Isochron Age Research Articles

Cretaceous intermediate sulfidation ore system in the Dongkeng volcanic Basin, SE China: Constraints from the Dongji and Maluntou gold deposits

• Using pyrite Re–Os dating to confirm the timing of gold mineralization . • Investigating the occurrence and deposition mechanism of visible and invisible gold related to pyrite. • Gold deposits within the Dongkeng volcanic basin have an intermediate-sulfidation epithermal origin. The Dongji (>12.5 t Au @ 4.27 g/t) and Maluntou (>5.0 t Au @ 3.70 g/t) gold deposits are the two largest ones in the Dongkeng Volcanic Basin (DVB), SE China, that are hosted by volcanic rocks. Mineralization is represented by three stages (i.e., stage 1, 2, and 3) that are characterized by four types of unzoned hydrothermal pyrite (i.e., pyrite1, 2a, 2b, and 3). Hydrothermal fluids responsible for pyrite1 deposition are moderate temperatures (308–377 °C) and low salinity (4.6–9.1 wt% NaCl equiv.). The deposition of pyrite2a and 2b is related to the stage 2 fluids featured by moderate-low temperatures (253–341 °C) and low salinity (3.2–9.1 wt% NaCl equiv.). Pyire3 is deposited from the stage 3 fluids with low temperatures (220–250 °C) and salinities (1.0–6.5 wt% NaCl equiv.). Hydrothermal fluids potentially have a magmatic origin and experience fluid boiling and mixing of meteoric water. Scanning electron microscopy and laser ablation inductively coupled plasma-mass spectrometry were used to investigate the occurrence of visible gold and the distribution of invisible gold in pyrite from different generations. Visible gold mainly exists as native gold and electrum within crystal interstices, fractures, and hollows of pyrite2b, and precipitates directly from the fluids during fluid boiling at ca. 329 °C with pressure of ca.120 bar. Invisible gold mainly presents in the form of solid solution in all pyrite types. Solid solution gold is also closely related to pyrite2b as the measured Au concentrations in pyrite2b (up to 27 ppm) exceed those in other pyrite types (<0.6 ppm) by one or two orders of magnitude. The elevated content of invisible Au (together with As, Ag, Zn, and Sb) in pyrite2b is potentially associated with lattice dislocations caused by rapid precipitation of Py2b during boiling process. The Re–Os isochron age of pyrite2b (99 ± 10 Ma) and the zircon U–Pb dating of the volcanic and subvolcanic rocks (95.1–104 Ma) indicate that the formation of gold mineralization and the igneous activity in the DVB were coeval during the Turonia–Albian. Geochronology, fluid characteristics, together with low Ni concentrations (<98 ppm) and high mean Co/Ni ratios (≥2.0) of pyrite from different generations, support that the Dongji and Maluntou gold deposits formed in a magmatic-hydrothermal ore system and have an intermediate-sulfidation epithermal origin.

Neoarchean to Paleoproterozoic tectonothermal evolution of the North China Craton: Constraints from geological mapping and Th-U-Pb geochronology of zircon, titanite and monazite in Zanhuang Massif

The North China Craton (NCC) consists of Neoarchean to Paleoproterozoic basement, with rare Eo-Paleoarchean crustal relicts. However, the Precambrian tectonothermal evolution of the Central Orogenic Belt (COB) of the NCC is still hotly debated. In this contribution, we present new results of field-based litho-structural analysis, Th-U-Pb geochronology of accessory minerals including zircon, titanite and monazite from paragneiss and marble in a passive margin sequence in the Zanhuang Massif of the COB. Field studies show that two generations of deformation events occurred in the Zanhuang Massif. The earlier deformation is widely distributed in the entire Zanhuang Massif and developed with a top-to-SE shear sense resulting in foliation of the micaschist, metavolcanic rock, marble and paragneiss. The latter deformation is locally occurred in the central Zanhuang Massif and developed with a top-to-SW shear sense represented by the faulting and folding events which are similar to the fold-and-thrust structures of accretionary wedge during the formation of Zanhuang mélange. Zircons from paragneiss yield single-peak age populations with mean ages of 2.52 and 2.51 Ga, and a metamorphic age of 1.84 Ga. Zircon ages from marble are divided into four groups including a main peak of 2.54 Ga, two older groups of 2.77–2.82 and 2.60–2.70 Ga, and a younger metamorphic age of 1.81 Ga. Titanites from paragneiss and marble yield mean metamorphic ages between 1.94 and 1.88 Ga, representing an earlier thermal event. Monazites from paragneiss yield an isochron age of 1.80 Ga, representing a later thermal event. Based on our new litho-structural observations and geochronological data, coupled with previous studies, we suggest that the paragneiss and marble from the marble-siliciclastic unit represent the passive margin sequence, and were mainly deposited at 2.51–2.50 Ga. In addition, the Zanhuang Massif has experienced two tectonothermal events at ca. 1.94–1.88 Ga and ca. 1.83–1.80 Ga, which are widely distributed in the entire NCC, but gradually become weaker from north to south.

Genesis of the Yanghuidongzi Cu deposit, NE China: Constraints from H-O-Pb isotopic compositions and geochronological study

• The Yanghuidongzi Cu deposit classified as a porphyry type Cu deposit. • H-O isotopes indicate a mixing of magmatic water and meteoric water for ore fluids. • Pb-Re isotopes indicated a deep seated magma mixed with crustal materials for ore metals. • The Yanghuidongzi Cu deposit formed during Early Jurassic . The newly-discovered Yanghuidongzi porphyry Cu deposit is located in the southeastern part of Heilongjiang Province in NE China and represents one of the rare Jurassic porphyry Cu deposits within the easternmost segment of the Central Asian Orogenic Belt (CAOB). The Cu mineralization occurs as veins and disseminations hosted by granodiorite porphyry, associated with typical porphyry copper deposit alteration zoning, which is characterized by from an inner K-silicate zone through to a quartz–sericite zone to an outer propylitic zone, the K-silicate alteration and quartz–sericite alteration are closely related to mineralization. LA-ICP-MS zircon U-Pb data, sericite Ar-Ar and Molybdenite Re-Os data, and H-O and Pb isotopic compositions are used to constrain the ore-forming age and genesis of the Yanghuidongzi Cu deposit. LA-ICP-MS U-Pb dating of zircons from three granodiorite porphyry samples yielded weighted mean 206 Pb/ 238 U ages of 194.3 ± 1.2 Ma, 192.0 ± 2.0 Ma, and 194.6 ± 2.1 Ma respectively, whereas zircon U-Pb age of granodiorite in the study area yielded weighted mean 206 Pb/ 238 U age of 199.5 ± 1.5 Ma that are interpreted to be the emplacement age of these intrusions. 40 Ar- 39 Ar dating of sericite from a quartz-pyrite vein and a quartz-polymetallic sulfide vein yielded plateau ages of 193.9 ± 2.1 Ma and 193.6 ± 1.9 Ma, respectively, whereas five molybdenite samples yielded a Re-Os isochron age of 194.8 ± 2.1 Ma, with a weighted mean model age of 193.6 ± 1.2 Ma. Mineralization ages are consistent with the emplaced ages of the granodiorite porphyry within the analytical uncertainties, indicating that the Cu mineralization is genetically related to the granodiorite porphyry. The fluid δ 18 O fluid and δ 18 D fluid values range from −4.22‰ to 6.5‰ and from −92.8‰ to −65.7‰, respectively, indicating that the ore fluids were mixtures of magmatic and meteoric water. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb values of the pyrite and chalcopyrite are 17.547–18.327, 15.446–15.505, and 37.408–38.113, respectively, suggesting a common, magmatic source for the sulfur. Taking Re (6.75–29.53 ppm) data into consideration, the sources of ore-forming materials may be from a mixture of crustal and mantle sources. Combining our new data with the Jurassic regional tectonic setting, the Yanghuidongzi Cu deposit and the related granitoids formed in a compressional tectonic setting are associated with the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Bitumen Sm-Nd, pyrite Rb-Sr and zircon U-Pb isotopes constrain timing of ore formation and hydrocarbon deposition in the Erdaokan Ag-Pb-Zn deposit, NE China

Abstract Dating of bitumen can potentially constrain both the timing of mineralization and hydrocarbon deposition in hydrocarbon-bearing hydrothermal deposits. Here we report for the first time direct Sm-Nd dating on bitumen from the Erdaokan Ag-Pb-Zn deposit, a newly discovered, large, hydrocarbon-bearing hydrothermal deposit with 2964 kt of ore and 1535 t Ag with an average grade of 517 g/t Ag, 0.6 wt% Pb and 0.41 wt% Zn in the Back-Arc basin of NE Great Xing’an Range, NE China. Dating results show that Sm-Nd isochron age of four bitumen samples is 234.6 ± 1.2 Ma and an isochron age that includes three galena and four bitumen samples is 234.9 ± 1.4 Ma. In order to verify these ages, we performed Rb-Sr dating on pyrite and galena from the same deposit and U-Pb dating of zircon from the adjacent diorite porphyrite dike, which yielded ages of 232.9 ± 2.3 Ma and 234.2 ± 2.8 Ma, respectively. The highly concordant ages from multiple techniques confirms that bitumen Sm-Nd isotopic dating is reliable, and that the ages obtained in our study represent the hydrocarbon deposition as well as mineralization timing in the deposit. We conclude that, in the Duobaoshan metallogenic belt, the Erdaokan Ag-Pb-Zn deposit is the first identified large, Middle Triassic, hydrocarbon-bearing silver-lead–zinc deposit and is of great significance to the regional metallogenesis research and prospecting work in the Great Xing’an Range.

The linkage between the Jiawula-Chaganbulagen Ag-Pb-Zn and adjacent porphyry Mo-Cu mineralization, Inner Mongolia, northeast China

Abstract The Jiawula-Chaganbulagen (Jia-Cha) super-large, magmatic-hydrothermal Ag-Pb-Zn ore field in the southern Erguna Block, northeast China, is characterized by a large and well-developed alteration and metallic zonation (~56 km2) that is centered on porphyry Mo-Cu mineralization. The ore field includes two intermediate-sulfidation epithermal Ag-Pb-Zn deposits, which are located at Jiawula and Chaganbulagen. A weighted-mean molybdenite Re-Os age of 135.1 ± 2.1 Ma (MSWD = 0.16) and a molybdenite 187Re-187Os isochron age of 135.4 ± 2.3 Ma (MSWD = 0.44) were obtained for the Mo-Cu mineralization, and the host quartz monzonite porphyry yielded a weighted-mean zircon U-Pb age of 141.9 ± 2.4 Ma (MSWD = 0.044). Geochronological data suggest that the epithermal mineralization occurred at 135 to 133 Ma, and the porphyry age is little older than the porphyry-epithermal mineralization. The porphyry Mo-Cu mineralization is interpreted as the hydrothermal center of the ore field, based on the metal zonation:Mo-Cu → Zn-Cu → Zn-Cu-Pb-(Ag) → Zn-Pb-Ag-(Cu) → Ag (Au)-Pb-Zn → Ag, with vertical ore minerals zonation from deeper molybdenite-chalcopyrite to shallower sphalerite-galena-chalcopyrite through molybdenite-chalcopyrite-sphalerite. Trace element analyses by LA-ICP-MS of pyrite and pyrrhotite show that micro-inclusions exhibit similar “ore field-scale” trace element zonation: Bi-Ag → Bi-Sb-Ag → Sb-Ag. Geological and geochronological evidence indicates that the Jia-Cha epithermal system is genetically related to the adjacent porphyry Mo-Cu mineralization. XRF element mappings of the replaced ores revealed that late ore stage oxidizing fluids dissolved the early chalcopyrite at depth, and then replaced the sulfides in the main ore stage. Porphyry-epithermal mineralization system in the Jia-Cha ore field is significant for further exploration for porphyry Mo-Cu and epithermal Ag-Pb-Zn deposits in northeast China.

Comparison of isotope data obtained with Sm-Nd and Re-Os methods for minerals and rocks from the Ozernoe ore occurrence, Salla-Kuolajarvi belt

Sm-Nd isochrone, drawn for rock-forming and sulfide minerals from the Ozernoe ore occurrence, indicates albitite age of 1,759 ± 11 Ma. It shows synchronous formation of albitite and sulfide mineralization, and fully corresponds to the earlier defined age of rutile in albitite (1,757 ± 7 Ma U-Pb, n = 3, MSWD = 0.2), and Rb-Sr isochrone age 1,754 ± 39 Ma for biotite, apatite, albite, and WR. Recently published Re-Os ages of molybdenite 1,872 ± 23 Ma and chalcopyrite 1,891 ± 230 Ma indicate more ancient age of sulfide mineralization. These figures are in conflict with the age of rock-forming minerals, defined with Sm-Nd and Rb-Sr methods. The possibility of use of molybdenite from the Salla-Kuolajarvi belt for rock dating has been considered, and low reliability of Re-Os method for it has been shown. The reasons are the following: 1) extremely uneven distribution of Re in molybdenite, where Re content varies 1 wt.% even within one and the same grain, and 2) openness of the Re-Os system after molybdenite crystallization, Re is mobylized and partly removed from the mineral in the zone of hypergenesis. Removal of Re from molybdenite promotes erroneous ancient age of the molybdenite. According to the equations of radioactive decay, the age would be 110-130 Ma bigger if 5-6 % of Re is taken away. The conclusion is that molybdenite must be studied in detail, proved to be homogenous and unaltered, before it is used for Re-Os dating. In the other case the results will be not reliable.

Mineralogy, petrochronology, geochemistry, and fluid inclusion characteristics of the Dardvay skarn iron deposit, Sangan mining district, NE Iran

The Sangan mining district, with a proven reserve of 1 Gt of 35 to 60% iron, is a world-class iron ore district in Iran, and located in the far eastern part of the Cenozoic Alborz Magmatic Arc. The east–west trending Sangan mining district consists of fourteen iron deposits, spatially associated with the Eocene Sarnowsar composite intrusion (syenite to syenogranite and granite), subvolcanic (quartz monzonite and syenite) and volcanic rocks (andesite and rhyolite). Syenogranite and quartz monzonite rocks in the Dardvay skarn are classified as oxidized I-type granites, with an average SiO2 content of 69.25–65.91 wt%, Al2O3 content of 13.71–13.98 wt%, total alkali (Na2O + K2O) content of 8.55–8.02 wt%, respectively. LA-ICP-MS zircon U-Pb dating on three syenogranite samples yield U-Pb ages of 39.6 ± 0.7 Ma, 39.3 ± 0.3 Ma, and 39.1 ± 0.4 Ma, whereas two zircon samples from skarn zones yield U-Pb ages of 39.7 ± 0.4 Ma and 39.5 ± 0.4 Ma, indicating a Middle Eocene mineralization event closely related to the Sarnowsar syenogranite. Zircon grains from a host rhyolite yield a U-Pb isochron age of 40.2 ± 0.4 Ma and previous published age for quartz monzonite is 42.3 ± 0.8 Ma. The Dardvay deposit is developed along the contact of the Sarnowsar syenogranite with dolomitic limestone with distinct exoskarn and endoskarn zones. The endoskarn zone is limited in extent, whereas the exoskarn is more extensive and includes pyroxene and garnet skarn zones close to the Sarnowsar syenogranite and epidote-phlogopite-actinolite skarn distal to it. The composition of clinopyroxenes plot along the diopside-hedenbergite join in the diopside and augite fields. The garnet composition ranges from Ad60Gr38 to Ad40Gr59. Three main paragenetic stages of skarn formation and ore deposition have been recognized at the Dardvay deposit: (1) a prograde stage developed at 380° to >550 °C with a fluid salinity between 16 and 66 wt% NaCl equivalent, (2) a retrograde stage which formed at 175°–325 °C with fluid salinity of 2.3 and 33.4 wt% NaCl equivalent, and (3) a post-ore-stage with quartz veins that developed at 180°–300 °C with salinity range of 8.3 to 17.7 wt% NaCl equivalent. Oxygen isotope analyses were conducted on magnetite from retrograde stage, resulting in a narrow variation of δ18O between 1.2 and 1.6 per mil with δ18O fluid of 10.3 to 10.7 per mil. Fluid inclusion and oxygen isotope analyses suggest that a possible mechanism for magnetite deposition is boiling and dilution with surface (meteoric) fluids, which together with increasing pH and decreasing T explains iron ore formation.

Geochronology and Isotope Geochemistry of the Yingfang Pb-Zn-Ag Deposit: Implications for Large-Scale Metallogeny along the Northern Flank of the North China Craton

The northern flank of the North China Craton (NCC) hosts a linear zone of gold, molybdenum, silver, lead, and zinc polymetallic ore deposits. Among these, the Yingfang Pb-Zn-Ag deposit is located in the central part of the Yanshan–Liaoxi metallogenic belt (YLMB) which extends for approximately 1000 km and forms part of the major mineralized zone. In this study, we characterize the mineralization and trace the ore genesis based on new sulfur and lead isotopic geochemistry and evaluate the timing of mineralization from Rb-Sr isotope dating of sulfides. The pyrite δ34S values range from +3.2‰ to +5.8‰ with a mean at +4.07‰, close to the values of mantle and meteorite sulfur. The 206Pb/204Pb values range from 16.833 to 18.956, 207Pb/204Pb from 15.374 to 15.522, and 208Pb/204Pb from 37.448 to 37.928. Five samples of sulfide, from the Yingfang deposit, yield a Rb-Sr isochron age of 135.7 ± 4.1 Ma. This age is close to the age of the adjacent Niujuan Ag-Au deposit and the associated Er’daogou granite, suggesting a close relationship between magmatism and metallogeny in this region. The S and Pb isotopes of the regional silver polymetallic deposits show similar sources of ore-forming materials. According to a compilation of the available age data on the Mesozoic ore deposits in the northern flank of the NCC, we divide the mineralization into the following four periods: 240–205 Ma, 190–160 Ma, 155–135 Ma, and 135–100 Ma. Mesozoic magmatism and mineralization in the Yingfang deposit mainly took place at 245 Ma and 145–135 Ma. We correlate the Pb-Zn-Ag mineralization to metallogeny associated with large-scale inhomogeneous lithosphere thinning beneath the NCC.

Effect of Si addition on the precipitation and mechanical/electrical properties of dilute Al–Zr-Sc alloys

Microstructural evolution and mechanical/electrical properties of aged Al-0.2 wt%Zr-0.05 wt%Sc alloys with different Si additions (0, 0.05, 0.10 and 0.15 wt%) were comparatively studied by using transmission electron microscopy, electrical conductivity measurements, and microhardness measurements. A significant Si addition effect was demonstrated that the Si addition accelerated the precipitation kinetics and enhanced the heterogeneous precipitation of Al 3 (Sc, Zr) nanoparticles. However, both the peak-aging temperature and peak microhardness during isochronal aging were insensitive to the Si addition. The underlying mechanism is related to a partitioning effect of Si among Sc, Zr and vacancy that produces a saturated concentration of Si-vacancy clusters to serve as heterogeneous nuclei. It was further manifested that dual-scale particles, i.e. , Al 3 Zr dispersoids and Al 3 (Sc 1-x Zr x ) nanoprecipitates in different length scales, could be produced by deliberately designing the heat treatment protocol. A coupling effect of the dual-scale particles led to an enhanced threshold stress in high-temperature creep testing, indicative of an improved creep resistance promoted by the minor Si addition. Moreover, the recrystallization resistance was seldom affected by Si addition, while slightly improved by introducing dual-scale particles.

Age constraints on the genesis of the Changkeng tungsten deposit, Nanling region, South China

Abstract The Changkeng quartz vein tungsten deposit is located in the eastern part of the Helong ore field, Nanling region, South China. It has a proven WO3 reserves of 9680 tons WO3, and an average grade of 2.98%. It contains 28 ore-bearing quartz veins, and the mineralization stages are divided as follows: (I) quartz-wolframite-cassiterite-scheelite stage; (II) quartz-wolframite-scheelite-molybdenite-pyrite-sphalerite-galena-flourite stage; (III) quartz-calcite-flourite stage. The intrusion within the deposit is concealed, and comprising medium- and coarse-grained biotite monzogranite. Zircon grains from the Changkeng pluton have a U-Pb age of 156.7 ± 0.54 Ma (MSWD = 2.9). Cassiterite grains from stage I yield a weighted mean 206Pb/238U age of 156.1 ± 3.4 Ma (MSWD = 3.4). Molybdenite separates from stage II have a well-defined 187Re/187Os isochron age of 156.9 ± 2.0 Ma (MSWD = 0.77), with model ages ranging from 155.6 ± 2.5 to 157.5 ± 2.2 Ma. The zircon eHf(t) values range from −14.1 to −11.1, and the Hf two-stage model ages range from 1.87 to 2.06 Ga, indicating a primary reworking of the Paleoproterozoic crust. The low rhenium contents (0.3238 to 8.763 μg/g) in molybdenite suggest that the ore-forming materials was derived from a crustal source. The age consistence between the intrusion and mineralization, together with the Hf and Re isotope signature, all indicate a close genetic link between magmatism and mineralization. The Changkeng deposit is an example of a Jurassic regional-scale tungsten-tin ore-forming event between 160 and 150 Ma in the Nanling region, South China.

Multi-stage garnet formation and destruction in Kimberley harzburgitic xenoliths, South Africa

Thirty-nine garnet harzburgites from Kimberley in the Kaapvaal Craton (South Africa) were studied to constrain the origin, age and evolution of sub-cratonic lithospheric mantle (SCLM). In order to avoid chemical overprinting by recent metasomatism, only garnet harzburgites that appeared clinopyroxene-free to the naked eye were sampled. The majority of garnets were, however, in equilibrium with clinopyroxene (24 of 39). Whole rock and mineral major–trace element geochemistry and garnet Sr–Nd–Hf isotope data are presented. Equilibration pressures range from 3.8–6.1 GPa, indicating the harzburgites were derived from a large portion of the SCLM (~115–185 km). High olivine Mg# (~93.4, n = 39) and low whole rock heavy rare earth elements (HREE) contents are consistent with large degrees of partial melting (>45%) and garnet exhaustion leaving a dunitic residue with olivine ≥90%, orthopyroxene ≤10% and HREE <0.01 times chondrite. Mineral modes, whole rock Al2O3 (0.5–3.2 wt%) and SiO2 (43.1–49.1 wt%), however, indicate heterogeneous re-introduction of garnet (≤13%) and orthopyroxene (≤50%). Harzburgites with high garnet and relatively low orthopyroxene modes (mostly ~7–13% and ~ 9–30%; n = 6) are characterised by mildly sinusoidal garnet REE patterns (Tb-Dy minimum and high HREE) and Archaean depleted Hf TDM ages (2.7–3.3 Ga; εHfe: +190 to +709). In contrast, harzburgites with high orthopyroxene and relatively low garnet and modes (~1.5–7.5% and ~ 25–50%; n = 19) are characterised by highly sinuous REE patterns (Ho-Yb minimum and low HREE) and Proterozoic enriched Hf TDM ages (0.7–1.6 Ga; εHfe: −16 to +6). It is inferred that Archaean G10 garnet re-introduction caused a significant increase in HREE, making melt depletion models based on HREE inaccurate. Orthopyroxene addition, a few hundred million years later, most likely at ~2.7 Ga and associated with Ventersdorp magmatic activity, caused partial consumption of garnet and olivine, and changed garnet compositions leading to: 1) Cr/Al ratio increase; 2) HREE decrease; 3) more sinusoidal REE patterns; and 4) un-radiogenic 176Hf/177Hf. Garnets define a Lu-Hf isochron age of 2702 ± 64 Ma (εHfi = +44, n = 31), which is interpreted as a consequence of partial isotopic equilibrium within the SCLM and mixing of the garnet- and orthopyroxene-rich metasomatic components. The low LILE contents and absence of Nb-Ta anomalies are consistent with modal metasomatism caused by intra-plate magmatism. In addition, the REE signatures of metasomatic agents in equilibrium with the garnets suggest that carbonatitic melts and SiO2-rich hydrous melts were responsible for re-introduction of garnet and orthopyroxene, respectively. Sr-Nd isotope systematics were disrupted associated with kimberlite magmatism (Nd isochron: 217 ± 58 Ma, εNdi = +4; n = 34), consistent with recent G10 garnet transformation into G9 garnets (Ca + Fe-enriched). This event may have caused garnet addition (up to 1%), suggesting that garnet was formed or destroyed in at least 4 different events: i) initial extensive polybaric melting, ii) asthenospheric melts re-introducing the bulk of the garnet, iii) orthopyroxene addition and garnet loss, all in the Archaean, and iv) minor garnet addition possibly related to recent kimberlite magmatism prior to eruption.

The Genesis of the Askartor Be-Mo Deposit in the North Xinjiang, Northwest China: Evidence From Geology, Geochemistry, U-Pb, and Re-Os Geochronology

The Askartor Be-Mo deposit is located in the southeastern area of the Chinese Altay orogenic belt in Xinjiang, NW China. Zircon U-Pb data show that there are two periods of magmatic activities in the Askartor Be-Mo ore district, namely, the Devonian granodiorite (386.8 ± 2.6 Ma) and biotite granite (385.4 ± 4.4 Ma), and the Triassic two-mica granite (247.5 ± 2.2 Ma) and muscovite granite (231.4 ± 2.0 Ma). The zircon U-Pb age of pegmatoid orebody is 220.6 ± 1.6 Ma which coincides with the molybdenite Re-Os isochron age of 228.7 ± 7.1 Ma. The two-mica and muscovite granites belong to the high-K Calc-alkaline series with peraluminous features, and are characterized by high SiO2 (71.92–75.41 wt%), and Al2O3 (13.43–15.98 wt%), and low TiO2 (0.01–0.25 wt%), Fe2O3 (0.11–1.14 wt%) and CaO (0.07–0.76 wt%). The highly fractionated element ratios of Y/Ho, Zr/Hf and Nb/Ta, and the rare earth element tetrad effect occur in the muscovite granite, indicating the fluid exsolution occurs at the late stage of magma evolution, and the muscovite granite experienced the strong self-metasomatism. Rayleigh fractional calculations show that the Askartor Be-Mo deposit is the product of multistage fractional crystallization of initial Be-enriched magma.

Metallogenic chronology and tectonic setting of the Erdaohe Pb–Zn–Ag deposit in Inner Mongolia, NE China: Constraints from sphalerite Rb–Sr dating, zircon U–Pb dating, and Hf isotope analysis

• The Erdaohe deposit is a large-scale Pb-Zn-Ag deposit in the Great Hinggan Range. • The granite porphyry and the sphalerite in the deposit formed at 133.6Ma and 130.5Ma. • The mineralization was affected by the Mongolia-Okhotsk and Paleo-Pacific Ocean. • The discovery of the Erdaohe deposit shows a good exploration prospect in the region. The Erdaohe deposit is a recently discovered large-scale Pb–Zn–Ag deposit in the northern section of the Great Hinggan Range, Inner Mongolia, China. The ore bodies are mainly located in the skarnized contact zone between the granite-porphyry and calcareous wall rock, and in the fissures of the wall rock of the outer contact zone. Several vein-type ore bodies are located in the fissures of the distal volcanic strata, with obvious magmatic–structural ore-control characteristics. In this study, seven sphalerite samples from ores in the quartz–sulfide metallogenic stage yielded an Rb–Sr isochron age of 130.5 ± 1.9 Ma (MSWD = 1.6), with initial 87 Sr/ 86 Sr ratios of 0.70967 to 0.70994. LA-ICP-MS zircon U–Pb dating of the granite-porphyry samples yielded a concordant age of 133.6 ± 0.46 Ma (MSWD = 1.2). Combined with the previous studies, dating results and the field relation between the granite-porphyry and the ore body, it is considered that the granite-porphyry is genetically related to the mineralization of the deposit. The zircon ε Hf (t) values of the granite-porphyry ranged from 6.8 to 8.5, and the two-stage Hf model age (t DM2 ) ranged from 575 to 668 Ma, revealing that the granite-porphyry originated from the partial melting of the juvenile crust accreted from the depleted mantle during the end of the Neoproterozoic. Extensive magmatism during the Early Cretaceous was caused by lithospheric delamination after the closure of the Mongolia–Okhotsk Ocean and the back-arc extension induced by the subduction of the Paleo-Pacific Plate towards Eurasia; thus, the Erdaohe deposit was formed under this favorable metallogenic structure and material system. By comparing the metallogenic age, tectonic setting, and ore-forming resources, we propose that the formation of Erdaohe Pb–Zn–Ag deposit belongs to the same regional metallogenic event as the Pb–Zn–Ag deposits in the southern segment of the Great Hinggan Range, potentially indicating further exploration prospects for Pb–Zn–Ag deposit in the middle segment of the Great Hinggan Range.

SULFIDE RECRYSTALLIZATION AND GOLD REMOBILIZATION DURING THE 2.0 GA STAGE OF THE MINAS OROGENY: IMPLICATIONS FOR GOLD MINERALIZATION IN THE QUADRILÁTERO FERRÍFERO AREA, BRAZIL

Abstract The historic Quadrilátero Ferrífero mining district in Brazil is a polydeformed Archean terrane recording several tectonic events. This history has hampered the accurate dating of its important gold deposits, given that most of the geochronological clocks have been affected by multiple thermal events. To determine an accurate time constraint for the mineralizing events in this region, we provide the first Re-Os dating for a gold deposit in the Quadrilátero Ferrífero, obtained from 13 mineral separates of disseminated sulfide phases (pyrrhotite, coarse-grained arsenopyrite, fine-grained arsenopyrite, and pyrite) at the recently discovered São Sebastião deposit (northwest Quadrilátero Ferrífero). Three distinct successive sulfidation stages are interpreted at the deposit. The final stage, texturally associated with gold, is marked by fine-grained arsenopyrite and pyrite (assemblage III) and is associated with high-temperature (~600°C) features. A Re-Os errorchron is obtained when considering the data of all samples collectively; however, a comprehensive Re-Os isochron age of 1987 ± 72 Ma (2σ; n = 4; IsoplotR model 3; initial 187Os/188Os = 2.1 ± 0.7) is obtained from assemblage III. A 187Re-187Osr isochron at 1988 ± 56 Ma (mean square of weighted deviates = 0.1, initial 187Osr = –0.1 ± 2.6 ppt; n = 4) is obtained if an initial 187Os/188Os composition of ca. 2.1, is used, this being consistent with the fact that the 187Os in the sulfides in assemblage III is largely radiogenic (187Osr). Earlier assemblages (I and II) individually show variable initial 187Os/188Os, indicative of disturbance to the Re-Os systematics at ~2.0 Ga. This age is considered the best estimate for the timing of gold mineralization at São Sebastião, being coincident with the waning stages of the Minas orogeny, the thermal effects of which are restricted to the southern Quadrilátero Ferrífero. Hence, we consider that the hot fluids from which assemblage III crystallized were responsible for resetting the Re-Os systematics in assemblages I and II and were central to Paleoproterozoic gold deposition in the region. This ~2.0 Ga age challenges previous notions of a late Archean, ~2.7 Ga mineralizing event in the Quadrilátero Ferrífero and makes it possible that gold remobilization occurred elsewhere in the region, particularly in areas representing deeper crustal levels.

Genesis of the Hebaoshan gold deposit in Fujian Province of Southeast China: constraints from a combined fluid inclusion, H-O-C-S-Pb-He-Ar isotope and geochronological study

The large Hebaoshan gold deposit (41.5 t Au, average grade: 3.5 g/t) is located in a hitherto poorly documented gold province in the northeastern part of the South China Block. It is hosted by Precambrian metasedimentary rocks that experienced Triassic greenschist- to amphibolite-facies metamorphism. Three hydrothermal stages can be distinguished: quartz + sericite + pyrite + pyrrhotite (stage I), quartz + magnetite + sulfide (stage II), and quartz + carbonate + pyrite + hematite (stage III). Auriferous pyrite samples yielded a Rb-Sr isochron age of 234 ± 3 Ma (2σ, MSWD = 0.24), and ore-related hydrothermal sericite yielded a 40Ar-39Ar plateau age of 226.4 ± 2.0 Ma (2σ, MSWD = 1.01), which are distinctly younger than ca. 430 Ma granitic rocks in the mining district and older than 170–130 Ma granites in the region, ruling out any genetic link between granite emplacement and gold mineralization. The mineralizing fluid system can be described as carbonic-aqueous with low to moderate salinity (2.2–10.1 wt% NaCl equiv.) and medium temperature of 290 to 350 °C (stage I) and 230 to 280 °C (stage II). The δ18O quartz ranges from 13.3 to 15.4 ‰, and the δD values for fluid inclusions in quartz range from − 97 to − 60 ‰. The calcite has C-isotopes ranging from − 6.1 to − 3.8 ‰ and O-isotopes from 2.8 to 13.6 ‰. H-O-C isotope data are consistent with a metamorphic fluid derived from devolatilization of Neoproterozoic basement rocks during regional metamorphism in the Late Triassic Indosinian period. The δ34S values of sulfides for stages I, II, and III are 0.1 to 7.8 ‰, − 10.6 to 5.1 ‰, and − 18.9 to − 14.1 ‰, respectively, indicating an increase in oxygen fugacity during fluid ascent which resulted in more negative sulfur isotope values of sulfides and precipitation of gold. Pyrite separates have 206Pb/204Pb ratios of 17.340 to 17.687, 207Pb/204Pb ratios of 15.539 to 15.604, and 208Pb/204Pb ratios of 37.749 to 38.094. The S-Pb isotope data suggest derivation of S and Pb from the Precambrian metasedimentary country rocks. Auriferous pyrite yielded elevated 3He/4He ratios (0.78–1.46 Ra), which suggest a mantle component as can be expected from fluids derived from a subduction setting. It is concluded that Hebaoshan is an orogenic gold deposit that formed during Triassic flat-slab subduction of the paleo-Pacific plate beneath the South China Block and thus constitutes the first Triassic deposit of this type recognized in the coastal area of Southeast China.

Petrogenesis and Geodynamic Implications of a Newly Discovered Basanite Dike in Zaolin, Jingdezhen City, South China

Abstract A recently discovered basanite dike in the Zaolin area of Jingdezhen, South China, contains mantle xenocrysts such as kink-banded olivines, olivines + orthopyroxenes assemblage, and chromites. In addition, polymorphic carbonates of the MgCO3–FeCO3 series occur as augens, either independently or interspersed with diopside and spinel in the matrix. The rock is characterized by high Cr and Ni contents, high whole-rock Mg# values (0.66–0.72), and high Ca/Al (0.72–1.03) and TFeO/MgO (1.1–1.3) ratios and is alkali-rich with Na2O &amp;gt; K2O. The trace-element partition patterns are similar to those of other basanites in eastern China as well as ocean island basalts. Whole-rock geochemical analyses show depleted Sr and Nd isotopic compositions (86Sr/87Sr=0.70358–0.703853, εNd=2.52–6.73). These data indicate that the rock has experienced negligible crustal contamination, should be derived from asthenospheric mantle, or mixed by the MORB with EMI/EMII mantle and have been carbonated. The calculated T–P conditions of the melt in equilibrium with xeno-olivine are 1160–1320°C at the mantle depth. The high Cr# values of the spinel xenocrysts indicate that the lithospheric mantle under the Jingdezhen area was probably relict Proterozoic mantle. The Ar–Ar plateau age and the isochron and inverse isochron ages for the matrix of the basanite are all 44 Ma. The basanite, as well as other alkaline basalt or lamprophyre dikes in southeastern China, formed in a rifting regime during the Eocene.

40Ar/39Ar ages of Northwest Africa 7034 and Northwest Africa 7533

AbstractThe Martian breccias NWA 7034, NWA 7533, and paired meteorites record events ranging in age from 4.47 Ga to &lt;200 Ma. Published ages indicate a period of major disturbance at ~1.4 Ga, examined in detail here through 40Ar/39Ar dating of handpicked grains and two small chips. Argon diffusion parameters were obtained for six samples. Also presented are He, Ne, Ar, Kr, and Xe contents of two small (&lt;100 µg), handpicked mineral separates, a felsic “Light” sample and a mafic/pyroxene‐rich “Dark” sample. The 40Ar/39Ar ages of five samples, four containing &gt;1 wt% K and thought to be rich in feldspar and one containing &lt;~1 wt% K, cluster near 1.4 Ga. The 40Ar/39Ar ages of nine grains with low K contents have a wide range of apparent ages from 0.3 ± 0.1 Ga to 2.9 ± 0.1 Ga for individual temperature steps, and from 0.74 ± 0.06 Ga to ~2.1 Ga for plateau ages. Isochron ages are less precise, but generally agree with plateau ages. Only two isochrons have the significantly positive intercepts expected in the presence of terrestrial or Martian atmospheric argon. At higher release temperatures, activation energies for diffusion obtained from 39Ar data for six samples are generally 160–200 kJ mol−1, consistent with published values for feldspathic minerals. For three of these samples, lower temperature data on Arrhenius plots are best fit with a much lower activation energy of &lt;100 kJ mol−1. We attribute the low values to the effects of varying degrees of shock on feldspathic minerals and/or the presence of phases in vitrophyric spherules produced by hydrothermal alteration. The low activation energies place an upper limit of ~14 ka on the terrestrial age of NWA 7034. Much lower concentrations of cosmogenic (c) 3He and 21Ne in the Light than in the Dark separate indicate substantial losses concurrent with or postdating cosmic ray irradiation. A one‐stage, cosmic ray exposure (CRE) age for the Dark separate from NWA 7034 is estimated to be between 7 and 10 Ma from the concentrations of 3Hec and 38Arc, and of close to 15 Ma from the concentration of 21Nec. Most of the 40Ar/39Ar and noble gas data are compatible with (1) a heating and alteration event ~1.40 Ga caused by contact metamorphism, an impact, and/or the infiltration of hydrothermal fluids; and (2) at least one later event at lower temperatures that led to either loss of He and Ar from phases with low activation energies, or to gain of K. Most of the 40Ar/39Ar ages are consistent with the assembly of NWA 7034 1.4 Ga ago or perhaps earlier followed more recently by selective alteration. A more recent time of assembly is also consistent with these ages provided that the temperature stayed low. The five most precise 40Ar/39Ar ages of the samples analyzed are all ~1.4 Ga, a value seen frequently in other NWA 7034 chronometers and very similar to crystallization ages of nakhlites and chassignites (NC). Some CRE ages based on noble gases in NWA 7034 agree within their considerable uncertainties with those of NC. These two chronometric coincidences suggest that the NWA 7034 clan and the NC share a launch date on Mars. We propose that K‐rich fluids derived from the nakhlite source area interacted with proto‐NWA 7034 and modified the K/Ar ratios and ages of previously shocked feldspar grains, with the degree of modification depending on the degree of shock. The NWA 7034 clan may therefore be considered components from a metamorphic aureole around a nakhlite massif.

Sr Isotope Chemostratigraphy and Pb–Pb Age of the Riphean Carbonate Deposits of the Kharaulakh Uplift (Northeastern Margin of the Siberian Platform)

Abstract —The least altered limestones of the Ukta and Eselekh formations in the Precambrian section of the Kharaulakh uplift have a minimum 87Sr/86Sr ratio of 0.70673–0.70715. The lowest 87Sr/86Sr ratio of the overlying Neleger and Sietachan formations is 0.70791–0.70817. Based on these data, along with the earlier obtained positive δ13С values (up to 8‰) for the Kharaulakh section, we have estimated the age of the Ukta and Eselekh formations at 800–670 Ma and the age of the Neleger and Sietachan formations at ~640–580 Ma. The Pb–Pb isochron age of the least altered limestones of the Eselekh Formation calculated from eight samples is 720 ± 30 Ma. This age permits us to define the lower part of the Kharaulakh section of the Ukta and Eselekh formations to be the late Tonian of the International Chronostratigraphic Chart or to the Upper Riphean of the General Stratigraphic Scale of Russia. The presence of reliably dated Upper Riphean sediments in the Kharaulakh uplift indicates a more complex structure of the Precambrian sedimentary cover on the Arctic margin of the Siberian Platform than assumed earlier.

Age disequilibrium between zircon and their granitoid hosts caused by intracrustal reworking: Nd-Hf-Ar isotope evidence of Archaean Granitoids from Barberton Mountain Land (Kaapvaal craton, South Africa)

The degree of early Archaean terrestrial differentiation into mantle and crust is long debated. A number of studies rely on bulk rock Hf and Nd isotope data to gain further insight. Here we present Hf-Nd data from two igneous basements suites of Barberton Mountain Land, namely a tonalite-trondhjemite-granodiorite (TTG) and granodiorite-granite-monzogranite (GGM) suite to further decipher crust formation and its timing in the Archaean. TTG show high initial ε(Hf) and ε(Nd) if corrected using their respective U/Pb zircon ages that can be interpreted in two ways: (i) early differentiation of bulk silicate Earth or (ii) overcorrection of these data caused by decoupling of zircon from the ‘zircon-free’ bulk rock due to metamorphic overprint. Indeed, regional metamorphism affected the early TTG ca. 250 Ma after their emplacement and disturbed their bulk rock Hf and Nd systematics. Correction of bulk rock Hf and Nd data to the age of metamorphism yields values between +0.9 and − 6.2 and from −1.3 to −4.3, respectively, thus indicating open-system behaviour. Lu-Hf and Sm-Nd mineral isochron ages of two TTG plutons are significantly younger than their corresponding U/Pb zircon age, but agree within uncertainty perfectly with external age constraints for the age of the metamorphic overprint. The same accounts for the 40Ar-39Ar systematics of the investigated plutons. Amphibole samples consist of a mixture of heterochemical generations that yield ages that are both older and younger than the metamorphic overprint at 3.2 Ga. The metamorphic minerals in each plutons provide a highly divergent age information from the coexisting zircon. Hence, the positive ε(Nd) and ε(Hf)-values that resulted from age-correction to the respective U/Pb zircon age are most probably over-corrected. Finally, if all data is age-corrected to the metamorphic overprint all samples agree with models for the Nd-Hf isotopic evolution in the mantle that do not require a large-scale early differentiation of Earth into enriched and depleted reservoirs. Mantle extraction ages (TDM) of TTG are interpreted as minimum ages for the generation of felsic Archaean crust whereas TDM of GGM are interpreted as maximum ages for the last major crust growth episode in the Barberton Mountain Land.

Re-Os Age and Stable Isotope (O-H-S-Cu) Geochemistry of North Eastern Turkey’s Kuroko-Type Volcanogenic Massive Sulfide Deposits: An Example from Cerattepe-Artvin

The eastern Pontide tectonic belt (EPTB) contains greater than 350 identified Kuroko type volcanogenic massive sulfide deposits/mineralization/occurrences (VMSD). The deposits are associated with Late Cretaceous felsic volcanics consisting mainly of dacitic and rhyolitic lavas and pyroclastics that outcrop within a narrow zone running parallel to the eastern Black Sea coast and represent the axial zone of a paleo-magmatic arc. The Cerattepe deposit is the second-largest and is a hybrid VMS system with some epithermal features. To date, no geochemical research constrains the origin and timing of mineralization in the Cerattepe VMS deposit. Here, we provide Cu, O, H and S, isotope analysis of ores and alteration minerals to understand the hydrothermal history of the deposit and date the massive ore with Re-Os geochronology. Secondary weathering mobilized and redistributed metals in the deposit. The copper isotope signatures of shallow ores in the gossan follow patterns resulting from oxidative weathering of copper minerals with gossan Fe oxides of δ65Cu = −2.59‰, enrichment zone copper sulfide of d65Cu = +2.23 and +1.73‰, and primary ores of δ65Cu = +0.71 and +0.41‰. At the boundary of the enrichment zone, further cycling and migration of enrichment zone copper are evidenced by two samples having larger ranges of the δ65Cu = +3.59‰, and −2.93‰. Evidence for a magmatic source for fluids and S are evidenced by the O and H isotope values from quartz veins (δ18O = +7.93‰ to +10.82‰, and δD = −78‰ and −68‰) and sulfides that possess δ34S ratios of –5 and 0‰ from drill core samples. 187Os/188Os–187Re/188Os ratios from drill core sulfide samples of Cerattepe VMS deposit yields a 62±3 Ma isochron age and a highly radiogenic Os initial ratio. This age is compatible with silicate alteration ages from a proximal deposit and clearly shows mineralization occurs at a much younger time than previously proposed for VMS mineralization in the eastern Pontides. The new Re-Os age and source of Os imply that mineralization in the area occurs at a distinctly younger interval in the back-arc basin and metals could be sourced from the surrounding host rocks.