REE Characteristics Research Articles

Catagenetic type of manganese ores: REE and isotope (δ13C, δ18O) geochemical features (on the example of the Usa deposit, Russia)

Catagenetic type of manganese ores: REE and isotope (δ13C, δ18O) geochemical features (on the example of the Usa deposit, Russia)

Genesis of the Tiemuli skarn W–Fe deposit in the Nanling region, South China: Constrains from multi-mineral U-Pb geochronology and scheelite geochemistry

The Tiemuli scheelite-magnetite (W-Fe) deposit is located in the central part of the Nanling region, South China and contains five skarn-type ore bodies that are associated with the Tiemuli porphyritic granite. The mineralization process of this deposit can be divided into three stages: the prograde skarn, the retrograde skarn, and the quartz-sulfide-calcite stages. Zircon grains from the porphyritic granite yielded a U-Pb age of 138±1 Ma. The vesuvianite and garnet grains from the prograde skarn stage yielded a weighted mean 206Pb/238U age range of 139–136 Ma. The apatite and cassiterite from the retrograde stage exhibited a weighted mean 206Pb/238U age range from 138 to 136 Ma. These U-Pb ages suggest that the Tiemuli granite emplacement and W–Fe skarn mineralization occurred during the Early Cretaceous period. Based on the cathodoluminescence (CL) characteristics, two generations of scheelite were identified. Sch-I showed homogeneous luminescence, whereas Sch-II exhibited a complex core-mantle-rim texture. REE was incorporated into scheelite following the substitution mechanism of 3Ca2+ = (2REE)3+ + □Ca, allowing the scheelite to effectively reflect the REE features of the ore-forming fluids. The Mo concentration and Eu anomaly in the scheelite reflected fluctuation in the fluid conditions during the scheelite crystallization. While transiting from the prograde to retrograde skarn stage, the hydrothermal fluid shifted from a relatively reducing environment to a relatively oxidizing environment. The Tiemuli deposit is characterized by the reductive skarn mineralization that occurred during the Early Cretaceous lithospheric expansion. Variations in the physicochemical characteristics of the fluids and fluid-rock interaction played crucial roles in the W–Fe mineralization of the Tiemuli deposit. Thus, this deposit serves as a unique example of a W–Fe mineralization event in the Nanling region of South China during the Early Cretaceous.

Geochemical records and environmental analysis of the Ediacaran-Cambrian boundary in Eastern Yunnan, South China

The Ediacaran–Cambrian transition is characterized by an unprecedented change in biosphere, lithosphere, and atmosphere. However, the identification of this transition and its global correlation remains debated. East Yunnan is a region of utmost importance in the study of the Ediacaran-Cambrian transition. Although strata from this region have been well studied, recognition of the base of the Cambrian continues to be controversial. This paper presents, a carbon chemostratigraphic isotope and trace element profile through the Dengying and Zhujiaqing formations in the Sujiawa section in East Yunnan. Through carbon isotope and trace element analyses of the Dengying and Zhujiaqing formations we attempt to regionally correlate the section and discuss the paleo-marine redox environment changes during this period. The Fe content of samples is low and the Mn/Sr ratios of the majority of the Sujiawa section samples is lower than 10 and shows no correlations with δ13Ccarb and δ18Ocarb values, indicating that the samples retain near-primary δ13Ccarb characteristics. The carbon isotope profile from the Sujiawa section, shows a prominent negative δ13Ccarb in the basal Zhongyicun Member (N1’, −7.3‰) that most likely represents the prominent negative excursion recorded in Ediacaran-Cambrian strata across the world. REE patterns and Y/Ho ratios were used to screen samples, to ensure that the primary seawater REE features and Ce anomalies were preserved. The samples satisfying this screening process show seawater-like REE distribution patterns in leached carbonates and may have recorded Ce anomalies of the seawater from which the carbonates precipitated. These data show that the Baiyanshao Member has Ce/Ce* values between 0.57 and 0.88, the Daibu Member between 0.56 and 0.83, and the Zhongyicun Member between 0.60 and 0.96. The Ce anomalies through the section indicates that the region experienced suboxic-oxic-anoxic conditions throughout the Ediacaran-Cambrian period. Carbon isotope and trace element analyses indicate that the strata record a negative carbon isotope excursion event and anoxic conditions during the period of deposition. This study complements the carbon chemostratigraphic information and trace element data in the region and provides valuable clues for understanding the lithofacies changes through the Ediacaran and Cambrian in different regions of South China.

Geochemical characteristics of hydrogen, oxygen, carbon isotopes and REE of cenozoic dolomites in Well Xike 1, Xisha Islands, South China sea and the significance for dolomitization in island-reef areas

Due to the special tectonic background and complex geological evolution characteristics of the South China Sea, reef dolomite reservoirs (sush as Well Xike 1) are widely developed. Based on the drilling core data of Well Xike 1, the structure and geochemical characteristics of dolomite reservoirs, including carbon, oxygen, hydrogen isotopes and REE were systematically studied using geochemical and petrological methods. It is found that the geochemical characteristics of REE show that the main diagenetic environment of dolomites is a low-temperature alkaline semi open oxidation environment; the carbon and oxygen isotopes of the dolomites are generally lack of correlation, the δD value is significantly lower than the hydrogen isotope value of seawater. Meanwhile, the oxygen isotope value of deep dolomites is negatively biased, which may be due to the increase or decrease of pore water temperature caused by deep thermal convection that related to the regional tectonic movements of the South China Sea. The δ18O value is also consistent with the geological reality of increasing saddle dolomite content in deep dolomites. The distribution of the δ13C value indicates that the dolomite inherited the carbon of the original limestone during dolomitization, while the characteristic of the δD value shows that it may be affected by the mixing of atmospheric precipitation and concentrated seawater in the quasi contemporaneous period. Based on the comprehensive analysis of the geochemical characteristics of the Well Xike 1, it is considered that the higher diagenetic temperature could be an important factor leading to the huge differences between the diagenetic model of deep and shallow dolomites. The geochemical characteristics of the shallow dolomites show that it is mainly reflux infiltration dolomitization under the micro evaporation and concentration sea water environments, while the deep dolomite is transformed by the hot water fluids in the epigenetic diagenetic evolution stage.

Geochemistry and origin of hydrothermal apatite in Carlin-type Au deposits, southwestern China (Gaolong deposit)

Apatite can record the evolution of magmatic–hydrothermal ore systems, but it is unclear whether it is suitable for investigating low-temperature, sediment-hosted hydrothermal ore systems. To fill the knowledge gap. we investigated the Gaolong Carlin-type Au deposit in the Guangxi Zhuang Autonomous Region, southwestern China (Youjiang Basin). Results show that the hydrothermal apatite occurs in quartz–sulfide ores, has an intact crystal form and homogeneous texture, and is F-rich (2.77–3.67 wt%) and Cl-poor (<0.01 wt%). The apatite is enriched in rare earth elements (REEs; ΣREE = 739–2481 ppm) and has middle-REE-enriched chondrite-normalized REE patterns with positive Eu anomalies (δEu = 1.35–2.00). These REE features were inherited from a reducing hydrothermal system in which anion complexation was dominant. The apatite has high Sr contents (up to 0. 9 wt%) and relatively uniform initial 87Sr/86Sr ratios (0.709055–0.709208), suggesting that Sr was leached or derived from the deep continental crust. In situ U–Pb dating of 156.8 ± 8.3 Ma for apatite shows that the Late Jurassic–Early Cretaceous was an important time for the formation of Carlin-type Au deposits in the Youjiang Basin. The Au mineralization was associated with far-field tectonism and deeply sourced hydrothermal ore-forming fluids. Hydrothermal apatite can be used to fingerprint low-temperature, sediment-hosted hydrothermal ore systems.

High-pressure/ultrahigh-temperature mafic and felsic granulites associated with magnetite deposits from the Jiaobei terrane, Jiao-Liao-Ji Belt, North China Craton

Garnet-bearing mafic and felsic granulites associated with magnetite deposits that have undergone high-pressure (HP) and ultrahigh-temperature (UHT) metamorphism occur in the Yangjiazhai area of the Jiaobei terrane within the Jiao-Liao-Ji Belt, North China Craton. In mafic granulite, the early prograde assemblage (M1) is preserved as inclusions in the core of garnet. Matrix minerals and the garnet mantle define the peak metamorphic assemblage (M2). Near-isothermal decompression (M3) is recorded by the garnet rim and symplectitic minerals. The final cooling process is defined by the late growth of amphibole (M4). Abundant magnetite occurring in some mafic granulite samples may be inherited from the protolith. The felsic granulite records the peak metamorphic stage (M2) defined by the matrix minerals and the garnet mantle, and the retrograde stage defined by the symplectite and the garnet rim (M3). P-T estimations using pseudosection techniques and two-feldspar thermometry yield ∼8–9.8 kbar/710–828 °C for the prograde stage M1, ∼14.5–15 kbar/1040–1073 °C for the peak P-T stage M2, 9.1–10 kbar/965–1015 °C for the post-peak stage M3 and 6.6–9.4 kbar/820–853 °C for the cooling stage M4 as recorded by the mafic granulite. P-T conditions of ∼14–16.5 kbar/985–1077 °C for the peak P-T stage M2 and ∼7.8–9 kbar/889–902 °C for the post-peak stage M3 were derived from the felsic granulite. Thus a clockwise trajectory can be defined by constraining peak-metamorphic high-pressure/ultrahigh-temperature (HP-UHT) conditions. Zircon U-Pb dating and REE characteristics show a contrasting information. Oscillatory zoned zircon domains yield an age of ∼2150 Ma with a steep HREE characteristic, which is interpreted to indicate a magmatic origin of the mafic granulite. The peak HP-UHT stage obtained from metamorphic zircon domains in felsic and mafic granulites is constrained by an age of ∼1950–1900 Ma, which possibly corresponds to a collisional event shortly after subduction. The decompression occurred at ∼1880 Ma following the subsequent cooling at ∼1820 Ma indicating a rapid exhumation of the HP-UHT granulite followed by slow cooling.

Geochronology and Petrogeochemistry of Metallogenic Intrusions in the Shanmen Large‐scale Silver Deposit in the Siping Area of Jilin Province, NE China

AbstractThe Shanmen deposit, located in the Siping area of Jilin Province, is one of large‐scale silver deposits in Northeast (NE) China. Due to its high Ag grade, associated gold resources and special tectonic location, this deposit has important theoretical and ore‐prospecting significance. To present new data on the ore genesis, mineralization time and tectonic settings, the relationship between silver mineralization and intrusions in this deposit has been studied and some analyses have been carried out, including LA‐ICP‐MS zircon U‐Pb dating, whole‐rock major and trace element analysis and Sr‐Nd‐Pb isotope analysis of the granitoids associated with silver mineralization. Studies on deposit geology indicate that the ore‐hosting granodiorite and monzogranite have intimate genetic relationships with silver mineralization. LA‐ICP‐MS zircon U‐Pb dating results of the both intrusions are 167.6 ± 1.9 Ma and 167.0 ± 1.5 Ma, respectively, implying that the two intrusions and associated silver mineralization in the Shanmen deposit formed during the Middle Jurassic. Major element analytical data suggest that the two intrusions are high‐K calc–alkaline series I‐type granites and belong to metaluminous and peraluminous rocks. Both intrusions have similar REE characteristics with regard to the relative enrichment of LREEs, depletion of HREEs, obvious fractionation of LREEs and HREEs (ΣLREE/ΣHREE = 8.68–14.09, (La/Yb)N = 12.51–21.96), moderately negative Eu anomalies as well as weakly negative Ce anomalies (δEu = 0.56–0.71, δCe = 0.93–1.09). Moreover, the samples are generally enriched in LILEs and depleted in HFSEs. The rock assemblages and geochemical characteristics of granitoids in the Shanmen deposit suggest that they formed in an active continental margin associated with the Paleo‐Pacific plate subduction. The Sr‐Nd‐Pb isotope compositions show that both the granodiorite and monzogranite have low Sr initial ratios and high Sm/Nd initial ratios. The εNd(t) values and young Nd‐model ages are –3.2 to –1.7 and 990 Ma to 1110 Ma, respectively. Considering the Sr‐Nd isotope compositions, in conjunction with the relatively high 206Pb, 207Pb and 208Pb, alongside the relatively low 204Pb, it can be concluded that the Middle Jurassic intermediate‐acidic magma represented by the studied monzogranite and granodiorite in the Shanmen deposit, was derived from the partial melting of juvenile lower crust enriched in mantle‐derived materials and affected by the mixing of ancient crustal materials during the ascension and intrusion processes.

Tracing the effects of fO2, pressure and H2O on the ore-forming magmas: Perspective from zircon REE composition

Efforts have been devoted to explore if there are independent zircon trace element ratios that could be proxy of oxidation state and magmatic water content, and can distinguish between mineralization and barren granites. In this study, I evaluated the difference of the zircon elemental signatures between fertile magmatic suites (which host porphyry Cu-Mo, Mo, Cu-Au, Cu, Cu-Mo-Au, Mo-Cu, and Au deposits) and infertile I-type granites. The “potential” magmatic suites, which means that they are supposed to develop porphyry deposits based on the previously proposed ore-forming criteria (e.g., zircon Eu/Eu* > 0.5) and yet no associated mineralization has been found, were also included in this comparison. Most of the trace elemental signatures cannot differentiate the I-type granite zircons from fertile zircons and “potential” zircons, except for the Eu anomaly (∼0.5 < Eu/Eu* < ∼0.8 for ore-related and “potential” suites; ∼0.2 < Eu/Eu* < ∼0.5 for I-type granites) and middle/heavy rare earth element variation diagrams (MREE and HREE; e.g., Eu/Gd versus Gd/Yb, Lu/Yb versus Gd/Yb, Sm/Ho versus Sm/Lu, and Yb/Lu versus Dy/Lu). The behavior of zircon REE possibly indicate a more MREE-depleted signature for the ore-forming and “potential” suites. Taken together, zircon REE features documented in this study highlight the fractional crystallization of hornblende and garnet, and the delay of plagioclase crystallization, revealing the combined effect of high overlying pressure, high fO2 and high H2O (or H+) concentration on the differentiation of fertile magmas.

Geochemical evolution of structure-bedding controlled hydrothermal dolomites of the Kinta Valley, Western Malaysia

Study of secondary dolomitization and documentation of the evolution of basinal fluids and host rock, especially that of fine-grained deepwater carbonate mudstone are of interest toward of reservoir characterization. From this perspective, field characteristics in relation to mineralogy, petrography and geochemistry of the Paleaozoic carbonates of the Kinta Valley, Peninsular Malaysia, were investigated and documented. A total of 33 samples were collected across the various degree of dolomitization and analyzed for their petrographic properties (n = 55), while mineralogical and geochemical compositions (n = 33) were analyzed using XRD, ICP and ICPMS analysis, respectively. These analyses were performed to elucidate the diagenetic events, episodic dolomitization and development of fluid evolution as a function of regional tectono-thermal events during the Permian to Cenozoic. It is interpreted that the host calcareous mudstones (HCMs) were deposited in oxygenated, open marine seawaters. The studied samples retain the original seawater characteristics despite negligible, minor alterations of REE characteristics due to diagenesis and detrital input. Though the concentrations and trends of REEs of the dolomitic facies are modified to a certain extent, their affinities with HCMs are preserved. Enrichments of Mg, Mn, Na and depletion of Sr are linked to the effect of dolomitization under the open system of diagenesis. The redox conditions fluctuated to show distinct, recognizable variations between different dolomite phases. The MREE enrichments with other trace elements such as Mn, Fe and low Sr content indicate the association of diagenetic fluids with deep-circulating crustal hydrothermal fluids. Hydrothermal influence has been recorded along the fractures, which might have originated from the sub-surface litho units and facilitated episodic dolomitization, especially during significant tectono-thermal events. This study affirms that the occurrence, geochemical and mineralogical evolution of the dolomites of the Kinta Valley were controlled by magmatic events, whereas the intensity of replacement dolomitization was controlled by host rock texture, varied bulk chemistry of host rock, circulating fluid and temperature.

Qia’erdunbasixi Fe–Cu Deposit in Sawur, Xinjiang: A Case Study of Skarn Deposit Hosted by Volcanic Rock

The Sawur Cu–Au belt, northern Xinjiang, China, is the eastward extension of the Zarma–Sawur Cu–Au belt in Kazakhstan, where Late Paleozoic volcanic rocks and intrusions are highly developed. The Qia’erdunbasixi Fe–Cu deposit in Sawur is a recently discovered deposit and is still under exploration. The intrusive rocks are syenite and diorite, and the wall rocks consist of andesite and minor basalt, lamprophyre, and tuff. The U–Pb SHRIMP zircon age of the Qia’erdunbasixi syenite intruding into the volcanic rocks is 345 ± 2.2 Ma (MSWD = 1.3), presenting as the lower limit of skarn Fe mineralization. The intrusives belong to the calc–alkaline to high-K calc-alkaline series with large ion lithophile element (LILE) enrichment, high LREE/HREE fractionation, and high field strength element (HFSE) depletion. The initial 87Sr/86Sr ratios of the Qia’erdunbasixi syenite range from 0.70403 to 0.70420, and the εNd(t) values are from +5.5 to +6.8, which are the typical characteristics of island arc igneous rocks. Diorites having similar REE features with syenite should share the same magma source. Magnetite and copper mineralization develop mostly along the contact zones of syenite and diorite, respectively. Fe mineralization develops along the contact zone of syenite, with typical skarn zonation. The metallogenesis event can be divided into the prograde skarn stage (diopside–augite–andradite–magnetite–calcite–quartz), retrograde skarn stage (epidote–chlorite–actinolite–K-feldspar–calcite–magnetite–quartz), and quartz–sulfide stage (quartz–magnetite–K-feldspar–calcite–sercite–chlorite–actinolite–prehnite–chalcopyrite–pyrite). The early–mid-stage magnetite with certain amounts of Ti and V was crystallized from magma, while the late-stage magnetite has the typical characteristics of hydrothermal calcic skarn magnetite. The temperature of mineralization is between 350 and 450°C based on mineral assemblages and phase diagrams. Copper mineralization is concentrated along the outer contact zone of the diorite. Paragenesis sequences of the four stages of mineralization could be identified for copper mineralization: 1) albite–quartz; 2) chalcopyrite–pyrite–gold–seriate–quartz; 3) chalcopyrite–pyrite–epidotic–reunite; and 4) sphalerite–galena–quartz–calcite. Qia’erdunbasixi is a composite deposit with skarn-type Fe mineralization and mesothermal Cu mineralization and has a genetic relationship with magmatism in an island arc setting.

REE and cathodoluminescence features of scheelites from the Tongshankou Cu–Mo deposit in Eastern China

The Tongshankou porphyry-/skarn-type Cu–Mo deposit is located in the Edong ore district, the largest district of the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in Eastern China. From the ore-causative granodiorite porphyry and skarn at Tongshankou, three types of scheelite are recognized (Sch1 to Sch3): Sch1-subhedral–anhedral, occurring in the chalcopyrite ores within the porphyry; Sch2-granular, occurring inside garnet in the endoskarn and Sch3-subhedral–anhedral, occurring in calcite veins in the exoskarn. Sch2 and Sch3 have different internal texture according to CL images: Sch2-Dark core and bright rim, with the heterogeneous dark core; Sch3-Dark core and narrow dark rim, showing patches of different brightness in core. In addition, three REE types have been differentiated: 1) REE-Type1: LREE-enriched, HREE-depleted, and negative Eu anomalies patterns (Sch1), which is related to the porphyry system; 2) REE-Type2: No apparent fractionation between LREE and HREE, with positive Eu anomalies (Sch2), which is related to the endoskarn; 3) REE-Type3: Higher LREE and lower HREE with extremely positive Eu anomalies (Sch3), which is related to the exoskarn. The negative MoO3 vs. WO3 correlation for the Tongshankou scheelites suggests Mo-W substitution in the scheelite crystal lattice. No humped pattern (MREEs-enriched) of REE, lower Na content and the lack of correlation between Na and REE at Tongshankou excluded the Na substitute into the Ca site. The positive correlation with Nb and REE for Sch 1 and 3, suggests equation [2] and [3] should be more plausible mechanism to generate the Tongshankou scheelite REE patterns.

Implications for Metallogenesis and Tectonic Evolution of Ore-Hosting Granodiorite Porphyry in the Tongkuangyu Cu Deposit, North China Craton: Evidence from Geochemistry, Zircon U-Pb Chronology, and Hf Isotopes

The Tongkuangyu copper deposit in Zhongtiaoshan at the southern margin of the North China Craton is one of the oldest porphyry Cu deposits in the world and its metallogenesis and tectonic evolution have been debated. Here, porphyritic intrusion geochemical and geochronological data are reported to identify the diagenetic age, mineralization, tectonic setting, and evolution of the deposit. Geochemical data show that granodiorite porphyry is a peraluminous rock, with low concentrations of Fe (~3.99%) and Ti (~0.29%) and high concentrations of alkali (~6.13%) and high Al (~15.42%) and Mg numbers (~51). The rocks show comparative enrichment of Na, K, and Mg; higher La/Yb ratios, no significant Eu anomaly, and obvious Nb–Ta–Ti negative anomaly, showing similar geochemical characteristics to Archean TTG and sanukitoid. ΣREE vary greatly, ranging from 33.47 × 10−6 to 277.81 × 10−6 (average 137.09 × 10−6). The characteristics of REE show obvious fractionation of LREE and HREE, enrichment of LREE, and depletion of HREE. Some of the LREE (La and Ce) and LILE (K, Rb, and Ba) are enriched, but some of the LILE (Th and U) are depleted. In addition, some of the HFSE (Nb, Ta, P, and Ti) are depleted while some (Zr and Hf) are enriched. High precision LA–MC–ICP MS zircon U–Pb dating yield concordant ages of 2159 ± 19 Ma, which is broadly coeval with ore formation (~2.1 Ga) in the area. Zircon εHf(t) values range from −3.8 to 1.13, with a model age of 2778 to 2959 Ma, indicating that the formation of porphyry is related to the partial melting of Archean crust (~2.7 Ga) with a minor amount of mantle material added. Tongkuangyu granodiorite porphyry formed in the tectonic setting of the post-orogenic extension in the Paleoproterozoic, and Tongkuangyu Cu deposit may be related to the extension of the North China Craton in the Paleoproterozoic.

U‐Pb Geochronology and Geochemistry of Stratiform Garnet from the Aqishan Pb‐Zn Deposit, Eastern Tianshan, Xinjiang, NW China: Constraints on Genesis of the Deposit

AbstractThe Aqishan lead‐zinc deposit, located in the Jueluotag metallogenic belt of eastern Tianshan, Xinjiang, Northwest China, has a stratiform occurrence in the marine volcanic tuff of the Yamansu Formation. The ore body has a typical double‐layer structure, having a stratified, stratoid, lenticular upper part and a veined, stockwork‐like lower part. The occurrence of the upper orebody is consistent with that of the volcanic tuff wall rock. The ore minerals are mainly chalcopyrite, pyrite, sphalerite, galena and magnetite, the altered minerals mainly being silicified, such as sericite, chlorite, epidote, garnet. The garnetized skarn, being stratiform and stratoid, is closely related to the upper part of the orebody. Geological observations show that the limestone in the ore‐bearing Yamansu Formation is not marbleized and skarnized. Spatially, it is associated with the ferromanganese deposits in the marine volcanic rocks of the Yamansu Formation. These geological features reflect the likelihood that the Aqishan lead‐zinc deposit is a hydrothermal exhalation sedimentary deposit. The results from the EPMA show that the garnet is mainly composed of grossular–andradite series, contents being in a range of 34.791–37.8% SiO2, 32.493–34.274% CaO, 8.454–27.275% FeO, 0.012–15.293% Al2O3, 0.351–1.413% MnO, and lower values of 0.013–1.057% TiO2. The content of SiO2 vs. CaO and FeO vs. Al2O3 has a significant positive correlation. The results of ICP‐MS analysis for the garnet show that the REE pattern is oblique to right in general. The total amount of rare earth elements is relatively low, ΣREE = 71.045–826.52 ppm, which is relatively enriched for LREE and depleted for HREE. LREE/HREE = 8.66–4157.75, LaN/YbN = 23.51–984.34, with obvious positive Eu and Ce anomalies (δEu = 2.27–76.15, δCe = 0.94–1.85). This result is similar to the REE characteristics of ore‐bearing rhyolite volcanic rocks, showing that the garnet was formed in an oxidizing environment and affected by clear hydrothermal activity. The U‐Pb isotopic dating of garnet by fs‐LA‐HR‐ICP‐MS gives an age of 316.3 ± 4.4 Ma (MSWD = 1.4), which is consistent with the formation time of the Yamansu Formation. According to the study of deposit characteristics and geochemical characteristics, this study concludes that the Aqishan lead‐zinc deposit is a hydrothermal exhalation sedimentary deposit, the garnet being caused by hydrothermal exhalative sedimentation.

Diagenetic evolution and its impact on reservoir porosity in Lower Triassic Jialingjiang Formation in eastern Sichuan Basin, SW China: Evidence from petrography, trace elements, and Isotopic Studies

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13CVPDB values of −3.4–+7.3 ‰ and δ18OVPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm)N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34SV-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13CVPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18OVPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34SV-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity.

Geochemical Characteristics of Early Permian Pyroclastic Rocks in the Jimunai Basin, West Junggar, Xinjiang (NW China): Implications for Provenance and Tectonic Setting

AbstractDuring the Late Paleozoic, West Junggar (Xinjiang, NW China) experienced a shift in tectonic setting from compression to extension. Ha'erjiao is an important area for investigating collisional structures, post–collisional structures, and magmatic activities. Based on the petrological and geochemical characteristics of pyroclastic and other volcanic rocks in the Permian Kalagang Formation from the borehole ZKH1205 in the Jimunai Basin, the main types of source rock for the pyroclastic rocks deposited in the basin are identified and their implications for the Early Permian tectonic setting examined. The abundance of basalt and andesite lithic fragments in the pyroclastic rocks, together with the REE characteristics and the contents of transition and high field strength elements show that the source rocks were chiefly intermediate–basic volcanic rocks. High ICV values, low CIA values, low Rb/Sr ratios, low Th/U ratios and the mineralogical features suggest weak chemical weathering of the source rocks; the geochemical patterns of the pyroclastic rocks might not only have been impacted by crustal contamination but also might be related to the nature of the magma from the source area. The geochemical properties of the pyroclastic rocks distinguish them from arc‐related ones, and such samples plot in the within‐plate basalt (WPB) field in some diagrams. This is consistent with the formation background of the Early Permian volcanic rocks in this region.

Mineralogy and mineral chemistry of carbonates from the Zhulingou Ge-Zn deposit in Guizhou Province and its geological significance

以碳酸盐岩为容矿岩石的后生热液铅锌矿床（Mississippi Valley-type，MVT矿床）是世界上一种重要的铅锌矿床类型，也是锗的重要工业来源之一。热液碳酸盐矿物是MVT矿床中最主要的脉石矿物，其形成贯穿整个MVT矿床成矿过程。因此，碳酸盐矿物携带丰富的成矿信息，是认识MVT矿床成因的重要补充。位于贵州省贵定县境内的竹林沟锗锌矿床，是近年来新发现的富锗锌矿床（平均品位97.9×10^-6 Ge，6.54% Zn），赋存于泥盆系碳酸盐岩中。本次工作发现该矿床不同期次热液白云石的矿物学和微量元素地球化学特征存在明显差异：成矿期前白云石（Dol1）主要呈细脉状穿插围岩，被成矿期白云石和硫化物脉穿插，部分呈细粒状被后期白云石包裹；成矿早期白云石（Dol2）主要呈粗粒状，与闪锌矿共生；主成矿期白云石（Dol3）主要呈脉状、团块状，与闪锌矿和黄铁矿共生；成矿晚期白云石（Dol4）呈团块状充填于闪锌矿矿石或者围岩中；成矿期后白云石（Dol5）呈脉状穿插或包裹其它期次白云石/闪锌矿-黄铁矿条带。C-O同位素研究表明，成矿期白云石主要来源于碳酸盐围岩的溶解，成矿流体中的C来源于围岩，而较低的δ¹⁸O值可能是亏损¹⁸O的成矿流体和围岩间水/岩反应过程中O同位素发生交换的结果。激光剥蚀电感耦合等离子体质谱（LA-ICPMS）白云石原位微量元素分析结果表明，Dol2-Dol4的Y/Ho比值相对稳定（30.5~47.9），结合区域成矿特征，认为形成成矿期白云石的流体主要为盆地卤水，与该矿床属于MVT矿床事实吻合。Dol1和Dol5具有相对较低的稀土总量（∑REE=3.97×10^-6~29.7×10^-6），很可能与白云岩围岩（∑REE=25.2×10^-6~61.3×10^-6）溶解作用有关，直接继承围岩的稀土元素组成特征；成矿期Dol2-Dol4的∑REE较高（∑REE=39.2×10^-6~117×10^-6），暗示成矿流体本身也携带了部分稀土元素。成矿各期次白云石均具有明显的Eu负异常（δEu=0.38~0.72）特征，指示成矿温度可能较高（>200℃），Ce正异常相对稳定（δCe=1.10~1.28），暗示其成矿流体pH值保持弱酸性。综上，本文认为竹林沟锗锌矿床的形成经历如下过程：在伸展背景下，深循环盆地卤水萃取下伏地层和基底岩石中的Zn、Ge和REE等元素，形成富金属流体，受构造作用驱动沿区域性断裂不断向上运移，在赋矿层位与富硫流体发生混合作用，导致闪锌矿等硫化物沉淀；在整个成矿过程，成矿环境经历了还原→氧化的转变，成矿元素发生了共生分异，在竹林沟形成富锗锌矿体，在牛角塘形成富镉锌矿体，而pH始终保持弱酸性，直至被围岩碳酸盐岩中和。;Carbonate-hosted epigenetic hydrothermal Pb-Zn deposits, i.e. Mississippi Valley-type (MVT) deposits, are important lead-zinc resources in the world and a major industrial source of germanium (Ge). Wherein hydrothermal carbonates are the most important gangue minerals and their formation runs through the whole metallogenic process of MVT deposits. Therefore, hydrothermal carbonates contain rich mineralization information and are an important supplement for understanding the genesis of MVT deposits. The Devonian carbonate-hosted Zhulingou Ge-Zn deposit (97.9×10^-6 Ge, 6.54% Zn) located in Guiding County, Guizhou Province, is newly discovered in recent years. The hydrothermal dolomite of the Zhulingou deposit have various mineralogical features:the pre-ore dolomite (Dol1) occurs mainly as veinlets or interspersed in host rocks and cross-cut by ore-stage dolomite and sulfides, or occurs as fine crystals enclosed by the late dolomite; the early ore-stage dolomite (Dol2) is mainly coarse-grained and coexists with sphalerite; the main ore-stage dolomite (Dol3) is mainly vein-like or clumpy, coexisting with sphalerite and pyrite; the late ore-stage dolomite (Dol4) fills into sphalerite or host rocks in the form of clumps; the post-ore dolomite (Dol5) crosscuts/encloses early stages of dolomite/sphalerite-pyrite bands. The C and O isotopic data show that the formation of Dol2-Dol4 is likely related to the dissolution of the carbonate host rocks, and the C in the ore-forming fluid derives from the country rocks, the lower δ¹⁸O may be contributed to the isotope exchange between the host rock and the ore-forming fluid with ¹⁸O depletion. The laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) in situ analytical results show that the Y/Ho ratios of Dol2-Dol4 are relatively constant (30.5~47.9). This indicates that the fluids formed metallogenic dolomite are basin brine, which is supported by the MVT genesis of the Zhulingou deposit. Dol1 and Dol5 with lower ∑REE values (3.97×10^-6~29.7×10^-6) may be related to the dissolution of the host rock dolostone (∑REE=25.2×10^-6~61.3×10^-6), and directly inherit the REE characteristics of the latter. The relatively high ∑REE values (39.2×10^-6~117×10^-6) of Dol2-Dol4 suggest an REE source from the ore-forming fluids. Hydrothermal dolomite at Zhulingou is characterized by significantly negative Eu anomalies (δEu=0.38~0.72), implying a relatively high ore-forming temperatures (>200℃). The positive Ce anomalies (δCe=1.10~1.28) suggest weakly acidic ore-forming fluid. In conclusion, this study proposed that the formation of the Zhulingou Ge-Zn deposit experienced the following process:under the regional extensional stress, the deep circulated brine extracted the ore-forming elements (e.g. Zn, Ge and REE) from underlying strata and basement rocks, and formed the metal-rich fluids, which continued to transport upward along the regional faults driven by tectonic action. The metal-rich fluids were mixed with the sulfur-rich fluids in the ore-bearing strata to form the sulfide such as sphalerite. During the ore-forming process, the metallogenic environment went through the transition from reduction to oxidation accompanied by symbiosis and differentiation of ore-forming elements, forming Ge-rich zinc ore body at Zhulingou, and Cd-rich zinc ore body at Niujiaotang, while the fluids remain weakly acidic until neutralized by the host rock carbonates.

Timing and characterization of multiple fluid flow events in the Beibuwan Basin, northern South China Sea: Implications for hydrocarbon maturation

The history of thermal and fluid flow events is of crucial importance to understand the mechanism of hydrocarbon maturation. The northern South China Sea (SCS) contains abundant hydrocarbon resources with high heat flow anomalies, but the mechanisms responsible for modification of thermal regimes are poorly understood. Here we conducted the first high-precision Rb-Sr isochron dating and geochemical analyses of authigenic illites separated from the Palaeogene sandstones in the Beibuwan Basin, northern SCS. Eight high-precision Rb-Sr isochron ages reveal three major thermal and fluid flow events in the Beibuwan Basin, northern SCS. The 34.5 ± 0.9 Ma and 23.6 ± 0.8 Ma events occurred in the western area are probably related to the Eocene-Oligocene transition characterised by a significant regional unconformity resulted from intensive uplift of Hainan Uplift and Oligocene-Miocene transition marked by a post-rift unconformity, respectively. By contrast, the 31.2 ± 0.6 Ma event affecting the eastern area appears to be closely associated with the igneous intrusion dated by zircon U/Pb to 32.3 ± 0.7 Ma. REE and stable isotope characteristics indicate that the illites from the western area formed within a meteoric-hydrothermal system, whilst those from the eastern area were likely precipitated from fluids heated by the large-scale igneous intrusion at higher water/rock ratios. Moreover, our data in conjunction with palaeo-temperature reconstructions show that the fluid flow events had profound effects on the rapid organic maturation in the northern SCS.

Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex, Central Mongolia: Petrogenesis and tectonic implications

The Tsoroidog Uul’ accretionary complex is hosted by the Tsetserleg terrane in the southwestern Khangay-Khentey orogenic system (Central Mongolia), which represents the segment of the Central Asian Orogenic Belt and has significant regional implications for its tectonic evolution. This paper reports the results of field investigations and petrography, bulk-rock major and trace element geochemical, as well as SmNd isotopes of Middle Paleozoic intermediate-mafic rocks from the Tsoroidog Uul’ accretionary complex. We investigate a wide range of rock types which can be divided into 4 groups on the basis of their TiO2 and REE characteristics. Group 1 is characterized by moderate TiO2, relatively flat chondrite-normalized REE patterns (La/Smn = 1.0; Gd/Ybn = 1.2). These basalts are divided into two subgroups: (1) Nb/Thpm = 3.6, Nb/Lapm = 0.8, Zr/Nb = 24.6, and Ce/Ybpm = 0.8 (N-MORB type); (2) Nb/Thpm = 1.3, Nb/Lapm = 1.1, Zr/Nb = 11.9, and Ce/Ybpm = 1.6 (E-MORB type). Group 2 shows high TiO2 and LREE (La/Smn = 3.0), differentiated HREE (Gd/Ybn = 2.5), positive Nb anomalies shown in primitive mantle-normalized multi-element patterns (Nb/Thpm = 1.2; Nb/Lapm = 1.1), and low Zr/Nbav. ratios (~6). Group 3 displays low TiO2, high LREE (La/Smn = 3.6), Zr/Nbav. = 24.4, and low Nb (Nb/Lapm = 0.2). Group 4 exhibits moderate TiO2, flat REE patterns (La/Smn = 0.8; Gd/Ybn = 1.1), negative Nb anomalies (Nb/Thpm = 0.3; Nb/Lapm = 0.6) and Zr/Nbav. = 33. The εNd(t) values are positive for Group 1 and Group 2, but negative for Group 3. Based on their petrological and geochemical features, we suggest that the Group 1 and 2 mafic volcanic rocks were formed in an oceanic environment, and represent mid-oceanic ridge basalt (MORB) and oceanic-island basalt (OIB), respectively. In contrast, Group 3 intermediate dikes probably have supra-subduction origin with calc-alkaline features, whereas Group 4 represents arc tholeiite basalt including remnants of the continental volcanic arc. Overall, the Middle Paleozoic intermediate-mafic rocks of the Tsoroidog Uul’ accretionary complex were probably generated from heterogenous mantle sources. Thus, we propose that spatial and temporal changes of the Paleo-Pacific Oceanic lithosphere, which subducted under the continental margin of the Siberian Craton, resulted in the variable composition of the intermediate-mafic rocks of this complex. The accretionary complex of the Tsetserleg terrane, which extends into Ulaanbaatar terrane, was formed by subduction of the Paleo-Pacific Oceanic lithosphere or Mongol-Okhotsk Ocean.

Burial dolomitization, the genesis of dolomite in the Dapu Formation (Upper Carboniferous), Guixinan area, Youjiang basin, Southwest China: petrologic and geochemical evidence

The genesis of dolomite is an intensively debated and extensively studied problem in the field of carbonate sedimentology. This study focuses on the dolomite of the Dapu Formation in the Guixinan area, with the aim of deciphering the mechanism of dolomitization in the Dapu Formation. Three types of sedimentary environment, including the open marine of inner ramp, the restricted marine of inner ramp, and the shoal of inner ramp, have been identified. Three types of dolostone (DI, DII and DIII) identified in the study area with massive structure had close relationship with fault structures, and contacted with limestone in curved diffuse way on the outcrop. Both of the above points indicated diagenetic dolomite. Three types of dolomite (D1, D2 and D3) displayed similar dark—light purple red luminescence, indicating that the characteristics of their diagenetic fluids were similar. The planar-s to nonplanar dolomite crystals indicated that the temperature was more than 50 °C. The mineral assemblage including D2, D3, and silicate minerals (feldspar, tremolite and actinolite) suggested contact metamorphism and tht the diagenetic fluid was rich in silicon and magnesium. The dolomitization process was not influenced by the leaching of meteoric water in surface or shallow buried environment and hydrothermal fluids, based on the corrected REE characteristics. The range of the δ18O of dolostone samples showed that the burial dolomitization had taken place in high-temperature environment. The lower 87Sr/86Sr ratio of the dolostone samples than the coeval seawater meant that the diagenetic fluid was derived from the alteration of mid-ocean ridge basalt. Based on the geologic setting, it was speculated that the burial dolomitization was driven by the hydro-tectonic processes in the passive margin burial diagenesis hydrologic realm. In brief, magnesium came from the low-temperature alteration of ocean basalts, the thermohaline and compactional flow supplied hydrodynamic condition, and the synsedimentary fault zones-supplied channels.

Age and genesis of the W-Bi-Cu-F (Au) Nui Phao deposit, Northeast Vietnam: Constrains from U-Pb and Ar-Ar geochronology, fluid inclusions study, S-O isotope systematic and scheelite geochemistry

Northeast Vietnam comprises several tectonic units, which host number of metallic deposits of different types from orogenic gold to intrusion hosted PGE deposits. The Lo Gam domain being part of the NE Vietnam, hosts the Lang Vai gold district, a several Pb–Zn deposits, and the recently discovered super-large W-Bi-Cu-F (Au) Nui Phao deposit. The Nui Phao deposit is 87.9 Mt at 0.19% WO3, 7.95% CaF2, 0.18% Cu, 0.19 g/t Au, and 0.09% Bi, while the measured resource is 25.1 Mt at 0.25% WO3, 8.4% CaF2, 0.23% Cu, 0.26 g/t Au, and 0.108% Bi. The Nui Phao deposit is a very interesting object for research due to location of the orebodies between of two granite massifs of the Cretaceous (Da Lien) and Triassic (Nui Phao) age. Consequently, the temporal and genetic relationships of mineralization with a certain magmatic events are unclear. Here we present mineralogical, compositional, and geochronological data of the Nui Phao deposit in conjunction with physical-chemical researches to understand the ore genesis and its genetic link to a certain magmatic events. According to our observations, at least three different mineral associations are distinguished at the Nui Phao deposit, which include greisens, skarns, and quartz-sulfide ore. Skarn and sulfide ore mineral assemblages generally contain abundant scheelite associated with pyrrhotite, chalcopyrite, and native bismuth. The greisens consist of fluorite, muscovite, and rare molybdenite. The results of fluid inclusions study indicate that at the Nui Phao deposit the initial ore-forming fluid was represented by the reduced (CO2-CH4)–H2O–NaCl(CaCl2) system characterized by high temperatures and moderate salinities. The fluid boiling is the dominant mechanism for ore deposition at the Nui Phao deposit. Relatively homogeneous isotopic compositions of oxygen (δ18OH2O = +6.73 to +9.24‰) and sulfur (δ34S = +1.6 to +3.5‰) that closely correspond to magmatic source, suggest that the ore-forming fluid was generated from cooling granite pluton without contribution meteoric water. The systematic studies of scheelite composition show that ΣREE contents decrease from early to late scheelite generations, which suggest gradual REE depletion of the ore-forming fluid without significant fractionation between LREE, MREE, and HREE. The REEs of scheelite are mainly controlled by the substitution mechanisms Ca2++W6+ ↔ REE3+ + Nb5+ and 3Ca2+ = 2REE3+ + □(Ca). By combining REE features, Y/Ho ratios and other characteristics of the Nui Phao deposit, we propose a single magmatic source for hydrothermal ore-firming fluids without fluid mixing; a significantly contribution of material during fluid/rock interaction is also unlikely. The combination of mineralogical and geochemical features as well as complex studies of the formation conditions allowed us to conclude that gold at the Nui Phao deposit occur as impurities in the native bismuth. The ages of the Da Lien and Nui Phao granite massifs and the Nui Phao deposit have been estimated to be ca. 84 Ma (Ar-Ar), 244 Ma (U-Pb) and ca. 85 Ma (Ar-Ar), respectively, indicating a genetic relationship between the mineralization and the Cretaceous magmatic event.