Rb-Sr Ages Research Articles

Detrital K-feldspar as a novel archive of continental crustal evolution using coupled in situ Rb[sbnd]Sr dating and Pb isotope analysis

Here we present in situ RbSr and Pb isotopic analysis of K-feldspar from the Shap Granite, (Cumbria, UK) and detrital K-feldspar from the associated Shap Wells Conglomerate Formation, to investigate the potential of the detrital K-feldspar record as an archive of crustal composition. We use a unique, collision cell – multi collector inductively coupled plasma tandem mass spectrometer to make the in situ isotope measurements, using ‘chemical resolution’ of 87Rb+ and 204Hg+ isobaric interferences on 87Sr+ and 204Pb+, achieved with SF6 and NH3 reaction gases, respectively. Using this technique, we successfully demonstrate that the majority of detrital K-feldspar RbSr ages, as well as initial Sr and Pb isotope ratios, from the Shap Wells Conglomerate are within uncertainty of those for parental Shap Granite K-feldspar (398 ± 1 Ma, n = 6). Some detrital K-feldspar grains record perturbed RbSr ages, reflecting post depositional alteration events. The majority of samples with disturbed RbSr ages carry distinct petrographic features evident in SEM imaging, prior to in situ dating, which indicates a means to help screen for reliable ages.We further illustrate the potential value of combined RbSr and Pb isotopic information preserved in K-feldspar to estimate the timing and chemical composition of juvenile continental crust formed in the past. Where UPb has been fractionated during juvenile crust formation, the initial Pb isotope ratio provided by the K-feldspar yields model crustal U/Pb (diagnostic of tectonic setting) and extraction age. The contrast in the U/Pb composition of juvenile crust formed in intraplate or subduction settings allows the model U/Pb ratios of the detrital K-feldspars to be used to constrain the style of juvenile crust formation. Additionally, the initial 87Sr/86Sr isotope ratio measured in detrital K-feldspar can be combined with the calculated crustal extraction age and RbSr K-feldspar age to model the Rb/Sr of the juvenile crustal protolith.

Mineralogical and Geochemical Response to Fluid Infiltration into Cambrian Orthopyroxene-Bearing Granitoids and Gneisses, Dronning Maud Land, Antarctica

Fluid infiltration into Proterozoic and Early Palaeozoic dry, orthopyroxene-bearing granitoids and gneisses in Dronning Maud Land, Antarctica, has caused changes to rock appearance, mineralogy, and rock chemistry. The main mineralogical changes are the replacement of orthopyroxene by hornblende and biotite, ilmenite by titanite, and various changes in feldspar structure and composition. Geochemically, these processes resulted in general gains of Si, mostly of Al, and marginally of K and Na but losses of Fe, Mg, Ti, Ca, and P. The isotopic oxygen composition (δ18OSMOW = 6.0‰–9.9‰) is in accordance with that of the magmatic precursor, both for the host rock and infiltrating fluid. U-Pb isotopes in zircon of the altered and unaltered syenite to quartz-monzonite indicate a primary crystallization age of 520.2 ± 1.0 Ma, while titanite defines alteration at 485.5 ± 1.4 Ma. Two sets of gneiss samples yield a Rb-Sr age of 517 ± 6 Ma and a Sm-Nd age of 536 ± 23 Ma. The initial Sr and Nd isotopic ratios suggest derivation of the gneisses from a relatively juvenile source but with a very strong metasomatic effect that introduced radiogenic Sr into the system. The granitoid data indicate instead a derivation from Mid-Proterozoic crust, probably with additions of mantle components.

Age, Conditions and Sources of Igneous Rocks of Gora Rudnaya (Southern Yakutia)

The paper presents the results of a complex petrogeochemical and isotope-geochemical (Rb–Sr and Sm–Nd systems) study of alkaline syenites and ore-bearing metasomatites (beresites) of the Gora Rudnaya (Southern Yakutia, Russia), as well as a comparison with compositionally similar massifs of the Aldan Mesozoic igneous province. The Gora Rudnaya, together with the recently discovered Morozkinskoye deposit, is located within the Central Aldan ore region. The massif is composed predominantly of alkaline syenites with a minor amount of alkaline porphyritic syenites, which are intruded by later dikes and sills of alkaline syenite porphyries and calc-alkaline lamprophyres. Industrial gold mineralization is confined to beresitization zones (Qz–Ser– Ank–Py) in areas of intense metasomatic reworking of rocks along steeply dipping fault structures within the intrusion. The obtained Rb–Sr age values for ore-bearing metasomatites (132±1 Ma) indicate subsynchronism of the ore process and crystallization of alkaline syenites, which corresponds to the time of manifestation of the main stage of magmatism in the Aldan Mesozoic igneous province (150–115 Ma). The rocks of the Gora Rudnaya do not differ in petrogeochemical and isotope-geochemical characteristics from rocks of similar composition from other massifs of the Aldan Mesozoic igneous province, indicating a wide lateral distribution of enriched EM I type mantle beneath the studied region. Variations in the initial isotopic composition of neodymium ((143Nd/144Nd) : from 0.511375 to 0.511636) in the studied samples are probably due to the heterogeneity of the source composition. At the same time, the calculated model ages indicate that the enriched mantle source was formed no later than 2.0–2.5 Ga.

T-t Evolution of the Early Proterozoic Rocks in the Northern Ladoga Region from the Data on U-Pb, Rb-Sr and Sm-Nd Systems in Minerals

This paper presents the results of a study of isotopic systems in minerals and rocks in southern margin of the epi-Archean Karelian craton in the zone of its junction with the Svecofennian mobile belt. U-Pb, Sm-Nd and Rb-Sr mineral ages of metamorphic rocks allowed reconstructing a T-t trend during ~1.88–1.61 Ga, which reflects a wide-ranging cooling history of metamorphic rocks from the peak values of about 650–700 °C at 1.88–1.79 Ga (U-Pb age of monazites and apparent oldest Sm-Nd age of amphiboles) to 300–400 °C at 1.61 Ga (model Rb-Sr age of biotites) in zones of low- and medium-temperature metamorphism. The specificity of removal of deep-seated rocks to the present-day erosion surface and the reconstructed T-t trend comply with the development of thrust-nappe structures during the exhumation of the Svecofennids. It is also assumed that differential vertical block movements played a significant role during the post-orogenic extensional collapse and neorifting.

Structural Evolution, Exhumation Rates, and Rheology of the European Crust During Alpine Collision: Constraints From the Rotondo Granite—Gotthard Nappe

AbstractThe rheology of crystalline units controls the large‐scale deformation geometry and dynamics of collisional orogens. Defining a time‐constrained rheological evolution of such units may help unravel the details of collisional dynamics. Here, we integrate field analysis, pseudosection calculations and in situ garnet U–Pb and mica Rb–Sr geochronology to define the structural and rheological evolution of the Rotondo granite (Gotthard nappe, Central Alps). We identify a sequence of four (D1–D4) deformation stages. Pre‐collisional D1 brittle faults developed before Alpine peak metamorphism, which occurred at 34–20 Ma (U–Pb garnet ages) at 590 ± 25°C and 0.9 ± 0.1 GPa. The reactivation of D1 structures controlled the rheological evolution, from D2 reverse mylonitic shearing at amphibolite facies (520 ± 40°C and 0.8 ± 0.1 GPa) at 18–20 Ma (white mica Rb–Sr ages), to strike‐slip, brittle‐ductile shearing at greenschist‐facies D3 (395 ± 25°C and 0.4 ± 0.1 GPa) at 14–15 Ma (white mica and biotite Rb–Sr ages), and then to D4 strike‐slip faulting at shallow conditions. Although highly misoriented for the Alpine collisional stress orientation, D1 brittle structures controlled the localization of D2 ductile mylonites accommodating fast (∼3 mm/yr) exhumation rates due to their weak shear strength (<10 MPa). This structural and rheological evolution is common across External Crystalline Massifs (e.g., Aar, Mont Blanc), suggesting that the European upper crust was extremely weak during Alpine collision, its strength controlled by weak ductile shear zones localized on pre‐collisional deformation structures, that in turn controlled localized exhumation at the scale of the orogen.

Geochemistry, Rb-Sr whole rock age and Sr-Nd isotopic constraints on the Variscan A1-type granite from Azegour area in the Marrakech High Atlas (Moroccan Meseta) and their geodynamic implications

In the northern part of the Marrakech High Atlas (MHA), along the southern Variscan segment of the Western Meseta, a Variscan granitic intrusion crops out, intruding metasediments and meta-volcanosedimentary rocks of Early Cambrian to Ordovician age. A new whole-rock Rb-Sr isochron age of 268 ± 9 Ma for the granite, combined with a previously published whole-rock Rb-Sr radiometric dating (271 ± 3 Ma), reveals a post-kinematic (tectonic) character with regard to the main Variscan deformational event, belonging within the tectonic context of the Moroccan Variscan orogenic belt. Geochemically, the Azegour intrusion is metaluminous to peraluminous and exhibits a calc-alkaline affinity with a ferruginous composition. The massif shows an extremely differentiated character (SiO2 = 77.53–78.14 per cent), K2O and high total alkali contents, FeOt/(FeOt + MgO) and Ga/Al ratios, which have typical characteristics of an A-type granite. In addition, the granite contains high concentrations of LREE (LaN/SmN= 7.9–13.67) relative to HREE (LaN/YbN= 4.81–11.61) and a well-defined Eu negative anomaly (Eu/Eu* = 0.44–0.75). The granitic samples exhibit a strong enrichment of the most incompatible elements (RbN/YbN = 69.84–159.98) and a strong depletion of Ba, Sr, Eu, Nb, P and Ti. These characteristics are similar to those of A1-type granites. The absence of mineralogy typical of an S-type granite, combined with its weakly peraluminous character [A/CNK (molar Al2O3/CaO+Na2O+K2O) = 1,013–1,045], suggest that there is little or no significant involvement of supracrustal sources in the petrogenesis of the intrusion studied. Despite the strongly differentiated character of Azegour granitic rocks samples, their multi-element patterns shows many similarities to those of I-type granitoids, which has led to postulate that the parental liquids of A1-type were derived from partial melting of mafic magmas. The representative samples studied show less depleted εNd(t = 270 Ma)values of –0.94 to –4.85 and lower positive to slightly negative εSr(t = 270 Ma) values of –1.45 to 9.32. The isotopic data suggest that the Azegour granite was emplaced 270 myr ago, apparently generated by partial melting of a mafic/intermediate magma source in the lower crust as a result of the underplating of the asthenosphere mantle-derived Oceanic Island Basalt-like magmas. Alternatively, their isotopic signatures also can be attributed to the interaction and/or hybridisation of basaltic liquids derived from the mantle with these lower crust materials. The generated parental magma probably occurred at deep structural levels and involved fractional crystallisation processes by the separation of a mineralogical association composed of plagioclase + potassium feldspar ± biotite ± amphibole ± sphene ± apatite. The whole-rock Rb-Sr age of 268 ± 9 Ma, whole-rock geochemistry and Sr-Nd isotopic compositions of εNd(t = 270 Ma) and εSr(t = 270 Ma), combined with fieldwork data, suggest that the Azegour granite was emplaced.

Tephra zircon U-Pb geochronology of kimberlite maar sedimentary fills in subarctic Canada: Implications for Eocene paleoclimate and Late Cretaceous paleogeography

The Wombat and Giraffe kimberlite pipes in the Lac de Gras kimberlite field (64°N, 110°W) of the Northwest Territories, Canada, preserve unique post-eruptive lacustrine and paludal sedimentary records that offer rare insight into high-latitude continental paleoclimate. However, depositional timing—a key datum for atmospheric CO2 and paleoclimatic proxy reconstructions—of these maar infills remains ambiguous and requires refinement because of the large range in the age of kimberlites within the Lac de Gras kimberlite field. Existing constraints for the Giraffe pipe post-eruptive lacustrine and paludal maar sedimentary facies include a maximum Rb-Sr age of ca. 48 Ma (Ypresian, Eocene) based on kimberlitic phlogopite and a glass fission-track age of ca. 38 Ma (Bartonian, Eocene). The age of the Wombat pipe lacustrine maar sediments remains unclear, with unpublished pollen-based biostratigraphy suggesting deposition in the Paleocene (66−56 Ma). In this study, we examine distal rhyolitic tephra beds recovered from exploration drill cores intersecting the Wombat and Giraffe maar facies. We integrate zircon U-Pb laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) and chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) geochronology, glass fission-track dating, palynology, and tephra glass geochemistry to refine chronological frameworks for these sedimentary deposits. The Giraffe maar CA-ID-TIMS tephra zircon U-Pb dating yielded a Bayesian model age of 47.995 ± 0.082|0.087 Ma (Ypresian) for the upper portion of the lacustrine sediments, while a single zircon grain from tephra in the lowermost lacustrine sediments had an age of 48.72 ± 0.29|0.30 Ma. The revised geochronology for the Giraffe maar provides a working age model for the ∼50 m record of lacustrine silt and indicates an age ∼10 m.y. older than previously thought. The Wombat maar LA-ICP-MS zircon U-Pb dating yielded an age of 80.9 ± 1.0 Ma (Campanian), which indicates deposition during the Late Cretaceous. This first radiometric age for the Wombat maar deposits is substantially older than earlier biostratigraphic inferences of a Paleocene age. This new age suggests that the Wombat maar sediments preserve evidence of some of the oldest known freshwater diatoms and synurophytes and provide key constraints for the paleogeography of the Western Interior Seaway during the Late Cretaceous.

Геологическое и изотопное обоснование возраста золотого оруденения Гонжинского центра Приамурской провинции

The geological and isotopic age of the deposits in gold-sulfide-quartz and gold-silver formations of the Gonzhinsky gold ore center in the Amur gold-bearing province is considered. The formation of the gold-sulfide-quartz deposits of Pioneer, Anatolyevskoye, and Alexandra is paragenetically associated with the Burindinsky monzodiorite complex of intrusions and dikes, with the estimated potassium-argon age of 92-116 Myr; the isotopic age of the Burindinsky complex diorite array was established by the U-Pb method by zircons as 117.8 ± 2.6 Myr within the Aptian stage of the Early Cretaceous. For gold-silver deposits (Pokrovskoye, Burinda), a paragenetic relationship of gold-silver mineralization with a complex of subvolcanic intrusions and dikes of the Early Cretaceous Taldan complex has been established. They were formed at the subvolcanic level and belong to the Barremian time. The Rb-Sr age of the most productive Pokrovsky gold-silver deposit is 131 ± 12 Myr.

Refining the understanding of Cretaceous subduction zone processes in the central Indonesian region: The Bantimala Complex (SW Sulawesi) revisited

A prominent occurrence of Cretaceous metamorphic rocks in the central Indonesian region is the Bantimala Complex, SW Sulawesi, where an eclogite- and blueschist-bearing sequence is part of an accretionary wedge with a complex internal structure. Zircon U–Pb dating of metamafic blocks indicate a significant protolith age range (c. 369–185 Ma) for the HP/LT rock suite. The HP/LT rocks include lawsonite-free and lawsonite-bearing samples. Lawsonite in eclogite is of retrograde origin. Assuming fast exhumation, multipoint Rb–Sr age data (c. 132–125 Ma) are interpreted to broadly approximate the time of peak HP/LT metamorphism. Existing and new K–Ar and Rb–Sr dates can be subdivided into two groups (c. 137–130 Ma and c. 126–113 Ma). Samples with post-eclogitic lawsonite growth belong to the older age group and all lawsonite-free samples are in the younger age group. The mineralogical and age diversity of various blocks indicates detachment of individual fragments from the subducting plate at different depths, followed by return flow in the subduction channel along different P–T–t trajectories. The role of serpentinite during exhumation remains ambiguous. Field observations offer little support for the suggestion that most HP/LT blocks were originally transported in a serpentinite matrix. Subduction-related magmatism is recorded by a U–Pb zircon age of 109.2 ± 1.1 Ma for a quartz dioritic mélange block. Detrital zircon U–Pb data from a sandstone indicate that tectonic stacking of the Bantimala Complex occurred only after partial erosion of already exposed Cretaceous basement.

The stability of cratons is controlled by lithospheric thickness, as evidenced by Rb-Sr overprint ages in granitoids

The ancient cores of modern continents, cratons, are the oldest blocks of “stable” lithosphere on Earth. Their long-term survival relies on the resistance of their underlying thick, strong, and buoyant mantle keels to subsequent recycling. However, the effect of substantial geographical variations in keel thickness on the post-assembly behaviour and mass movement within these continental cores remains unknown. Here, we demonstrate that the spatial distribution of fluid-reset in-situ Rb-Sr ages for Paleo-Mesoarchean (3.6–2.8 billion years ago; Ga) granitoids of the Pilbara Craton, Australia shows remarkable correlation with independently-constrained lithospheric thickness models. Without craton-wide heating/magmatic events, these anomalously young Rb-Sr ages document episodes of fluid infiltration into granitoid complexes as a response to lithospheric reactivation by far-field stresses. This correlation implies that craton-wide fluid mobilization triggered by extra-cratonic Neoarchean to Mesoproterozoic (2.8–1.0 Ga) tectonic events is facilitated by variations in lithospheric strength and thickness. Compared to areas of older overprints, the two-thirds of the craton comprised of younger reset ages is underlain by comparatively thin lithosphere with higher susceptibility to reactivation-assisted fluid flow. We propose that even the strongest, most pristine cratons are less stable and impermeable than previously thought, as demonstrated by the role of granitoid complexes and cratons as selective lithospheric “sponges” in response to minor tectonic forces. Therefore, variations in lithospheric thickness, likely attained before cratonization, exert a crucial control on billions of years of fluid movement, elemental redistribution and mineralization within ancient continental nuclei.

Metallogenic Setting and Temporal Evolution of Porphyry Cu-Mo Mineralization and Alteration in the Delamerian Orogen, South Australia: Insights From Zircon U-Pb, Molybdenite Re-Os, and In Situ White Mica Rb-Sr Geochronology

Abstract Paleozoic porphyry-style hydrothermal alteration and mineralization has previously been recognized within the Delamerian orogen, South Australia, where porphyry prospects include Anabama Hill, Netley Hill, and Bendigo. However, limited exploration due in part to thick postmineralization cover hinders the understanding of the temporal context, metallogenic setting, and mineral potential of the porphyry systems along the Proterozoic continental margin of Australia. In this study, we have characterized the hydrothermal alteration and mineralization of these porphyry occurrences. Zircon U-Pb, molybdenite Re-Os, and white mica Rb-Sr ages have been determined to constrain the timing for emplacement of magmatic intrusions, precipitation of metal-bearing sulfides, and duration of hydrothermal alteration in the Delamerian orogenic belt. Zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of nine granitoids reveal that the intrusive rocks were emplaced mostly between 485 and 465 Ma, whereas three intrusions at Bendigo have zircon U-Pb ages of 490 to 480 Ma. Molybdenite isotope dilution-negative thermal ion mass spectrometry (ID-NTIMS) Re-Os dating of the four prospects identifies two porphyry Cu-Mo mineralization events at 480 and 470 to 460 Ma, respectively. Nineteen white mica Rb-Sr LA-ICP-MS/MS (tandem mass spectrometers) analyses return an age range between 455 and 435 Ma for phyllic alteration at the Anabama Hill and Netley Hill prospects, whereas intense white mica-quartz-pyrite alteration at Bendigo prospect appears to have developed between 470 and 460 Ma. These geochronologic results indicate that the Delamerian porphyry systems postdated subduction-related magmatism in the region (514–490 Ma) but instead formed within an inverted back-arc regime, where mineralized magmas and fluids ascended along favorable lithospheric-scale structures, probably due to asthenospheric upwelling triggered by mafic delamination. Porphyritic stocks, dikes, and aplites with ages of 470 to 460 Ma are the most likely hosts to porphyry-style mineralization in the Delamerian orogen that appears to have formed simultaneously with the oldest known porphyry systems in the intraoceanic Macquarie arc (e.g., Marsden, E43, and Milly Milly; 467–455 Ma). These results emphasize the significance and potential of Early-Middle Ordovician intrusive systems to host such a type of magmatic-hydrothermal mineralization in the Delamerian orogen.

Multistage mineralization of the Liling goldfield (NE Hunan) in the Jiangnan Orogen: Constraints from hydrothermal rutile U–Pb dating and sericite Rb–Sr dating

The Liling goldfield located in northeastern (NE) Hunan contains >200 tons of proven gold reserves. The gold mineralization age in Liling has long been controversial due to the lack of datable minerals. In this study, petrographic observations indicate that hydrothermal rutile and sericite were precipitated with auriferous pyrite/arsenopyrite in the ore veins. The electron probe microanalysis (EPMA) results show that the sericite from ores and the sericite from Neoproterozoic slate display distinct compositions (Especially Al, K, and Ti). The high W (1,081–36,456 ppm) and low Zr (21.35–288.7 ppm) contents of the rutile suggests a hydrothermal origin. We reported new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb ages of ore-stage hydrothermal rutile (415.2 ± 8.3 Ma) for the Tuanshanbei gold deposit, and sericite Rb-Sr ages (417.6 ± 12.9 Ma and 216.9 ± 7.8 Ma) for the Tuanshanbei and Yanlinsi gold deposits in the goldfield. The new data confirm that multistage gold mineralization has occurred in the Liling goldfield, and falls within the generally accepted timing (ca. 430–400 Ma and 250–200 Ma) of gold mineralization in the Jiangnan Orogen. Integration of geological characteristics, tectonic setting, and geochronology of the Liling goldfield suggest two Au mineralization episodes, which occurred in the Late Silurian-Early Devonian intracontinental orogeny and Triassic intracontinental orogeny, respectively.

A Permian intermediate-sulfidation epithermal Pb–Zn–Ag deposit in the northern margin of North China Craton

Recent Pb–Zn–Ag exploration in the northern margin of North China Craton has made significant new discoveries. Here we report one Pb–Zn–Ag deposit in the Shaolanghe orefield with typical signatures of intermediate-sulfidation epithermal deposits. Zhangjiagou Pb–Zn–Ag mineralization occurs within the Early Permian quartz diorite and Late Permian andesite. Three mineralization stages were identified: white quartz–rhodochrosite–sulfide Pb–Zn–mineralization stage (I), smoky quartz–sulfide Pb–Zn–Ag–mineralization stage (II), and calcite–quartz non–mineralization stage (III). Four sphalerite samples yield the Rb–Sr age of 265.3 Ma, indicating that Pb–Zn–Ag mineralization was associated with the Late Permian andesite. The liquid-rich fluid inclusions of Stage I show low–medium temperatures of 190–270 °C and low salinities (1–3 wt%), and those of Stage III are slightly lower (190–250 °C, 0.5–2 wt%), suggesting a low-salinity, low-temperature H2O–NaCl fluid system for the Zhangjiagou deposit. The H–O–C isotopic signatures indicate a mixed fluid of magmatic water with meteoric water. S and Pb isotopes suggest magmatic source for Zhangjiagou Pb–Zn–Ag mineralization, most likely shallow hydrous intermediate magma. Collectively, these geological, geochronological, and geochemical features indicate that Zhangjiagou is an intermediate-sulfidation epithermal Pb–Zn–Ag deposit.

Uranium-Lead Systematics of Lunar Basaltic Meteorite Northwest Africa 2977.

Northwest Africa (NWA) 2977 is a lunar basaltic meteorite that was found in 2005 and has been classified as an olivine cumulate gabbro. This meteorite contains a shock melt vein (SMV) induced by an intense shock event. We report herein on an in-situ analysis of phosphates in the host gabbro and the shock vein for the U-Pb dating of NWA 2977 using an ion microprobe, NanoSIMS. The majority of the analyzed phosphates, in both the SMV and host-rock, lie on a linear regression in 238U/206Pb-207Pb/206Pb-204Pb/206Pb three-dimensional space, indicating a total Pb/U isochron age of 3.15±0.12 Ga (95% confidence level), which is consistent ages determined in previous isotopic studies of NWA 2977 (Sm-Nd age of 3.10±0.05 Ga, Rb-Sr age of 3.29±0.11 Ga, and Pb-Pb baddeleyite age of 3.12±0.01 Ga), and identical to the age of the U-Pb phosphate in a paired meteorite NWA 773, 3.09±0.20 Ga, derived from our dataset. There was no clear difference in the formation age between the phosphates found in the SMV and host-rock, although the shape and size of the grains and the Raman spectra show the evidence of intense shock metamorphism. Based on these findings, the cooling rate of the phosphate was very rapid, constrained to be larger than 140 K/s.

Carbonate U[sbnd]Pb and illite Rb[sbnd]Sr geochronology of sediment-hosted gold: A case study of Yata gold deposit

In situ U–Pb dating of carbonate minerals and Rb–Sr dating of illite was carried out on the Yata sediment-hosted disseminated-type gold deposit in the Youjiang basin, South China. The new study determines the timing of gold mineralization and clarifies the geological conditions required for the successful application of in situ U–Pb dating of alteration-related carbonate minerals. The results suggest that the Yata gold deposit likely formed during two metallogenic episodes. Illite Rb–Sr ages of 178.6 ± 5.8 Ma (MSWD = 1.2) for the 2–0.5 μm fraction and 171.3 ± 3.4 Ma (MSWD = 0.62) for the 0.5–0.2 μm fraction constrain an earlier episode of gold mineralization to >178.6 Ma. Hydrothermal carbonate minerals within the Yata gold deposit are associated with organic matter. In situ U–Pb dating of paragenetically early ankerite and later calcite veins yields ages of 149.0 ± 10.4 Ma (MSWD = 2.0) and 147.3 ± 32.6 Ma (MSWD = 1.8), respectively. These mineral ages and their petrographic relationships provide unequivocal evidence that another episode of gold mineralization occurred in the Early Cretaceous. Importantly, this study demonstrates in situ U–Pb dating is applicable to low-temperature deposits consisting of organic-rich hydrothermal carbonates characteristic of relatively high common Pb. Such deposits potentially include hydrothermal antimony, Carlin-type gold deposits, gold‐uranium, and carbonate-hosted uranium deposits.

New insights into the genesis of the Sipingshan gold deposit in eastern Heilongjiang, north-eastern China: Evidence from geology, U-Pb and Rb-Sr geochronology, geochemistry, and H[sbnd]O[sbnd]S-Pb isotopes

The Sipingshan gold deposit is intermediately sized and located in the eastern Wandashan Terrane. It has attracted much attention for its uncommon genetic type of hot spring. Gold orebodies mainly occur in the upper and lower siliceous rocks, siliceous cemented breccia is found between the upper and lower siliceous rocks, and silicified rhyolite porphyry is observed below the lower siliceous rocks. Two main mineralisation periods (i.e. exhalative sedimentation and hydrothermal superimposition) have been identified at Sipingshan based on mineral paragenesis and crosscutting vein relationships. In this study, analyses of whole-rock geochemical compositions, pyrite Rb–Sr age, zircon U–Pb–Hf contents, and S–PbOH isotopes were conducted to determine the Au mineralisation time and ore genesis of this deposit. The ore-bearing siliceous rock revealed weighted mean 206Pb/238U ages of 121.8 ± 1.1 Ma and 117.6 ± 1.0 Ma, representing the mineralisation age of the exhalative sedimentation period. The Rb–Sr isochron age of pyrites (106 ± 3 Ma) was consistent with the Au–Cu mineralisation age in the Wandashan area and represented magmatic–hydrothermal mineralisation. The siliceous rocks were characterised by high SiO2 (97.92–99.18 wt%), low TiO2 (0.01–0.03 wt%), low Al2O3 (0.22–0.55 wt%), negative δCe (0.78–1.00), positive δEu (1.12–4.04), and low Sb, As, Cs, Cd and Li contents, indicating the genesis of hydrothermal sediments. The OH isotope results suggested that the ore-forming fluids were mainly derived from magmatic water and mixed with meteoric water in the later stage. The δ34S values of pyrites from the siliceous rocks (–25.8 to –8‰) indicated a biogenic sulfur-dominated source, corresponding to the exhalative sedimentation period. The δ34S values of pyrites (9.8 to 29.7 ‰ and –1.9 to –4.7 ‰) formed in the magmatic–hydrothermal period indicated that sulfur was most likely derived from sedimentary rock and magmatic sulfur sources. The Pb isotope data also implied a mixture of upper crust and mantle sources. Therefore, we propose that the Sipingshan gold deposit is a continental exhalative sedimentation-type gold deposit superimposed by later magmatic–hydrothermal fluids, and formed during the subduction of the Paleo-Pacific Plate during the Early Cretaceous.

Contribution of Triassic Tectonomagmatic Activity to the Mineralization of Liziyuan Orogenic Gold Deposits, West Qinling Orogenic Belt, China

The Western Qinling orogenic belt (WQOB) is one of the most important prospective gold districts in China, with widely distributed Indosinian intermediate–acidic intrusions. The Liziyuan Au deposit is a representative orogenic deposit in the northern WQOB, hosting several sections spatially associated with igneous rocks. The Au deposit is hosted by meta-sedimentary volcanic rocks of the Cambrian–Ordovician Liziyuan Group and the Tianzishan monzogranite. Two periods, including five stages of mineralization, are recognized in this area: an early metamorphic mineralization period (PI), including quartz–pyrite (Stage I) and banded quartz–polymetallic sulfide (Stage II) veins, and a later magmatic mineralization period (PII) including quartz–K-feldspar–pyrite–molybdenite veins (Stage III), quartz–polymetallic sulfide–chlorite ± calcite veinlets and stockwork (Stage IV), and late calcite–quartz veinlets (Stage V). Geochronological studies indicate a SHRIMP zircon U-Pb age of 236.1 Ma for the Tianzishan monzogranite, and our published ages of ore-bearing diorite porphyrite of the Suishizi section and granite porphyry of the Jiancaowan section being 213 and 212 Ma, respectively. Pyrites formed in association with PI and PII mineralization have well-defined Rb–Sr ages of 220 ± 7.5, 205.8 ± 8.7, and 199 ± 15 Ma, with close temporospatial coupling between mineralization and magmatism. The δ18O and δD values of fluid inclusions in Stage IV auriferous quartz veins range from −0.03‰ to +5.24‰ and −93‰ to −75‰, respectively, suggesting that mineralizing fluid was likely of magmatic origin. Three distinct ranges of δ34S values are identified in the studied sections (i.e., 7.04‰–9.12‰, −4.95‰ to −2.44‰, and 0.10‰–3.08‰), indicating a source containing multiple sulfur isotopes derived from magmatic and metamorphic fluids. The Liziyuan Au deposit is thus likely an orogenic deposit closely related to magmatism. Geochemical characteristics indicate that Tianzishan monzogranite is adakitic and was derived from thickened lower crust during Triassic orogenesis. The ore-bearing diorite porphyrite and granite porphyry formed in a post-collision extensional setting. Together with previous geological and geochemical data, our results indicate that the Liziyuan orogenic Au deposit was formed by early collisional–compressional metamorphism and late post-collision extensional magmatic fluids related to the evolution of the WQOB.

Magmatism and mineralization of the Taolin Pb–Zn–Cu deposit in the Mufushan area, central Jiangnan Orogen (South China): Insightfvs from zircon U–Pb and sphalerite Rb–Sr geochronology, and H–O–S–Pb isotope geochemistry

The large Taolin Pb–Zn–Cu deposit contains 0.98 Mt Pb + Zn @ 2.35 % and 0.04 Mt Cu @ 0.1 %, and is associated with the Mufushan granitic complex in the central Jiangnan Orogen (JNO), South China. It consists of sulfides in hydrothermal veins, resulting from three successive mineralization stages (I–III). However, the geochronology of mineralization and ore-related granite, and sources of ore-forming fluids and materials are not well defined. Here we present zircon U–Pb dating for granite, Rb–Sr age of sphalerite, and H–O–S–Pb isotopes for ores that served to constrain the magmatism and mineralization at Taolin better. Zircon U–Pb laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) dating of the biotite monzogranite gave an age of 136 ± 0.8 Ma (MSWD = 1.8). Furthermore, an Rb–Sr isochron of sphalerite separates yielded an age of 135.4 ± 2.6 Ma (MSWD = 1.0). These results suggest that the intrusion of biotite monzogranite and Pb–Zn–Cu mineralization are contemporaneous with a close temporal relationship. δDH2O values of quartz samples from stage II range from –67.4 % to –62.5 % and calculated δ18OH2O values from –4.1 % to –1.8 ‰, indicating a mixing source of magmatic fluids and meteoric water for ore-forming fluids. S and Pb isotopic compositions of sphalerite and galena from stage II support the genetic relationship between the Pb–Zn–Cu mineralization and biotite monzogranite at Taolin and probably indicate a common source. However, previous high δ34S values (12.6 ‰ to 17.7 ‰) for barite from stage III suggest the involvement of high sulfur isotopes, which could be derived from gypsum layers within the Early-Middle Triassic sedimentary strata. The ore-related biotite monzogranite, with εHf(t) values ranging from –6.8 to –3.0 and two-stage Hf model ages ranging from 1625 to 1383 Ma, was probably derived from Neoproterozoic lower crust with a minor injection of the asthenospheric mantle. Geochronological and isotopic data demonstrate a magmatic-hydrothermal origin for the Taolin Pb–Zn–Cu deposit.

The Middle Jurassic Sanhe Pb–Zn–Ag deposit in NE China: Constraints from geochronology, geochemistry, fluid inclusion and multi-isotope (S–Pb–He–Hf) systematics

The Sanhe is a typical medium-size Pb–Zn–Ag deposit in the Argun Massif, NE China. Its Pb–Zn–Ag orebodies are mainly hosted in the Jurassic Tamulangou Formation volcanic rocks. The orebodies occur as quartz vein clusters. The ore-forming quartz porphyry is LA–ICP–MS zircon U–Pb dated to be 165.2 ± 1.2 Ma, and the ore sphalerite yielded a Rb–Sr age of 162.5 ± 4.3 Ma. These ages indicate that the Sanhe represents the oldest Pb–Zn–Ag mineralization in the Argun Massif. Combined with published age data, the Pb–Zn–Ag mineralization of the Argun Massif occurred in three phases at ca. 160 Ma, 140 Ma and 130 Ma. The quartz porphyry shows right-inclining REE ((La/Yb)N = 22.92–40.69) patterns and negative Eu anomalies (δEu = 0.68–0.81), together with enrichments in K, La, Ce, Nd, Zr and Hf but depletions in Ba, Ta, Nb, Sr, P and Ti. Whole-rock geochemistry and Hf isotopes suggest that the quartz porphyry was derived from partial melting of juvenile lower crustal materials during the post–collision extension of the Mongol–Okhotsk orogenic belt. Sulfide S–Pb isotope compositions indicate that the ore-forming materials were derived from the deep magma, whilst He isotopes indicate that the ore fluids were dominantly crustal-derived. Fluid chemical compositions obtained from LA–ICP–MS single fluid inclusion analyses support that mixing of magmatic fluids with meteoric water was an important mineralization trigger at Sanhe. Our new data show that the involvement of unusually metal-rich fluids is a prerequisite to form abundant Pb–Zn–Ag mineralization at Sanhe.

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.