Re-Os Isotopic Dating of a W-Be Polymetallic Deposit in the Southern Qinling Region, China

Objective In recent years, a series of tungsten prospecting breakthroughs have been made in the southern Qinling Mountains. Especially, a new deposit type with a scheelite -beryl-molybdenite assemblage in the Zhen＇an area of Shaanxi Province was firstly discovered. This deposit is currently in a detailed investigation stage, and no detailed study has been yet conducted. This work selected one molybdenite sample from the Be （W） ores in this deposit for Re-Os isotope measurements to define the time limit of tungsten and beryllium mineralization, and to further reveal the ore-forming geological setting of rare metals in the southern Qinling region.

Diffusion of Sm-Nd in Scheelite and its Significance to Isotopic Dating and Tracing

As the principal ore mineral in various tungsten(-gold)deposits,scheelite(CaWO_(4))plays an important role in directly dating the timing of ore formation,and in tracing associated material sources through the study of its Sm-Nd geochronology and Nd isotopic characteristics.Since the retention of Sm-Nd systematics within scheelite is presently unconstrained,equivocal interpretations for isotopic data resulting from this method have occurred quite often in previous studies that apply these isotopic data.In order to better elucidate the closure of Sm-Nd in scheelite,the kinetics of Sm and Nd within this mineral lattice were investigated through calculation of diffusion constants presented herein.The following Arrhenius relations were obtained:D_(Nd)=4.00exp(-438 kJ·mol^(–1)/RT)cm^(2)/s D_(Sm)=1.85exp(-427 kJ·mol^(–1)/RT)cm^(2)/s showing diffusion rate of Nd is near identical to Sm in scheelite when at the same temperature.However,compared to other rare earth elements(REEs),which have markedly different atomic radii to either Nd or Sm,these are shown to exhibit a great variation in diffusivities.The observed trends in our data are in excellent agreement with the diffusion characteristics of REEs in other tetragonal ABO4 minerals,indicating that ionic radius is a key constraint to the diffusivity of REEs in the various crystal lattices.With this in mind,the same substitution mechanism and a very slight discrepancy in radii will allow us to infer that significant Sm/Nd diffusional fractionation in scheelite is unlikely to occur during most geological processes.Based upon the diffusion data determined herein,Sm and Nd closure temperatures and retention times in scheelite are discussed in terms of diffusion dynamics.Those results suggest that closure temperatures for Sm-Nd within this mineral are relatively high in contrast to the temperature ranges of ore-formation responsible for scheelite-related deposits,and any later thermal environments.It is likely,therefore,that relevant isotopic information could be easily retained under most geological conditions,since initial crystallization of the scheelite.In addition,comparison of this mineral-element pair over a range of temperatures with some other common minerals used as geochronometers(e.g.,zircon and apatite)indicates that Sm-Nd system has a slower diffusive rate in scheelite than for Sr in apatite or Ar in quartz,and only a little faster than for Pb in zircon.It should be noted,within most hydrothermal deposits where zircon has crystallized,its size is typically no more than 100μm,whereas scheelite commonly occurs as macroscopic grains.For this reason,the larger dimensions of scheelite would provide a robust Sm-Nd system more able to resist perturbations,relating to any later thermal process.As such Sm-Nd investigations of scheelite are akin to U-Pb within zircon samples used in isotopic dating.These observations indicate that Sm-Nd age and isotopic information can provide reliable data in all but the most extreme case,especially when data are extracted from macroscopic grains of scheelite that are chosen to be“pristine”(i.e.,free of surface alteration and/or fractures).

Sm-Nd,Pb-Pb and Rb-Sr Isotopic Dating and Its Dynamic Implications for the Proterozoic Augen Granite in the Yunkai Area,Western Guangdong Province

The samples of ductile-rheologic deformational augen granite from the Yunkai uplift area, western Guangdong province, were determined by the whole-rock Sm-Nd, Pb-Pb and Rb-Sr isotopic dating to have an Sm-Nd isochron age of 1414±68 Ma, a Pb-Pb isochron age of 1388±90 Ma and a Rb-Sr isochron age of 490±36 Ma. The first two ages are interpreted as the formation age of this suite of granite and the last age represents the timing of the tectono-thermal event of Caledonian ductile-rheologic shear partial melting. It is indicated that in the study area not only an orogeny took place in the Caledonian, but also a more important tectono-magmatic activity occurred in the Meso-proterozoic there, which may be related to the subduction-collision between the Yangtze block and Cathaysia block.

Timing of Magma Mixing in the Gangdisê Magmatic Belt during the India-Asia Collision: Zircon SHRIMP U-Pb Dating

Abundant mafic microgranular enclaves (MMEs) extensively distribute in granitoids in the Gangdise giant magmatic belt, within which the Qüxü batholith is the most typical MME-bearing pluton. Systematic sampling for granodioritic host rock, mafic microgranular enclaves and gabbro nearby at two locations in the Qüxü batholith, and subsequent zircon SHRIMP II U-Pb dating have been conducted. Two sets of isotopic ages for granodioritic host rock, mafic microgranular enclaves and gabbro are 50.4±1.3 Ma, 51.2±1.1 Ma, 47.0±l Ma and 49.3±1.7 Ma, 48.9±1.1 Ma, 49.9±1.7 Ma, respectively. It thus rules out the possibilities of mafic microgranular enclaves being refractory residues after partial melting of magma source region, or being xenoliths of country rocks or later intrusions.Therefore, it is believed that the three types of rocks mentioned above likely formed in the same magmatic event, i.e., they formed by magma mixing in the Eocene (c. 50 Ma). Compositionally, granitoid host rocks incline towards acidic end member involved in magma mixing, gabbros are akin to basic end member and mafic microgranular enclaves are the incompletely mixed basic magma clots trapped in acidic magma. The isotopic dating also suggested that huge-scale magma mixing in the Gangdise belt took place 15-20 million years after the initiation of the India-Asia continental collision, genetically related to the underplating of subduction-collision-induced basic magma at the base of the continental crust. Underplating and magma mixing were likely the main process of mass-energy exchange between the mantle and the crust during the continental collision, and greatly contributed to the accretion of the continental crust, the evolution of the lithosphere and related mineralization beneath the portion of the Tibetan Plateau to the north of the collision zone.

Re–Os Dating of Bitumen from Paleo–Oil Reservoir in the Qinglong Antimony Deposit, Guizhou Province, China and Its Geological Significance

Abundant organic inclusions are present in the Qinglong antimony deposit. However, the source rocks of these organic matters have not been reliably identified. Recently, a paleo--oil reservoir was found in the Qinglong antimony deposit. In view of similar components of gaseous hydrocarbon, we propose that the organic matters observed in inclusions in Qinglong antimony deposit would come from this paleo-oil reservoir. We used the Re-Os dating method to determine the age of the bitumen from this paleo-oil reservoir, and obtained an isochron age of 254.3~2.8 Ma. The age indicates that the oil- generation from source rock occurred in the early Late Permian, earlier than the Sb mineralization age （-148~8.5 Ma） in the Qinglong antimony deposit area. After oil generation from Devonian source rock, first and secondary migration, the crude oil have probably entered into the fractures and pores of volcanic rocks and limestone and formed a paleo-oil reservoir in the western wing of Dachang anticline. As burial process deepened, the crude oil has turned into natural gas, migrates into the core of Dachang anticline and formed a paleo-gas reservoir. The hydrocarbons （including CH4） in the reservoirs can serve as reducing agent to provide the sulfur required for Sb mineralization through thermal chemical reduction of sulfates. Therefore, the formation of oil-gas in the area is a prerequisite for the Sb mineralization in the Qinglong antimony deposit.

Re-Os Isotopic Age of Molybdenite of the Jingren Deposit and its Mineralogical Significance of Magnetite,Pyrite and Chalcopyrite

The Jingren deposit is part of the Qimantage metallogenic belt within the eastern Kunlun orogenic belt,the largest metallogenic belt in Qinghai Province,northwestern China.Exploration data show that the metal resources of the Jingren deposit are greater than 93000 t in a mining area of 76.15 km2,which indicates significant exploration potential in the near future.Three W–E-trending faults,F1-3,dominate the extension of the mineralization zone,which consists of chalcopyrite,pyrite,magnetite,galena,sphalerite,and molybdenite as well as bismuth-bearing minerals.The deposit contains a large amount of late Triassic intrusive rocks,however,previous research did not reach a consensus on the timing or the origin of the mineralization owing to a lack of geochronological data and poor exposure conditions.In the present study,Re-Os isotopic dating from six molybdenite samples collected from a borehole of the granodiorite in the Jingren deposit using negative thermal ionization mass spectrometry(NTIMS)showed 187 Re and 187 Os concentrations of 0.26–4.40 ppm and 1.03–16.46 ppb,respectively,with an initial 187 Os/188 Os value of 0.06±0.19.This proves that the Jingren deposit has a metallogenic age of(225±4)Ma and is the product of united mineralization of the Qimantage metallogenic belt and that the Jingren deposit might actually be an Indosinian metallogeny.In addition,the Re content of these samples,at 0.42 ppm to 7.00 ppm shows that the mineralization was derived mainly from a crustal source.Furthermore,electron probe microanalysis(EPMA)conducted on chalcopyrite obtained from 22 metallic mineral samples revealed(Fe+Cu)/S ratios of 1.801–1.947 with an average of 1.852,which is lower than the ideal value(1.875).Besides,the main ore body formed in a relatively higher temperature environment than the surrounding rocks in the Jingren deposit.These data indicate that the Jingren deposit formed in a metallogenic environment at lower temperature.Moreover,according to the TiO_(2)-Al_(2)O_(3)-(MgO+MnO)and TiO_(2)-Al_(2)O_(3)-MgO genetic classification diagram for magnetite,the Jingren deposit most likely belongs to the skarn family.In addition,the Co-Ni-As genetic classification diagram of the pyrite indicates sedimentary and skarn genetic characteristics.

The Ore-forming Mechanism of the Jiajika Pegmatite-Type Rare Metal Deposit in Western Sichuan Province:Evidence from Isotope Dating

Granitic pegmatites are commonly thought to form by fractional crystallization or by liquid immiscibility of granitic magma; however, these proposals are based mainly on analyses of fluid and melt inclusions. Here, we use the Jiajika pegmatite deposit, the largest spodumene deposit in Asia, as a case study to investigate ore forming processes using isotope dating. Dating of a single granite sample from the Jiajika deposit using multiple methods gave a zircon U-Pb SHRIMP age of 208.4 ~ 3.9 Ma, an 4~Ar/39Ar age for muscovite of 182.9 ~ 1.7 Ma, and an 4~Ar/39Ar age for biotite of 169.9 ＋ 1.6 Ma. Based on these dating results and the 4~Ar/39Ar age of muscovite from the Jiajika pegmatite, a temperature-time cooling track for the Jiajika granite was constructed using closure temperatures of the different isotope systems. This track indicates that the granite cooled over ^-40 m. y., with segregation of the pegmatite fluid from the granitic magma at a temperature of ~700~C. This result suggests that the Jiajika pegmatite formed not by fractional crystallization, but by segregation of an immiscible liquid from the granitic magma. When compared with fractional crystallization, the relatively early timing of segregation of an immiscible liquid from a granitic magma can prevent the precipitation of ore-forming elements during crystallization, and suggests that liquid immiscibility could be an important ore-forming process for rare metal pegmatities. We also conclude that isotope dating is a method that can potentially be used to determine the dominant ore-forming processes that occurred during the formation of granite-related ore deposits, and suggest that this method can be employed to determine the formation history of the W-Sn ore deposits found elsewhere within the Nanling Metallogenic Belt.

The dating and temperature measurement technologies for carbonate minerals and their application in hydrocarbon accumulation research in the paleouplift in central Sichuan Basin, SW China

A new method for reconstructing the geological history of hydrocarbon accumulation is developed, which are constrained by U-Pb isotope age and clumped isotope((35)47) temperature of host minerals of hydrocarbon-bearing inclusions. For constraining the time and depth of hydrocarbon accumulation by the laser in-situ U-Pb isotope age and clumped isotope temperature, there are two key steps:(1) Investigating feature, abundance and distribution patterns of liquid and gaseous hydrocarbon inclusions with optical microscopes.(2) Dating laser in-situ U-Pb isotope age and measuring clumped isotope temperature of the host minerals of hydrocarbon inclusions. These technologies have been applied for studying the stages of hydrocarbon accumulation in the Sinian Dengying gas reservoir in the paleo-uplift of the central Sichuan Basin. By dating the U-Pb isotope age and measuring the temperature of clumped isotope((35)47) of the host minerals of hydrocarbon inclusions in dolomite, three stages of hydrocarbon accumulation were identified:(1) Late Silurian: the first stage of oil accumulation at(416±23) Ma.(2) Late Permian to Early Triassic: the second stage of oil accumulation between(248±27) Ma and(246.3±1.5) Ma.(3) Yanshan to Himalayan period: gas accumulation between(115±69) Ma and(41±10) Ma. The reconstructed hydrocarbon accumulation history of the Dengying gas reservoir in the paleo-uplift of the central Sichuan Basin is highly consistent with the tectonic-burial history, basin thermal history and hydrocarbon generation history, indicating that the new method is a reliable way for reconstructing the hydrocarbon accumulation history.

Isotope Dating of the Potash and Rock Salt Deposit at Bamnet Narong, NE-Thailand

Bamnet Narong is located in northeastern Thailand (Chaiyaphum Province). It is the largest salt mine in the country and has been mined for decades. The landscape in this part of Thailand is characterised by a low plateau, which is called the Khorat Plateau. The plateau is divided into two basins by the Phu Phan Range, the Sakhon Nakhon Basin in the north and the Khorat Basin in the south. The analysed potashes and rock salts are deposited in the Maha Sarakham Formation, which represents the salt-bearing strata of the Khorat Basin. The stratigraphic age of this deposit has been debated since the late 1960’s. The assigned ages range from Mid-Cretaceous to Late Cretaceous and up to the Eocene. In this study different isotopic dating systems (Rb-Sr, Sr-Sr, K-Ar and K-Ca) were applied. The stratigraphic age for the time of deposition was confirmed to be Mid Cretaceous (Cenomanian). Furthermore, the homogeneity of the carnallites was investigated in order to trace a possible redistribution of rubidium.

A ca. 2.2Ga Acidic Magmatic Event at the Northern Margin of the Yangtze Craton: Evidence from U-Pb Dating and Hf Isotope Analysis of Zircons from the Kongling Complex

Objective The Yangtze craton collisional orogeny at ca. extensional events at ca. 1 experienced Paleoproterozoic 1.95-2.0 Ga and post-orogenic 85 Ga related to amalgamation of the Columbia （Nuna） supercontinent （Zhao and Cawood, 2012）. A ca. 2.15 Ga suprasubduction zone ophiolitic melange was recongized in the Archean- Paleoproterozoic Kongling Complex of the northern Yangtze craton （Han et al., 2017）. However, the tectonic evolution in early Paleoproterozoic from 2.4 Ga to 2.2 Ga remains unclear. We report here the presence of a suite of Paleoproterozoic （2.2 Ga） granites in the Huangling dome, northern Yangtze craton, which may provide important insights into crustal growth processes in the craton prior to the assembly of Columbia.

Chronology and Genesis of S-type Granites in Hetai District, Guangdong Province: Constraints from LA-ICP-MS Zircon U-Pb Dating and Tourmaline Boron Isotope In-situ Analyses

1 Introduction Hetai district,which is a mountainous area,situated on Guangning and Zhaoqing city,west Guangdong Province.Hetai district is generally located on southwest of South China Caledonian fold belt,east margin of Yunkai post-Caledonian uplift.Multiple type granites are widely distributed in Hetai district,including Caledonian,Indosinian and Yanshanian granites.Based on different

Krypton-85 dating of shallow aquifer in Hebei Plain

The inert gas radioactive isotope ^(85)Kr(with a half-life of 10.74 years), due to its stable physical and chemical properties, is an ideal tracer for shallow groundwater dating. In such a dating application, first the dissolved gas is extracted from groundwater in the field, then krypton is separated from the gas sample, and finally the isotopic abundance ^(85)Kr/Kr will be determined by an ATTA instrument. According to the atmospheric input curve of ^(85)Kr, the ^(85)Kr age of groundwater is determined. We conducted ^(85)Kr analysis in three wells in Zhengding County on the plains in front of the Taihang Mountains, and made a comparison with tritium(~3H) method.

Geological Characteristics and Ore-forming Time of the Dexing Porphyry Copper Ore Mine in Jiangxi Province

The Dexing porphyry copper ore mine is located in the Qin-Hang metallogenic belt between the Yangtze block and the Cathaysia block. It is a giant porphyry copper mine in China, including 3 ore districts： Tongchang, Fujiawu and Zhushahong. Our analyses of Re in molybdenite indicate that the ore-forming material of the copper ore deposits in Dexing should be mainly mantle-derived. Our study fills in a gap in the study of formation time of the Dexing copper mine, and further proves that the copper ore deposits in the three ore districts should be formed simultaneously, about 170 Ma, belonging to the early Yanshan period, and that the formation time of the copper ore deposits should be consistent with the formation time of granodiorite porphyry in which the copper ore deposits are hosted. Promising areas for seeking porphyry copper ore deposits is predicated to be the west or southwest of Dexing.

Identification of Five Stages of Dike Swarms in the Shanxi-Hebei-Inner Mongolia Border Area and Its Tectonic Implications

Dike swarms are generally ascribed to intrusion of mantle-source magma result from extension. Basic dike swarms around the Shanxi-Hebei-Inner Mogolia borders in the northern peripheral area of the North China Craton can be divided into five age groups according to isotopic dating: 1800-1700 Ma, 800-700 Ma, 230 Ma, 140-120 Ma, and 50-40 Ma. Geological, petrological and isotope geochemical features of the five groups is investigated in order to explore the variation of the mantle material composition in the concerned area with time. And the various extensional activities reflected by the five groups of dike swarms are compared with some important tectonic events within the North China Craton as well as around the world during the same period.

Petrogenesis of Early Carboniferous Alkaline Basalt from the Wusun Mountain:Implications for Tectonic Evolution of the Western Yining Block,NW China

Late Paleozoic volcanic rocks are well exposed in the Yining Block,NW China,and are predominately composed of andesites,rhyolites and volcaniclastics as well as minor basalts.Study of the petrology,whole-rock geochemistry and zircon U-Pb dating for the Early Carboniferous alkaline basalts from Wusun Mountain,western Yining Block,constrains their petrogenesis and tectonic evolution.The alkaline basalts consist mainly of plagioclases,mostly albite and labradorite,as well as clinopyroxenes and olivines;zircon U-Pb dating indicates their formation at ca.350 Ma.Geochemically,the basaltic samples have low SiO_(2)contents,and high TiO_(2),Al_(2)O_(3)and alkaline contents,coupled with high Na_(2)O/K_(2)O ratios,displaying an alkaline basalt affinity.They show remarkable LILE enrichment and HFSE depletion.Meantime,these samples have relatively high TFe_(2)O_(3),MgO,and Mg#values as well as Ni and Cr,relatively high Sm/Yb and U/Th,suggesting origination from a mantle source metasomatized by slab fluids.They formed in a transitional tectonic setting from arc to intraplate,showing a typical affinity of back-arc basin basalts.The alkaline basalts were likely generated in a nascent back-arc extension setting resulting from slab rollback of the southern Tianshan oceanic lithosphere.A bidirectional subduction model seems more reasonable for the evolution of the southern Tianshan Ocean.These new data will provide a new tectonic model for Late Paleozoic tectonic evolution of the western Yining Block.

Three Gorges Dam Controls Sediment Coarsening of the Mud Patch on the Inner East China Sea Shelf

The well-known Three Gorges Dam(TGD) within the Yangtze catchment launched its operation in 2003. The effect of the TGD operation on the sediment size on the East China Sea shelf is rarely known. High resolution(0.5 cm sampling) grain size analysis and 137 Cs and 210 Pb dating of the DH8-1 core were conducted with core collected from the distal part of a main sink for the modern Yangtze sediment entering the sea, the Min-Zhe Coastal Mud Deposits(MZCMD) on the inner East China Sea shelf. The 137 Cs dating results show that the core DH8-1 formed during 1946–2012 with a mean deposition rate of 0.65 cm yr^(-1), indicating that the 0.5 cm sampling for grain size analysis in this local area could reflect environmental changes generally on a one-year time scale. The mean grain size of DH8-1 core sediment that deposited after 2003 is significantly larger than that deposited during 1988–2002. After ruling out other possible factors, we infer that the sediment coarsening of DH8-1 core after 2003 is attributed to the TGD operation which causes the erosion of the Yangtze subaqueous delta. Specifically, the TGD operation significantly intensifies the declining trend of the Yangtze sediment loads to the sea despite no decreased water discharge, which results in extensive erosion of the Yangtze subaqueous delta. The relatively coarse sediment of the subaqueous delta is eroded and resuspended by ocean dynamics and then transported by coastal current, finally depositing on the MZCMD area. In addition, the general sediment fining of core DH8-1 that deposited during 1988–2002, comparing with 1946–1987, is mainly caused by dam construction and soil and water conservation within the Yangtze catchment. Our findings are helpful for better understanding the effects of such a huge dam as the TGD on a sediment sink like the MZCMD of such a large river as the Yangtze River.

An Early Aged Ophiolite in the Western Kunlun Mts., NW Tibetan Plateau and Its Tectonic Implications

The early aged ophiolites have attracted attention of many geologists in recent decades, because the early aged ophiolites can provide the information about the ancient oceanic processes relevant to the evolution of plate tectonics in the early period of the earth, and also concern such problems as whether there existed a ＂Proto-Tethys＂ and the break-up and convergence of the Rodinian Supercontinent. This paper reveals a definite complete ophiolite of Neoproterozoic-Early Paleozoic, named Kuda ophiolite in the western Kunlun Mrs., NW Tibetan Plateau, and reports the recent reasonable SHRIMP zircon U-Pb ages of 510±4 Ma, and 502±13 Ma for the cumulates of the Kuda ophiolite, using the most powerful dating tool, the SHRIMP-Ⅱ. The geochemical and geochronology data integrating with the geological setting suggest that the Kuda ophiolite might have formed in an archipelago oceanic basin, not in a vast ocean, the so-called ＂Proto-Tethys＂, and was tectonically emplaced during the Early Paleozoic.

Petrogenesis of Kejie Granite in the Northern Changning-Menglian Zone, Western Yunnan: Constraints from Zircon U-Pb Geochronology, Geochemistry and Hf Isotope

The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80-77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 （71.68%-72.47%）, K2O （4.73%-5.54%）, total alkali （K2O ＋ Na2O = 8.21%-8.53%）, K2O/Na2O ratios （1.36-1.94） and low P2O5 （0.13%-0.17%）, with A/CNK of 1.025-1.055; enriched in U, Th, and K, depleted in Ba, Nb, St, Ti, P and Eu. They are highly fractionated, slightly peraluminous 1-type granite. The two samples of the Kejie pluton give a large variation of εHf（t） values （-5.04 to 1.96） and Hf isotope crustal model ages of 1.16-1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock （801℃-823℃） show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.

Geochronology,geochemistry and Hf isotope of monzogranite in Niubiziliang of Qinghai

Zircon U-Pb dating, whole-rock geochemical analyses and Hf isotope are undertaken for the monzogranite in Niubiziliang area with the aim of constraining its formation time,petrogenesis and the regional tectonic setting. The zircons from monzogranite are euhedral-subhedral in shape,and display rhythm growth zoning,indicating a magmatic origin. LA-ICP-MS zircon U-Pb dating indicates the monzogranite formed most probably in the Late Devonian( 359. 0 ± 2. 6 Ma). The monzogranite has Si O2= 74. 69%--76. 11%,Al2O3=12. 07%--12. 81%,Na2 O + K2 O = 8. 24%--8. 70%,Na2 O / K2 O = 0. 60--0. 68,A / CNK > 1,which shows that it belongs to high-Si and high-K weakly peraluminous calc-alkaline series. The monzogranite is enriched in K,Rb,Th( LILEs) and La,Ce,Sm,Nd( LREEs); and depleted in Ba,U( HREEs) and Ta,Nb( HFSEs).Their zircon εHf( t) values range from 1. 21 to 3. 46,in response to their two-stage Hf model ages( TDM2) ranging from 1 034 Ma to 1 159 Ma,respectively,indicating that the primary magma was derived from the young crust in Meso-Neoproterozoic. Combined with the regional geological evolution background,it is considered that the Niubiziliang monzogranite formed the closure of North Qaidam ocean,which was the stretching stage product after the collision between Qaidam block and Qilian block.

Re-Os Dating of Galena and Sphalerite from Lead-Zinc Sulfide Deposits in Yunnan Province, SW China

Dating of lead-zinc deposits is of critical importance for better understanding of ore genesis, but has long been a big challenge due to the lack of suitable minerals that can be unequivocally linked to the ore genesis and that can be used for tradition radiometric methods. This kind of deposits have simple mineralogy dominated by galena and sphalerite commonly associated with calcite and other gangue minerals. Both galena and sphalerite have low and high variable Re concentrations and thus Re-Os dating of these minerals have been less promising. In addition, the recovery of Re is extremely low for galena when conventional method was applied, lending additional difficulty in precisely dating galena. In this study, we investigate the recovery of Re using different media for anion exchange separation and reporte a revised preparation method for Re-Os dating of galena and sphalerite. By using the new protocol, two reliable Re-Os isochron ages of galena and sphalerite from the Fule（20.4±3.2 Ma） and Laochang（308±25 Ma） Pb-Zn deposits in Yunnan Province, SW China, are achieved.