Constraints on the formation of Carlin-type gold deposits in Sichuan and Gansu provinces, China
Franz NeubauerSibila Borojević ŠoštarićAlbrecht von QuadtIrena PeytchevaGertrude FriedlJohann GenserZhihui Zeng
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ABSTRACT Using the ion microprobe SHRIMP we have analysed zircons from the Ben Vuirich, Glen Kyllachy, Inchbae and Vagastie Bridge granites from the Scottish Caledonides, in an attempt to resolve the ages of inherited zircons shown to be present in these granites by previous conventional multigrain analyses. Middle Proterozoic age components were found in inherited zircons from all four granites. Late Proterozoic (900–1,100 Ma) components have been identified in zircons from the Glen Kyllachy and Ben Vuirich granites in the Grampian Highlands. A Late Archaean age has only been detected in one zircon from the Glen Kyllachy granite. The distribution of inherited components in the granite zircon populations could reflect fundamental divisions in the age composition of granite source rocks; however, detailed assessment of this possibility must await further ion microprobe analyses on zircons from many more granites. SHRIMP isotopic and U, Th and Pb analyses were made on successive shells of zoned zircon surrounding inherited cores from the Glen Kyllachy granite to monitor chemical changes during magmatic zircon growth. Results show that zircon shells have characteristic but significantly different Th, U and Pb concentrations. Magmatic zircon from the Vagastie Bridge granite also forms as clearly defined oscillatory zoned shells around unzoned nuclei of inherited zircon. However, the distinction between magmatic and inherited zircon in zircons from the Inchbae granite is less clear. Zircons from the Ben Vuirich granite occur as euhedral, magmatic zircons, or as rounded, subhedral, inherited zircon grains. A SHRIMP age of 597 ± 11 (2σ) Ma for euhedral magmatic zircon from this granite is identical, within the uncertainty, to the conventional multigrain zircon age of 590 ± 2 (2σ) Ma reported by Rogers et al. (1989) and confirms the conclusions of those authors that sedimentation of the Dalradian sequence took place in the Precambrian.
Geochronology
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Abstract The Epembe Complex is one of the Mesoproterozoic (~1200 Ma) carbonatite alkaline complexes situated along the southern margin of the Congo Craton in northwestern Namibia. Nepheline syenites and minor syenites constitute the main lithologies, cross-cut by a calcite-carbonatite dyke. In order to constrain zircon forming-processes and magma sources, cathodoluminescence (CL) imaging combined with trace elements (including REE) as well as Hf isotope compositions of zircon grains extracted from one syenite, five nepheline syenite samples and one carbonatite sample are presented. Syenite zircons are generally unaltered and are characterised by positively sloping REE patterns in a chondrite-normalised diagram, with positive Ce anomalies. Syenite zircon further displays significant negative Eu anomalies attributed to earlier plagioclase formation and fractionation. These features are consistent with zircon formation in a magmatic environment. In the nepheline syenite samples, two zircon types are recognised. Type 1 zircon is magmatic, with homogeneous-grey, unzoned and oscillatory-zoned domains in CL, while type 2 zircon underwent low temperature fluid alteration and displays a cloudy appearance. Type 2 zircon is characterised by enrichment in LREE, Nb and Ti when compared to magmatic type 1 zircon. Carbonatite zircon displays a variety of textures and variable chemical compositions suggestive of the presence of both xenocrystal, altered and magmatic zircon. The Hf concentration and Hf isotope composition of type 1 and type 2 zircon are similar suggesting that zircon alteration did not affect the Hf isotope systematics. The similarity of ƐHf(t) values in zircon from syenite (+0.5 ± 0.4 to +1.5 ± 0.4), nepheline syenite (+1.6 ± 0.3 to +2.7 ± 0.5) and carbonatite (+1.5 ± 0.2 to +1.9 ± 0.1) is consistent with the melts having been derived from a moderately Depleted Mantle.
Carbonatite
Nepheline syenite
Baddeleyite
Nepheline
Trace element
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Detrital zircon grains preserved within clasts and the matrix of a basal diamictite sequence directly overlying the Carrapateena IOCG deposit in the Gawler Craton, South Australia are shown here to preserve U–Pb ages and geochemical signatures that can be related to underlying mineralisation. The zircon geochemical signature is characterised by elevated heavy rare-earth element fractionation values (GdN/YbN ≥ 0.15) and high Eu ratios (Eu/Eu* ≥ 0.6). This geochemical signature has previously been recognised within zircon derived from within the Carrapateena orebody and can be used to distinguish zircon associated with IOCG mineralisation from background zircon preserved within stratigraphically equivalent regionally unaltered and altered samples. The results demonstrate that zircon chemistry is preserved through processes of weathering, erosion, transport, and incorporation into cover sequence materials and, therefore, may be dispersed within the cover sequence, effectively increasing the geochemical footprint of the IOCG mineralisation. The zircon geochemical criteria have potential to be applied to whole-rock geochemical data for the cover sequence diamictite in the Carrapateena area; however, this requires understanding of the presence of minerals that may influence the HREE fractionation (GdN/YbN) and/or Eu/Eu* results (e.g., xenotime, feldspar).
Diamictite
Overprinting
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Baddeleyite
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Granitoids play a key role in the geological structure of the Ros-Tikych megablock. Supercrustal rocks of the Ros-Tikych series have been preserved in the granitoids only in the form of isolated fragments such as elongated remains, small skialites and even smaller "melted" xenoliths. In particular, in the Ostrivsky quarry, located on the right bank of the Ros River east of Bila Tserkva, granitoids are found (even-grained, porphyry-like granites) among which, as a rule, small bodies of granodiorites, plagiogranites and amphibolites occur. In order to determine the source of the parent magmas of rocks the properties of zircon crystals and the isotopic composition (87Sr/86Sr ratio) of apatite were studied. An analysis of the zircon crystals of the crystalline rocks exposed at the Ostrivsky quarry allows us to propose that the and plagio- and difeldspar granites were formed from one protolith. This is because they contain similar virtually identical zircon relics as nucleus. In addition, none of the granitoids contain zircon crystals whose internal structure is similar to zircon crystals found in amphibolite. This suggests that the granitoids were not derived by melting of amphibolites. Most likely, amphibolites are relicts of the protolith that were not assimilated during granite formation. The occurrence of heterogeneous zircon crystals (relic zircon cores of the protolith) in the protolith of the various studied granitoids indicates that they formed from volcanic-sedimentary rocks. Apatites in plagiogranitoids and porphyry granite contain strontium of similar isotopic composition. Their 87Sr/86Sr isotopic ratio is 0.70680 in apatite granodiorite and 0.70822 in granite. A high ratio of 87Sr/86Sr = 0.77940 was measured for apatite from monazite-bearing granite, thus indicating a different source for its parent magma.
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Diorite
Radiogenic nuclide
Hadean
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The petrogenesis of the Pridoli to Early Lochkovian granites in the Miramichi Highlands of New Brunswick, Canada, is controversial. This study focuses on the Pridoli Nashwaak Granite (biotite granite and two-mica granite). In situ trace elements and O and Hf isotopes in zircon, coupled with O isotopes in quartz, are used to reveal its magmatic sources and evolution processes. In the biotite granite, inherited zircon cores have broadly homogenous δ18OZrc ranging from +6.7‰ to 7.4‰, whereas magmatic zircon rims have δ18OZrc of +6.3‰ to 7.2‰ and εHf(t) of −0.39 to −5.10. The Hf and Yb/Gd increase with decreasing Th/U. Quartz is isotopically equilibrated with magmatic zircon rims. The biotite granite is interpreted to be solely derived by partial melting of old basement rocks of Ganderia and fractionally crystallized at the fO2 of 10−21 to 10−10 bars. The two-mica granite has heterogeneous inherited zircon cores (δ18OZrc of +5.2‰ to 9.9‰) and rims (δ18OZrc of +6.2‰ to 8.7‰), and εHf(t) of −11.7 to −1.01. The two-mica granite was derived from the same basement, but with supracrustal contamination. This open-system process is also recorded by Yb/Gd and Th/U ratios in zircon and isotopic disequilibrium between magmatic zircon rims and quartz (+10.3 ± 0.2‰).
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Zircon megacrysts occur in association with mafic alkaline volcanic fields worldwide and have been used as indicators for the chemical characteristics of their mantle sources. However, their origins from magmas that are strongly undersaturated in zircon remain enigmatic. To resolve this conundrum, better constraints on the temporal and chemical relations between zircon megacrysts and associated mafic alkaline magmas are required. For six volcanoes from the West and East Eifel Volcanic Fields (WEVF, EEVF), Germany, we report concordant middle to late Pleistocene zircon megacryst crystallization ages from (230Th)/(238U) disequilibrium and disequilibrium-corrected 206Pb/238U geochronology, which generally agree with independently constrained eruption ages. Trace elements in Eifel zircon megacrysts indicate crystallization from highly fractionated melt pockets in which zircon competed with other accessory minerals (e.g. apatite, titanite, pyrochlore) for incompatible elements enriched in residual melts, such as the rare earth elements, Th, and U. Eifel zircon megacrysts display systematic covariation between indices of differentiation (Eu/Eu*, Zr/Hf) and isotopic signatures of continental crustal contamination, revealing magmatic differentiation of parental mafic melts via coupled assimilation and fractional crystallization (AFC). Isotopic compositions of εHf and δ18O in Eifel zircon megacrysts are consistent with mid- to upper-crustal AFC end-members, which are represented by xenolithic ejecta in WEFV and EEVF volcanic deposits, although not necessarily the same ones that yielded zircon megacrysts. Lower-crustal mafic granulites, by contrast, are a poor match for the isotopic trends displayed by the Eifel zircon megacrysts. These lines of evidence support that the zircon megacrysts in the Eifel originated from mantle melts that differentiated in the mid- to upper crust where they fractionated and partially solidified as syenitic intrusive bodies. Mafic magma recharge en route to the surface then scavenged and disintegrated syenitic rock fragments, in some cases liberating zircon crystals as the only recognizable survivors of their plutonic hosts. Zircon megacrysts in mafic alkaline magmas thus should be treated cautiously as tracers for mantle isotopic compositions. Mixing between mafic magmas and accessory-mineral rich syenites can selectively enrich incompatible trace elements, and potentially compromise the genetic interpretation of trace element patterns in mafic rocks.
Baddeleyite
Trace element
Fractional crystallization (geology)
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Hunandong granite located on the border of Hunan and Jiangxi,intruded into the Linyang granite.Based on the investigation of field geology,detailed zircon U-Pb age measurements were done on the Hunandong granite.These zircons have high Th/U ratio with zonal structure,which indicate magmatic zircons.The zircon U-Pb age is(447.2±1.8) Ma,which shows that the Hunandong granite was formed in the late Ordovician period,reflecting the important stage of Caledonian magmatism.According to the result together with other information published,the authors think that the formation of Hunandong granite may be related to the amalgamation and collision between Cathaysian block and Yantze block,providing strong evidence for further understanding the Caledonian magmatism and its geodynamic process in South China.
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