Rb-Sr Isochron Age Research Articles

The polymetallic magmatic-hydrothermal Xiangdong and Dalong systems in the W–Sn–Cu–Pb–Zn–Ag Dengfuxian orefield, SE China: constraints from geology, fluid inclusions, H–O–S–Pb isotopes, and sphalerite Rb–Sr geochronology

Vein-type deposits, including the Xiangdong W–Sn and Dalong Pb–Zn deposits, occur in or near the Dengfuxian composite granite pluton, comprising predominantly Triassic and minor Jurassic intrusions. Liquid-rich NaCl-aqueous inclusions, vapor-rich NaCl-aqueous inclusions, liquid-rich CaCl2-NaCl-aqueous inclusions, two-phase CH4-rich inclusions, three-phase CO2-H2O inclusions, and three-phase calcite-bearing inclusions occur in the quartz veins at Xiangdong, whereas only liquid-rich NaCl-aqueous inclusions occur at Dalong. The Xiangdong veins formed at temperatures near 241 °C, from NaCl-CaCl2-H2O-CH4-CO2 fluids averaging 11.4 wt% NaCl eq. The Dalong deposit formed at temperatures near 186 °C from NaCl-H2O(-CH4) fluids averaging 6.2 wt% NaCl eq. The ore-forming mechanisms at Xiangdong include fluid immiscibility during stage I, fluid mixing during stage II, and mixing with meteoric water accompanied by cooling during stage III. The ore-forming mechanisms at Dalong include cooling and mixing with meteoric water. Oxygen and hydrogen isotopes suggest that the ore-forming fluids from both deposits originated as mixtures of magmatic water with various amounts of meteoric water. Sulfur and strontium isotopes suggest an igneous origin for both deposits and possibly mixing with S and Sr from sedimentary rock for Dalong. Lead isotopes indicate that ore metals originated mainly from the upper crust with minor mantle contributions. Sphalerite from Dalong gives a Rb-Sr isochron age of 151.6 ± 7.1 Ma, consistent with the mineralization age of Xiangdong. Both the W–Sn and Pb–Zn ore-forming events are closely related to Late Jurassic magmatism, which occurred in an environment of lithospheric extension and thinning.

Geochronological systematics of the Huayna Potosí, Zongo and Taquesi plutons, Cordillera Real of Bolivia, by the K/Ar, Rb/Sr and U/Pb methods

The Huayna Potosi, Zongo and Taquesi are Triassic plutons located at the core of the Real Cordillera of Bolivia. In this paper, several Rb-Sr and K-Ar ages obtained in the past at the Sao Paulo Geochronology Laboratory, yet unpublished, will be presented, along with newer U-Pb Sensitive High-Resolution Ion Microprobe (SHRIMP) determinations made in the same laboratory, allowing us to redefine the geologic history of this part of the Central Andes. Rb/Sr analyses of some low grade metapelitic country rocks of the early Paleozoic (Amutara and Cancaniri Formations) yielded a Rb-Sr isochron age of 344 ± 38 Ma, indicating the action of an early Gondwanide regional event. A five-point Rb-Sr isochron from a granite outcrop of the Huayna Potosi pluton yielded an age of 224 ± 28 Ma. In addition, an important Ar loss in micas was detected in the Zongo granitoids and their country rocks, recording a thermal event that opened this isotopic system in the Oligocene. Newer U-Pb SHRIMP zircon ages of ca. 221 Ma were obtained in two other granitic outcrops of the Huayna Potosi granite. They confirmed its Triassic crystallization age, and a similar U-Pb SHRIMP age of 221.9 ± 1.5 Ma was obtained for one sample of the Taquesi pluton. For the Zongo pluton, many of the zircon grains obtained from one sample of its Kuticucho facies yielded extremely high uranium content, which produced reverse discordant apparent ages. However, due to the fair alignment of the analytical points in the Concordia diagram, possibly corresponding to a linear correlation, we made a regression calculation and the interception of the Concordia curve resulted in a rather imprecise age of 220 ± 20 Ma. Our conclusion was that the final magmatic crystallization and the intrusion of plutons in the central part of the Cordillera Real of Bolivia have occurred close to 221.5 ± 2.0 Ma, in late Triassic times. Finally, the U-Pb SHRIMP ages obtained in inherited zircon xenocrysts from the four available granitic rocks yielded very different ages, and many of them are related to previous magmatic episodes of the Andean Tectonic System. A few other age measurements indicated sources related to much older Proterozoic magmatic events associated with rocks from the Andean basement.

Tectonic significance of the Cretaceous granitoids along the south‐east coast of continental China

We present the results of our study on 16 Cretaceous granitoid plutons along the south‐east coast of continental China. The zircon U–Pb ages and the bulk‐rock Rb–Sr isochron age (R2 = 0.935) indicate that the granitoids represent the last episode of magmatism (119–92 Ma) associated with the paleo‐Pacific Plate subduction beneath continental China. These granitoids show large compositional variation that is to a first‐order consistent with varying extents of magma evolution, which is best expressed by a large SiO2/MgO range and correlated trends of SiO2/MgO with the abundances and ratios of major and trace elements. The correlated Nd (εNd(t) = −6.1 to −1.2) and Hf (εHf(t) = −4.7 to +3.4) isotopic variation reflects parental magma compositional differences as a result of varying sources and processes. The Nd–Hf isotope data indicate that these granitoids were produced by mature continental crust melting with significant mantle input (~20–60%). Rhyolite‐MELTS modelling shows that relative to the less evolved (i.e., low SiO2/MgO) granitoid plutons, the progressively more evolved (i.e., varying larger SiO2/MgO) plutons can be explained by varying extents (~24% to 67%) of fractional crystallization. The origin of the magmas parental to the granitoids is best explained by a two‐stage process: (a) subducting slab dehydration‐induced mantle wedge melting for basaltic magmas and (b) ascent and underplating/intrusion of the basaltic magmas caused the mature crustal melting for the granitoid magmas. The systematic northward decrease in εNd(t) and εHf(t) suggests progressively more enriched crustal material towards the north, but it may very well indicate northward crustal thickening, permitting a greater extent of crustal assimilation.

The distinctive tectonic and metamorphic history of the Barru Block, South Sulawesi, Indonesia: Petrological, geochemical and geochronological evidence

This study describes the field relationships, petrography and geochemistry of various metamorphic and magmatic rocks from the Barru Block, South Sulawesi, Indonesia, and reports Rb-Sr isochron ages for clastic metasedimentary rocks. The Barru Block is composed of a tectonic stack of various lithological units, including cherts, radiolarian mudstone, breccias, shales, phyllites, mica schist, gneisses, amphibolite and ultramafic rocks. Dacitic rocks intrude the metamorphic and ultramafic sequences. Greenschist- to amphibolite-facies metasedimentary rocks represent detritus from nearby continental blocks that have been redistributed by turbidity currents. Quartzofeldspathic gneisses are derived from andesitic and dacitic protoliths, whereas amphibolites have basaltic precursors. The clastic metasedimentary rocks show enrichment in LILE and HFSE, particularly Nb, but depletion in Sr. The amphibolites display trace element patterns typical of N-MORB rocks, whereas compositional characteristics of gneisses and dacites suggest that they originated in an arc-related environment.Geothermobarometric calculations and mineral assemblages of greenschist facies-rocks suggest metamorphic equilibration at 250–300 °C and a maximum pressure of 5 kbar. Temperature estimates for amphibolites are 640 ± 20 °C at a pressure of 1.3–3 kbar, whereas the temperature estimates for quartzofeldspathic gneisses range from ca. 630–660 °C for peak conditions to 430–520 °C for the retrograde stage. Internal Rb-Sr mineral isochrons for three medium-pressure mica schists indicate metamorphic ages of ca. 70.7 ± 2.4 Ma, 107.3 ± 0.7 Ma and 110 ± 2.4 Ma, respectively. This age range confirms the importance of Early Cretaceous (Albian) metamorphism within the region, but also indicates a previously unknown Late Cretaceous (Maastrichtian) metamorphic or deformational event at ca. 70 Ma.The Barru rocks show a range of tectonic environments as diverse as that of the adjacent Bantimala Block, but overall, the Barru rock association is more felsic in composition. Although the two blocks accreted approximately at the same time (Early Cretaceous), differences in protoliths, geochemistry and metamorphic P–T conditions imply that they were not geographically close together at that time, but were subsequently juxtaposed tectonically.

Age of theYongxin Audeposit in theLesser Xing'an Range: Implications for anEarly Cretaceousgeodynamic setting for gold mineralization inNE China

The newly discovered Yongxin Au deposit in the Lesser Xing'an Range of north‐eastern Heilongjiang Province, NE China, has a resource of ~20 t with a grade of 4.1 g/t Au. The deposit is hosted by Carboniferous syenogranite and mylonite, and Cretaceous diorite porphyry and granite porphyry. The syenogranite has a LA‐ICP‐MS206Pb/238U zircon age of 316 ± 2 Ma, and both the granite porphyry and diorite porphyry yield an age of 119 ± 1 Ma. The Rb–Sr isochron age of Au‐bearing pyrite from hydrothermal breccia‐type ore is 107 ± 4 Ma, and this is interpreted as the mineralization age of the Yongxin deposit. In combination with published geochronological data, we argue that epithermal Au mineralization in NE China formed during the period ca. 124 to 107 Ma and was produced during rollback of Palaeo‐Pacific oceanic slab. The Cretaceous epithermal Au deposits are distributed in the Erguna Massif, Lesser Xing'an Range, Zhangguangcai Range, and Jiamusi and Nadanhada terranes. In combination with the coeval porphyry Mo deposits present in these areas, we propose that the Erguna Massif, Lesser Xing'an Range, north‐eastern Great Xing'an Range, and eastern Jiamusi and Nadanhada terranes, where only epithermal gold deposits were discovered up to date, might be prospective for concealed porphyry Cu or Mo deposits.

The Hämmerlein skarn-hosted polymetallic deposit and the Eibenstock granite associated greisen, western Erzgebirge, Germany: two phases of mineralization—two Sn sources

The Hammerlein polymetallic deposit is hosted in skarn, schists, and gneisses that reached their metamorphic peak at ~ 340 Ma during the Variscan orogeny. The deposit is spatially closely associated with one of the most voluminous granites of the Erzgebirge, the Eibenstock granite which intruded the metamorphic units at ~ 320 Ma, and locally also developed greisen mineralization. Cassiterite is the main ore mineral in the Hammerlein skarn and in greisen mineralizations associated with the Eibenstock granite. The age of skarn formation is bracketed by multi-mineral Rb-Sr isochron ages of the gneisses (~ 340 Ma) and the end of ductile deformation (> 330 Ma). The dated calc-silicate minerals of the skarn have elevated Sn contents, which implies that some Sn was present in the system during regional metamorphism, i.e., well before the emplacement of the Eibenstock granite. Tin in the > 330 Ma old skarn silicates possibly was mobilized from the metamorphic wall rocks. Retrogression of the skarn mineral assemblage may have released some Sn that formed cassiterite in an assemblage with chlorite and fluorite. The Sr isotope signatures of fluorite indicate that this late assemblage is not related to cooling of the metamorphic rocks, but to the emplacement of the Eibenstock granite, which introduced additional Sn into the skarn. Thus, mineralization in the Hammerlein deposit includes Sn that was introduced during two different events from different sources.

Re–Os dating of molybdenite and in-situ Pb isotopes of sulfides from the Lamo Zn–Cu deposit in the Dachang tin-polymetallic ore field, Guangxi, China

The Dachang tin-polymetallic district, Guangxi, China, is one of the largest tin ore fields in the world. Both cassiterite-sulfide and Zn–Cu skarn mineralization are hosted in the Mid-Upper Devonian carbonate-rich sediments adjacent to the underlying Cretaceous Longxianggai granite (91–97 Ma). The Lamo Zn–Cu deposit is a typical skarn deposit in the district and occurs at the contact zone between the Upper Devonian limestone and the granite. The ore minerals mainly consist of sphalerite, arsenopyrite, pyrrhotite, galena, chalcopyrite, and minor molybdenite. However, the age of mineralization and source of the metals are not well constrained. In this study, we use the molybdenite Re–Os dating method and in-situ Pb isotopes of sulfides from the Lamo deposit for the first time in order to directly determine the age of mineralization and the tracing source of metals. Six molybdenite samples yielded a more accurate Re–Os isochron age of 90.0 ± 1.1 Ma (MSWD = 0.72), which is much younger than the reported garnet Sm–Nd isochron age of 95 ± 11 Ma and quartz fluid inclusions Rb–Sr isochron age of 99 ± 6 Ma. This age is also interpreted as the age of Zn–Cu skarn mineralization in the Dachang district. Further, in this study we found that in-situ Pb isotopes of sulfides from the Lamo deposit and feldspars in the district’s biotite granite and granitic porphyry dikes have a narrow range and an overlap of Pb isotopic compositions (206Pb/204Pb = 18.417–18.594, 207Pb/204Pb = 15.641–15.746, and 208Pb/204Pb = 38.791–39.073), suggesting that the metals were mainly sourced from Cretaceous granitic magma.

Geochemistry and Petrogenesis of 2.8 Ga Old Rare Metal Bearing Fertile Granite at Allapatna, Mandya District, Karnataka

ABSTRACT This paper reports petrography, geochemistry and Rb-Sr age data on the rare metal bearing Neoarchean fertile (Nb-Ta) granite at Allapatna and elucidates its petrogenesis and role in Nb-Ta-Li-Be mineralization. The Allapatna granite (AG) intrudes the Tonalitic-Trondhjemitic - Granodioritic (TTG) Peninsular Gneiss and analysed SiO2 (72.3-75.6 wt%), K2O (4.0-5.7wt%), Na2O (3.0-4.4wt%), CaO (0.7-1wt%), MgO (0.13-0.25wt%) and K2O/Na2O (>1) indicating evolved nature. The presence of muscovite, biotite and garnet in the mode, peraluminous nature and high initial 87Sr/86Sr ratio (0.7284±0.0083) attest to their S-type characteristics. Varying Nb/Ta ratio and high Li with moderate abundance of Cs further indicate affinity to Li-Cs-Ta (LCT) type granite-pegmatite system. The AG showing whole rock Rb-Sr isochron age of 2803± 68 Ma, is the oldest reported fertile granite in India parental to rare metal pegmatites hosting Nb-Ta, Be, and Li resources. Partial melting of a mixed source consisting of both basement TTG rocks and metapelites has generated such type of granitic magma. Fractionation of such granitic magma possibly has given rise to the rare metal (Ta-Nb-Li-Be) bearing pegmatites intruding the nearby schist belt.

Petrogenesis of peralkaline granite dykes of the Straumsvola complex, western Dronning Maud Land, Antarctica

Peralkaline syenite and granite dykes cut the Straumsvola nepheline syenite pluton in Western Dronning Maud Land, Antarctica. The average peralkalinity index (PI = molecular Al/[Na + K]) of the dykes is 1.20 (n = 29) and manifests itself in the presence of the Zr silicates eudialyte, dalyite and vlasovite, and the Na–Ti silicate, narsarsukite. The dykes appear to have intruded during slow cooling of the nepheline syenite pluton, and the petrogenetic relationship of the dykes and the pluton cannot be related to closed-system processes at low pressure, given the thermal divide that exists between silica-undersaturated and oversaturated magmas. Major and trace element variations in the dykes are consistent with a combination of fractional crystallization of parental peralkaline magma of quartz trachyte composition, and internal mineral segregation prior to final solidification. The distribution of accessory minerals is consistent with late-stage crystallization of isolated melt pockets. The dykes give an Rb–Sr isochron age of 171 ± 4.4 Ma, with variable initial 87Sr/86Sr ratio (0.7075 ± 0.0032), and have an average e Nd of − 12.0. Quartz phenocrysts have δ18O values of 8.4–9.2‰, which are generally in O-isotope equilibrium with bulk rock. Differences in the δ18O values of quartz and aegirine (average Δquartz−aegirine = 3.5‰) suggest aegirine formation temperatures around 500 °C, lower than expected for a felsic magma, but consistent with poikilitic aegirine that indicates subsolidus growth. The negative e Nd (< − 10) and magma δ18O values averaging 8.6‰ (assuming Δquartz−magma = 0.6‰) are inconsistent with a magma produced by closed-system fractional crystallization of a mantle-derived magma. By contrast, the nepheline syenite magma had mantle-like δ18O values and much less negative e Nd (average − 3.1, n = 3). The country rock has similar δ18O values to the granite dykes (average 8.0‰, n = 108); this means that models for the petrogenesis of the granites by assimilation are unfeasible, unless an unexposed high-δ18O contaminant is invoked. Instead, it is proposed that the peralkaline syenite and granite dykes formed by partial melting of alkali-metasomatised gneiss that surrounds the nepheline syenite, followed by fractional crystallization.

Genesis of the Dianfang breccia-hosted gold deposit, western Henan Province, China: Constraints from geology, geochronology and geochemistry

The Dianfang gold deposit, located in Songxian, western Henan province, China, is one of two breccia-hosted gold deposits in the Xiong’ershan–Waifangshan region. Previous studies suggest that the breccia and host volcanic rocks of the Xiong’er Group formed coevally. The deposit consists of seven vein-type orebodies, occurring within the southern and hanging wall margins of the breccia pipe, as well as in fractures in the rhyolites of the Jidanping Formation, and are structurally controlled by ENE-trending faults. Their mineral assemblage includes K-feldspar, quartz, sericite, chlorite, epidote, carbonate, adularia, and sulfides formed from hydrothermal alteration related to mineralization. Pyrite is the dominant ore mineral, and is associated with minor galena, sphalerite, and chalcopyrite. The trace minerals include magnetite, rutile, cervelleite, matildite, and bismuthinite. Gold occurs as native gold, and electrum inclusions in pyrite, or along microfractures in sulfides and quartz. Four stages of hydrothermal mineralization have been identified: quartz–K-feldspar (I), quartz–pyrite (II), polymetallic sulfide (III), and carbonate–adularia (IV). Zircon U–Pb geochronology of the granitic matrix of the breccia constrains the timing of breccia pipe formation to 145.8Ma, indicating it formed at the end of the Late Jurassic, rather than the Paleoproterozoic, as suggested by previous studies. Hydrothermal sulfides from the main orebody yield a Rb–Sr isochron age of 121.5±1.7Ma, placing gold mineralization at the end of the Early Cretaceous. The sulfur and lead isotope data suggest that the ore-forming materials have a close connection with magmatic fluid. The initial 87Sr/86Sr value of the sulfides is 0.71056±0.00012, suggesting an ancient crustal source, likely the Archean Taihua Group, mixed with a juvenile mantle component. The trace element characteristics of pyrite indicate the mineralizing fluid was magmatic in origin, sourced from a magma derived from the partial melting of the Taihua Complex, and may have been affected by water–rock interaction at high temperatures under reducing conditions. Geological, geochronological, and geochemical data suggest the Dianfang ore deposit is genetically related to a hidden late Mesozoic granite, and forms part of an intrusion-related auriferous system. Gold deposits of similar ages are widespread at the southern margin of the North China Craton, and are coeval with the emplacement of felsic and mafic intrusions, exhumation of a metamorphic core complex, and formation of extensively faulted basins. This suggests that the Dianfang gold deposit formed under conditions of lithospheric extension caused by crustal thinning, and the destruction of the lithosphere mantle beneath the craton during the Late Jurassic to Early Cretaceous.

The most continent-sided occurrence of the Phanerozoic subduction-related orogens in SW Japan: Zircon U-Pb dating of the Mizoguchi gneiss on the western foothill of Mt. Daisen volcano in Tottori

Understanding the pre-Cenozoic geotectonic framework along the Japan Sea coast in southwest Japan is critical in clarifying the relationship between oceanic subduction-related and collision-related geotectonic belts along the continental margin of East Asia. Although a thick Neogene-Quaternary volcanic-sedimentary cover often conceals the pre-Cenozoic units, the Western Honshu is one of the pivotal regions especially the Mizoguchi gneiss (high-grade gneisses up to amphibolite facies with Rb-Sr isochron age of 185Ma) exposed near Mt. Daisen in western Tottori prefecture. This unit, together with a neighboring 199Ma granitoid, was previously correlated with the Hida gneiss in Central Japan, located nearly 250km to the east. The present study examines this interpretation using zircon U-Pb geochronology. Ages of three gneissose rocks of the Mizoguchi gneiss reveals that the protolith is composed of and represented by granitoid, accretionary complex, and/or high pressure/low temperature (HP/LT) type metamorphosed units. In terms of protolith rock type and zircon age spectra, the Mizoguchi gneiss is different from the Hida gneiss that is interpreted to have been derived from sedimentary rocks/volcanics with continental shelf, and also from the Oki gneiss characterized by its preservation of abundant Precambrian detrital zircon grains. The high-grade metamorphism of the Mizoguchi gneiss is interpreted to have occurred after middle Jurassic as constrained by the youngest detrital grain. The protolith of the Mizoguchi gneiss belongs to the Suo belt (Jurassic HP/LT type metamorphosed accretionary complex) in southwest Japan; however, the unit is interpreted to have been metamorphosed by a Jurassic thermal event, which overlaps that of the Jurassic felsic magmatism in the Hida belt (the Funatsu episode). The Mizoguchi gneiss in western Honshu is unique, because it marks the northernmost or the most proximal occurrence of Phanerozoic subduction-accretion complex in Southwest Japan, clearly on the south of non-accretionary Oki and Hida belts.

Different stages of chemical alteration on metabasaltic rocks in the subduction channel: Evidence from the Western Tianshan metamorphic belt, NW China

To understand the geochemistry of subduction zone metamorphism, especially the large-scale mass transfer at forearc to subarc depths, we carried out a detailed study of a ∼1.5m size metabasaltic block with well-preserved pillow structures from the Chinese Western Tianshan high- to ultrahigh-pressure metamorphic belt. This metabasaltic block is characterized by omphacite-rich interiors gradually surrounded by abundant channelized (veins) glaucophane-rich patches toward the rims. The glaucophane-rich rims share the same peak metamorphic conditions with omphacite-rich interiors, but have experienced stronger blueschist-facies overprinting during exhumation. Representative samples from the glaucophane-rich rims and omphacite-rich interiors yield a well-defined Rb-Sr isochron age of 307±23Ma, likely representing this overprinting event. Both glaucophane-rich rims and omphacite-rich interiors show elevated K-Rb-Cs-Ba-Pb-Sr contents relative to their protolith, reflecting a large-scale enrichment of these elements and formation of abundant phengite during subduction. Compared with the omphacite-rich interiors, the glaucophane-rich rims have gained rare earth elements (REEs, >25%), U-Th (∼75%), Pb-Sr (>100%) and some transition metals like Co and Ni (25–50%), but lost P (∼75%), Na (>25%), Li and Be (∼50%); K-Rb-Cs-Ba show only 10% loss. These chemical changes would be caused by serpentinite-derived fluids during the exhumation in the subduction channel. Therefore, there are two stages of fluid action in the subduction channel. As the formation of phengite stabilizes K-Rb-Cs-Ba at the first stage, the residual fluids released from the phengite-rich metabasaltic rocks would be depleted in these elements, which are unlikely to contribute to elevated contents of these elements in arc magmas if phengite remains stable at subarc depths. In addition, the decrease of U/Pb ratios as the preferred enrichment of Pb over U in the eclogitic rocks during the first stage chemical alteration may further lead to the lower radiogenic Pb isotope component of the deeply subducted ocean crust with time, which is inconsistent with the high radiogenic Pb isotope component of high µ (=238U/204Pb) basalts.

Early Cretaceous magmatism and associated polymetallic mineralization in South China: the Tiantang example

ABSTRACTThe Tiantang Cu–Pb–Zn polymetallic deposit in western Guangdong, South China, is hosted in the contact zone between the monzogranite porphyry and limestone of the Devonian Tianziling Formation. Orebodies occur in the skarn and skarnized marble as bedded, lenses, and irregular shapes. In this study, we performed LA-ICP-MS zircon U–Pb dating, zircon trace elements, and Hf isotopic analyses on the Tiantang monzogranite porphyry closely related to Cu–Pb–Zn mineralization. Twenty-two zircons from the sample yield excellent concordia results with a weighted mean 206Pb/238U age of 104.5 ± 0.7 Ma, which shows that the emplacement of the monzogranite porphyry in the Tiantang deposit occurred in the Early Cretaceous. The zircon U–Pb age is largely consistent with the sulphide Rb–Sr isochron ages, indicating that both the intrusion and Cu–Pb–Zn mineralization were formed during the Early Cretaceous in South China. The εHf(t) values of three inherited zircons from the monzogranite porphyry are 13.1, 11.9, and 12.9, respectively, and the two-stage Hf model ages are 1096 Ma, 1087 Ma, and 1055 Ma, respectively. Except for the three inherited zircons, all εHf(t) values of zircons are negative and have a range of −7.6 to −3.4, with the two-stage model ages (TDM2) of 1380–1643 Ma, which indicates the rock-forming materials were mainly derived from the partial melting of Mesoproterozoic to Neoproterozoic crust rocks, and probably included some Neoproterozoic arc-related volcanic-sedimentary materials. In this study, the monzogranite porphyry from the Tiantang deposit has calculated Ce4+/Ce3+ ratios of zircon ranging from 91 to 359, indicative of a more oxidized signature and significant prospecting potential for ore-related magmatism. Based on ore deposit geology, isotope geochemistry, and geochronology of the Tiantang Cu–Pb–Zn deposit and regional geodynamic evolution, the formation of Early Cretaceous magmatism and associated polymetallic mineralization in South China is believed to be related to large-scale continental extension and subsequent upwelling of the asthenosphere.

Permian copper–polymetallic mineralization in the southern Great Xing’an Range, Northeast China: the Daolundaba copper–polymetallic deposit example

ABSTRACTThe Daolundaba Cu–polymetallic deposit is a newly discovered Cu–W–Sn deposit on the western slopes of the southern Great Xing’an Range, and its mineralization was related to an early Permian coarse-grained biotite granite. However, there is little information on the age of formation of the deposit. In this article, we present the results of our investigation into the age of the Daolundaba Cu–polymetallic deposit, which involved the selection of chalcopyrite and pyrrhotite samples for Rb–Sr isochron dating. A Rb–Sr isochron defined by the chalcopyrite samples yielded a Rb–Sr isochron age of 290.0 ± 11 Ma (MSWD = 1.2) with an initial Sr isotopic composition (ISr) of 0.71446. The pyrrhotite samples yielded a Rb–Sr isochron age of 283.0 ± 2.6 Ma (MSWD = 1.16) with an initial Sr isotopic composition (ISr) of 0.71447. The Rb–Sr isochron age determined from the chalcopyrite and pyrrhotite is 282.7 ± 1.7 Ma (MSWD = 1.13). These results indicate that the Daolundaba Cu–polymetallic deposit formed during the early Permian (282.7–290.0 Ma). The Rb and Sr contents of the chalcopyrite and pyrrhotite range from ~0.1325 to ~3.6810 ppm and from ~0.1219 to ~9.5740 ppm, respectively, and the initial Sr isotope ratios (ISr) range from 0.71047 to 0.71869, with an average of 0.714723. These isotopic characteristics indicate the ore-forming minerals of the Daolundaba Cu–polymetallic deposit originated mainly from the crust, but with small amounts of mantle material involved. The copper was derived from the associated magma whereas the W and Sn was derived from the surrounding strata. The Permian mineralization of the Xing’an–Mongolia region occurred in an active continental margin setting during subduction of the Palaeo-Asian oceanic plate beneath the Siberian Plate.

Late Oligocene–Miocene mantle upwelling and interaction inferred from mantle signatures in gabbroic to granitic rocks from the Urumieh–Dokhtar arc, south Ardestan, Iran

ABSTRACTThe south Ardestan plutonic rocks constitute major outcrops in the central part of Iran’s Cenozoic magmatic belt and encompass a wide compositional spectrum from gabbro to granodiorite. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating of zircon three granodiorites yielded ages of 24.6 ± 0.1, 24.6 ± 0.1, and 24.5 ± 0.1 Ma. For tonalitic rocks, internal Rb–Sr isochron ages (biotite, feldspars) indicate cooling ages of 20.4 ± 0.1, 20.5 ± 0.1, and 22.3 ± 0.1 Ma, which are slightly younger than the zircons’ ages. The limited variations in their Sr–Nd isotope ratios indicate derivation from an asthenospheric mantle source. A geodynamic model is presented in which late Oligocene–Miocene rollback of the Neotethyan subducting slab triggered asthenospheric upwelling and partial melting in the south Ardestan. These melts were subsequently modified through fractional crystallization and minor crustal contamination en-route to the surface. Plagioclase + orthopyroxene-dominated fractional crystallization accounts for differentiation of gabbro to gabbroic diorite, whereas fractionation of clinopyroxene, titanomagnetite, and orthopyroxene led to differentiation of gabbroic diorite to diorite. Amphibole fractionation at deeper levels led to the development of tonalites.

Rb-Sr geochronology of single gold-bearing pyrite grains from the Katbasu gold deposit in the South Tianshan, China and its geological significance

The newly discovered Katbasu gold deposit in the South Tianshan region has a gold reserve of 76t at an average grade of 3.84g/t. It is the first large gold deposit in the central segment of the South Tianshan. Gold mineralization is hosted within granite, and is associated with potassic, phyllic and chloritic alteration. Gold-bearing pyrite occurs as disseminations in the host rock and as quartz-sulfide veins cross-cutting the potassic and phyllic alteration assemblage. Rubidium (Rb) - strontium (Sr) analyses on sericite within five individual pyrite grains from a phyllic alteration sample yield an isochron age of 322.5±6.8Ma (MSWD=3.2). The host granite has a SIMS U-Pb zircon age of 351.4±1.1Ma (MSWD=0.13). The Rb-Sr isochron age represents the mineralizing age of the Katbasu gold deposit, which significantly postdates the ore-hosting granite but is consistent with the age of regional porphyry type mineralization at ~320Ma in the South Tianshan. Discovery of the Katbasu gold deposits indicates huge potential for gold exploration in the central segment of the South Tianshan as it is geologically similar to the western and eastern segments of the South Tianshan, which both host many world-class gold deposits.

Pyrite Re-Os isotope systematics at the Zijinshan deposit of SW Fujian, China: Constraints on the timing and source of Cu-Au mineralization

Zijinshan is the largest high-sulfidation epithermal Cu–Au deposit on mainland China. The primary mineralization at the deposit is characterized by pyrite, digenite and covellite. Although some RbSr isochron ages of the fluid inclusions in quartz and the apparent K-Ar age of the Cu-bearing alunite alteration zone have been previously interpreted as the ore-forming age, the Rb-Sr and K-Ar dating systems are usually readily reset due to their closure temperature, and thus document the latest thermal event. In order to precisely determine the age of the Cu mineralization, eight-pyrite separates were Re-Os dated, and seven yielded an isochron age of 103±4Ma with an initial 187Os/188Os ratio of 0.45±0.14. This date is interpreted as the age of Cu mineralization, which is in agreement with geochronological data from molybdenite and the porphyritic granodiorite that hosts Cu-Mo mineralization at Luoboling (located 2km NE of Zijinshan), suggesting that these two deposits were formed during the same metallogenic event. The relatively low initial 187Os/188Os ratio indicates that the source of the Cu at Zijinshan is predominantly the crust with input from the mantle.

Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell

The Rb-Sr beta-decay dating system is one of the most attractive tools in geochronology, as Rb is sufficiently abundant in common K-bearing minerals like biotite, muscovite and K-feldspar. This allows dating of a wide variety of rocks (e.g., volcanic, magmatic, metamorphic, sedimentary and hydrothermal environments) without the need of preconcentration, as is often required for U-Pb dating of zircon. However, this advantage was to date negatively counteracted by the lack of a suitable in-situ technique, as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope.A reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer (ICP-MS) allows exactly this, the online chemical separation of two different elements. Coupled to a laser ablation (LA) system, in-situ Rb-Sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates Sr from Rb.We present here a simple procedure in which Rb-Sr ages can be obtained from a suite of individual phases in regular thin sections. Using the most established reaction gas, oxygen (O2), it is possible to react part of the ablated Sr to SrO+ while no RbO+ is formed. Measurements of feldspars and micas with a spot size of 80μm were calibrated against glass standards NIST SRM 610, BCR-2G and biotite from La Posta (California; 91.6Ma). Results are presented for a variety of magmatic rocks with well-established thermal records: a sample each from the Klokken syenodiorite (Greenland; 1161Ma), the Ulvö alkaligabbro (Sweden; 1256Ma) and a pegmatite from the Bohus granite (Sweden; 920Ma). Obtained in situ Rb-Sr isochron ages are accurate <1.5%, while initial 87Sr/86Sr ratios are accurate <0.2% compared to published data.The methodology outlined in this study has significant implications for Rb-Sr studies due to the high spatial resolution: (1) quality of measurements can be better controlled by avoiding alterations and inclusions, (2) large sample numbers can be investigated simply by using regular thin sections, (3) several mineral generations can be targeted that would not be distinguishable in mineral separates, and (4) isotope zonation within single crystals can be revealed. This will open the Rb-Sr dating system to new fields of study. For example, thermochronologic studies can use 87Sr/86Sr and 87Rb/86Sr zonation within crystals to better work out cooling and/or reheating paths. Further applications are in the field of provenance studies and temporal evolution of shear zones.

Rb-Sr Isotopic Geochronology and Geological Implications of Dongfeng Gold Deposit in Jiaodong Area

&lt;p&gt;The superlarge Dongfeng gold deposit is located in the Potouqing faults-alteration belt of the eastern part of the ‘Zhao-Lai-gold ore belt’, which belongs to the northwestern part of the Jiaodong area. Tectonically, ore bodies are controlled by faults and gold mainly occurs in the pyrite and polymetallic sulfide-bearing quartz vein. In this paper, Rb–Sr isotopic analysis is carried out with the beresite, which formed by hydrothermal metasomatism, and the Rb–Sr isochron age is 125.5±6.7Ma, indicating this deposit set up in the early Cretaceous of the late Yanshanian. Based on the relationship between the Dongfeng gold deposit and the Mesozoic granite, it is suggested that the formation of the gold deposit is a complex geological process of gradual enrichment and precipitation of the ore-forming elements. The initial &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr ratio of the beresite is 0.711502±0.000069, which indicates the ore-forming materials mainly come from the crust. Combined with the complex mineralization process of the Dongfeng gold deposit and the reported H-O isotopic data, it is suggested that the ore-forming materials are mainly derived from the crust with some mantle materials, while the ore-forming fluids are originated primarily from magmatic hydrothermal and mantle with some precipitate water.&lt;/p&gt;

Apatite fission track thermochronology of Khibina Massif (Kola Peninsula, Russia): Implications for post-Devonian Tectonics of the NE Fennoscandia

The thermal history of the Kola Peninsula area of NE Fennoscandia remains almost fully unknown because of the absence of any thermochronological data such as apatite and/or zircon fission track or (UTh)/He ages. In order to fill this gap and to constrain the post-Devonian erosion and exhumation history of this region, we present the results of apatite fission track (AFT) dating of eleven samples selected from the cores taken from different depths of the northern part of the Khibina intrusive massif. The RbSr isochron age of this alkaline magmatic complex which is located at the center of Kola Peninsula is 368+6Ma (Kramm and Kogarko, 1994). Samples were analyzed from depths between +520 and −950m and yielded AFT ages between 290 and 268Ma with an age uncertainty (1σ) of between ±19Ma (7%) and ±42Ma (15%). Mean track lengths (MTL) lie between 12.5 and 14.4μm. Inverse time–temperature modeling was conducted on the age and track length data from seven samples of the Khibina massif. Thermal histories that best predict the measured data from three samples with the most reliable data show three stages: (1) 290–250Ma—rapid cooling from >110°C to 70°C/50°C for lower/upper sample correspondingly; (2) 250–50Ma—a stable temperature stage; (3) 50–0Ma—slightly increased cooling rates down to modern temperatures. We propose that the first cooling stage is related to late-Hercynian orogenesis; the second cooling stage may be associated with tectonics accompanying with opening of Arctic oceanic basin. The obtained data show that geothermal gradient at the center of Kola Peninsula has remained close to the modern value of 20°C/km for at least the last 250Myr. AFT data show that the Khibina massif has been exhumed not more then 5–6km in the last 290Myr.