Pb Isotope Systematics Research Articles

Post-Ore Processes of Uranium Migration in the Sandstone-Hosted Type Deposits: 234U/238U, 238U/235U and U\u2013Pb Systematics of Ores of the Namaru Deposit, Vitim District, Northern Transbaikalia

To assess the nature of the post-ore behaviour of uranium in the Namaru deposit (Khiagda ore field), U–Pb isotope systems and the isotopic composition of uranium (234U/238U and 238U/235U) were studied. The studied samples represent different ore zones of the deposit and were collected along cross-sections both vertically and horizontally. Wide variations in the isotopic composition of uranium and U–Pb isotopic age have been established. Deviations of the 234U/238U ratio from equilibrium values, which for some samples exceed 50%, along with significant variations in the isotopic age, indicate that permafrost layer, which covered the catchment areas of paleovalleys with meteoric oxygen-containing waters ca. 2.5 Ma ago, did not lead to preserving uranium ores at the deposit. Uranium migration took place during the Quaternary period. The effective combining the U–Pb dating and 234U/238U data in assessing the post-ore redistribution of uranium made it possible to recognize: removal of uranium from some zones of the ore body and its accompanying redeposition in others. Wide variations in the 238U/235U (137.484–137.851) ratios throughout the entire studied cross-sections can be explained by the different locations of samples relatively to the ore deposition front and change in redox conditions as this front advanced. Depletion of the light isotope 235U in the lower zone of the ore body may be associated with the influence of ascending carbonic waters established in the regional basement. The effect of such waters on uranium-bearing rocks causes predominant leaching of light 235U.

Fluid Inclusions and H–O–C–S–Pb Isotopic Systematics of the Jinba Gold Deposit, NW China: Implications for Ore Genesis

The Jinba gold deposit is located in the Maerkakuli Shear Zone of the south Altay Orogenic Belt, NW China. Mineralization types are classified as altered rock–and quartz vein–type. Orebodies occur as veins or lenses controlled by NW–trending faults, and are hosted in phyllite (Early–Middle Devonian Ashele Formation) and plagiogranite (Early Devonian Habahe Pluton). Three paragenetic stages were identified: early quartz–pyrite–gold (Stage 1), middle quartz–chalcopyrite (Stage 2), and late calcite–quartz–galena–sphalerite (Stage 3). Fluid inclusions within the deposit are liquid–rich aqueous (LV–type), vapor–rich aqueous (VL–type), carbonic–aqueous (LC–type), and purely carbonic (C–type) FIs. Homogenization temperatures for stages 1–3 FIs were 373–406 °C, 315–345 °C, and 237–265 °C, respectively. Fluid salinities for stages 1–3 were 2.1–13.6 wt%, 3.2–6.1 wt% and 3.9–6.0 wt% NaCl equivalent, respectively. The ore–forming fluids evolved from a CO2–NaCl–H2O ± CH4 to a NaCl–H2O system from stage 1–3. Oxygen, hydrogen, and carbon isotopic data (δ18Ofluid = 1.7‰–8.1‰, δDfluid = –104.1‰ to –91.7‰, δ13Cfluid = –0.4‰–6.3‰) indicate that ore–forming fluids were metamorphic hydrothermal origin with the addition of a late meteoric fluid. Sulfur and lead isotope data for pyrite (δ34Spy = 3.3‰–5.3‰, 206Pb/204Pb = 17.912.3–18.495, 207Pb/204Pb = 15.564–15.590, 208Pb/204Pb = 37.813–38.422) show that the ore–forming materials were mainly derived from diorite and the Ashele Formation. Mineralization, FIs, and isotope studies demonstrate that the Jinba deposit is an orogenic gold deposit.

Neoarchean A-type acid metavolcanics in the Keivy Terrane, northeastern Fennoscandian Shield: Geochemistry, age, and origin

The Lebyazhka acid metavolcanics occupying 40% of the Keivy Terrane in the Arctic Region of the Fennoscandian Shield belong to mainly alkali-calcic and calc-alkaline series and are classified as peraluminous to metaluminous metadacites, metarhyodacites, metarhyolites and metatrachyrhyolites. Geochemical characteristics of most of these metavolcanics are very close or identical to those discriminative of A-type granites. Compositionally these rocks are similar and often identical to the Keivy A-type peralkaline granites. On variation plots and geochemical spider and discriminant diagrams the Lebyazhka acid metavolcanics and Keivy peralkaline granites share the same trends and patterns and fall into fields of post-orogenic, within-plate and A-type granites. A U–Pb (SHRIMP II) zircon age of the Lebyazhka acid metavolcanics is 2678 ± 7 Ma. The first thermal event recorded by the U–Pb isotope system of magmatic zircon occurred at ca. 1.90 Ga, an age corresponding to the termination of the Paleoproterozoic Lapland-Kola collisional orogeny. The youngest zircon generation is dated at 1805 ± 23 Ma ago and is assumed to have grown during metasomatic or hydrothermal alteration.The finding of relics of baddeleyite, a mineral crystallizing only in mafic and ultramafic igneous rocks, in zircon from the Lebyazhka acid metavolcanics along with the chemical similarity of these metavolcanics and the Keivy peralkaline granites and their emplacement simultaneously with the Keivy gabbro-anorthosites 2.68–2.66 Ga ago, obviously suggests that these three types of rocks should have a common origin. Based on new data and existing tectonic models, the origin of the Lebyazhka acid metavolcanics as well as the Keivy peralkaline granites, gabbro-anorthosites and nepheline syenites is concluded to be linked with underplating by basaltic magma of the lower crust of the eastern Kola Province 2.68 Ga ago. This underplating led to anorogenic bimodal magmatism including the 2678 Ma Lebyazhka acid metavolcanics, the 2674–2663 Ma Keivy peralkaline granites, the 2674–2659 Ma Keivy gabbro-anorthosites, and the 2645 Ma Sakharjok nepheline syenites and subalkaline gabbro. The underplating also caused partial melting of the Mesoarchean continental crust and involvement of this older crustal component into felsic melts parental for the Lebyazhka acid metavolcanics and the Keivy peralkaline granites.

Genesis of the Linghu Au deposit in Xiaoqinling Region, Henan province, China: Constraints from fluid inclusions and isotope systematics

The Linghu Au deposit is located within the northeastern part of the Xiaoqinling region in the southern margin of the North China Craton. Field investigations, cross-cutting relationships, and mineral paragenetic associations have identified three stages of hydrothermal activity. To determine the origin and evolution of ore-forming fluids, and better understand genesis of the Linghu Au deposit, we have undertaken a series of studies including fluid inclusion and He–Ar–H–O–S–Pb isotope systematics. Fluid inclusion study indicates that the early stage of gold mineralization was characterized by a high-temperature and low-salinity H2O–CO2–NaCl hydrothermal fluids that have δD values of – 82.0 to – 78.6‰ and δ18OH2O values of 5.3 to 6.4‰. The intermediate stage of gold mineralization involved moderate temperature H2O–CO2–NaCl hydrothermal fluids with δD and δ18OH2O values of – 92.7‰ to – 87.9‰ and – 1.4‰ to 0‰, respectively. Pyrite samples from this mineralization stage have 3He/4He and 40Ar/36Ar ratios of 0.26–0.31 Ra and 2834.7–4300.6, respectively. The last mineralization stage involved (low-temperature and low-salinity NaCl–H2O system of hydrothermal fluids with relatively lower δD values of – 93.1 to – 92.3‰ and δ18OH2O values of – 5.7 to – 5.2‰. Moreover, sulfides in ores yielded δ34S values of – 3.6 to 5.9‰, with an average of 2.65‰. The sulfides have 206Pb/204Pb ratios of 17.101–17.804, 207Pb/204Pb ratios of 15.438–15.554, and 208Pb/204Pb ratios of 37.438–38.654. The results of fluid inclusions and H–O, S–Pb, and He–Ar isotopes indicate that hydrothermal fluids associated with the early stage of mineralization originated from magmatic fluids, with sulfur and lead being derived from both units of the Taihua Group and a Late Triassic magmatic–hydrothermal system. All of these data indicate that the Linghu gold deposit formed as a result of magmatic–hydrothermal mineralizing processes.

Carbonatites from the southern Brazilian Platform: A review. II: Isotopic evidences

Abstract Early and Late Cretaceous alkaline and alkaline–carbonatitic complexes from southern Brazil are located along the main tectonic lineaments of the South America Platform. Calcium-, magnesium-, and ferrocarbonatites are well represented and frequently associated even in the same complex. Primary carbonates present significant variations in C–O isotopic compositions, which are mainly due to isotope exchange with H2O–CO2-rich hydrothermal fluids, whereas fractional crystallization or liquid immiscibility probably affects the δ18O and δ13C values by no more than 2δ‰. Our isotope exchange model implies that the most significant isotopic variations took place in a hydrothermal environment, e.g., in the range 400–80°C, involving fluids with the CO2/H2O ratio ranging from 0.8 to 1. Sr–Nd–Pb isotope systematics highlight heterogeneous mixtures between HIMU and EMI mantle components, similar to the associated alkaline rocks and the flood tholeiites from southern Brazil. In spite of the strong variation shown by C–O isotopes, Sr–Nd–Pb–Os isotopic systematics could be related to an isotopically enriched source where the chemical heterogeneities reflect a depleted mantle “metasomatized” by small-volume melts and fluids rich in incompatible elements. These fluids are expected to have promoted crystallization of K-rich phases in the mantle, which produced a veined network variously enriched in LILE and LREE. The newly formed veins (enriched component) and peridotite matrix (depleted component) underwent a different isotopic evolution with time as reflected by the carbonatites. These conclusions may be extended to the whole Paraná–Etendeka system, where isotopically distinct parent magmas were generated following two main enrichment events of the subcontinental lithospheric mantle at 2.0–1.4 and 1.0–0.5 Ga, respectively, as also supported by Re–Os systematics. The mantle sources preserved the isotopic heterogeneities over a long time, suggesting a nonconvective lithospheric mantle beneath different cratons or intercratonic regions. Overall, the data indicate that the alkaline–carbonatitic magmatism originated from a locally heterogeneous subcontinental mantle.

Petrographic and Sr–Nd–Pb–Li isotope characteristics of a complex lamproite intrusion from the Saxo-Thuringian Zone: A unique example of peralkaline mantle-derived melt differentiation

Variscan orogenic lamproites in the Bohemian Massif predominantly occur as 1 to 2 m wide and petrographically uniform dykes along the eastern borders of the Moldanubian and Saxo-Thuringian zones. Variscan orogenic lamproites were derived by preferential melting of subduction-related olivine-free metasomatic vein assemblages stabilised in the lithospheric mantle. These lamproitic melts may subsequently undergo extensive differentiation. In this study, we present the first combined petrographic and Sr–Nd–Pb–Li isotope characteristics of a complex lamproite exposed at ca 100 m long profile near Horní Rokytnice (Czech Republic) in the Saxo-Thuringian Zone. This lamproite is characterised by the primary mineral assemblage of K-amphibole + K-feldspar ± aegirine and quartz that petrographically varies from relatively primitive (fine-grained, mafic) to more differentiated (medium- to coarse-grained, felsic) pegmatitic lamproite domains. These domains may represent the product of crystallisation of immiscible liquids that had separated from the mafic melt. The primitive lamproite zone is characterised by the typomorphic minerals – baotite, benitoite, and henrymeyerite. The more differentiated pegmatitic domains are free of aegirine and show replacement of primary red-luminescent (Fe3+-rich) K-feldspar by blue-luminescent (Fe-poor) K-feldspar. Residual fluids rich in Ca, Ti, and HFSE in combination with the decreasing peralkalinity of the lamproite system resulted in the local formation of secondary zircon, titanite and quartz at the expense of the primary Ti–Ba–Zr–K lamproitic mineral assemblages. Lamproites from the Moldanubian and Saxo-Thuringian zones fall on separate mixing trends in the 87Sr/86Sr(t) - εNd(t) diagram, which indicates that the mantle beneath these two zones had been metasomatised by different crustal material. The scatter in the peralkalinity index vs. δ7Li diagram indicates that the Li isotope composition is not controlled by mixing of two end members metasome and ambient depleted mantle alone, but may also be affected by late-stage magmatic and hydrothermal processes. The compositionally zoned Horní Rokytnice dyke is special as the petrographically different types show a variation of about 4 δ-units in δ7Li due to dyke-internal processes, such as fractionation, which increases δ7Li in late-stage lamproitic melts, and post-emplacement interaction with fluids that reduced δ7Li in samples that have lost Li. Post-emplacement alteration also led to the disturbance in the Pb isotope systematics of the differentiated orogenic lamproite as indicated by variable over-correction of in situ radiogenic Pb ingrowth.

Deciphering garnet genesis in meta-igneous rocks: An example from the Jiao-Liao-Ji Belt, North China Craton

Garnet is common in metamorphic rocks and can be used to efficiently constrain pressure–temperature conditions experienced during orogenesis; however, it may also crystallize as a primary phase in many types of magma, which can complicate deciphering the petrogenesis of meta-igneous rocks. In this study, geochronological and geochemical analyses of garnet and zircon were integrated to constrain garnet genesis in plagioclase-absent mafic granulite from the Jiao-Liao-Ji Belt, North China Craton. The rare earth element patterns show an equilibrium relationship between garnet and metamorphic zircon, indicating that garnet has a metamorphic origin. U–Pb dating shows that garnet has a whole-grain age of c. 1.83 Ga, with no difference between core and rim domains, which is slightly younger than metamorphic ages recorded by zircon (c. 1.87 Ga). The age difference between garnet and zircon is likely a result of differences in their closure temperature for the U–Pb isotope system. The mineral assemblage garnet + clinopyroxene + K-feldspar + titanite shows textural equilibration and was used to determine peak pressure–temperature conditions of 0.8–1.2 GPa at 830–880 °C. Given the age of high-pressure metamorphism (c. 1.87 Ga) calculated here being younger than c. 1.95–1.90 Ga proposed by previous studies for the west Liaodong peninsula, we suggest that c. 1.95–1.87 Ga represents the timing of progressive collision between the Longgang Block and the Nangrim Block, with the western part having collided slightly earlier than the eastern part.

Highly accurate dating of micrometre-scale baddeleyite domains through combined focused ion beam extraction and U–Pb thermal ionization mass spectrometry (FIB-TIMS)

Abstract. Baddeleyite is a powerful chronometer of mafic magmatic and meteorite impact processes. Precise and accurate U–Pb ages can be determined from single grains by isotope dilution thermal ionization mass spectrometry (ID-TIMS), but this requires disaggregation of the host rock for grain isolation and dissolution. As a result, the technique is rarely applied to precious samples with limited availability (such as lunar, Martian, and asteroidal meteorites and returned samples) or samples containing small baddeleyite grains that cannot readily be isolated by conventional mineral separation techniques. Here, we use focused ion beam (FIB) techniques, utilizing both Xe+ plasma and Ga+ ion sources, to liberate baddeleyite subdomains directly, allowing their extraction for ID-TIMS dating. We have analysed the U–Pb isotope systematics of domains ranging between 200 and 10 µm in length and from 5 to ≤0.1 µg in mass. In total, six domains of Phalaborwa baddeleyite extracted using a Xe+ plasma FIB (pFIB) yield a weighted mean 207Pb∕206Pb age of 2060.1±2.5 Ma (0.12 %; all uncertainties 2σ), within uncertainty of reference values. The smallest extracted domain (ca. 10×15×10 µm) yields an internal 207Pb∕206Pb age uncertainty of ±0.37 %. Comparable control on cutting is achieved using a Ga+-source FIB instrument, though the slower speed of cutting limits potential application to larger grains. While the U–Pb data are between 0.5 % and 13.6 % discordant, the extent of discordance does not correlate with the ratio of material to ion-milled surface area, and results generate an accurate upper-intercept age in U–Pb concordia space of 2060.20±0.91 Ma (0.044 %). Thus, we confirm the natural U–Pb variation and discordance within the Phalaborwa baddeleyite population observed with other geochronological techniques. Our results demonstrate the FIB-TIMS technique to be a powerful tool for highly accurate in situ 207Pb∕206Pb (and potentially U–Pb in concordant materials) age analysis, allowing dating of a wide variety of targets and processes newly accessible to geochronology.

Geochronology, fluid inclusions, and isotopic characteristics of the Xiaoshan gold deposit, Henan Province, China

The Xiaoshan gold deposit of western Henan Province is located in the southern margin of the North China craton (NCC). Ore bodies of the deposit occur as veins in the fault zones. The ore-forming process can be divided into three stages: an early quartz–pyrite stage, a middle quartz–polymetallic sulfide stage, and a late quartz–carbonate stage. The LA-ICP-MS zircon U–Pb age of the Houhe granite, which is related to Au mineralization of the Xiaoshan deposit, was 139–137 Ma. The initial 176Hf/177Hf values of the Houhe intrusion are 0.282126–0.282368, with the εHf(t), TDM1(Hf), and TDM2(Hf) values being −19.9 to −11.3, 1242–1583 Ma, and 1912–2454 Ma, respectively, indicating that the Houhe granite is formed by partial melting of the lower crust and that the source is mainly the Taihua Group. Two types of fluid inclusions (FIs) have been distinguished in various quartz veins including liquid-rich (WL-type) and H2O–CO2 (C-type) inclusions. The FIs in the quartz of the quartz–pyrite stage comprise WL- and C-type inclusions; their homogenization temperatures, salinities, and densities vary from 300 °C to 393 °C, 1.6 wt% to 11.0 wt% NaCl eqv., and 0.57 to 0.82 g/cm3, respectively. The FIs in the quartz of the quartz–polymetallic sulfide stage consist of WL- and C-type inclusions; their homogenization temperatures, salinities, and densities range from 261 °C to 298 °C, 1.1 wt% to 11.8 wt% NaCl eqv., and 0.74 to 0.89 g/cm3, respectively. Only WL-type inclusions are observed in the quartz of the quartz–carbonate stage; their homogenization temperatures, salinities, and densities vary from 193 °C to 258 °C, 2.2 wt% to 12.7 wt% NaCl eqv., and 0.87 to 0.97 g/cm3, respectively. The ore-forming fluids of the Xiaoshan gold deposit are generally characterized by medium temperature, moderate–low salinity, low density and belonging to an H2O–NaCl ± CO2 system. The δ18OH2O values calculated for ore-bearing quartz vary from 0.7‰ to 4.5‰, and the δDV-SMOW values from bulk extraction of fluid inclusion water vary from −69.5‰ to −47.8‰, suggesting that the ore-forming fluid consists dominantly of magmatic water. The δ34SV-CDT values range from 0.37‰ to 3.9‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of the ore minerals are in the ranges of 17.391–17.728, 15.420–15.577, and 37.420–37.923, respectively. The data from the S and Pb isotopic systems indicate that the ore-forming materials came from granitic magma. The Xiaoshan gold deposit is a mesothermal lode-gold deposit of magmatic-hydrothermal origin. Fluid phase separation and temperature decrease were the dominant mechanisms for the deposition of ore-forming materials.

Trace-element remobilisation from W–Sn–U–Pb zoned hematite: Nanoscale insights into a mineral geochronometer behaviour during interaction with fluids

AbstractPreferential removal of W relative to other trace elements from zoned, W–Sn–U–Pb-bearing hematite coupled with disturbance of U–Pb isotope systematics is attributed to pseudomorphic replacement via coupled dissolution reprecipitation reaction (CDRR). This hematite has been studied down to the nanoscale to understand the mechanisms leading to compositional and U/Pb isotope heterogeneity at the grain scale. High-Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF STEM) imaging of foils extracted in situ from three locations across the W-rich to W-depleted domains show lattice-scale defects and crystal structure modifications adjacent to twin planes. Secondary sets of twins and associated splays are common, but wider (up to ~100 nm) inclusion trails occur only at the boundary between the W-rich and W-depleted domains. STEM energy-dispersive X-ray mapping reveals W- and Pb-enrichment along 2–3 nm-wide features defining the twin planes; W-bearing nanoparticles occur along the splays. Tungsten and Pb are both present, albeit at low concentrations, within Na–K–Cl-bearing inclusions along the trails. HAADF STEM imaging of hematite reveals modifications relative to ideal crystal structure. A two-fold hematite superstructure (a = b = c = 10.85 Å; α = β = γ = 55.28°) involving oxygen vacancies was constructed and assessed by STEM simulations with a good match to data. This model can account for significant W release during interaction with fluids percolating through twin planes and secondary structures as CDRR progresses from the zoned domain, otherwise apparently undisturbed at the micrometre scale. Lead remobilisation is confirmed here at the nanoscale and is responsible for a disturbance of U/Pb ratios in hematite affected by CDRR. Twin planes can provide pathways for fluid percolation and metal entrapment during post-crystallisation overprinting. The presence of complex twinning can therefore predict potential disturbances of isotope systems in hematite that will affect its performance as a robust geochronometer.

The effect of host magma infiltration on the Pb isotopic systematics of lower crustal xenolith: An in-situ study from Hannuoba, North China

The Pb isotopic compositions of granulite xenoliths are crucial to constrain the formation and evolution of the lower continental crust (LCC) and are potentially the key to resolve the first Pb paradox of the Earth. However, it remains unclear how host magma infiltration affects the Pb isotopic compositions of LCC xenoliths and how different this effect is for different xenoliths in composition (from mafic to felsic). To address these issues, here we provide an in-situ chemical and PbSr isotopic study on feldspars and clinopyroxenes for a suite of fresh mafic and felsic granulite xenoliths from the Hannuoba region in the northern margin of the North China Craton (NCC), which have been previously well characterized for bulk-rock chemical and Sr–Nd–Pb isotopic compositions. Our study shows that the in-situ plagioclase and bulk-rock Pb isotopic compositions of the Hannuoba mafic granulite xenoliths are significantly decoupled. Such decoupling is demonstrated to have been caused by host magma infiltration through contributing the main Pb budget into the grain boundaries and cracks. However, this process appears to have relatively less or negligible effect on the Pb isotopic systematics of feldspar-rich intermediate-felsic granulite xenoliths, which is reflected by their consistent in-situ (feldspar) and bulk-rock Pb isotopic compositions. Such different effects on Pb isotopic systematics for different LCC xenoliths in composition are highly related to the Pb contents or abundances of feldspar in LCC xenoliths. Furthermore, our data suggest that the effect of host magma infiltration on the Pb isotopic systematics of LCC xenoliths may be not easily eliminated by the leaching technique and thus it should be prudent to use the bulk-rock Pb isotopic data of low-Pb LCC and potentially upper mantle xenoliths, such as peridotite, pyroxenite and garnet−/pyroxene-rich mafic granulite. Instead, in-situ analyses of minerals (e.g., feldspar, clinopyroxene) could provide a more convenient and reliable method to obtain the pristine Pb isotopic compositions of these low-Pb xenoliths.

Insights into the chemical diversity of the martian mantle from the Pb isotope systematics of shergottite Northwest Africa 8159

Shergottite Northwest Africa (NWA) 8159 is a basaltic rock derived from a mantle source with chemical characteristics that are unique in the martian meteorite suite. To further investigate this source reservoir, the Pb isotope compositions of plagioclase/maskelynite, pyroxene, phosphates, and shock melt-glass in NWA 8159 have been measured in situ by Secondary Ion Mass Spectrometry (SIMS). Due to the limited spread in Pb isotope data, these Pb isotope compositions have been used to calculate an imprecise PbPb isochron age of 3.4 ± 2.1 Ga (2σ), which is broadly consistent with the crystallization age of 2.37 ± 0.25 Ga determined previously by 147Sm143Nd. The lack of radiogenic in-growth within individual minerals since 2.4 Ga means that this sample is depleted in U, which is in agreement with NWA 8159's positive initial ε143Nd. An initial Pb composition was calculated using an x-y weighted average of the least radiogenic Pb isotope population measured in the sample. This initial Pb composition is not consistent with the model for Pb growth in the shergottite mantle at 2.4 Ga. This composition is, however, consistent with the model for the Nakhla-Chassigny mantle. Using the latter model, a source μ (238U/204Pb) of 2.6 ± 0.6 has been calculated. This μ-value is in contrast with the other depleted shergottites (1.4-1.5) and falls significantly off the array of source ε143Nd vs. μ defined by the rest of the martian meteorite suite and thus, necessitates a differentiation history distinct from the other martian meteorites. Sequestering Pb in sulphides during differentiation is the only mechanism to fractionate U from Pb and create a low-μ reservoir. Consequently, the relatively high μ-value of the source of NWA 8159 is in contrast with the positive initial ε143Nd and indicates that its mantle source region likely lacked significant sulphur. This is consistent with the lack of sulphides in the sample itself and could have played a role in its complicated oxidation history.

Ore genesis of skarn mineralization in continental collision orogens: A case study from the Pusangguo Co-bearing Cu–Pb–Zn deposit in Tibet

The Gangdese belt in southern Tibet, is renowned for hosting numerous collision-related porphyry Cu-Mo deposits. The recent discovery of Pusangguo Co-bearing Cu–Pb–Zn skarn deposits is the first reported Co-bearing deposit in Gangdese belt as well as in Tibet. Researches on the geological and metallogenic characteristics of Pusanguo indicate that this Co-bearing Cu-Pb-Zn deposit was formed in a collisional setting. We document mineralogy, fluid inclusion studies, and H–O–S–Pb isotope systematics of the skarns and ores of the Pusangguo deposit. Intense endoskarn alteration, dominated by green and pink epidote and quartz, is widely disseminated throughout the host granodiorite as well as in veinlets. Exoskarn alteration is characterized by the presence of prograde grandite (andradite32–100 and grossular0–68) with minor pyroxene (diopside0–46, hedenbergite38–74, and johannsenite1–62). Retrograde epidotes from both the endoskarn and exoskarn show a narrow range of Fe3+/(Fe3+ + Al) ratios (0.25–0.34). Ore minerals include chalcopyrite, sphalerite and galena, with lesser pyrite, pyrrhotite, bornite, Co minerals and falkmanite.Four paragenetic stages of skarn formation and ore deposition are recognized in the Pusangguo Cu–Pb–Zn deposit: prograde (stage I), retrograde (stage II), early sulfide (stage III), and late sulfide (stage IV). Fluid inclusions in all stages are dominated by two-phase liquid-rich (type L) inclusions. During the major phase of mineralization (stage III), fluid inclusions formed at low temperatures (180–240 °C) and with low salinities (0.5–8.6 wt% NaCl equiv.). This is in contrast to other skarn deposits within the Gangdese porphyry copper belt (e.g., the Zhibula, Jiama, and Bangpu deposits) that formed from highly saline fluids at high temperatures. Hydrogen-Oxygen isotopic values obtained from garnet, epidote, quartz and calcite are δDH2O = −179‰ to − 213‰ and δ18OH2O = −9.1‰ to 9.3‰, whereas S-Pb isotopic values for sphalerite, chalcopyrite, pyrite and galena are δ34S = −1.0‰ to 1.2‰, estimated δ34Sfluid = −0.05‰. Such data are indicative of ore-forming fluids and metals that originated from a magmatic–hydrothermal source related to granodiorite emplacement during the Miocene post-continental collision. Extensive mixing between the magmatic-hydrothermal and meteoric fluids led to the co-precipitation of Cu, Pb and Zn minerals from low-salinity fluids at relatively low temperatures. The Pusangguo Co-bearing Cu-Pb-Zn skarn deposit shows consistent features with other (porphyry-related) skarn deposits in the Gangdese belt, except for relatively low temperature and salinity. The research is of great significance for understanding cobalt as an associated mineral in the Gangdese sulfide skarn deposit in Tibet.

Fluid inclusions and C–H–O–S–Pb isotope systematics of the Caixiashan sediment-hosted Zn-Pb deposit, eastern Tianshan, northwest China: Implication for ore genesis

The Caixiashan sediment-hosted Zn-Pb deposit (131 Mt at 3.95% Zn + Pb) is located in the western segment of eastern Tianshan, on the southern margin of the Central Asian Orogenic Belt, Xinjiang, northwest China. Zinc and lead mineralization is mainly hosted in the dolomite marble of the Mesoproterozoic Kawabulake Group. Four stages (I to IV) of hydrothermal activity have been identified, i.e., calcite + dolomite + quartz + pyrite stage I, calcite + dolomite + quartz + spahlerite + pyrrhotite ± arsenopyrite stage II, calcite + dolomite + quartz + galena + pyrite ± chalcopyrite stage III, and late quartz + calcite stage IV. Sphalerite and galena mainly occur in the vein ores of stage II and III. Five types of fluid inclusions are distinguished in the calcite- and quartz-bearing veins, i.e., liquid-rich two-phase (L-type), pure-liquid phase (PL-type), vapor-rich two-phase (V-type), pure-vapor phase (PV-type), and halite-bearing (H-type) inclusions. Fluid inclusions of stages I to IV were homogenized at temperatures of 336–494 °C, 240–357 °C, 140–297 °C, and 71–156 °C, with salinities of 4.0–18.0 wt% NaCl equiv., 4.2–17.3 wt% NaCl equiv., 0.2–13.5 wt% NaCl equiv., and 1.2–7.6 wt% NaCl equiv., respectively. The ore-forming fluids at the Caixiashan deposit are characterized by high- to moderate temperatures, moderate salinities, and low densities, belonging to the H2O–NaCl system. Hydrogen and oxygen isotope data indicate that the ore-forming fluids at Caixiashan have a dominantly metamorphic signature and were diluted by meteoric water. Carbon and oxygen isotope compositions of calcite, dolomite, and marble demonstrate that the ore-forming fluids were primarily sourced from the dissolution and low-temperature alteration of carbonates. Sphalerite, galena, pyrrhotite, and pyrite samples from stage I to III record high δ34SV-CDT values between 11.2 and 16.1‰, indicating a predominant sulfur source from the Precambrian marine sulfates by thermochemical sulfate reduction. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of sulfide samples are similar to those of marble and carbonaceous slate of the Kawabulake Group, whereas they differ from those of granitoid rocks, suggesting that ore-forming metals may have been primarily derived from the Precambrian basement. All of these observations combined with the stable and radiogenic isotope results reveal that the formation of the Caixiashan Zn-Pb deposit was dominated by a metamorphic system, and the ore-forming components were sourced from the Precambrian basement.

Geochemically heterogeneous Martian mantle inferred from Pb isotope systematics of depleted shergottites

Radiogenic isotopic compositions of shergottite meteorites suggest that early planetary differentiation processes, which are related to the crystallization of the Martian Magma Ocean (MMO), resulted in the geochemically heterogeneous Martian mantle. In order to understand the early geochemical evolution of Mars, we investigated the Pb isotope systematics in the depleted Martian mantle on the basis of the analyses of two geochemically depleted shergottites, Dar al Gani (DaG) 476 and Yamato 980459 (Y-980459). Their initial Pb isotopic compositions were estimated from geochemical analyses of highly leached acid residues and age-correction calculations using reference crystallization ages. This yielded μ-values (238U/204Pb) for the DaG 476 and Y-980459 source reservoirs of 2.33 ± 0.07 and 2.32 ± 0.06, respectively. These μ-values are distinct from those of other depleted shergottite source reservoirs (e.g., 1.4 ± 0.1 for the Tissint meteorite) and show a negative correlation with corresponding 147Sm/144Nd, 176Lu/177Hf, ɛ182W, and ε142Nd compositions. Such correlations between long- and short-lived isotopic signatures suggest that a geochemically heterogeneous depleted shergottite source mantle was formed on the early Mars. This geochemical heterogeneity would have been formed by variable mixing of depleted and enriched end-member components that originally formed by fractional crystallization in the MMO. Local remelting in the geochemically depleted Martian mantle after the crystallization of the MMO is another possible explanation for the formation of a geochemically heterogeneous depleted shergottite source mantle.

Mineralogy, Geochemistry, and Sr–Nd–Pb Isotope Systematics of Late Cenozoic Basanites of the Borozdin Bald Mountain (Khentei Ridge, Southern Transbaikalia)

Abstract —We have estimated the P–T conditions of formation of basaltoid melts: P = 1.15–1.06 GPa and T = 1379–1293 ºC. Olivine pyroxenites (Ol + Cpx + Grt) are assumed to be a mantle source for nepheline-normative basanitic melts. During ascent, the melt trapped mantle xenoliths, which were disintegrated into olivine and augite xenocrysts. A decrease in pressure and temperature led to the crystallization of highly magnesian (Mg# = 86) olivine and diopside and Timgt + Ilm ± Pl phenocrysts. Then, the Ol + Cpx + Timgt + Pl microlite paragenesis formed. Alkaline aluminosilicates (acid plagioclase + nepheline + leucite) were the last to crystallize in the rock interstices. The presence of residual glass indicates that the last stage of crystallization took place under subsurface conditions. The volcanic area of southern Transbaikalia (Khentei Ridge) resulted, most likely, from the mantle plume impact on the lithosphere. The age of this area is estimated at 3.51 Ma. PREMA was the main mantle source for these volcanics, and the contribution of HIMU was strongly subordinate. In geochemical features the studied volcanics correspond to mafic rocks of oceanic islands. They are similar in composition to alkali basalts of the South Baikal volcanic area.

The Metallogenic Setting of the Jiangjiatun Mo Deposit, North China: Constraints from a Combined Zircon U–Pb and Molybdenite Re–Os Isotopic Study

The Jiangjiatun Mo deposit is a recently discovered molybdenum deposit in the easternmost area of the Yan-Liao metallogenic belt, North China Craton. Quartz vein-type Mo mineralization at Jiangjiatun is associated with the granitic porphyry stock. In this study, we performed a combined zircon U–Pb and molybdenite Re-Os dating study on the Jiangjiatun Mo deposit to constrain its mineralization age and metallogenic setting. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) zircon U–Pb analyses suggest that the granitic porphyry was formed during the Late Jurassic, with a weighted mean 206Pb/238U age of 154 ± 1 Ma (2σ). Seven molybdenite samples from the Jiangjiatun deposit yield a 187Re–187Os isochron age of 157.5 ± 0.5 Ma (2σ). The discrepancy between the U–Pb and Re–Os ages may be explained (1) by the “2 sigma” measurement uncertainty, or (2) by the different closure temperature of the Re–Os isotopic system of molybdenite and the U–Pb isotopic system of zircon. Even though there is a small difference between the zircon U–Pb and molybdenite Re–Os ages, we can clearly identify a Late Jurassic Mo mineralization event at Jiangjiatun in the easternmost area of the Yan-Liao metallogenic belt. The moderate Re concentrations (13 to 73 ppm) in molybdenite from the Jiangjiatun Mo deposit are indicative of the involvement of the mantle materials into the Mo mineralization. The Jiangjiatun Mo deposit is likely the result of the subduction of the paleo-Pacific plate beneath the North China Craton during the Late Jurassic. Combined with the available published regional robust geochronological data, we proposed that the Mo mineralization in the Yan-Liao belt is in good agreement with the tectonic transition from Late Triassic post-collision extensional setting due to the closure of the paleo-Asian ocean to the Yanshanian (J–K1) continental arc setting in response to the subduction of the paleo-Pacific Plate. The study highlights that regional mineralization may provide an excellent constraint on tectonic change.

Fluid inclusions, C–H–O–S–Pb isotope systematics, geochronology and geochemistry of the Budunhua Cu deposit, northeast China: Implications for ore genesis

The Budunhua Cu deposit is located in the Tuquan ore-concentrated area of the southern Great Xing’an Range, NE China. This deposit includes the southern Jinjiling and northern Kongqueshan ore blocks, separated by the Budunhua granitic pluton. Cu mineralization occurs mainly as stockworks or veins in the outer contact zone between tonalite porphyry and Permian metasandstone. The ore-forming process can be divided into four stages involving stage I quartz–pyrite–arsenopyrite; stage II quartz–pyrite–chalcopyrite–pyrrhotite; stage III quartz–polymetallic sulfides; and stage IV quartz–calcite. Three types of fluid inclusions (FIs) can be distinguished in the Budunhua deposit: liquid-rich two-phase aqueous FIs (L-type), vapour-rich aqueous FIs (V-type), and daughter mineral-bearing multi-phase FIs (S-type). Quartz of stages I–III contains all types of FIs, whereas only L-type FIs are evident in stage IV veins. The coexisting V- and S-type FIs of stages I–III have similar homogenization temperatures but contrasting salinities, which indicates that fluid boiling occurred. The FIs of stages I, II, III, and IV yield homogenization temperatures of 265–396 ​°C, 245–350 ​°C, 200–300 ​°C, and 90–228 ​°C with salinities of 3.4–44.3 ​wt.%, 2.9–40.2 ​wt.%, 1.4–38.2 ​wt.%, and 0.9–9.2 ​wt.% NaCl eqv., respectively. Ore-forming fluids of the Budunhua deposit are characterized by high temperatures, moderate salinities, and relatively oxidizing conditions typical of an H2O–NaCl fluid system. Mineralization in the Budunhua deposit occurred at a depth of 0.3–1.5 ​km, with fluid boiling and mixing likely being responsible for ore precipitation. C–H–O–S–Pb isotope studies indicate a predominantly magmatic origin for the ore-forming fluids and materials. LA-ICP-MS zircon U–Pb analyses indicate that ore-forming tonalite porphyry and post-ore dioritic porphyrite were formed at 151.1 ​± ​1.1 ​Ma and 129.9 ​± ​1.9 ​Ma, respectively. Geochemical data imply that the primary magma of the tonalite porphyry formed through partial melting of Neoproterozoic lower crust. On the basis of available evidence, we suggest that the Budunhua deposit is a porphyry ore system that is spatially, temporally, and genetically associated with tonalite porphyry and formed in a post-collision extensional setting following closure of the Mongol–Okhotsk Ocean.

Major/trace elements and Sr–Nd–Pb isotope systematics of lavas from lakes Barombi Mbo and Barombi Koto in the Kumba graben, Cameroon volcanic line: Constraints on petrogenesis

This study presents major elements, trace elements and Sr–Nd–Pb isotope data for fifteen (15) representative volcanic rock samples from volcanic edifices of Lakes Barombi Mbo (n = 9) and Barombi Koto (n = 6), located in the Kumba graben along the Cameroon Volcanic Line (CVL). The samples are dominantly basalts and trachy-basalts with MgO ranging from 7 to 9 wt.%. The lavas from the edifice of Lake Barombi Mbo are slightly richer in TiO2 (mean 3.3 wt%) and lower in CaO (mean 9.2 wt%) relative to those from the edifice of Lake Barombi Koto (2.9 wt% and 10.3 wt%, respectively). Trace element characteristics of the lavas suggest an enrichment of the source by carbonatitic metasomatism and melting of a heterogeneous source at varying depth in the garnet-spinel transition zone. Trace elements and isotopic composition of the lavas suggest insignificant crustal contamination. Modelling of the degree of partial melting suggest that the magmas were formed at varying depth (60–80 km) and by ~2% partial melting of a source containing less than 4% garnet. Samples from Lake Barombi Mbo formed at a higher degree of partial melting (2%–0.5%) and at a shallower depth relative to those from the volcanic edifice of Lake Barombi Koto (mostly <0.5%) and Mt. Cameroon. Interestingly, the relationship of Zr/Hf and 206Pb/204Pb suggests that the magma was affected by carbonatitic metasomatism whose effect decreased from Mt. Cameroon to Lake Barombi Mbo. Our results suggest that Lakes Barombi Mbo and Barombi Koto lavas are typical of lavas with HIMU-like signature and involved contributions from a dominantly sublithospheric mantle (SLM) and minor contributions from the sub-continental lithospheric mantle (SCLM). The SLM material rose and was hosted in the SCLM prior to melting.

Modelling U-Pb discordance in the Acasta Gneiss: Implications for fluid–rock interaction in Earth's oldest dated crust

The U–Pb isotopic system in zircon is the tool of choice to interrogate high-temperature geological processes, yet this system has potential to investigate lower temperature fluid–rock interaction as well. In some cases, removal of radiogenic Pb is incomplete, potentially allowing regression of discordant U–Pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid-driven isotopic disturbance. However, in rocks preserving more complex histories, simple regression is not effective at resolving multiple Pb loss events. Here, we use a ‘concordant–discordant comparison’ (CDC) test to establish the times of U–Pb disturbance in the Acasta Gneiss Complex (AGC), Canada. AGC c. 4.03 to c. 3.40 Ga orthogneisses experienced a long and complex post-crystallization history, for which U–Pb zircon data reflects not only the heterogeneous nature of the rock, but also the varying degrees and duration of crustal reworking that inevitably involved open system processes. The CDC test calculates the similarity between the concordant age structure and a modelled age structure, the latter inferred from discordant analyses, over a wide range of potential disturbance times. Our analysis reveals concordant zircon components implying new growth and/or recrystallization at 3992 ± 5, 3501 ± 6, 3442 ± 5 and 3126 ± 6 Ma. In addition, we establish episodes of radiogenic-Pb loss driven by fluid–rock interaction at 3150 ± 50 Ma, and probably at 2875 ± 50 Ma and c. 2590 Ma. These Pb-loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism, granite emplacement, and unroofing. Pb-loss within the AGC shows an antithetic relationship in different samples that are in close geographic proximity. We suggest that zircon alteration and associated new growth effectively rendered those grains that underwent Pb-loss at a particular time less susceptible to alteration during the next episode of fluid interaction.