Rb-Sr Ages Research Articles

Episodic alteration within a gold-bearing Archean shear zone revealed by in situ biotite Rb–Sr dating

Many economic gold deposits formed in the Neoarchean under greenschist facies conditions in association with ductile–brittle shear zones that were conduits for sulfur-bearing hydrothermal fluids. Although the large-scale architecture and formation history of Neoarchean structurally controlled gold deposits has been the subject of many studies, the timing, rates, and durations of mineralization-related alteration remain challenging to deconvolve owing to a lack of event-specific geochronometers and the potential multitude of overprinting processes. In an effort to better understand the alteration history along the mineralized Zuleika Shear Zone in the Eastern Goldfields Superterrane of the Yilgarn Craton in Western Australia, we present the results of biotite Rb–Sr from 13 drill core samples (seven with apatite Sr isotopes) from the Kundana gold camp and the Carbine area. The Kundana district is a lode gold deposit that represents an ideal case study area to test the longevity of mineralization-associated alteration systems. We use in situ laser ablation collision cell triple quadrupole mass spectrometry on biotite from a range of characteristic alteration assemblages with well-defined relative growth ages. Biotite Rb–Sr ages constrain the timing of four different alteration stages between ∼2700 and 2550 Ma. The correlation between relative alteration chronology and Rb–Sr age and the lack of any relationship between grain size and age supports an interpretation of Rb–Sr ratios reflecting biotite growth during hydrothermal alteration with subsequent rapid cooling. Initial 87Sr/86Sr ratios of biotite and apatite are similar and indicate compositions that are more radiogenic than chondritic values and generally slightly more radiogenic than modelled Neoarchean Bulk Silicate Earth values, consistent with a mixture of mantle and crustal fluid sources. Ti-in-biotite thermometry yields temperatures between 540 and 615 °C, peaking towards the main gold mineralization phase. Although it is not possible to link individual Rb–Sr ages to specific faulting events, the absolute age dating for the various alteration assemblages indicates a prolonged, multiphase alteration system lasting ∼150 Ma. Long-term permeability networks with episodic short-lived fluid pulses that outlast terrane- or camp-scale deformation may not only apply to Neoarchean lode gold deposits, but to any tectonically driven zone of weakness in the continental crust.

Formation of the Miaoan Au-Polymetallic Deposit in the Northern Taihang Mountain, North China Craton: Ore Geology, Geochronological and Geochemical Perspectives

Several gold ore-concentrated areas have been recognized in the destruction zone of the North China Craton (NCC). However, the deposits in the western part of the destruction zone have received less attention. Miaoan, a typical Au-polymetallic deposit in the northern Taihang Mountain, provides a good sample for deepening our understanding of the genesis of gold deposits in the western destruction zone. In this study, detailed ore geology, pyrite Rb-Sr age, trace element and S-C-O isotopes of Au-bearing ores were conducted to constrain the source of ore-forming materials and their tectonic setting. The pyrites obtain an Rb-Sr isochron age of 129.5 ± 2.5 Ma, consistent with those of magmatic rocks in this deposit, suggesting their genetic relationship. The δ34S values ranging from −5.5‰ to 1.6‰ and the high Co/Ni and Y/Ho ratios of pyrites indicate the mantle-crust mixing characteristics of ore-forming fluids. The δ13C (−6.3‰ to −2.0‰) and δ18O (9.3‰ to 17.6‰) values of Au-bearing ores and calcites suggest mixing characteristics as well. Geochronologically, the Miaoan Au-polymetallic deposit was formed during the destruction of the NCC. We propose that the Miaoan Au-polymetallic deposit is a decratonic gold deposit and that its ore-forming materials have a mixed source of mantle and crust.

Advances in in-situ Rb-Sr dating using LA-ICP-MS/MS: applications to igneous rocks of all ages and to the identification of unrecognized metamorphic events

Age determinations of rocks by in-situ Rb-Sr dating of minerals has the advantage of providing spatially resolved analysis without time-consuming preparation, but is hampered by isobaric and polyatomic interferences on Sr isotopes, especially 87Rb. LA-ICP-MS/MS using N2O as a reaction gas allows online chemical separation of 87Rb from 87Sr by measuring SrO+. Here, the method for in-situ Rb-Sr is optimized and applicability demonstrated for geological samples ranging from late Archean to 20 Ma in age.Rb-Sr isotopes of multiple reference materials and geological samples have been analyzed by LA-ICP-MS/MS to evaluate the method for dating rocks throughout the geological record. The quality of Sr isotopic data can be greatly improved by external calibration of mass bias. For internal calibration, better accuracy is obtained with the addition of H2 as a trace molecular gas. The elemental fractionation of Rb and Sr during LA-ICP-MS/MS analysis varies for different reference materials. We find that the matrix of NIST SRM 610 renders it unsuitable for correction of 87Rb/86Sr. The Mica-Mg reference material is preferred for calibration of 87Rb/86Sr of micas despite showing significant Rb-Sr fractionation.In-situ Rb-Sr ages were determined for igneous samples ranging in age from 20 Ma to Mesoproterozoic, confirming accurate application of the method to samples of widely differing ages. The most accurate isochron ages were obtained with 87Rb/86Sr calibrated against Mica-Mg. Application to two metamorphic gneisses with late Archean protoliths from Labrador and Finland reveal thermal events at 1.66–1.70 Ga when they were part of a large Paleoproterozic active margin along the southern edge of Laurentia-Baltica. Data collected for minerals with differing closure temperatures for the Rb-Sr system allow detection of thermal events not recorded by other geochronological systems, indicating a huge potential of this technique not only for accurately dating samples, but also for providing previously inaccessible information on metamorphic histories.

Crustal contribution for the formation of the Walali Au deposit and implications on the Early Cretaceous Au mineralization in the northern Great Xing’an Range

The northern Great Xing’an Range (NGXR) gold (Au) metallogenic belt is characterized by the wide occurrence of the Early Cretaceous orogenic and arc-magma related epithermal Au deposits. Information of the newly discovered granite-hosted Walali Au deposits in this area could shed new light on the Early Cretaceous Au metallogenesis in NE China. The ore-hosting granite at Walali shows zircon U-Pb ages of 310.6 ± 2.2 Ma, which are significantly different from the ore-bearing pyrite Rb–Sr age (119.9 ± 1.7 Ma). The distinct ages indicate that the Au mineralization occurred at the Early Cretaceous and therefore preclude the genetic link between the Late Carboniferous magmatism and Au ore formation. The fluid inclusion studies show that the ore precitpitated from the hydrothermal fluids with a temperature range of 150–330 °C and a salinity range of 3.4–11.6 wt% equiv. H–O isotopic compositions indicate the involvement of both magmatic and meteoric water in the hydrothermal system. The pyrite and galena S (δ34SV–CDT values: 7.7–10.0‰) and Pb (206Pb/204Pb ratios of 18.177–18.217; 207Pb/ 204Pb ratios of 15.614–15.633) isotopic compositions suggest a crustal contribution for the ore-forming materials at Walali. By comparison, the Walali Au deposits are more comparable with the strata-hosted orogenic Au deposits in the north NGXR, but different from the epithermal- and porphyry-type Au deposits in this area. Combined with the Late Mesozoic tectonic evolution in NE China, we conclude that the Early Cretaceous extensional setting caused the large scale magmatism, which drove the remobilization of crustal materials and resulted in the formation of the granite- and strata-hosted Au deposits in the NGXR.

Emplacement ages of diamondiferous kimberlites in the Wafangdian District, North China Craton: New evidence from LA-ICP-MS U-Pb geochronology of andradite-rich garnet

This contribution presents new U-Pb geochronological data and attempts to elucidate the complex evolution history of various garnet types identified from two kimberlite pipes in the Wafangdian diamond mining district, southern Liaoning Province. These calcic garnets are dominated by andradite with relatively low proportions of schorlomite, grossular and pyrope. Abundant euhedral to subhedral, highly brecciated andradite phenocrysts hosted by LN30 “carbonatite-like” kimberlite samples yield a lower-intercept age of 459.3 ± 3.4 Ma, which is in excellent agreement with the previously reported phlogopite Ar-Ar (463.9 ± 0.9 Ma) and Rb-Sr ages (461.7 ± 4.8 Ma). Based on their trace element and C-O isotopic compositions of associated groundmass carbonate, we infer that these primary magmatic andradites probably originated from kimberlitic magmas. By comparison, three compositionally and texturally distinct groups of Ti-bearing andradites from LN42 hypabyssal kimberlites separately define three well-fitted regression lines with lower intercept ages at 581 ± 12 Ma, 414.9 ± 9.3 Ma and 292.0 ± 5.7 Ma, respectively. Relict andradite xenocrysts implies that ancient lower crust of the North China Craton (NCC) might have been affected by a significant but less-known tectonothermal event to varying degrees at ∼ 0.6 Ga. By contrast, fresh grains of magmatic Ti-andradites with chemical zoning produce a relatively young age of ∼ 415 Ma, which can still provide minimum age estimates for the most recent pulses of Paleozoic kimberlite magmatism in this study area. Noteworthily, a yet unrecognized local-scale hydrothermal alteration event at ∼ 292 Ma has been recorded in the texturally distinct population of secondary hydroandradites, whose age reported here for the first time is geologically meaningful. To sum up, this study further highlights andradite U-Pb dating as a potential robust geochronometer for constraining the late-stage evolution of kimberlite magmas as well as post emplacement hydrothermal alteration.

Thermal history of the Siberian Traps Large Igneous Province revealed by new thermochronology data from intrusions

We present results of apatite fission-track analysis of seven intrusions located within the Permian-Triassic Siberian Traps Large Igneous Province (LIP): (1) alkaline-ultramafic central type plutons of Odikhincha, Yessey and Magan, (2) intrusions of Norilsk-1 and Kontay, (3) Padunsky sill, and (4) Kotuy dike. Additionally, we also present a set of new geochronological data for some of studied intrusions: (a) LA-ICPMS UPb apatite ages of the Odikhincha pluton (266 ± 29 Ma) and the Padunsky sill (241 ± 12); (b) RbSr ages from the Odikhincha (258.0 ± 0.6 Ma), Magan (242.8 ± 6.8 Ma) and Yessey (243.1 ± 2.7 Ma) plutons; and (c) 40Ar/39Ar mica ages from the Odikhincha (264.3 ± 3.0 Ma) and Magan (254.7 ± 3.1 Ma) plutons. Most of these intrusions, probably with the exception of the Odikhincha, were emplaced during the most voluminous phase of the Siberian Traps magmatism ca. 252–251 Ma, but their AFT ages are distributed from 207 ± 17 to 173 ± 13 Ma and much younger than Late Permian to Early Triassic isotopic ages from these and other Siberian Traps LIP intrusions. Available AFT, UPb, RbSr and 40Ar/39Ar data and time-temperature modeling has allowed us to create the first model of the post-Paleozoic tectono-thermal history of the Siberian Traps. After their emplacement ca. 251 Ma the studied rocks underwent a later phase of rapid cooling to below 120–60 °C during the time interval ~207–173 Ma and have remained near the surface until present. We propose this later cooling event is linked to exhumation associated with coeval Late Triassic-Early Jurassic large-scale uplift of the Siberian platform caused by collisional processes at its periphery. Our new results also indicate that at least a 1–2 km thickness of Siberian Traps lavas have been removed by erosion in the north of the Siberian platform since eruption, implying the total volume of the Permian-Triassic lavas was much greater than that currently preserved today.

Unravelling the histories of Proterozoic shales through in situ Rb-Sr dating & trace element laser ablation analysis

Authigenic components in marine sediments are important archives for past environment reconstructions. However, defining reliable age constraints and assessing the effects of post depositional overprints in Precambrian sequences are challenging. We demonstrate a new laserbased analytical approach that has the potential to rapidly and accurately evaluate the depositional and alteration histories of Proterozoic shales. Our study employs a novel application of in situ Rb-Sr dating coupled with simultaneous trace-element analysis using reaction-cell laser ablation– inductively coupled plasma–tandem mass spectrometry (LA-ICPMS/MS). We present results from shales sourced from two wells in the Proterozoic McArthur Basin, northern Australia. These rocks have been widely used by previous studies as a key section for ancient biogeochemical and paleoredox reconstructions. Shales from well UR5 yielded initial ⁸⁷Sr/⁸⁶Sr ratios, Rb-Sr ages, and rare earth element plus yttrium (REEY) patterns similar to those of a dolerite sampled from the same core. We propose that the UR5 samples chronicle hydrothermal alteration instigated by the dolerite intrusion. In contrast, a correlative shale from well UR6 yielded an age consistent with the expected depositional age (1577 ± 56 Ma) with REEY and initial ⁸⁷Sr/⁸⁶Sr ratios similar to ca. 1.5 Ga seawater. We suggest that this sample records the minimum depositional age and early marine diagenetic history for this unit. This new technique can date Proterozoic shales quickly, cheaply, and with minimum sample preparation. Importantly, ages are triaged to differentiate between those recording primary marine versus secondary processes. This novel approach provides a potentially powerful tool for dating and fingerprinting the vast array of ancient marine shales for further studies of Earth systems through deep time.

40Ar-39Ar and Rb-Sr age constraints on the formation of Sukhoi-Log-style orogenic gold deposits of the Bodaibo District (Northern Transbaikalia, Russia)

The Bodaibo District (the Irkutsk Region of Russia) is a world-class gold-mining subprovince comprising a total of 13 large- and medium-scale black-shale-hosted orogenic gold deposits known as the Sukhoi Log-style gold deposits (Ivanov, 2014). The gold mineralization of the Sukhoi Log-style deposits is represented by early veinlet-disseminated sulfide ores, which have the main economic significance, and late native gold-bearing quartz veins with only minor gold reserves. This paper presents the results of Rb-Sr и 40Ar-39Ar dating of the gold mineralization in four Sukhoi Log-style deposits: Verninsky, Golets Vysochaishy, Ozherelie, and Ikan, with the estimated gold reserves of 143, 86, 6.5, and 7.0 metric tons, respectively. The Rb-Sr and 40Ar-39Ar data obtained indicate the existence of two ore-forming events in the district. The early sulfide veinlet-disseminated ores formed in the Silurian (450–430 Ma) and postdate the peak of the regional metamorphism in the region.. During the middle Carboniferous (ca. 330 Ma), the rejuvenation of ore-forming processes resulted in the formation of native gold-bearing quartz veins. The constraints on the ages of gold mineralization of the Sukhoi Log-style deposits are of key importance for understanding their genesis and the features of evolution of the Bodaibo District as a gold-mining subprovince.

Genesis of the Maoling gold deposit in the Liaodong Peninsula: Constraints from a combined fluid inclusion, C-H-O-S-Pb-He-Ar isotopic and geochronological studies

The large tonnage Maoling gold deposit (25 t @ 3.2 g/t) is located in the southwest Liaodong Peninsula, North China Craton. The deposit is hosted in the Paleoproterozoic metamorphic rocks. Four stages of mineralization were identified in the deposit: (stage I) quartz-arsenopyrite ± pyrite, (stage II) quartz-gold- arsenopyrite-pyrrhotite, (stage III) quartz-gold- polymetallic sulfide, and (stage IV) quartz-calcite-pyrrhotite. In this paper, we present fluid inclusion, C-H-O-S-Pb-He-Ar isotope data, zircon U-Pb, and gold-bearing sulfide (i.e. arsenopyrite and pyrrhotite) Rb-Sr age of the Maoling gold deposit to constrain its genesis and ore-forming mechanism. Three types of fluid inclusions were distinguished in quartz-bearing veins, including liquid-rich two-phase (WL type), gas-rich two-phase (GL type), and daughter mineral-bearing fluid inclusions (S type). Fluid inclusions data show that the homogenization at temperatures 197 to 372 °C for stage I, 126 to 319 °C for stage II, 119 to 189 °C for stage III, and 115 to 183 °C for stage IV, with corresponding salinities of 3.7 to 22.6 wt.%, 4.7 to 23.2 wt.%, 5.3 to 23.2 wt.%, and 1.7 to 14.9 wt.% NaCl equiv., respectively. Fluid boiling was the critical factor controlling the gold and associated sulfide precipitation at Maoling. Hydrogen and oxygen stable isotopic analyses for quartz yielded δ18O = −5.0‰ to 9.8‰ and δ D = −133.5‰ to −77.0‰. Carbon stable isotopic analyses for calcite and ankerite yielded δ13C = −2.3‰ to −1.2‰ and O = 7.9‰ to 14.1‰. The C-H-O isotope data show that the ore-forming fluids were originated from magmatic water with meteoric water input during mineralization. Hydrothermal inclusions in arsenopyrite have 3He/4He ratios of 0.002 Ra to 0.054 Ra, and 40Ar/36Ar rations of 1225 to 3930, indicating that the ore-forming fluids were dominantly derived from crustal sources almost no mantle input. Sulfur isotopic values of Maoling fine-grained granite range from 6.‰1 to 9.8‰, with a mean of 7.7‰, δ34S values of arsenopyrite from the mineralized phyllite (host rock) range from 8.9‰ to 10.6‰, with a mean of 10.0‰, by contrast, δ34S values of sulfides from ore vary between 4.3‰ and 10.6‰, with a mean of 6.8‰, suggesting that sulfur was mainly originated from both the host rock and magma. Lead radioactiveisotopic analyses for sulfides yielded 206Pb/204Pb = 15.830–17.103, 207Pb/204Pb = 13.397–15.548, 208Pb/204Pb = 35.478–36.683, and for Maoling fine-grained granite yielded 206Pb/204Pb = 18.757–19.053, 207Pb/204Pb = 15.596–15.612, and 208Pb/204Pb = 38.184–39.309, also suggesting that the ore-forming materials were mainly originated from the host rocks and magma. Zircon U-Pb dating demonstrates that the Maoling fine-grained granite was emplaced at 192.7 ± 1.8 Ma, and the host rock (mineralized phyllite) was emplaced at some time after 2065.0 ± 27.0 Ma. Arsenopyrite and pyrrhotite give Rb–Sr isochron age of 188.7 ± 4.5 Ma, indicating that both magmatism and mineralization occurred during the Early Jurassic. Geochronological and geochemical data, together with the regional geological history, indicate that Early Jurassic magmatism and mineralization of the Maoling gold deposit occurred during the subducting Paleo-Pacific Plate beneath Eurasia, and the Maoling gold deposit is of the intrusion-related gold deposit type.

Origin of the Shanggong gold deposit, the southern margin of the North China Craton: Constraints from Rb-Sr ages of sericite, and trace elements and sulfur isotope of pyrite

The Shanggong Au deposit is a world-class Au deposit in the southern margin of the North China Craton. Pyrite, as the primary Au carrier in this deposit, is an ideal mineral for dissecting the complex ore-forming process and origin of this deposit. Four types of pyrite were identified by EPMA, SEM, LA-ICP-MS, and NanoSIMS analyses: the coarse-grained cubic, disseminated pyrite (Py1) in the early ore stage (S1) and three types of the fine-grained pentagonal dodecahedron, disseminated pyrite (Py2-1, Py2-2, and Py2-3) with zoning textures in the main ore stage (S2). Py1 is relatively depleted in most metals but enriched in Co and Bi compared with other types of pyrite in S2. On the other hand, both Py2-1 and Py2-3 contain relatively high contents of As, Au, and other metals, whereas Py2-2 contains highest Au, As, Ag, Cu, Sb, Te, Tl, and Pb. Oscillatory zoning in Py2-1 resulted from repeated local fluid phase separation at the near-surface of pyrite crystal due to sharp fluid pressure fluctuation. The Au and As-rich Py2-2 was probably formed under both extrinsic fluctuations in fluid pressure and composition and intrinsic local crystal-fluid interface kinetic effects (e.g., diffusion-limited self-organization, surface electrochemistry-driven adsorption). Crystallization of Py2-1 and Py2-2 has exhausted the metals in the residual fluids, wherein the trace element-poor Py2-3 crystallized and locally replaced Py2-2 through a dissolution-reprecipitation process. Our new data and previous results suggest the ore-forming fluids were likely initial produced by degassing of the concealed pluton, and the metals were sourced from both the crust (e.g., the Taihua complex and Xiong’er Group) and the mantle. The negative δ34S values for pyrite in the S2 stage might result from both the isotopic fractionation between sulfide and sulfate minerals via oxidation of the ore-forming fluid and the input of biogenic sulfur.

Unraveling the histories of Proterozoic shales through in situ Rb-Sr dating and trace element laser ablation analysis

Abstract Authigenic components in marine sediments are important archives for past environment reconstructions. However, defining reliable age constraints and assessing the effects of post-depositional overprints in Precambrian sequences are challenging. We demonstrate a new laser-based analytical approach that has the potential to rapidly and accurately evaluate the depositional and alteration histories of Proterozoic shales. Our study employs a novel application of in situ Rb-Sr dating coupled with simultaneous trace-element analysis using reaction-cell laser ablation–inductively coupled plasma–tandem mass spectrometry (LA-ICP-MS/MS). We present results from shales sourced from two wells in the Proterozoic McArthur Basin, northern Australia. These rocks have been widely used by previous studies as a key section for ancient biogeochemical and paleo-redox reconstructions. Shales from well UR5 yielded initial 87Sr/86Sr ratios, Rb-Sr ages, and rare earth element plus yttrium (REEY) patterns similar to those of a dolerite sampled from the same core. We propose that the UR5 samples chronicle hydrothermal alteration instigated by the dolerite intrusion. In contrast, a correlative shale from well UR6 yielded an age consistent with the expected depositional age (1577 ±56 Ma) with REEY and initial 87Sr/86Sr ratios similar to ca. 1.5 Ga seawater. We suggest that this sample records the minimum depositional age and early marine diagenetic history for this unit. This new technique can date Proterozoic shales quickly, cheaply, and with minimum sample preparation. Importantly, ages are triaged to differentiate between those recording primary marine versus secondary processes. This novel approach provides a potentially powerful tool for dating and fingerprinting the vast array of ancient marine shales for further studies of Earth systems through deep time.

Ultrasonic Shaking of Glauconite Pellets with Diverse Reagents for a Comparison of Their K–Ar with Already Published Rb–Sr Results

A combined ultrasonic treatment, with de-ionized H2O, dilute HAc or dilute HCl, of three Mid-Miocene glauconite samples was applied to K–Ar date the different separates in order to compare the results with those obtained by the Rb–Sr method using the same three samples and that were analyzed strictly in the same way. Two aliquots yield opposite elemental and K–Ar trends, which suggests different initial mineral compositions for the various pellets. The K–Ar data of two untreated and leached L7 and L8 aliquots are almost within analytical uncertainty from 17.3 ± 0.6 Ma to 19.6 ± 0.7 Ma (2σ), while those of the third L10 sample are slightly higher at 22.1 ± 1.2 Ma (2σ). Comparatively, the earlier published Rb–Sr ages of the three untreated samples and of the leached aliquots gave similar data for the L7 aliquots by an isochron at 18.1 ± 3.1 (2σ) Ma and for the sample L8 by an isochron with an age of 19.6 ± 1.8 (2σ) Ma, while the untreated L10 aliquot yields a very high Rb–Sr date of 42.1 ± 1.6 (2σ) Ma. This untreated L10 glauconite fraction contains blödite, a Sr-rich carbonate that impacted the two isotopic systems differently. Generally, dilute HCl or HAc acids dissolve carbonates, sulfates, sulfites and oxides, while they do not affect the clay-type crystals such as glauconites. These soluble minerals can be identified indirectly, as here, by X-ray diffraction and the amounts of leached Na2O, CaO and Fe2O3 contents. Together with the leaching of some metallic trace elements, those of NaO confirm the leaching of metals and of blödite that are both hosted by the glauconite pellets. The occurrence of this Sr-enriched mineral explains the age differences of the non-treated aliquots and suggests a systematic leaching of any glauconite separate before isotope determination and, possibly, a comparison of the Rb–Sr and K–Ar results. Ultrasonic shaking appears appropriate for physical disaggregation of any contaminating grains that may remain hosted within the pellets, even after a preliminary H2O wash, which may dissolve and remove the soluble minerals but not the H2O-insoluble silicates. The K–Ar study completed here as a complement to a previous Rb–Sr study highlights, again, the importance of the preparation step in isotopic studies of glauconite-type and, by extension, of any clay material, as all occurring minerals can interfere in the final age determinations and, therefore, differently in the mineral assemblages. All those not in isotopic equilibrium need to be removed before analysis, including the soluble Sr or alkali-enriched ones.

Apatite and biotite thermochronometers help explain an Arctic Caledonide inverted metamorphic gradient

Biotite generates significant radiogenic Sr and is thus suitable for Rb–Sr dating, which can be achieved directly in thin section via laser ablation and provides a means to directly date growth of this important fabric-forming mineral or its cooling. In contrast, apatite has an extremely low Rb/Sr ratio and hence offers a useful tool to estimate source 87Sr/86Sri. Plagioclase also contains significant Sr and may further assist in understanding the rocks Sr isotopic system. However, the fluid source of growing or recrystallizing apatite, plagioclase, and biotite need not be the same. Compounding the complexity, apatite may gain radiogenic Sr from biotite during recrystallization or dissolution–reprecipitation that accompanies metamorphism. In this contribution, we present a case study from the Kalak Nappe Complex in the Scandinavian Caledonides, Arctic Norway, where a combined geochronology program, critically supported by biotite Rb–Sr dating, helps to resolve the conspicuous inverted metamorphic field gradient found upwards through the nappe stack in this orogen. Apatite U–Pb ages of 417–413 Ma in the upper nappes are interpreted as cooling through the Pb retention zone. Such ages contrast with the suggestion of older, partially reset apatite ages in the lower nappes. Biotite Rb–Sr ages of c. 413 Ma in the upper nappes are similar to U–Pb ages of coexisting apatite but conspicuously different from c. 433 Ma biotite Rb–Sr ages in the lower nappes. Ages of c. 433 Ma are comparable to muscovite 40Ar/39Ar and titanite and monazite U–Pb ages throughout the complex and may directly date mineral growth at peak thermal conditions during the Scandian orogenic event. Apatite 87Sr/86Sr ratios also track the influence of metamorphism and imply more radiogenic signatures relative to relic plagioclase in samples which have greater overprinting in the upper nappes of the complex. Phase equilibrium modelling establishes Palaeozoic peak P–T conditions of 610–750 °C at <12 kbar in the upper nappes, comparable to peak conditions previously established for the lower nappes. Combined thermal modelling of the post-peak Scandian orogenic history based on published 40Ar/39Ar, paired with new Rb–Sr, and U–Pb data, implies a slower cooling pathway in the upper nappes relative to the lower nappes. We reconcile this thermal history with a geodynamic model of obduction of a hot back-arc, as represented by the Silurian Magerøy Nappe, onto the upper nappes of the Kalak Nappe Complex at c. 430 Ma. These results demonstrate that in-situ laser ablation Rb–Sr dating of biotite provides a powerful tool for resolving orogenic tectonothermal histories in orogens flushed with excess argon.

Sequential leaching of silicified Archaean carbonates: A Rb-Sr, Sm-Nd and Pb-Pb isotopic contribution to their tectonic-thermal history (Kaapvaal Craton, South Africa)

Sequential leaching was completed by successive interactions of ultra-pure H2O, HAc, HCl and HNO3 with silicified/carbonated Archaean whole rocks from Barberton Greenstone belt of South Africa. The purpose of this experiment was an identification of the minerals interacting with each reagent and, therefore, a detailed succession of isotopically dated tectonic-thermal episodes recorded in these minerals. The contents of the major, trace and rare-earth elemental, together with the Sr, Nd and Pb isotopic compositions of untreated, leachate and residue triplets allowed identification and analysis of various types of carbonates and sulfates mixed with insoluble silicates. The samples yield two overall geological age milestones at 2.9 ± 0.1 and 2.1 ± 0.1 Ga, as well as varied age values, some with large uncertainties. A further event at about 1.6 ± 0.1 Ga is suggested probably in relation with the emplacement of the Bushveld complex or of further intrusive complexes of the Kaapvaal Craton.While the basics of the different used isotopic systems can be considered to be similar, the Rb-Sr method appears to be the best suited to detail the studied rocks, especially their sensitive soluble minerals. The Sm-Nd method is less flexible because of its inherent need of widely ranging Sm/Nd ratios that depend on the mineral assemblage. In the case of the Rb-Sr ages well constrained by isochron arrays, more similar ages were obtained by tentatively called isotrends that consist in data points slightly scattered along the arrays giving higher uncertainties, especially with the Sm-Nd method. The fact that these isotrends gave similar ages in addition to those of the isochrons comforts the historic evolution of the studied material.These scatters most probably correspond to slight changes in the chemical characteristics of minerals leached by the successive reagents, but analytical uncertainties cannot be completely excluded, especially in the case of the HNO3 leaching step. Finally, the Pb-Pb method appears to be of a more limited application, possibly because of possible metal contamination of the host rocks, either during their evolution in a region characterized by metal-rich concentrations or more recently during discrete surficial to sub-surficial alteration/weathering processes. In terms of leaching efficiency, that with H2O removed expectedly the lowest amounts of soluble components followed by HNO3, the two efficient removers being dilute HAc and HCl. In turn, the obtained geochronological ages consolidate previously published ages that concentrate on post-depositional tectonic-thermal events within the Kaapvaal Craton during about 1.0 Ga. However, they also suggest some reconsideration for other ages of the literature.

Vertical evolution of Ag-Pb-Zn-(Cu)-Mo in porphyry system: A case study from the Laochang deposit, SW China

Distal massive sulfide Ag-Pb-Zn mineralization is commonly associated with proximal porphyry Mo-(Cu) systems, but the vertical evolution of Ag-Pb-Zn-(Cu)-Mo distribution within porphyry systems remains poorly understood and documented. The Laochang deposit located in the southeastern Sanjiang metallogenic belt, SW China contains both the massive sulfide Ag-Pb-Zn-(Cu) and porphyry-skarn Mo-(Cu) mineralization (1737 t Ag@222 g/t, 0.51 Mt Pb@5.1 wt%, 0.34 Mt Zn@3.1 wt%, 117, 800 t Mo@0.15 wt%, and 0.1 Mt Cu@0.2 wt%), thus representing an excellent case study. The Laochang deposit is mainly hosted in early Carboniferous volcanic rocks, late Carboniferous to early Permian carbonate rocks, and links with Eocene granite porphyry. Here, we report Rb-Sr age of sphalerite and pyrite from the massive sulfide Ag-Pb-Zn-(Cu) ores, zircon U-Pb age for the granite porphyry, C-O isotopes of hydrothermal calcite, and S-Zn isotopes of sulfide minerals to reveal the vertical distribution of Ag-Pb-Zn-(Cu)-Mo mineralization in porphyry system. The sulfides Rb-Sr age (45.1 ± 3.6 Ma) is similar to the zircon U-Pb age (44.6 ± 0.2 Ma), indicating the Ag-Pb-Zn-(Cu) mineralization has a genetic link with the granite porphyry and formed in the same system as the porphyry-skarn Mo-(Cu) mineralization. Furthermore, the C-O isotopic compositions of the hydrothermal calcite suggest a diminishing contribution of the host rocks and an increasing input of the meteoric water in the hydrothermal fluids from deep to shallow. The S-Zn isotopic compositions of the sulfide minerals indicate a magmatic source of the ore-forming elements. The δ34S values decrease, and the δ66Zn values increase with elevated levels of sampling depth, implying a Rayleigh fractionation process. This means fluids and late-stage sulfides evolved toward isotopically lighter sulfur and heavier zinc isotopes. Hence, we propose that the Laochang deposit is a porphyry system comprising distal Ag-Pb-Zn-(Cu) and proximal Mo-(Cu) mineralization. In addition, the sulfur and zinc isotopic systematics observed in this study provides a new insight into the mineralization processes of the porphyry-distal systems.

Petrochronological constraints and tectonic implications of Tonian metamorphism in the Embu Complex, Ribeira Belt, Brazil

The Embu Complex of the Ribeira Belt, southeast Brazil, provides records of orogenic processes within the time period between Rodinia break-up and West Gondwana assembly. Recent studies have questioned the accretionary setting for the belt, suggesting an intracontinental setting. In this paper, we present new petrological, U–Pb and Rb–Sr data from metasedimentary rocks of the Embu Complex, discussing the implications for the tectonic evolution from Rodinia to West Gondwana in the Ribeira Belt. Sillimanite, sillimanite–K-feldspar and cordierite zones represent the high-grade metamorphic zones for the metamorphic event identified in the southern portion of the Embu Complex. Petrological and thermodynamic modelling data indicate that the high-grade metamorphic rocks of the complex attained conditions between 750–805 °C and 3.6–9.2 kbar. Monazite and zircon U–Pb data indicate that the high-grade metamorphic event occurred between 810–760 Ma, concomitant with igneous activity recorded during this period. In addition, zircon and apatite U–Pb ages, as well as, biotite and muscovite Rb–Sr ages, between 675–550 Ma record a slow cooling rate of 4.4 °C/Ma for the period between 675–640 Ma, and a late stage of shear zones development. These results reinforce the accretion of distinct units within the Ribeira Belt during the Neoproterozoic Era.

Unravelling metamorphic ages of suture zone rocks from the Sabzevar and Makran areas (Iran): Robust age constraints for the larger Arabia–Eurasian collision zone

AbstractIn this study we establish a time frame for metamorphic processes affecting Neotethyan suture zones at the northern (Sabzevar) and southern (Makran) margin of the Central Iranian Microcontinent. For this purpose, we have applied Rb–Sr and U–Pb geochronology to blueschist‐ and amphibolite facies rocks from these occurrences. So far, no isotopic ages are available for high‐P/low‐T rocks from the Sabzevar area which are exposed in the Govain and Chili mountains near Soltanabad. Three newly dated blueschists from the Govain mountain yielded Rb–Sr ages of 56.52 ± 0.32, 57.20 ± 0.32, and 55.12 ± 0.33 Ma respectively. These ages provide the first unambiguous evidence for late Palaeocene to early Eocene high‐P metamorphism in the Sabzevar zone and confirm the accuracy of earlier age assignments that were derived indirectly. The clearly confirmed age difference between late Palaeocene–early Eocene blueschists of the Govain mountain and kilometre‐scale dismembered HP granulites with Early Cretaceous (Albian) metamorphic ages from outcrops c. 60 km to the west of Soltanabad further corroborates interpretations that these rocks are not related to the same subduction zone complex. The blueschists record closure of an Albian to Late Cretaceous back‐arc basin, whereas the granulites formed due to subduction of older oceanic lithosphere. Uranium–Pb zircon dating of an amphibolite from the Iranshahr region in the Makran Zone yielded an intercept age of 87.8 ± 1.8 Ma and weighted mean age of 88.1 ± 1.7 Ma. Rb–Sr mineral isochrons for seven blueschist‐ and amphibolite facies samples from the same area generated very consistent ages of c. 84 Ma (± &lt;1–3 Ma), regardless of metamorphic grade. The apparent contradiction that rocks recording different P–T conditions have identical metamorphic ages either indicates tectonic mixing of rocks during exhumation that originally had formed at the same time at different depths of the subduction system, or very rapid exhumation so that differences in closure temperatures became indistinguishable. The new data are similar, but much more precise, than previously reported K–Ar ages and suggest that most of the existing data had not been significantly compromised by contamination with extraneous Ar. Potassium–Ar indications for older blueschist‐ and/or amphibolite facies metamorphism (&gt;85 Ma) have not yet been confirmed but can be reconciled with the larger geological frame implying that northward subduction of the Arabian plate beneath the Bajgan–Durkan continental block started shortly after c. 120 Ma.

Subgrain 40Ar/39Ar dating of museum-quality micas reveals intragrain heterogeneity

Museum-grade mica megacrysts of the Phalaborwa phlogopite (ca. 2 Ga) and the Rubikon pegmatite (ca. 0.5 Ga) were screened for intra-grain compositional and chronological heterogeneities by electron probe microanalysis and 40Ar/39Ar dating, respectively. Both micas were known to have RbSr ages indistinguishable from UPb ages. Even though step heating plateaus were obtained for nearly a hundred 100 μm scale subgrain chips, age variations of several percents were observed among individual chips. The age variations were unrelated to the position within the megacryst and to the position in the irradiation canister. Element mapping parallel and perpendicular to the (001) plane showed significant variations in concentrations of major elements Ti, Fe, Mn, K, Al. Cation transport, element redistribution and retrograde mineral formation that escaped visual detection require open-system, fluid-assisted chemical reactions. These recrystallization processes occurred during one or several post-magmatic hydrothermal event(s). The spatial distribution of sub-grain ages show that 40Ar/39Ar dating does not provide a simple, diffusion-controlled cooling age, but rather apparent ages controlled by mineral retrogression and recrystallization. The result was variable loss of Ar at the subgrain scale. These observations show that retrogression events cannot be detected by the presence or absence of plateau ages. Instead, recrystallization can be diagnosed (1) by a thorough petrological investigation based on microchemical maps, and (2) by Cl/K and Ca/K constraints from 40Ar/39Ar systematics. The quest for a natural mineral sufficiently homogeneous to act as a precise calibrator for the 40K half-life remains unsatisfactory.

Genesis of the vein-type Gaocheng Ag-Pb-Zn deposit in the western Guangdong Province of China and its implication for regional exploration

Abstract The Daganshan region in the western part of the Guangdong Province is an important mineralised area containing W, Sn, and Ag–Pb–Zn occurrences. Although vein-type Ag-Pb-Zn deposits are usually spatially related to W-Sn and Sn deposits, the genetic links between them are still not understood. The newly discovered Gaocheng deposit is a typical vein-type Ag-Pb-Zn deposit in this region, which is hosted predominantly by a Silurian biotite monzogranite and to a lesser extent by a Late Cretaceous quartz porphyry . Here, we present new Rb-Sr dating of sphalerite, fluid inclusion microthermometry, and H-O-S isotopic geochemistry of the deposit, and whole-rock major and trace elements geochemistry of the mineralised quartz porphyry. The aim is to better understand the genesis of the Ag-Pb-Zn mineralisation and its link with W, and Sn deposits in the region. The sphalerite Rb-Sr age is 90 ± 6 Ma, indicating that mineralisation is Late Cretaceous similar to that of the quartz porphyry. Three vein sets have been recognised around the quartz porphyry comprising early arsenopyrite–pyrite–quartz veins crossed by sulphides–quartz–Ag veins and late (non-mineralised) calcite–quartz–fluorite veins. The fluid inclusions have homogenisation temperatures of 360–300 °C with salinities of 8.2–6.4% NaCl equivalent for the first vein set, 320–240 °C with salinities of 6.5–4% NaCl equivalent for the second vein set, and 220–160 °C with a corresponding salinity of 4.2–3.4% NaCl equivalent for the late vein set. This indicates a gradual decrease of temperature and salinity of the mineralising fluids. The composition of the vapour accompanying the saline fluid in the inclusions is H 2O with minor amounts of CO2 and CH4, which indicates that the mineralising fluid included H2O–NaCl(–CO2–CH4). The fluid inclusions in the early vein set assays +6.52 - +6.13‰ δ18OH2O and –91 - –85‰ δDH2O, and the fluid inclusions in the mineralised second vein set contains +5.72 to +2.41‰, δ18OH2O and −72 to −42‰ δDH2O. These data indicate that the mineralised fluids represented by the stage I and II vein sets included magmatic fluids that were gradually contaminated by meteoric fluids. The magmatic source of the mineralising fluid is also indicated by the narrow range of δ34 S values between −2.2 and −2.4‰. The whole-rock analyses show that the Late Cretaceous quartz porphyry is a highly fractional peraluminous granite , similar to the W- and Sn-related granites in the Daganshan region. The Gaocheng vein-type Ag-Pb-Zn deposit is therefore genetically related to the Late Cretaceous quartz porphyry and shares similar geochemical characteristics at Sn-bearing granites in the Daganshan region and is also prospective for Sn mineralisation.

40Ar/39Ar and LA-ICP-MS U–Pb geochronology for the New England portion of the Early Cretaceous New England-Quebec igneous province: Implications for the postrift evolution of the eastern North American Margin

The Early Cretaceous New England-Quebec igneous province is a classic example of postrift magmatism along the eastern North American passive margin. Although a suite of ⁴⁰Ar/³⁹Ar ages has been available for the Monteregian Hills lobe in the Quebec portion of the New England-Quebec igneous province for many years, only a single high accuracy radiometric age has been published for the Burlington lobe and none for the Taconic lobe in the New England portion of the province. As a result, the timing of and driving mechanisms behind the magmatism have remained unresolved, and a hotspot origin for the entire province persists in the literature. We have dated four dikes and one pluton in the Burlington and Taconic lobes using ⁴⁰Ar/³⁹Ar and U–Pb geochronology to improve understanding of the age of magmatism in the New England portion of the province. In the Burlington lobe, ⁴⁰Ar/³⁹Ar plateau ages include a 137.55 ± 1.80 Ma biotite age and a 136.9 ± 4.2 Ma amphibole age for a lamprophyre dike from Charlotte, Vermont, and a 133.6 ± 2.2 Ma biotite age for a lamprophyre dike from Colchester, Vermont. In the Taconic lobe, ages include an ⁴⁰Ar/³⁹Ar plateau amphibole age of 107.09 ± 1.32 Ma for a lamprophyre dike from Castleton, Vermont, a 122 Ma minimum ⁴⁰Ar/³⁹Ar biotite age for a lamprophyre dike from Poultney, Vermont, and a 103.13 ± 0.53 Ma LA-ICP-MS U–Pb zircon age from the quartz syenite of the Cuttingsville complex. These results show that magmatism spanned at least 35 Ma, from ∼138 to 103 Ma, which is broadly consistent with the span of magmatism suggested by workers in the 1970s and 1980s based on K–Ar and Rb–Sr ages. This extended span of magmatism for the Burlington and Taconic lobes is in contrast to the brief 1 to 2 Ma episode of magmatism at ∼124 Ma inferred for the Monteregian Hills lobe. The New England-Quebec igneous province has traditionally been attributed to passage of the Great Meteor hotspot. However, given the close proximity of the Burlington and Taconic lobes, the magmatism in these lobes should span only a few Ma if the product of a hotspot. The age data are also difficult to reconcile with a more complex expression of hotspot magmatism in continental lithosphere related to either plume head magmatism or long-distance migration of plume material. Instead, the extended duration of Early Cretaceous New England-Quebec igneous province magmatism in New England may represent an expression of edge-driven convection, a process known to occur along passive margins and inferred to be operating beneath the eastern North American margin today.