craton Margin, Moroccan Sahara)

The Upper Cretaceous Twihinate carbonatite in Moroccan Sahara, which is the focus of ongoing exploration for Nb-P-Fe ± LREE ± U-Th resources, consists of a crescent-like intrusion made of a central sövite body encircled by an annular ring of vuggy siliceous breccia. Drill core logging coupled with petrographic observation, whole-rock and mineral chemistry reveal that the metasomatic interaction between the carbonatite melt and the silicate wall-rocks led to development of calc-silicate paragenesis within the sövite itself at depths ranging from ∼60 to >200 m from the surface for which the term antiskarn is attributed. We provide the first petrographic and textural descriptions, and report the whole-rock major- and trace-element geochemistry and chemical compositions of the main antiskarn-forming minerals and related strategic metal-bearing ore. Geochemically, the antiskarn lithotypes are characterized by a compositional range of 7–17 wt% SiO2, 25–72 wt% CaO, 1–2 wt% Al2O3, and 12–26 wt% loss on ignition, and enrichment in large-ion lithophile elements (LILE), particularly Sr (3125–7018 ppm ppm), Ba (393–1300 ppm), U (10–787 ppm) and light REEs (LREEs) (1058–1569 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, Ta and Hf. Nb and V show, however, much higher concentrations ranging from 140 ppm to >1000 ppm, and 603 to 1008 ppm, respectively. Mineralogically, the antiskarn lithotypes consist predominantly of diopsidic (Di66-82 Hd18-34) to aegirine-augite clinopyroxene (Di38-55 Ae21-35) and andradite (And86-94 Gro1-9) with subordinate amounts of micas (biotite/phlogopite) and titanite. The paragenetically later mineral assemblages involve amphibole, chlorite, epidotes, baryte, fluorite, kaolinite, carbonates and quartz. The main Nb-P-Fe ± LREE ± U-Th ore minerals consist of Fe oxides (titanomagnetite principally), pyrochlore group minerals, phosphates (apatite, monazite-(Ce)), and REE-fluorcarbonates (bastnäsite-(Ce)) with subordinate sulfides (chalcopyrite, sphalerite, and galena), fluorite and baryte. Development of the antiskarn calc-silicate paragenesis and related strategic metal-bearing mineralization is consistent with derivation from a carbonatite melt through assimilation of silicate country rocks including the Silurian-Devonian gneissic granitoid of the Laknouk suite. From an economic perspective, the widespread occurrence of high-Ti (0.7–16. 9 wt% Ti) and V- (0.2–0.7 wt% V) magnetite opens new opportunities in regard of exploration for V; an emerging strategic commodity whose presence has to be reassessed in the Twihinate prospect. The currently described antiskarn paragenesis may be therefore used as reliable exploration metallotect to target prospective occurrences in the search for critical metals including V as by product.

Rare earth elements as new biogeochemical proxies in deep-sea mussels

We report on the abundances of REE in a comprehensive suite of shells of deep-sea chemosynthetic mussels from hydrothermal vents and cold seeps. Except for mussel shells from oceanic hydrothermal sites that often show extremely pronounced Eu anomalies ((Eu/Sm)sn = 2–200), and abundances for this element that can occasionally exceed 200 ng/g, REE concentrations are usually low and typically between 10−4 and 10−3 times the shale reference.In addition to exhibiting commonly high Eu anomalies, mussel shells from hydrothermal vents are depleted in light-REE and heavy-REE compared to reference shales (e.g., Pr/Sm)sn < 1, (Tb/Yb)sn = 1–6). These features are inherited from hydrothermal fluid. Mussel shells from cold seeps have very different REE concentrations, which also reflect the compositions of the waters they filter: their (Eu/Sm)sn ratios are much lower (<2) and are much less heavy-REE fractionated ((Tb/Yb)sn = 0.5–2.7). Furthermore, the REE distributions show a clear dichotomy between thiotrophic and methanotrophic mussels. The latter show marked enrichments in light-REE and even sometimes La enrichments much larger than those classically observed in deep-sea waters, leading to huge positive La anomalies (La/La* > 10). These light-REE enrichments are likely related to REE-dependent methanol dehydrogenase enzymes used by the symbionts that these mussels host in their gills. These data show that REE chemistry is a promising tool to study chemosynthetic faunas living near hydrothermal vents or cold seeps. Furthermore, REE chemistry, coupled with stable isotopes, should reveal the footprint of aerobic methanotrophy in carbonates formed in cold seeps, but also potentially in ancient sediments.

Distribution, source and behavior of rare earth elements in surface water and sediments in a subtropical freshwater lake influenced by human activities

As tracers, rare earth elements (REEs) can reflect the influence of human activities on the environmental changes in aquatic systems. To reveal the geochemical behavior of REEs in a water–sediment system influenced by human activities, the contents of REEs in the surface water and sediment in the Chaohu Lake Basin were measured by inductively coupled plasma mass spectrometry (ICP–MS). The results show that the ΣREE contents in the surface water are 0.10–0.850 μg L−1, the ΣREE contents in the sediments are 71.14–210.01 μg g−1, and the average contents are 0.24 μg L−1 and 126.72 μg g−1, respectively. Almost all water and sediment samples have obvious light REE (LREE) enrichment, which is the result of the input of LREE-rich substances released by natural processes and human activities (industrial and agricultural production). Under the alkaline water quality conditions of Chaohu Lake, REEs (especially LREEs) are easily removed from water by adsorption/coprecipitation reactions with suspended colloidal particles, which leads to the enrichment of LREEs in sediments. The Ce anomaly of the water–sediment system is related to the oxidation environment, while the Eu anomaly is related to the plagioclase crystallization. Significant Gd anomalies was observed in the downstream of rivers flowing through urban areas, which was related to the anthropogenic Gd wastewater discharged by hospitals. The ∑REE–δEu and provenance index (PI) discrimination results are consistent, indicating that the sediments in Chaohu Lake mainly come from rivers flowing through the southwest farmland. Furthermore, the spatial distribution of REEs shows that these tributaries are significantly affected by agricultural activities. The distribution and accumulation of REEs in Chaohu Lake are the result of the interaction of natural and human processes. The results can provide a scientific reference for the distribution and environmental behavior of REEs in aquatic environments disturbed by human beings.