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1.
Isotopes (RbSr, C, O, S, and Pb) were investigated from the Zhenzigou PbZn deposit in the Qingchengzi mineral field (QMF) of the North China Craton as an aid to determine the genesis of stratiform PbZn deposits in the Liao-Ji Rift. A step-dissolution RbSr age of 1798 ± 8 Ma with 206Pb/204Pb ratios of 17.7477–17.8527 were obtained from sphalerite. Sulfur isotopic ratios for pyrite (5–14.4‰), sphalerite (2.4–8.6‰), and galena (− 0.3–8.6‰) from Zhenzigou have a narrower range than those from the host Paleoproterozoic Dashiqiao Formation, and granite in the area. Calcite and limestone from ore and wallrocks at the deposit have similar C and O isotope compositions, with δ13CPDB ranging from − 6.0 to − 2.3‰ and δ18OSMOW from 9.8 to 13.7‰, which are similar to those of carbonatite and the mantle.Comprehensive analysis of the Pb isotopic composition of the sulfide from the Zhenzigou deposit and PbZn deposits in adjacent area show that the Pb originated from the upper crust and mixed with Pb from the mantle. Sulfur isotopes from Zhenzigou deposit indicate that the mineralization has a volcanic eruption source. The δ13CPDB and δ18OSMOW values indicate that the CO2 originated from a mixed mantle, marine carbonate and organic source.Combined with the study of regional metallogenic background, this paper proposes that deposition of stratiform PbZn mineralization in the QMF began ca. 2052 Ma during development of the Liaoji Rift. The mineralization extended to ca. 1798 Ma prior to deformation associated with the Lvliang Movement, which dismembered the stratiform PbZn mineralization. The veined mineralization in the region cross-cuts the stratiform deposits and represents remobilized and redeposited deposits associated with the emplacement of Triassic plutons such as the Xinling and Shuangdinggou granites. 相似文献
2.
The Zapug–Duobuza magmatic arc (ZDMA), located along the southern edge of the south Qiangtang terrane in western Tibet, extends east–west for ~ 400 km. Small scattered granite and porphyry intrusions crop out in the ZDMA, but a large amount of granite may be buried by Late Cretaceous to Paleogene thrusting. Two stages of magmatism have been identified, at 170–150 Ma and 130–110 Ma. The widely distributed Middle–Late Jurassic granite intrusions in the ZDMA exhibit SrNd isotopic characteristics similar to those of ore-bearing porphyries in the Duolong giant CuAu deposit, and their εHf(t) values mostly overlap those of other porphyry CuMo deposits in the ZDMA and the Gangdese zone. The SrNdHf isotopic geochemistry suggests variable contributions of mantle and Qiangtang crustal sources, and indicates the presence of two new ore districts with potentials for CuAu, Fe, and PbZn ores, located in the Jiacuo–Liqunshan and Larelaxin–Caima areas. Except for the Duolong ore-forming porphyries, which show significant contributions of mantle components intruded into an accretionary mélange setting, the Early Cretaceous granites in other areas of the belt are of mostly crustal origin, from sources in Qiangtang felsic basement and Permo-Carboniferous strata, indicating the weak ore-forming potential of skarn-type Fe and PbZn deposits. The ephemeral but deep Bangong Co–Nujiang ocean in the Early Jurassic evolved into a shallow compressional marine basin in the Middle–Late Jurassic, possibly transitioning to northward flat subduction of oceanic crust at this time. The subducted slab broke off in the Early Cretaceous, initiating a peak in arc magmatism and metallogenesis at 125–110 Ma. 相似文献
3.
Although the sources of the ore metals remain problematic in most Iron-oxide Cu and Au (IOCG) deposits, external sulphur, either from surficial basinal brines and seawater (e.g., Central Andean and Carajás deposits) or from formation water and metamorphic fluids (e.g., the Cloncurry deposits), or introduced by magmatic assimilation of metasedimentary units (e.g., Phalaborwa), has been documented in many major Cu-rich IOCG centres. However, only the evaporite-sourced fluids yield diagnostically high δ34S values (i.e., > 10‰), while sedimentary formation water or metamorphic fluids commonly have lower values and are less clearly distinguishable from magmatic fluids, as in the Cloncurry deposits in which the involvement of external fluids is revealed by other evidence, such as noble gas isotopes. On the basis of these arguments, IOCG deposits could be redefined as a clan of Cu (AuAgU) deposits containing abundant hypogene iron oxide (magnetite and/or hematite), in which externally-derived sulphur probably plays an important role for the Cu (AuAgU) mineralization. In this definition, all “Kiruna-type” magnetite deposits, hydrothermal iron deposits (e.g., skarn Fe deposits) and magnetite-rich porphyry CuAu and skarn CuAu deposits are excluded. Two subtypes of IOCG deposits are recognized on the basis of the predominant iron oxide directly associated with the Cu (Au) mineralization, whether magnetite or hematite. Neither magnetite- nor hematite-rich IOCG deposits show any preference for specific host rocks, and both range in age from Neoarchean to Pleistocene, within a broad tectonic environment. 相似文献
4.
The Sailauf MnFeAs vein mineralization, located in the Spessart district (central Germany), is characterized by complex hydrothermal carbonate and oxide assemblages. The mineralization is hosted by a Permian rhyolite body and is structurally related to the Variscan unconformity that separates Permian sedimentary rocks from the underlying Variscan crystalline basement. The hydrothermal vein system has been studied by optical microscopy, electron-microprobe and LA-ICPMS analysis of major and trace elements (including the REE). Four distinct mineralization stages that are characterized by diagnostic carbonate-oxide assemblages are identified, which are (1) pre-ore stage, (2) ore stage 1, (3) ore stage 2, and (4) the replacement stage. Hydrothermal carbonates show complex compositional trends in CaMn (Fe + Mg) space, and comprise calcite, Mn-calcite, FeMg calcite, Ca-rhodochrosite, and Ca-kutnahorite. Oxide assemblages are dominated by braunite and hematite, with minor amounts of manganite and hausmannite. The mineralization is enriched in a distinct suite of trace elements, including As, W, Pb, Zn and Cu. Analysis of the paragenetic evolution, in conjunction with the major and trace element data, allows to reconstruct the fluid evolution of the hydrothermal system. The first order change in mineralogy between the two main ore stages (Mn oxides and calcite evolving into hematite and Mn-rich calcite) records a pronounced shift in fluid pH and silica activity of the system. This interpretation is also supported by variations in the behavior of Ce in different carbonate generations. The late stage replacement carbonates relate to destabilization of the primary ore assemblages. The distinct geological setting immediately below the Permian unconformity, in conjunction with the mineralogical and chemical data, suggests that dynamic fluid mixing processes involving basement-derived brines and more shallow groundwaters were important in the formation of the Sailauf MnFeAs mineralization. The significant enrichment in the AsWPbZnCu element suite resembles that of other MnFe deposits, and points to felsic lithologies as the main metal source of the mineralization. 相似文献
5.
《Gondwana Research》2013,23(3-4):799-809
Externally derived, pure CO2 that mixes with a carbon-(under)saturated C-O-H fluid in lower crustal granulites may result in graphite precipitation if the host-rock oxygen fugacity (fO2rock) is below the upper fO2 limit of graphite. The maximum relative amount of graphite that can precipitate varies between a few mol% up to more than 25 mol%, depending on pressure, temperature, and host-rock redox state. The maximum relative amount of graphite that can precipitate from an infiltrating CO2 fluid into a dry granulite (CO fluid system) varies between zero and a few mol%. Thermodynamic evaluation of the graphite precipitation process shows that CO2 infiltration into lower crustal rocks does not always result in a carbon (super)saturated fluid. In that case, graphite precipitation is only possible if carbon saturation can be reached as a result of the reaction CO2 → CO + ½ O2. Graphite that has been precipitated during granulite facies metamorphic conditions can subsequently be absorbed by a COH fluid during retrograde metamorphism. It is also possible, however, that significant amounts of graphite precipitate from a COH fluid during retrograde metamorphism. This study shows that interpreting the presence or absence of graphite in granulites with respect to CO2 infiltration requires detailed information on the P–T–fO2rock conditions, the relative amount of CO2 that infiltrates into the rock, and whether H2O is present or not. 相似文献
6.
The present paper reports the first detailed petrological and geochemical study of non-sulfide Zn–(FePb) deposits in the Riópar area (Prebetic Zone of the Mesozoic Betic Basin, SE Spain), constraining the origin and evolution of ore-forming fluids. In Riópar both sulfide and non-sulfide Zn–(FePb) (“calamine”) ores are hosted in hydrothermally dolomitized Lower Cretaceous limestones. The hypogene sulfides comprise sphalerite, marcasite and minor galena. Calamine ores consist of Zn-carbonates (smithsonite and scarce hydrozincite), associated with abundant Fe-(hydr)oxides (goethite and hematite) and minor Pb-carbonates (cerussite). Three smithsonite types have been recognized: i) Sm-I consists of brown anhedral microcrystalline aggregates as encrustations replacing sphalerite; ii) Sm-II refers to brownish subhedral aggregates of rugged appearance related with Fe oxi-hydroxides in the surface crystals, which replace extensively sphalerite; and iii) Sm-III smithsonite appears as coarse grayish botryoidal aggregates in microkarstic cavities and porosity. Hydrozincite is scarce and appears as milky white botryoidal encrustations in cavities replacing smithsonite. Also, two types of cerussite have been identified: i) Cer-I cerussite consists of fine crystals replacing galena along cleavage planes and crystal surfaces; and ii) Cer-II conforms fine botryoidal crystals found infill porosity. Calcite and thin gypsum encrustations were also recognized. The field and petrographic observations of the Riópar non-sulfide Zn–(FePb) revealed two successive stages of supergene ore formation under meteoric fluid processes: i) “gossan” and “red calamine” formation in the uppermost parts of the ore with deposition of Fe-(hydr)oxides and Zn- and Pb-carbonates (Sm-I, Sm-II and Cer-I), occurring as direct replacements of ZnPb sulfides; and ii) “gray calamine” ore formation with deposition of Sm-III, Cer-II and hydrozincite infilling microkarst cavities and porosity. The stable isotope variation of Riópar smithsonite is very similar to those obtained in other calamine-ore deposits around the world. Their CO isotope data (δ18O: + 27.8 to + 29.6‰ V-SMOW; δ13C: − 6.3 to + 0.4‰ V-PDB), puts constrains on: i) the oxidizing fluid type, which was of meteoric origin with temperatures of 12 to 19 °C, suggesting a supergene weathering process for the calamine-ore formation under a temperate climate; and ii) the carbon source, that resulted from mixing between two CO2 components derived from: the dissolution of host-dolomite (13C-enriched source) and vegetation decomposition (13C-depleted component). 相似文献
7.
Thick hydrogenetic ferromanganese (FeMn) crusts from the northwest and central Pacific seamounts often show a distinct dual structure composed of a typical hydrogenetic porous, friable upper part of FeMn oxides (Layer 1) and the underlying dense, hard phosphatized growth generation of FeMn oxides (Layer 2 in this study). Layer 2 always appears above the substrate rock and composes the lower part of the crust; it is never found as the upper crust layer in contact with seawater. The chemical composition of Layer 2 clearly differs from the younger Layer 1 hydrogenetic FeMn oxides, and is depleted in Fe, Al, Ti, and Co, and enriched in Ni, Cu, and Zn relative to Layer 1. The Be isotope age models of the crusts were refined with paleomagnetic and paleontological information, and applied to selected crust samples. The age model indicates fairly continuous growth from the substrate to the surface and fairly constant growth rates during the past 17 Ma. The growth rate from the Miocene to the present has varied by a factor of two, about 2–4 mm/Myr in Layer 1, while Layer 2 has similar but more variable growth rates than Layer 1.The calculated age for the base of Layer 1, and possibly the age of termination of phosphatization, is never younger than the late Miocene. The age seems to vary with water depth, shallower-water crusts (between 991 and 1575 m) showing a younger age of about 10 Ma whereas the deeper-water (2262 m) crusts have extrapolated ages for the base of Layer 1 of be 17.1 ± 2.5 Ma. This trend indicates that phosphatization took place in a less-oxidizing environment during growth of Layer 2, followed by a weakened oxygen-minimum zone or intensified AABW during growth of Layer 1. 相似文献
8.
The age and origin of the past-producing Nanisivik carbonate-hosted ZnPb deposit in Nunavut, Canada, have been controversial for decades. Various direct and indirect dating methods have produced results ranging from Mesoproterozoic to Ordovician in age, and previous studies of the mineralising fluids have suggested that the fluids were anomalously hot (> 150 °C). This study combines ReOs (pyrite) geochronology, in-situ sulphur isotope analysis, and fluid inclusion analysis to refine both the timing of mineralisation and the nature of mineralising fluids. ReOs pyrite analysis shows that the Nanisivik deposit formed ca. 1.1 Ga, broadly similar to the depositional age of the host rock and with the Grenville orogeny, making it one of few known Precambrian carbonate-hosted ZnPb deposits. In-situ sulphur isotope measurements from Nanisivik show a narrow δ34S range of 27.54 ± 0.72, very similar to what has been reported before in bulk sample analyses. New fluid inclusion data show that the mineralising fluids were ~ 100 °C, which is not anomalous in the context of carbonate-hosted base-metal deposits. The fluids exhibit no significant spatial variation in homogenisation temperature in the 2-km-long ‘upper lens’ of the ore deposit, but recrystallisation and modification of fluid inclusions took place in the immediate vicinity of the cross-cutting ~ 720 Ma “mine dyke”. The deposit is broadly inferred to have formed during late Mesoproterozoic assembly of supercontinent Rodinia, when regional hydrostatic head developed under the influence of far-field stresses originating in the developing Grenville orogen. The Nanisivik deposit remains anomalous only in its age; most other aspects of this ore deposit are now shown to be quite typical for carbonate-hosted ore deposits. 相似文献
9.
《Gondwana Research》2014,25(3):1120-1138
Phosphatized microfossils from ca. 580 Ma from the Doushantuo Formation in the Weng'an region of South China were analyzed by Fourier transform infrared (FTIR) microspectroscopy for their chemical characterization. Two morpho-types of phosphatized embryo-like fossils (Megasphaera and Megaclonophycus) were analyzed, together with algal fossils. Transmission IR spectra of the microfossils have absorption bands of around 2960 cm− 1 and 2925 cm− 1, indicating the presence of aliphatic hydrocarbon (anti-symmetric aliphatic CH3 and aliphatic CH2), and have an additional band of around 1595 cm− 1, probably derived from aromatic moieties (aromatic CC). In addition, IR microscopic mapping shows that aliphatic hydrocarbon and aromatics are distributed inside the embryo-like fossils. The embryo-like fossils appear to show three types of CH3/CH2 peak height ratios (R3/2) and aromatic CC/CH2 peak height ratios (RCC/2 values): (1) high-R3/2/low-RCC/2 type (R3/2 = ~ 0.2–1.0 and RCC/2 ~ 0–2), (2) low-R3/2/medium-RCC/2 type (R3/2 = ~ 0.2–0.6 and RCC/2 = ~ 1–4); and (3) low-R3/2/high-RCC/2 type (R3/2 = ~ 0.2–0.6 and RCC/2 ~ 1–8). All three types are contained in both Megasphaera and Megaclonophycus. Raman spectra for the carbonaceous matter within the rock sample show a similar degree of thermal alteration, indicating that the organics were embedded in situ prior to thermal maturation. The IR spectroscopic differences might reflect differences in original organic compositions of microorganisms, and/or immediate post-mortem alteration. This suggests that the origins of phosphatized embryo-like fossils are more diverse than was previously recognized on the basis of their morphology. A comparison of R3/2 and RCC/2 values in the embryo-like fossils with those of the algal fossils and the extant microorganisms indicates the possible interpretation that some of the embryo-like fossils belong to animal embryo, others are algae, but none of them originate from bacteria. 相似文献
10.
Understanding the formation mechanism of the South China Sea has important implications for research on plate rupture and continent-ocean transition globally. Granitoids dredged from the Xiaozhenzhu Rise provide new perspectives on lithosphere evolution processes of this region. Zircon UPb (127–122 Ma) and amphibole/K-feldspar 40Ar/39Ar (123–115 Ma) ages indicate high cooling rates of 55–64 °C/myr and thus rapid magma emplacement and uplift in the Early Cretaceous. These calc-alkaline granitoids with intermediate Mg# (44–53) and slightly negative Eu anomalies (Eu/Eu* = 0.63–1.00) have highly variable and well-correlated Cr (4.89–531 ppm) and Ni (2.27–258 ppm) contents, which indicate melt mixing. The low CrNi sample (19.4 ppm Ni) displays much higher Sr (847 ppm), Sr/Y (93.4), and overall stronger crustal signatures than the high CrNi samples (107–258 ppm Ni) which have more mantle-like characteristics. Despite these differences, all studied samples show relatively similar and moderately enriched SrNd isotopic compositions ((87Sr/86Sr)i = 0.7055–0.7064, εNd(t) = −0.6 to −1.7) and enriched Pb isotopic compositions that are comparable with those of marine sediments. They also show mantle-like depleted zircon O (δ18O = 4.5–6.3‰) and mostly positive zircon Hf (εHf(t) = −0.4–4.1) isotopic compositions that indicate limited upper crustal contribution in the melt source. Their compositional features are best explained by magma mixing between partial melts of a delaminated lower arc crust and partial melts of a metasomatized arc mantle wedge. Combining our new results with literature studies of magmatism, metamorphism, sedimentary records and crustal structures from the region, we propose a new model of the Late Mesozoic–Early Cenozoic lithosphere deformation of the South China continental margin where lower arc crust delamination generated a tectonic weak zone that is essential for the rifting of the South China Sea. 相似文献
11.
《Chemie der Erde / Geochemistry》2014,74(4):641-651
Groundwaters in the crystalline aquifers are the major source of drinking water in Vaniyambadi area of Vellore district. Geochemical methods in collaboration with statistical methods were applied in this industrial area to understand the natural and anthropogenic influences on groundwater quality. To accomplish this objective, groundwater samples were collected and analyzed for physicochemical parameters and the results showed a dominance in the order of Na+ > Mg2+ > Ca2+ > K+ and HCO3− > Cl− > SO42− > NO3− for anions and cations, respectively. In contrast to this anion dominance were changed to Cl− > HCO3− > SO42− > NO3− in samples collected near the tannery industries. Groundwater quality evaluation using TDS and TH suggested that 57% of the total samples are hard-brackish type, indicating its unsuitability for drinking purpose. Generally the water type is Na+Cl− to Ca2+Mg2+HCO3− type with an intermediate Ca2+Mg2+Cl−, suggesting the mixing of fresh groundwater with tannery effluent and cation exchange. Factor analysis and bivariate plots of major ions suggests that both natural and anthropogenic inputs are equally influencing the groundwater quality. Further investigations proved that silicate weathering is the dominant geogenic source of groundwater solute content, whereas tannery effluent is the anthropogenic source. Saline water mixing index (SWMI) and Cl− vs NO3− bivariate plot were employed to differentiate the tannery contamination from the other anthropogenic inputs such as agricultural fertilizers, municipal sewages, etc. This analysis shows that samples 2, 4, 8 and 9 (located within the tannery cluster) have a SWMI value greater than 1, representing the groundwater–tannery effluent mixing. This study infers that groundwater in the Vaniyambadi area is under serious threat from both natural and anthropogenic contamination. However, the controlling discharge of untreated tannery effluents must be regulated to reduce the further deterioration of this vital resource in this part of the country. 相似文献
12.
Although garnet UPb dating method has been reported recently, yet the accurate concordia 206Pb/238U ages and growth histories of multi generation of garnets based on ages were still lacked. LA-ICP-MS UPb dating on multi-generational grandite (grossular-andradite) garnet from the large Tonglvshan Cu-Fe-Au skarn deposit was applied in this study. Based on petrographic observation, in chronological order, three generation garnets have been distinguished, namely homogeneous Grt1-exo (in the exoskarn zone) and Grt1-endo (in the endoskarn zone), oscillatory zoning Grt2 and vein-type Grt3 cutting magnetite ores. LA-ICP-MS UPb dating on four grandite samples from the Grt1-exo, Grt1-edno, Grt2 and Grt3 yields Tera-Wasserburg lower intercept 206Pb/238U ages of 139.1 ± 1.0 Ma (2σ, MSWD = 0.79), 134 ± 11 Ma (2σ, MSWD = 2.5), 143.4 ± 8.3 Ma (2σ, MSWD = 2.3) and 140.3 ± 1.4 Ma (2σ, MSWD = 0.95), respectively. More importantly, two concordia 206Pb/238U ages of 139.2 ± 0.6 Ma (2σ, MSWD = 1.4) and 139.8 ± 1.5 Ma (2σ, MSWD = 0.13) were firstly obtained from the sample of Grt1-exo with highest U concentrations ([U]avg > 80 ppm) contents. The precision UPb ages of 139–140 Ma from Grt1-exo and Grt3 can be considered as the timing of Cu-Fe-Au skarn mineralization, and consistent with the majority of published zircon UPb ages of the quartz dioritic stock and 40Ar39Ar plateau ages of phlogopite at Tonglvshan (142–140 Ma). The precision grandite UPb ages also indicate that the entire metasomatic hydrothermal mineralization activity in the Tonglvshan Cu-Fe-Au skarn deposit occurred within a relatively short time span of <1 (or 2.5 considering errors) Myr. In addition, we found that the grandite garnet is more easily to be enriched in U and can obtain the high-precision concordia UPb ages with higher andradite Mol%, euhedral and larger crystals, and relevant oxidized magmatic rocks or skarns. 相似文献
13.
The North China Craton (NCC) is one of the most important regions hosting abundant banded iron formations (BIFs). The ~ 2.54 Ga Sijiaying BIF, the best-preserved and most extensive deposit in Eastern Hebei, is intercalated and closely associated with meta-volcanic rocks of the Luanxian Group. In this context, major and trace element and SmNd isotopic analyses of individual mesobands of a Sijiaying BIF specimen were conducted to characterize the source and depositional environment over a transient period.Low Al2O3, TiO2 and high field strength elements (HFSEs) concentrations show that the BIF is relatively detritus-free. Shale-normalized rare earth and yttrium distributions (REE + Y) of individual BIF mesobands exhibit positive La anomalies, enrichment in HREE relative to LREE and MREE and suprachondritic Y/Ho ratios, which are typical features of marine waters throughout the Archean and Proterozoic. The presence of consistently positive Eu anomalies indicates a significant high-T hydrothermal input, while the absence of true Ce anomalies suggests deposition from an anoxic water column. By comparison with other typical BIFs (e.g., the Isua BIF from Greenland; the Kuruman BIF from South Africa), the Sijiaying BIF is depleted in HREE, and appears to record variations in solute fluxes related to changing intensities of hydrothermal activity. These features, coupled with SmNd isotopic relations and Ge/Si distributional patterns, point to two decoupled sources controlling the depositional environment of the BIF and thus reveal source heterogeneity for silica and iron of the Sijiaying BIF. High Fe fluxes were associated with seafloor-vented hydrothermal fluids, which received their SmNd isotopic signature from alteration of depleted oceanic crust; whereas significant amounts of silica were associated with ambient seawater whose REE signature was controlled by solutes derived from weathering of nearby Mesoarchean continental landmasses. The old (up to ~ 3.0 Ga) Nd (TDM) model ages of Si-rich mesobands of the BIF support such a scenario. 相似文献
14.
The Chorukh-Dairon deposit is part of the metallogenic belt of WMo, CuMo, AuW, and Au deposits along the Late Paleozoic active continental margin of the Tien Shan. It is related to the Late Carboniferous multiphase pluton, with successive intrusive phases of early monzogabbro through monzonite-quartz monzonite to monzogranite and leucogranite, and the latest lamprophyre dikes. The deposit is an example of complex W–Mo–Cu magmatic-hydrothermal system related to magnetite-series shoshonitic igneous suite. It contains zones of W–Cu–Mo oxidized prograde and retrograde skarns, with abundant scapolite, plagioclase, K-feldspar, andradite garnet, magnetite, as well as molybdoscheelite and minor chalcopyrite, and molybdenite. Skarns are overprinted by hydrosilicate alteration assemblages, with amphibole, chlorite, epidote, quartz, calcite, scapolite, albite, scheelite, and chalcopyrite, and are cut by quartz-carbonate-barite-fluorite-sulfide veins.The fluid evolution included a release of high temperature (~ 400–500 °C), high pressure (900–1100 to 700–800 bars), high salinity magmatic-hydrothermal aqueous chloride fluid, with its direct separation from crystallizing magma and formation of prograde and retrograde skarns. Fluid enrichment in Ca (up to 15–22 wt.% CaCl2) at the retrograde skarn stage was possibly related to magmatic differentiation and provided intense molybdoscheelite deposition from a homogenous fluid. In contrast, hydrosilicate alteration assemblages were formed at lower temperatures (~ 350–400 °C) initially from a homogenous and then from a boiling Ca-rich (20–22 wt.% CaCl2) magmatic-hydrothermal fluid, with the latter contributing to the most intense scheelite deposition. The stable isotope data (δ13CCO2 = − 3.0 ± 0.5‰ and δ18ОH2O = + 6.5 ± 0.5‰, δ34S = + 7.5 to + 7.7‰) obtained for the hydrosilicate stage minerals suggest significant fluid sourcing from magmatic and meteoric waters as well as from sedimentary rocks enriched in seawater sulfate, possibly evaporites, although a strongly homogenous character of the isotopic composition reveals intense isotope homogenization in a magmatic chamber. Some light sulfur isotope enrichment of sulfides from the quartz-carbonate-barite-fluorite-sulfide veins (δ34S = + 6.0 to + 6.1‰) may be linked to the evolution of the magmatic source toward more mantle-related sulfur species, as these veins were formed after emplacement of the late mafic (lamprophyre) dikes. 相似文献
15.
Large Zn isotope variations in the NiMo polymetallic sulfide layer in the lower Cambrian,South China
In South China, black organic-rich shales in the lower Cambrian Niutitang Formation host a NiMo polymetallic sulfide layer that discontinuously extends over ~1600 km. Seawater and hydrothermal origins are among the many suggested hypotheses and are still under debate. In order to discriminate Zn sources, we report Zn isotopes in NiMo polymetallic sulfide layers and their host shales from the Nayong and Zunyi locations in Guizhou province and the Zhangjiajie section in Hunan province. In each section, host organic-rich shales show homogeneous Zn isotope compositions which likely resulted from quantitative scavenging of dissolved Zn from seawater under euxinic conditions. The difference in the average δ66Zn values of organic-rich shales between the two sections in Guizhou (0.76 ± 0.09‰) and one section in Hunan (0.59 ± 0.10‰) might reflect variations of Zn isotope gradient with the depth of seawater. Therefore, the organic-rich sediments need not always represent an isotopically light Zn sink, which is dependent on Zn isotope fractionation in the local basin. However, the δ66Zn values in the NiMo polymetallic sulfide layers are different from those of their host shales, indicating that these sulfide layers did not inherit the Zn isotope signal of seawater. Based on the regular increasing trend in δ66Zn values from Nayong (0.54 ± 0.06‰) to Zhangjiajie (1.34 ± 0.09‰) and the presence of PbZn mineralization in the Dengying/Doushantuo Formations, we argue that hydrothermal fluids associated with PbZn mineralization could be a major source of Zn in NiMo sulfide layers, especially in the Nayong location. A possible model is that the hydrothermal fluids related to MVT-type mineralization got overprinted on a multiple-sourced synsedimentary sulfide-rich layer. We provide additional evidence that Zn isotopes have great potential as a tracer of metal source and can be applied to similar types of mineralization as e.g., the late Devonian Ni-Zn-PGE Nick deposit (Selwyn Basin, Canada) or elsewhere. 相似文献
16.
《Comptes Rendus Geoscience》2014,346(11-12):317-325
Major features of the Earth's structure and dynamics originate in the contrast between the rigidity of SiO bonds and the softness of SiOSi linkages. Because this contrast results from orbital hybridization, a real understanding of bonding relies on ab initio quantum-mechanical principles. As investigated with first-principles interatomic potentials, the α–β transitions of SiO2 polymorphs illustrate how soft SiOSi linkages give rise to dynamical structures at rather low temperatures and yield the low melting temperatures of SiO2-rich minerals that are at the roots of SiO2 enrichment in magmatic differentiation. The increasing concentration of alkalis throughout this process is another aspect that must also be studied in terms of molecular orbitals in relation with the presence of aluminum in tetrahedral coordination. Finally, calculations of noble gas solubility show that some important features can be treated with “hybrid” calculations when, in addition to quantum-mechanical effects, the energy needed to create a cavity in the silicate melt is dealt with in a classical manner. 相似文献
17.
Geochemical, SrNd isotopic and geochronological methods were used to reveal the protoliths and geodynamic implications of the newly discovered Triassic Baqing eclogites, eastern Qiangtang terrane, central Tibet. We assessed the mobility of trace elements, and it turned out that high field strength elements (HFSEs) and rare earth elements (REEs) were immobile in the high-pressure (HP) metamorphic process and reliably employed to discriminate the protoliths of the Baqing eclogites. Whole-rock geochemical data (especially immobile elements) suggested both arc-related (negative NbTa anomalies, high light (L) REEs/heavy (H) REEs ratios) and mid-oceanic ridge basalt (MORB)-related (without negative NbTa anomalies, relatively low REEs contents) characteristics, indicating the contribution of different proportions of subduction material with a backarc basin origin. According to the SrNd isotopic ratios (initial 87Sr/86Sr ratios: 0.7078–0.7086; εNd(t) values: −0.58 to +1.96), the protoliths of the Baqing eclogites originated from mantle which experienced continental crustal contamination in the subduction zone. Compared with nearby subduction-related magmatic rocks from eastern Qiangtang terrane, backarc basin was the most likely tectonic environment for the Baqing eclogite protoliths. The Jinsha Paleo-Tethyan Ocean southward subduction underneath East Qiangtang block (EQB) and rollback led to the formation of this backarc basin between the Late Permian and Early Triassic, and subsequent northward subduction of the backarc basin formed the Baqing eclogites in early Late Triassic (227–221 Ma). 相似文献
18.
In order to understand and mitigate the deterioration of water quality in the aquifer system underlying Guadalajara metropolitan area, an investigation was performed developing geochemical evolution models for assessment of groundwater chemical processes. The models helped not only to conceptualize the groundwater geochemistry, but also to evaluate the relative influence of anthropogenic inputs and natural sources of salinity to the groundwater. Mixing processes, ion exchange, water–rock–water interactions and nitrate pollution and denitrification were identified and confirmed using mass-balance models constraint by information on hydrogeology, groundwater chemistry, lithology and stability of geochemical phases. The water–rock interactions in the volcanic setting produced a dominant NaHCO3 water type, followed by NaMgCaHCO3 and NaCaHCO3. For geochemical evolution modeling, flow sections were selected representing recharge and non-recharge processes and a variety of mixing conditions. Recharge processes are dominated by dissolution of soil CO2 gas, calcite, gypsum, albite and biotite, and Ca/Na exchange. Non-recharge processes show that the production of carbonic acid and Ca/Na exchange are decreasing, while other minerals such as halite and amorphous SiO2 are precipitated. The origin of nitrate pollution in groundwater are fertilizers in rural plots and wastewater and waste disposal in the urban area. This investigation may help water authorities to adequately address and manage groundwater contamination. 相似文献
19.
The Archaean Yilgarn Craton (Western Australia) is a world-class metallogenic province, hosting considerable resources of Au, Ag, Ni, Cu, Zn and Fe. Here we present trace element compositions of pyrite from > 30 orogenic Au and 5 volcanic hosted massive sulphide (VHMS) deposits across the Yilgarn. Pyrites from VHMS deposits tend to have higher Sn, Se, Cu, Pb, Bi and lower Ni relative to orogenic deposits. VHMS deposit pyrites commonly have Co > Ni, As > 100Au, Te > Au, Se > Te. Orogenic gold deposits could be subdivided based on association of Au with As or Te. Pyrites from AuAs ores generally have Pb/Bi > 5, Se/Te > 5, Pb/Sb < 5 and Tl/Te > 100 and the majority of Au is refractory (in pyrite structure). At the same time AuTe association pyrites are characterised by lower values of Pb/Bi, Se/Te and Tl/Te, higher values of Ag/Au, Pb/Sb and Au generally resides in inclusions of different compositions. Our data can be used at the exploration stage to distinguish between VHMS vs Orogenic Au signatures. For all studied deposits inclusion populations are summarised with implications for Au and Ag deportment. Orogenic Au deposits from the Yilgarn mostly have multistage formation histories reflected in the presence of multiple generations of pyrites. However, only some deposits record multiple high Au mineralisation events. 相似文献
20.
Shengyao Yu Jianxin Zhang Huaikun Li Kejun Hou C.G. Mattinson Jianghua Gong 《Gondwana Research》2013,23(3):901-919
In this study, we link zircon UPb SHRIMP and LA-ICP-MS geochronology and the LuHf isotopic composition of eclogites and their host gneisses/schists with whole-rock geochemistry of eclogites in the Dulan area to constrain their protoliths and metamorphic relationships. UPb dating suggests that the protolith of one of the eclogites was a Neoproterozoic mafic intrusive rock (828 ± 58 Ma) and the protolith of enclosing orthogneiss was an early-Neoproterozoic granitoid (923 ± 12 Ma). Detrital zircons from Grt-bearing mica-schists yield ages of 0.9–2.5 Ga, with a dominant range of 1.0–1.8 Ga, indicating sedimentary sources from Neoproterozoic to Neoarchean crust and a depositional age ≤ 0.9 Ga. The matching metamorphic ages of eclogites (438 ± 5 Ma, 436 ± 4 Ma) and their country rocks (Grt-bearing mica-schists: 438 ± 4 Ma, 439 ± 8 Ma; orthogneiss: 427 ± 8 Ma) indicate that all studied samples experienced coeval Early Paleozoic HP/UHP metamorphism. The UPb ages and Hf isotopic compositions of the inherited magmatic zircon cores of an eclogite sample (εHf (800) = 2.6–9.2, TDM1 = 1.0–1.3 Ga, TDM2 = 1.1–1.4 Ga) suggest that the protolith may be derived from Neoproterozoic depleted mantle with variable proportions of an older crustal component. The magmatic zircon cores of the orthogneiss (εHf (900) = ? 7.3 to ? 0.2; TDM2 = 1.8–2.1 Ga) suggest that the parental magma was derived from a Paleoproterozoic crustal source. Hf isotopic compositions of the detrital zircons from the metasediments (εHf(t) = ? 19.4 to + 10.6) suggest three crust formation and reworking events: (1) Archean (TDM2 = 2.7–2.9 Ga) juvenile crust reworked at ~ 2.5 Ga; (2) early Paleoproterozoic (TDM2 = 2.3–2.5 Ga) juvenile crust reworked at ~ 1.8 Ga; and (3) late Paleoproterozoic (TDM2 = 1.5–1.9 Ga) juvenile crust reworked in the Neoproterozoic.Whole-rock geochemical data suggest that the protoliths of the Dulan eclogites were probably derived from a continental rift or an incipient oceanic basin rather than a large, long-lived ocean basin. Thus, combined with field relationships, petrology, geochemistry, zircon UPb dating and the LuHf isotopic analysis presented in this paper and reported from previous studies, we suggest that the Dulan eclogites and their country rocks experienced a common UHP metamorphism during Late Ordovician deep continental subduction. 相似文献