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... For marble whose protolith is sedimentary carbonates, the composition of paleoseawater determines the trace element characteristics of its protolith; second, the metamorphic diagenesis and the mixing of terrigenous clastic materials in the later period will influence the element content of marble (Zhen, 2014). It is believed that the total amount of REEs in sedimentary rocks increases with seawater depth, which can reflect the depth of seawater when the paleo-sediments form to some extent (Yang et al., 2008). The low total amount of rare earth elements (12.18-75.64 ...
... Shen et al. (1997) interpreted the U-Pb age of detrital zircons as 2.5 Ga by using single-grain zircon evaporation. The Sm-Nd isotope data constrained that the metamorphic complex of the Douling group was formed iñ 2,000 Ma (Yang et al., 2008). The study on gneiss of supracrustal origin found that the protolith formed in~2,123 Ma, while sedimentation occurred very shortly followed by intense metamorphism at~1,878 Ma (Zhang et al., 1996). ...
As a tectonic block of the Qinling Orogenic Belt, Central China, the South Qinling Orogen is generally believed to have had a tight affinity with the Yangtze Block during the Precambrian. Knowledge on the South Qinling Orogen is important to constrain the tectonic evolution of the northern margin of the Yangtze Block basement. Previous studies mainly focused on the east segment of the South Qinling Orogen. This paper presented petrological, geochronological, and geochemical studies on the amphibolite-facies rocks newly discovered in the middle section of the South Qinling Orogen. Geochemical studies indicate that the protolith of these amphibolites is calc-alkaline island arc basalt, which was formed during the reconstruction of the Paleo-Middle Archean crust on the northern margin of the Yangtze Block 2,362 ± 100 Ma ago (equivalent to metamorphic rocks of the Yudongzi group). Later, these arc basalts experienced amphibolite-facies metamorphism at 1,500–1,800 Ma, which is consistent with the metamorphism identified in the Yangtze Block, confirming the association of the South Qinling Orogen with the Yangtze Block basement. Subsequently, these amphibolite-facies rocks further experienced three stages of metamorphism at c. 800 Ma, 440 Ma, and 200 Ma in the context of the northward subduction of the Yangtze Block.
... Rare earth elements (REEs) serve as tracers and indicators of sources, weathering, diagenesis, and mineralization processes in the terrestrial critical zone [19]. Their contents, total amount, and combinational patterns objectively reflect the geological body's evolutionary process, the physical and chemical conditions, as well as the source [20,21]. As such, studies of the geochemical behavior of REEs in terra rossa can contribute to reconstructing the pedogenesis effects, weathering processes, and the supergene mineralization [22][23][24]. ...
... Geochemical parameters such as Ce and Eu of sedimentary rocks can be used to judge material source and sedimentary environment [21]. Normally, Ce > 0.78 is a positive anomaly, and Ce < 0.78 is a negative anomaly; Eu > 1 is a positive anomaly, and Eu < 1 is a negative anomaly [27]. ...
Research on weathered crusts on carbonate rock is essential for paleoenvironmental studies in karst areas. Terra rossa, widely distributed in tropical karst areas, has not been studied in terms of its material sources and geochemistry. Two typical terra rossa profiles on dolomite (SC profile located at Sang Cai, Hoa Binh province) and limestone (TG profile located at Tong Gia, Lao Cai province) in Northern Vietnam were selected to examine the geochemical characteristics and the evolutionary processes of rare earth elements (REEs). Chondrite and bedrock normalized patterns indicated that these two profiles are in situ weathering crusts, meaning they are the residual material remaining after chemical weathering of the lower carbonate rocks. The average value of total REE in the SC profile is 381.19 ppm, which is 30 times higher than the bedrock. In the TG profile, the value is 386.26 ppm, 13 times higher than the bedrock. Compared with the profiles in nearby subtropical areas in Southeast China, the REE enrichment coefficients of terra rossa in Northern Vietnam are much higher. The REE depletion was also different between the SC and TG profiles. The light and heavy REE fractionations in the SC profile are higher than in the TG profile. Paleoclimate inversion analysis shows that the SC profile experienced a stable oxidation condition, whereas the TG profile was subjected to several reducing environments since a weathering crust formed.
... Ce and Eu are commonly used to assess the sedimentary environment [60][61][62], and most studies indicate that if Ce/Ce * > 1, there is Ce enrichment that possibly reflects an oxidation environment in seawater. Values of Ce/Ce * < 1 may represent Ce loss at the time of precipitation of metallic oxides, indicating an anoxic reducing environment [63][64][65]. Eu is a multivalent element, occurring as Eu 2+ and Eu 3+ . Under relatively oxidizing conditions, Eu can exist as Eu 3+ , and this can persist in solution for a long time. ...
The recently discovered middle-sized Bojitian Carlin-type Au deposit is located in southwestern Guizhou Province, China, near the well-known Shuiyindong super-large-sized deposit. To improve the understanding on this deposit, here we investigate the minerals that host Au and the occurrence of Au in the deposit, using a combination of microscopic work and electron probe microanalysis (EPMA). Based on the results, the formation of the deposit was addressed. Results indicate that the dominant minerals that host Au include arsenian pyrite and arsenopyrite. Au in the cores of zoned pyrite exists mainly as natural nanoscale Au (Au ⁰ ), while Au in the rims exists mainly as solid solution Au (Au ⁺ ), but it likely also exists in the rims as natural nanoscale Au. The framboidal, coarse-grained, and banded pyrite types contain both natural nanoscale Au ⁰ and solid solution Au ⁺ . The arsenopyrite is of hydrothermal origin, and Au within the arsenopyrite exists as gold solution Au ⁺ . The Bojitian deposit was formed from As-bearing, H 2 S-rich, low-to-medium-temperature fluids that migrated along faults and other channels. Au that was already present in the strata or source beds migrated with the fluids in the form of Au(HS) ⁻ and ore-forming fluids were then formed in the reducing environment. The ore-forming fluids interacted with Fe-rich carbonates to form an abundance of Au-hosting arsenian sulfides.
The Niutitang Formation in the Upper Yangtze Region comprises a set of the Early Cambrian black shales containing abundant organic matter. We investigated the relationship between the depositional environment and organic matter enrichment based on core description, slice observation, and geochemical analysis of the main trace elements. Additionally, we reconstructed the depositional environment of the Lower Cambrian Niutitang Formation shales and analyzed the main factors controlling the organic matter enrichment of shales during different deposition periods. According to the division of lithofacies, seven types of lithofacies have been identified in the Niutitang Formation. It is believed that the Niutitang Formation in northern Guizhou comprises a set of silicon-rich shales deposited on the continental shelf slope and formed under the control of different hydrodynamic forces. The main factors controlling its formation mechanism changed many times, from the initial hydrothermal deposition and enrichment controlled by oxygen deficiency to the enrichment accelerated by the high productivity at the surface of the water body. Subsequently, the change in sea level caused the change in the sea depth, which made it difficult for lower productivity to promote organic matter enrichment. Multiple main factors cooperated with the evolution of the depositional environment and jointly controlled the formation and distribution of organic-rich shales.
There is an urgent need to analyze the forming environment, the metal enrichment process and ore-forming rules of the Zhuxi vanadium ore field in eastern Guizhou Province. This paper systematically studies the major and trace element compositions of ore-bearing rocks and ore strata from three typical black shale sections in the ore field. The results show that ore-bearing rocks contain high contents/values of SiO 2 , MnO, Ce/Ce*and Eu/Eu*, while ore strata have higher concentrations of Th, U and V. The major and trace element geochemical characteristics of ore-bearing rocks and ores indicate that the ore-forming source of the Zhuxi vanadium ore field had a certain degree of continental material input, whilst both hydrothermal and biological processes played a significant role during the ore-forming process. In addition, the intensities of hydrothermal process decreased from southern to northern sections and the sedimentary environment of both ore-bearing rocks and ores was anoxic. The ores from the Pingshao lithological sections were formed in the most reducing environment than those from other sections. Therefore, the V enrichment and mineralization inthe Zhuxi ore field has mainly been controlled by reducing paleoenvironment and hydrothermal activity, and the reducing environment has been more significant for V mineralization. It can be inferred that ores from the Pingshao section should have been deposited in the deepest seawater environment while the ores from northern Zhongdong section had the shallowest seawater level. Exhalative fluid brought abundant V and other metal elements depositing in the vent and adjacent areas, leading to the large-scale mineralization.
We studied the states of rare earth elements in ore of the Xianglushan rare earth deposit. Rare earth ore samples were tested and examined by scanning electron microscope, electron probe, and chemical leaching. No independent rare earth minerals were detected by scanning electron microscope. Elements detected by the electronic probe for the in situ micro-zone of the sample included: O, Al, Si, Ca, Mg, Fe, Ti, K, Na, S, Cl, C, Cu, Cr, V, and Pt. Rare earth elements were not detected by electron probe. (NH4)2SO4, (NH4)Cl, NaCl, and H2SO4 were used as reagents in chemical leaching experiments that easily leached out rare earth elements under the action of 10% reagent, indicating that the rare earth elements in ore are mainly in the ionic state rather than present as rare earth minerals.
It has been established that important changes in the marine environment and the biosphere occurred during the Cambrian. However the relationships between the so-called “ Cambrian Explosion” and the concomitant environmental changes are not yet fully understood. This study presents new geochemical data from the black shale successions from different facies belts of the Yangtze Platform in South China. Variations in the concentrations of REE and trace elements (varying Ce/Ce*, Th/U, V/Sc and V/Cr ratios) in kerogen as well as in bulk rocks from different depositional environments along a transect from platform to basin indicate two oxidation events, which led to the oxygenation of the water column in shallow-marine environments and euxinic conditions (weak correlation between TOC, V, U and Mo) in the deeper sea. During the first oxidation event in the late Terreneuvian, anoxic conditions in bottom waters rapidly changed to euxinic conditions. Subsequently, the second oxidation event during the early Epoch 2 of the Cambrian led to oxic-suboxic conditions in deeper seawater.
The barite–fluorite metallogenic belt in southeastern Sichuan is one of the most important barite–fluorite ore-concentrated areas in China. In order to find the ore-forming source and the mineralization age of the Fengjia and Langxi barite–fluorite deposit, we systematically evaluated the rare earth element (REE) contents and isotopic characteristics of S, Sr, and Sm–Nd. The results show that the δ34S values of barites and pyrites are high and similar to S isotopic values of gypsum in the Cambrian Qingxudong Formation (∈1q), suggesting that the sulphur source was from Cambrian evaporite strata. The initial 87Sr/86Sr ratios of fluorites and barites range from 0.708800 to 0.712999, and these values are similar to Cambrian carbonate rocks and Lower Ordovician limestone. Except for black shales of the Lower Cambrian Niutitang Formation, the Ba content of other strata was generally low. The REE characteristics of barite and fluorite have close relationships, in regard to the source of ore-forming materials, to the Lower Cambrian Niutitang Formation black shales. Hence, we have concluded that the Cambrian carbonate rocks and Lower Ordovician limestone provide the source of calcium for mineralization and that the source of barium comes from the Lower Cambrian Niutitang Formation black shales. The fluorine contents of the Upper Sinian Doushantuo Formation and the Lower Cambrian Mingxinshi Formation are much higher than other strata, and these formations could provide the source for fluorine. The Sm–Nd isochron age of fluorites is 104 ± 11 Ma, which demonstrates that the mineralization mainly occurred during the late Yanshanian. Copyright
The authors analyzed REE contents of the Cambrian carbonaceous rock series in the Mian-Lüe region by using ICP-MS method in order to investigate their geochemical characteristics and genetic implications. Some conclusions can be reached from the acquired analytical data: the carbonaceous rock series show narrow variation in the total contents of rare earth elements (∑REE), from 167.88×10-6 to 210.50×10-6 with an average at 192.20×10-6; the ratios of LREE/HREE vary from 6.64 to 16.25 with an average at 11.52, showing relative enrichment of LREE and depletion in HREE; the carbonaceous rock series show remarkable negative Ce anomalies, varying from 0.58 to 0.90 (0.78 on average ), suggesting that the carbonaceous rock series formed in a dry and anoxic sedimentary environment close to a marginal sea of continental slope; the carbonaceous rock series also show distinctly negative Eu anomalies, δEuN=0.47-0.83(0.62 on average), suggesting that they formed in a passive continental margin environment as products of normal marine sediments under low temperature(<250°C) diagenesis, consistent with the their geotectonic location in the northern margin of the Yangtze platform; according to the diagrams of La/Yb-Ce/La and La/Yb-REE, it is also confirmed that the carbonaceous rock series not formed in a hydrothermal sedimentary condition but in a normal marine condition with their main source materials coming from a continental crust with high quartz content.
Located on the southern margin of Echeng batholith in the southeastern Hubei ore concentration area, the Chengchao iron deposit is the largest skarn iron deposit in the Middle - Lower Yangtze River Valley metallogenic belt. The ore deposit occurs along the contact between granitoids and carbonates or between granitoids and diorites. Ore bodies, granitoids and skarns have close contact relationships. REE analyses show that the chondrite-normalized REE patterns of garnets, epidote and magnetite ores in the Chengchao iron deposit are similar to each other and enriched in LREE (light REE) with Eu positive anomaly and without Ce anomaly, implying that they were formed in an oxidation environment by magmatic fluids characterized by high temperature and richness of Eu. The REE characteristics of granitoids, skarns and ores indicate their close genetic relationships. Geological and REE geochemical features show that granitoids, skarns and ore bodies are closely related to each other in time, space and genesis.
This paper studied the geochemistry of REEs in the anoxic and oxic sediments formed during the Cretaceous in the southern Tibet, and discussed the correlations of the geochemical behavior of REEs and the paleoceano-graphic redox state. Our data showed that the total concentrations of REEs in the sediments of the latest Cenoma-nian anoxic event increased 30% , while elements, Ce and Eu, increased 40% and 114% , respectively. High REEs content of the red mudstones was attributed to the increase of every REE, 107% for Eu, 38 ∼ 52% for La, Nd, Sm, Tb, Yb and Lu, and 17% for Cc. Compared with other REEs, Ce is relatively depleted. The shale-normalized patterns in the anoxic event sediments are characterized by enhanced positive anomalies of both Ce and Eu elements, with LREEs are heavier than HREEs. Shale-normalized patterns in the red marlstones have a distinctively negative Ce-anomaly and a large positive Eu-anomaly. SCe values within the latest Cenomanian black shales are 1. 04 in average without Ce-anomaly, showing a prevent anoxic condition. SCe decreased from 0. 96 to 0. 66 in the Santonian-Campanian red shales indicates a major redox state change from anoxic to oxic environment.
This paper studied the occurrence of platinum group elements (PGE) in the polymetallic layer of black shales, Zunyi, Guizhou using sequential chemical extraction(SCE), heavy liquid separation and inductively coupled plasma mass spectrometry (ICP-MS), and discussed the evolution of PGE in black shales. The results of SCE indicated that PGE mainly concentrates in sulfide minerals. The occurrences of Pt and Pd in the layer are different: a part of Pt is adsorbed in clay minerals while Pd is likely to form independent minerals in addition to adsorption portion in clay minerals. Combined with previous research results, we proposed that PGE transported in the hydrothermal fluid as different complexes, such as chlorine, bisulfide and ammonia complexes. The clay minerals, organic matter and sulfide served as geochemical barriers in the mineralization of PGE.
According to the outdoor profile survey, collection of Permian-Triassic marine source rock samples in systematic, and the organic matter abundance, type, maturity and chloroform bitumen "A", also study the relationship among them, the results show that: TOC, S1+S2, chloroform bitumen "A" have a good correlation, S1+S2 and chloroform bitumen "A" range of marine carbonates rocks and mudstone are stable,the value of mudstones are better than carbonates rock; Evaluation of P2c, T2d carbonates are favorable district two sets of source rocks. Make use of trace elements and rare earth elements of mudstone, trace elements of sandstone, the results show that the marine of source rock ancient water deposited environment were anaerobic and hypoxia, environment from oxidizing to restore continuous transition from Permian to Triassic. The wholly palaeosalinity over all lower, shallow water, influenced more by land than sea from Permian to Triassic. Provenance from source area under oxidizing environment, far away from the basin.
Niutitang Formation is one of the most potential strata for exploration in the upper Yangtze platform, therefore, we select Nangao section as an example in Danzhai county of Guizhou Province. Through various analytical methods such as field survey, thin section identification, geochemical analysis, X-ray diffraction mineral analysis, porosity measurement, scanning electron microscopy (SEM) observation and isothermal adsorption methods, studies on Niutitang Formation shale gas characteristics including geology, geochemistry, reservoir and adsorbed gas content were performed. Furthermore, the correlation between organic matter and mineral association was discussed, as well as the controlling factors of gas adsorption. For the section in the lower part, it is composed by mudstone deposited in deep-water environment while in the upper part it is silty shale and/or calcareous siltstone deposited in shallow-water shelf environment. Characteristic trace element ratio shows that black shale was formed in the depositional environment of oxygen reduction with obvious characteristics of hot water sedimentation. The black shale is characterized by the occurrence of type-I kerogen. The TOC values range from 0.38% to 15.31%, with an average of 5.2%. RO values range from 1.60% to 4.15%, with an average of 2.8%. The mineral composition of black shale is predominantly clay minerals (e. g. illite and chlorite), quartz, feldspar, barite, and gypsum. Quartz content is relatively high with an average of 53%, and decreases upwards in the section. Quartz content is too high or too low, which are not conducive to the enrichment of organic matter, and only when the quartz content of between 36.4% and 71.9% it has positive correlation with TOC values. Clay content is relatively low and increases upwards with an average of 34%, but it is negatively correlated with TOC values, which is opposed to the quartz's. There are a few carbonate minerals near the top of the section. The porosity of black shale and siliceous rocks in the lower section ranges from 0.99% to 4.66% with an average of 2.39%, and the density ranges from 1.99 g/cm3 to 2.62 g/cm3 with an average of 2.47 g/cm3. Four types of pores are present in the shale, such as interparticle pores, intraparticle pores, fracture pores and organic-matter pores. The first two are more common in gas shale. The adsorption isotherm experiments suggest that adsorbed gas in black shale is from 1.71m3/t to 2.62m3/t with an average of 2.33m3/t, which indicate a strong adsorption capacity. The adsorption ability of Nangao shale and TOC shows positive correlation, however, shale and Tmax show negative correlation. TOC content is also positively correlated with nanopore distribution frequency. This study shows that, TOC content, quartz content, and nanopore distribution play leading roles in influencing the adsorption capacity of gas in the Nangao shale.
The abundances of rare earth elements (REE) of the black shale on the slope facies from Lower Cambrian in Sansui and Huangping were measured by using inductively coupled plasma mass spectrometry (ICP-MS). The results show that the total REE content is 68.28×10 -6-252.00×10 -6, the average one is 133.16×10 -6; the ratio of ΣLREE to ΣHREE is 2.38-5.70, which shows that LREE are relatively more intensive than HREE. δEu=0.49-0.75, which shows that the deposition temperature is below 250°C. δCe=0.34-0.85, which reflects that a rock deposits under the condition of relatively hypoxic sea. The ratio of rock (La/Sm) N is between 1.77 and 3.31, which represents that it has added with the materials from deep resource. The chondrite normalized patterns of black shale incline to the right, the North American shale normalized REE patterns of siliceous rocks are nearly flat or left. Combined with the discrimination diagrams of La/Ce and La/Yb-REE, the black shale of Southeast Qian formed in a dry, hypoxia and shallow sea, with the participation of some hydrothermal fluids in the formation process.
Organic matter from Early Cambrian black rock series in western Hunan was derived from organisms in the euphotic zone. For not interfered by the detrital component, REE geochemistry of organic matter can demonstrate paleoenvironmental characteristics of the upper seawater. As the main part of organic matter, kerogen is fully consistent with the modern seawater-like REE pattern which shows a progressive enrichment towards the heavier REE, depletion of Ce, positive anomalies of Y and no obvious anomalies of Eu. The characteristics of kerogen may trace paleoredox condition of upper seawater. The euxinic environment may have arised in the lower part of sections in the Yangtze area, and the depositional condition may have shifted to gradual oxidization in the upper part of sections. The negative Ce anomalies of bulk rocks are most probably derived from organic matter input. Gradual increase of Ce/Ce* values indicates increasing oxygen content of upper seawater, and weakening of euxinic environment of bottom seawater may result in rising of oxygen content of upper seawater.
The living environment of the early cambrian Niutitang biota that is buried in situ is characterized by shallow continental shelf environment beneath the wave base with warm temperature, normal salinity, weak hydrodynamics, soft muddy substrates, and overall hypoxia but partial oxygenation conditions. From the bottom of the profiles towards the upper direction, both hydrodynamics and oxygen content would increase gradually, with the highest hydrodynamics found at the top of the profiles where oxygen content is close to the normal level. Niutitang biota could be classified as either benthic sessile types or swimming/floating types. As a result, corresponding ecological communities could be divided as the floating communities in the upper layer of the water body and the benthic sessile communities below, with the development of both communities controlled by oxygenation incidents. The food chain structure of Niutitang biota is relatively simple, with algae as primary producers, animals from higher trophic level as filter feeders, and Sphenothallus as the only predatory species. Although the palaeoecosystem of Niutitang biota is simple and unstable, it represents the prototype of the modern ecosystem. Hypoxia incident caused by the thriving of algal species is one of the most important reasons that lead to the mass mortality of creatures and the formation of fossils.
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