Os Geochronology Research Articles

The Longmala and Mengya’a skarn Pb–Zn deposits, Gangdese region, Tibet: evidence from U–Pb and Re–Os geochronology for formation during early India–Asia collision

The Longmala and Mengya’a deposits are two representative skarn Pb–Zn deposits of the Nyainqêntanglha Pb–Zn–(Cu–Mo–Ag) polymetallic belt in the Gangdese region, Tibet, China. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the mineralization-related biotite monzogranite from the Longmala deposit yielded a weighted mean age of 55.7 Ma, which can be interpreted as the emplacement age of the pluton. Re–Os dating of three molybdenite samples from the Longmala deposit yielded model ages of 51.8–54.3 Ma, with a weighted mean age of 53.3 Ma, which is interpreted as the mineralization age of the deposit and overlaps the age of the causative intrusion. The Re–Os dating of four molybdenite samples from the Mengya’a deposit yielded model ages of 60.4–65.8 Ma, with a weighted mean age of 63.6 Ma, which represents the mineralization age of this deposit. Our new precise age data for these two deposits are consistent with the existing ages of ca. 65–51 Ma for other skarn polymetallic deposits in the Nyainqêntanglha metallogenic belt. In addition, these new age data, combined with existing information on the geological evolution history of the Lhasa terrane, indicate that the belt of skarn deposits is closely related to initial collision between India and the Asian continents.

Temporal–spatial distribution and tectonic setting of porphyry copper deposits in Iran: Constraints from zircon U–Pb and molybdenite Re–Os geochronology

Porphyry copper deposits (PCDs) in Iran are dominantly distributed in Arasbaran (NW Iran), the middle segment of the Urumieh–Dokhtar Magmatic Arc (UDMA), and Kerman (central SE Iran), with minor occurrences in eastern Iran and the Makran arc. This paper provides a temporal–spatial and geodynamic framework of the Iranian porphyry Cu (Mo–Au) systems, based on geochronologic data obtained from zircon U–Pb and molybdenite Re–Os dating of host porphyritic rocks and molybdenites in 15 major PCDs. The dating results define a long metallogenic duration (39–6Ma), and suggest a long history of tectonic evolution from the accretionary orogeny related to early Cenozoic closure of the Neo-Tethys Ocean to subsequent collisional orogeny for the Iranian porphyry copper systems.The oldest porphyry mineralization occurred in the eastern part of Iran after the closure of a branch of the Neo-Tethyan (Sistan) Ocean between the Lut and Afghan blocks in the late Eocene (39–37Ma). This was followed by mineralization in the Kerman porphyry copper belt over a time interval of about 20m.y., where two metallogenic epochs have been recognized, including late Oligocene (29–27Ma) and Miocene (18–6Ma). The Bondar-e-Hanza deposit formed in the late Oligocene, while and the remaining dated deposits belong to Miocene epoch. According to the deposits' characteristics and their ages, the Miocene epoch can be divided into early, middle, and late stages. The Darreh Zar, Bakh Khoshk, Chah Firouzeh and Sar Kuh deposits formed during the early–middle Miocene. The largest porphyry deposits occur in the middle stage during the middle Miocene (14–11Ma) and include the Sar Cheshmeh, Meiduk, Dar Alu and Now Chun deposits. These deposits were formed during crustal thickening, uplift, and rapid exhumation of the belt. The final stage of porphyry mineralization occurred during the late Miocene (9–6Ma), and formed the Iju, Kerver, Kuh Panj and Abdar deposits.There were two porphyry mineralization stages in the Arasbaran porphyry copper belt in NW Iran, including an older late Oligocene (29–27Ma) and a younger early Miocene (22–20Ma) events. The Haft Cheshmeh deposit belongs to the older stage, and the world-class Sungun and Masjed Daghi deposits formed during the early Miocene.In the middle segment of the UDMA (Saveh–Yazd porphyry copper belt), PCDs formed during middle Miocene time (17–15Ma). The geochronological results reveal that the porphyry mineralization moved from the northwest to southeast of UDMA over the time.Our dating results, combined with the possible late Eocene–Oligocene timing for collision between the Arabian and Iranian plates, support a model for Iranian PCD formation by partial melting of previously subduction-modified lithosphere in a post-subduction and post-collisional tectonic setting.

Zircon U–Pb and molybdenite Re–Os geochronology, Hf isotope analyses, and whole-rock geochemistry of the Donggebi Mo deposit, eastern Tianshan, Northwest China, and their geological significance

The geodynamic setting of Mesozoic magmatic rocks and associated mineralization in eastern Tianshan, Northwest China, are attracting increasing attention. The newly discovered giant Donggebi molybdenum deposit (0.508 Mt at 0.115% Mo) is located in the central part of eastern Tianshan, Xinjiang. The molybdenum mineralization was genetically associated with the Donggebi stock, comprised of porphyritic granite and granite porphyry. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating constrains that the porphyritic granite and granite porphyry emplacement occurred at 233.8 ± 2.5 Ma and 231.7 ± 2.6 Ma, respectively. The Re–Os model ages of six molybdenite samples range from 235.2 to 237.0 Ma, with a weighted mean age of 236.1 ± 1.4 Ma, which is roughly consistent within errors with the zircon U–Pb ages, suggesting a Middle Triassic magmatic–mineralization event at Donggebi. Geochemically, the Donggebi granitoids are characterized by high SiO2 and K2O contents, with low MgO contents, belonging to high-K calc-alkaline granites. These rocks show pronounced enrichment in K, Rb, U, and Pb, and depletion in Sr, Ba, P, and Ti, with negative Eu anomalies (Eu/Eu* = 0.20–0.38). In situ Hf isotopic analyses of zircon from the porphyritic granite and granite porphyry yielded εHf(t) values ranging from +6.6 to +10.5, and from +5.5 to +10.1, respectively. The geochemical and isotopic data imply that the primary magmas of the Donggebi granitoids could have originated by partial melting of a juvenile lower crust that involved some mantle components. Combined with the regional geological history, geochemistry of the Donggebi granitoids, and new isotopic age data, we thus propose that the Donggebi molybdenum deposit was formed in the Middle Triassic, and occurred in an intracontinental extension setting in eastern Tianshan.

Re–Os age for the Lower–Middle Pennsylvanian Boundary and comparison with associated palynoflora

The Betsie Shale Member is a relatively thick and continuous unit that serves as a marker bed across the central Appalachian basin, in part because it includes an organic-rich shale unit at its base that is observable in drill logs. Deposited during a marine transgression, the Betsie Shale Member has been correlated to units in both Wales and Germany and has been proposed to mark the boundary between the Lower and Middle Pennsylvanian Series within North America. This investigation assigns a new Re–Os date to the base of the Betsie and examines the palynoflora and maceral composition of the underlying Matewan coal bed in the context of that date. The Matewan coal bed contains abundant lycopsid tree spores along its base with assemblage diversity and inertinite content increasing upsection, as sulfur content and ash yield decrease. Taken together, these palynologic and organic petrographic results suggest a submerged paleomire that transitioned to an exposed peat surface. Notably, separating the lower and upper benches of the Matewan is a parting with very high sulfur content (28wt.%), perhaps representing an early marine pulse prior to the full on transgression responsible for depositing the Betsie. Results from Re–Os geochronology date the base of the Betsie at 323±7.8Ma, consistent with previously determined age constraints as well as the palynoflora assemblage presented herein. The Betsie Shale Member is also highly enriched in Re (ranging from 319.7 to 1213ng/g), with high 187Re/188Os values ranging from 3644 to 5737 likely resultant from varying redox conditions between the pore water and overlying water column during deposition and early condensing of the section.

SIMS zircon U–Pb and molybdenite Re–Os geochronology, Hf isotope, and whole-rock geochemistry of the Wunugetushan porphyry Cu–Mo deposit and granitoids in NE China and their geological significance

The geodynamic setting of magmatic rocks and geodynamic mechanism of Cu–Mo–Fe–Sn–Pb–Zn–Ag polymetallic mineralization in northeastern (NE) China are attracting increasing attention. This study explores these issues by providing SIMS zircon U–Pb dating, whole-rock geochemical, Hf isotopic data of magmatic rocks, and molybdenite Re–Os dating exposed in the Wunugetushan porphyry Cu–Mo deposit, NE China. This deposit is located in the western part of the Great Xing'an Range, on the southeastern margin of the Mongol–Okhotsk Orogenic Belt. Molybdenite Re–Os and SIMS zircon U–Pb dating of the host monzogranitic porphyry and the wall rock of biotite granite in the Wunugetushan porphyry Cu–Mo deposit indicate that the ore-formation, host porphyry, and wall rock-emplacement occurred at 180.5±2.0Ma, 180.4±1.4Ma, and 203.5±1.6Ma, respectively, and the mineralization of the Wunugetushan porphyry Cu–Mo deposit occurred during the same period as that of the host monzogranitic porphyry. Geochemically, the Wunugetushan granitoids are characterized by strong LREE/HREE fractionation, and pronounced negative Nb, Ta, and Ti anomalies, with slightly negative Eu anomalies (Eu/Eu*=0.71–0.97) of the host monzogranitic porphyry and pronounced negative Eu anomalies (Eu/Eu*=0.29–0.32) of the wall rock of biotite granite. In situ Hf isotopic analyses of zircons from the host monzogranitic porphyry and the wall rock of biotite granite yielded εHf(t) values ranging from 0.5 to 8.2, and from −6.9 to 5.9, respectively. The geochemical and isotopic data for the Wunugetushan granitoids imply that the primary magmas of the host monzogranitic porphyry could have originated by partial melting of a thickened lower crust, with input of mantle components, while the primary magmas of the wall rock of biotite granite could have been derived by partial melting of a thickened lower crust that mixed with ancient crustal materials. Based on the regional geological history, geochemistry of the Wunugetushan granitoids, and new isotopic age data, we suggest that the formation of the Wunugetushan porphyry Cu–Mo deposit was possibly induced in the Early Jurassic during the period of the southeastward subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna Massif.

Neoproterozoic Re–Os systematics of organic-rich rocks in the São Francisco Basin, Brazil and implications for hydrocarbon exploration

The São Francisco Basin contains a remarkable archive of Neoproterozoic strata and its hydrocarbon-bearing strata are receiving increasing attention as global oil and gas exploration targets progressively deeper and older rocks. New Re–Os geochronology for the Paracatu Slate Formation of the Canastra Group, Brazil, yields a depositional age of 1002±45Ma. This age represents the first successful application of the Re–Os system to rocks of this group and indicates excellent agreement with a previously published U–Pb detrital zircon age (Rodrigues et al., 2010). Together with TOC values of ca. 2wt.% (despite greenschist metamorphism), it might be argued that the São Francisco Basin has had the potential for hydrocarbon generation since the Tonian (1000–850Ma). In addition, we also report an imprecise Re–Os age (1304±210Ma) for the Serra do Garrote Formation, a further potential source rock of the Vazante Group. We suggest, based on petrological evidence, that the Re–Os systematics were disturbed by post-depositional fluid flow that was most likely associated with Vazante ore deposit mineralization. An attempt to determine a Re–Os date for the Sete Lagoas Formation, a putative post-Sturtian cap carbonate, is precluded owing to low Re abundances (≤100ppt). Major environmental changes in the aftermath of the Jequitaí glaciation, particularly the development of palaeotopography such as subglacial tunnel valleys, may account for the apparent random distribution of TOC enrichment in these Cryogenian/Ediacaran post-glacial deposits. This scenario might thus have major implications for the hydrocarbon prospectivity of this post-glacial succession.

Isotope geochemistry and Re–Os geochronology of the Yanjiagou Mo deposit in the central North China Craton

The Yanjiagou deposit, located in the central North China Craton (NCC), is a newly found porphyry‐type Mo deposit. The Mo mineralization here is spatially associated with the Mapeng batholith. In this study, we identify four stages of ore formation in this deposit: pyrite phyllic stage (I), quartz–pyrite stage (II), quartz–pyrite–molybdenite stage (III), which is the main mineralization stage, and quartz–carbonate stage (IV). We present sulphur and lead isotope data on pyrite, and rhenium and osmium isotopes of molybdenite from the porphyry deposit and evaluate the timing and origin of ore formation. The δ34S values of the pyrite range from ‐1.1‰ to −0.6‰, with an average of −0.875‰, suggesting origin from a mixture of magmatic/mantle sources and the basement rocks. The Pb isotope compositions of the pyrite show a range of 16.369 to 17.079 for 206Pb/204Pb, 15.201 to 15.355 for 207Pb/204Pb, and 36.696 to 37.380 for 208Pb/204Pb, indicating that the ore‐forming materials were derived from a mixture of lower crust (or basement rocks) and mantle. Rhenium contents in molybdenite samples from the main ore stage are between 74.73 to 254.43 ppm, with an average of 147.9 ppm, indicating a mixed crustal‐mantle source for the metal. Eight molybdenite separates yield model ages ranging from 124.17 to 130.80 Ma and a mean model age of 128.46 Ma. An isochron age of 126.7 ± 1.1 Ma (MSWD = 2.1, initial 187Os = 0.0032 ± 0.0012 ppb) is computed, which reveals a close link between the Mo mineralization and the magmatism that generated the Mapeng batholith. The age is close to the zircon U–Pb age of ca. 130 Ma from the batholith reported in a recent study. The age is also consistent with the timing of mineralization in the Fuping ore cluster in the central NCC, as well as the peak time of lithosphere thinning and destruction of the NCC. We evaluate the spatio‐temporal distribution of the Mo deposits in the NCC and identify three important molybdenum provinces along the northern and southern margins of the craton formed during three distinct episodes: Middle to Late Triassic (240–220 Ma), Early Jurassic (190–175 Ma), and Late Jurassic to Early Cretaceous (150–125 Ma). The third period is considered to mark the most important metallogenic event, coinciding with the peak of lithosphere thinning and craton destruction in the NCC. Copyright © 2014 John Wiley & Sons, Ltd.

The nature and timing of ore formation in the Budunhua copper deposit, southern Great Xing'an Range: Evidence from geology, fluid inclusions, and U–Pb and Re–Os geochronology

The Budunhua copper deposit is located in the southern Great Xing'an Range where the stockwork ore bodies are mainly hosted in the Lower Permian Dashizhai Formation. The mineralization occurs in two blocks – the Jinjiling and Kongqueshan – and both are characterized by four main stages: arsenopyrite–pyrite–(molybdenite)–quartz, chalcopyrite–pyrrhotite–quartz, galena–sphalerite–chalcopyrite–quartz and calcite–fluorite–quartz. Wall-rock alteration, which is related to mineralization, is dominated by silicification, sericitization, and chloritization. Fluid inclusion studies show that the ore-forming fluids from Cu stages in both blocks are broadly identical with trapping temperatures of 280 to 400°C for the Kongqueshan, and 300 to 420°C for the Jinjiling. Estimated trapping pressures for the Kongqueshan are 15.0 to 26.6MPa and for the Jinjiling are 14.9 to 29.4MPa, corresponding to entrapment depth of 1.5 to 2.7km and 1.5 to 2.9km below the paleowater table, respectively. Zircon grains from the tonalite porphyry in the deposit yielded a weighted 206Pb/238U mean age of 152±0.7Ma, which is consistent with a molybdenite Re–Os model age of 150±2.2Ma, indicating that both the porphyry intrusion and the copper deposit are of Late Jurassic age. Our data suggest that the Budunhua deposit is an atypical porphyry deposit related to a Late Jurassic magmatic-hydrothermal system.

A porphyry-skarn metallogenic system in the Lesser Xing’an Range, NE China: Implications from U–Pb and Re–Os geochronology and Sr–Nd–Hf isotopes of the Luming Mo and Xulaojiugou Pb–Zn deposits

The Luming porphyry Mo deposit and the Xulaojiugou skarn Pb–Zn deposit are located in the southeast Lesser Xing’an Range, NE China. They are about 15km apart, and are both related to monzogranite. Mo orebodies in the Luming deposit are hosted within the medium- to fine-grained monzogranite, while Pb–Zn orebodies in the Xulaojiugou deposit are hosted by the contact zone between the medium-grained monzogranite and the marbles of the early Cambrian Qianshan Formation.LA-ICP-MS zircon U–Pb dating of the ore-related monzogranite in the Luming deposit yields crystallization age of 180.7±1.6Ma, and the medium-grained and porphyritic monzogranites from the Xulaojiugou deposit yield crystallization ages of 181.2±1.1Ma and 179.9±1.0Ma, respectively. Analyses of seven molybdenite samples from the Luming deposit display Re–Os isochron age of 177.9±2.6Ma. These results indicate that the mineralization in the Luming and Xulaojiugou deposits occurred at about 181–178Ma. These two deposits are genetically linked and belong to a porphyry-skarn metallogenic system. Combined with the previously reported geochronological data for ore deposits in adjacent areas, we consider that the early Jurassic is an important epoch for Mo and Pb–Zn mineralization in the Lesser Xing’an Range.The monzogranites from the Luming and Xulaojiugou deposits are enriched in and Rb, Th, U, Pb and light rare earth elements (LREEs), and are depleted in Ba, Nb, Ta, P, Ti and Eu. They have positive εHf(t) values of 1.0–4.0 with two-stage Hf model ages (TDM2) of 868–1033Ma. Whole-rock Sr and Nd isotopes show restricted ranges of initial compositions, with (87Sr/86Sr)i between 0.706346 and 0.707384 and εNd(t) between −3.5 and −1.8. These data indicate that their primary magmas originated from the partial melting of a depleted lithospheric mantle which had been metasomatized by subducted slab-derived fluids/melts. The early Jurassic magmatic–metallogenic events in the Lesser Xing’an Range are interpreted as a response to the subduction of the Paleo-Pacific Plate.

Re–Os geochronology and Os isotope fingerprinting of petroleum sourced from a Type I lacustrine kerogen: Insights from the natural Green River petroleum system in the Uinta Basin and hydrous pyrolysis experiments

Rhenium–osmium (Re–Os) geochronology of marine petroleum systems has allowed the determination of the depositional age of source rocks as well as the timing of petroleum generation. In addition, Os isotopes have been applied as a fingerprinting tool to correlate oil to its source unit. To date, only classic marine petroleum systems have been studied. Here we present Re–Os geochronology and Os isotope fingerprinting of different petroleum phases (oils, tar sands and gilsonite) derived from the lacustrine Green River petroleum system in the Uinta Basin, USA. In addition we use an experimental approach, hydrous pyrolysis experiments, to compare to the Re–Os data of naturally generated petroleum in order to further understand the mechanisms of Re and Os transfer to petroleum.The Re–Os geochronology of petroleum from the lacustrine Green River petroleum system (19±14Ma – all petroleum phases) broadly agrees with previous petroleum generation basin models (∼25Ma) suggesting that Re–Os geochronology of variable petroleum phases derived from lacustrine Type I kerogen has similar systematics to Type II kerogen (e.g., Selby and Creaser, 2005a,b; Finlay et al., 2010). However, the large uncertainties (over 100% in some cases) produced for the petroleum Re–Os geochronology are a result of multiple generation events occurring through a ∼3000-m thick source unit that creates a mixture of initial Os isotope compositions in the produced petroleum phases. The 187Os/188Os values for the petroleum and source rocks at the time of oil generation vary from 1.4 to 1.9, with the mode at ∼1.6. Oil-to-source correlation using Os isotopes is consistent with previous correlation studies in the Green River petroleum system, and illustrates the potential utility of Os isotopes to characterize the spatial variations within a petroleum system.Hydrous pyrolysis experiments on the Green River Formation source rocks show that Re and Os transfer are mimicking the natural system. This transfer from source to bitumen to oil does not affect source rock Re–Os systematics or Os isotopic compositions. This confirms that Os isotope compositions are transferred intact from source to petroleum during petroleum generation and can be used as a powerful correlation tool. These experiments further confirm that Re–Os systematics in source rocks are not adversely affected by petroleum maturation. Overall this study illustrates that the Re–Os petroleum geochronometer and Os isotope fingerprinting tools can be used on a wide range of petroleum types sourced from variable kerogen types.

Two epochs of magmatism and metallogeny in the Cuihongshan Fe-polymetallic deposit, Heilongjiang Province, NE China: Constrains from U–Pb and Re–Os geochronology and Lu–Hf isotopes

The large-sized Cuihongshan Fe-polymetallic deposit is located in the north segment of the Lesser Xing'an range, NE China. The Fe orebodies are dominantly hosted in the contact zone between the alkali–feldspar granite and the dolomitic crystalline limestones or skarns, whereas the Pb–Zn (Cu) and W–Mo orebodies are mostly hosted in the contact zone between the syenogranite and skarns, as well as within the syenogranite. The alkali–feldspar granite and syenogranite yield zircon U–Pb ages of 491.1±2.4Ma and 199.8±1.8Ma, with εHf(t) values of −3.7 to −1.3 and 2.5 to 3.9, respectively. Both of them are characterized by high SiO2 and Na2O+K2O content, enrichment in Rb, Th, U and Pb, and depletion in Ba, Sr, Nb, Ta, P, Ti and Eu, indicating an A-type affinity. The Re–Os model ages of the molybdenite range from 198.0 to 202.1Ma. These data suggest that the Fe-related alkali–feldspar granite was formed by partial melting of the Mesoproterozoic crust in an extensional setting after the final collision between the Xing'an and Songnen Blocks, while the Pb–Zn (Cu) and W–Mo-related syenogranite was probably generated by crystal fractionation from depleted-mantle-derived magmas, which are products of lithospheric delamination.

Re–Os geochronological constraints on the Dabolava mesothermal gold occurence, Madagascar: Implications for the Ikalamavony sub-domain deposition age

New Re–Os ages for early concordant veins and later discordant veins in the Ikalamovony sub-domain of west-central Madagascar require respectively a Paleoproterozoic age for metasedimentary rocks and a Pan-African age for the orogenic type occurrences. Indeed, this paper focuses on Re–Os geochronology of Dabolava gold occurrences, located in the Ikalamavony sub-domain, western central part of Madagascar. Two types of gold veins have been identified in this region; (i) type 1, centimetre-thick quartz veinlets, with diffuse contacts and concordant to the main regional foliation of the host amphibolites, part of the upper-Group of the Ikalamavony sub-domain; (ii) type 2, 30–50cm-thick quartz veins variously transposed in shear zones affecting the Dabolava granodioritic orthogneisses, part of the Dabolava magmatic suite. These two types of veins with Au–Py assemblages have been sampled for Re–Os geochronology. The type 1 pyrite and electrum assemblage yields a Palaeoproterozoic age (1961±79Ma) whereas the pyrite and electrum assemblage of discordant type 2 veins yields a Pan-African age (533±23Ma). These ages record two main gold mineralization events that affected this crustal segment. Assuming that the Re–Os systematics were not perturbed by more recent events, the 1961±79Ma age obtained in this study on type 1 gold vein concordant in amphibolites favours a Palaeoproterozoic deposition age for the whole Ikalamavony sub-domain, both lower- and upper Groups. This age contrasts with the previously proposed Mesoproterozoic age for the deposition of the upper Group of the Ikalamavony sub-domain but is consistent with the Paleoproterozoic age (∼1800Ma) deduced from detrital zircon ages in the Ikalamavony quartzites located in the Ikalamavony lower Group. The Pan-African age obtained on the type 2 gold deposits is much younger than the emplacement age of the host granodiorite and precludes a porphyry-type model for this gold mineralization and rather favours an orogenic type with a deposition during the Pan-African orogeny.

Re–Os geochronology of the El Salvador porphyry Cu–Mo deposit, Chile: Tracking analytical improvements in accuracy and precision over the past decade

Sulfide minerals from the El Salvador porphyry Cu–Mo deposit, Chile, were dated by Re–Os geochronology to clarify the timing and duration of mineralization. As these data are collected over the past 10years, they chronicle the evolution of Re–Os analytical procedures and improvements in spike-sample equilibration, mass spectrometry and data reduction. Included in the data is the first tennantite–pyrite Re–Os isochron along with tennantite–enargite–pyrite Os tracing of ore metals based on initial osmium systematics (187Os/188Osi). Porphyry-associated mineralization follows the traditional early (A), transitional (B), and late (D) stage classification of relative timing of vein formation. Most Cu- and Mo-bearing sulfides were deposited during stages A and B. Field relationships clearly show the B-stage veins are older than the D-stage veins. Samples analyzed with Carius tube digestion and mixed-double spike indicate B-stage molybdenite mineralization between 41.8 and 41.2Ma. These ages best represent the timing of main-stage mineralization at El Salvador. A five-point 187Re–187Os isochron for pyrite–tennantite from the latest D-stage veins yields an age of 42.37±0.45Ma. The isochron age, while older, is within error of early analyses of molybdenite by alkaline fusion digestion using single Re and Os spikes (42.4–42.0Ma). A separate pyrite and tennantite–enargite pair containing significant common Os yields an initial 187Os/188Os ratio of 0.134±0.042, compatible with a predominantly mantle origin. We appreciate the early analyses for their historical significance while emphasize the geological implications of the 41.8–41.2Ma molybdenite ages. Temperature estimates by sulfur thermometry and fluid inclusions are 390–510°C for the B-stage molybdenites and 230–240°C for the latest D-stage pyrite and tennantite.Analyses based on Carius tube digestion and updated spike calibrations suggest a ∼0.6m.y. duration (∼41.8 to 41.2Ma) in-line with published deposit geochronology. The timing and duration of mineralization from Re–Os dating of ore minerals is more precise than estimates from previously reported 40Ar/39Ar and K–Ar ages on alteration minerals. The Re–Os results suggest that the mineralization is temporally distinct from pre-mineral rhyolite porphyry (42.63±0.28Ma) and is immediately prior to or overlapping with post-mineral latite dike emplacement (41.16±0.48Ma).Based on the Re–Os and other geochronologic data, the Middle Eocene intrusive activity in the El Salvador district is divided into three pulses: (1) 44–42.5Ma for weakly mineralized porphyry intrusions, (2) 41.8–41.2Ma for intensely mineralized porphyry intrusions, and (3) ∼41Ma for small latite dike intrusions without major porphyry stocks. The orientation of igneous dikes and porphyry stocks changed from NNE-SSW during the first pulse to WNW-ESE for the second and third pulses. This implies that the WNW-ESE striking stress changed from σ3 (minimum principal compressive stress) during the first pulse to σHmax (maximum principal compressional stress in a horizontal plane) during the second and third pulses. Therefore, the focus of intense porphyry Cu–Mo mineralization occurred during a transient geodynamic reconfiguration just before extinction of major intrusive activity in the region.

Cause of Upper Triassic climate crisis revealed by Re–Os geochemistry of Boreal black shales

The Triassic Period is bracketed by two of the ‘big five’ Phanerozoic mass extinctions. Though long viewed as a period of climatic stability, emerging data suggest multiple climatic swings and at least one severe ecological crisis. Linking these climatic instabilities with probable causes is hampered by poor age control within the Triassic time scale. Here we present new Re–Os ages for shale sections straddling Middle–Upper Triassic stage boundaries. Nominal Re–Os isochron ages of 236.6 and 239.3Ma for the top and base of the Ladinian (upper Middle Triassic) bring absolute time into the contentious Triassic time scale, and place the beginning of the Late Triassic about 12m.y. earlier than previously assigned. A marked decrease in initial 187Os/188Os in Upper Ladinian shale records input from Wrangellian flood basalts — an instigator in the Carnian (Late Triassic) Pluvial Event and accompanying radiation of key fossil groups (e.g., dinosaurs and calcareous nanoplankton). An absolute time scale is proposed for the Anisian–Ladinian–Carnian boundaries based on Re–Os geochronology.

S–Pb isotopic geochemistry, U–Pb and Re–Os geochronology of the Huanggangliang Fe–Sn deposit, Inner Mongolia, NE China

The Huanggangliang Fe–Sn deposit is located in the eastern section of the Central Asian Orogenic Belt (CAOB) and in the southern segment of the Great Hinggan Range polymetallic belt. This deposit is the largest tin-based polymetallic ore deposit in the northern part of the North China plate. The Fe–Sn mineralization principally occurs in skarn zones, which formed in the contact areas between granite intrusions and marbles. Twenty-one δ34SV-PDB values for sulfides from the Huanggangliang Fe–Sn deposit range from −2.3 to +2.7‰, with an average of +0.38‰, and these values are interpreted to reflect a deep magmatic source of the sulfur contained within the ore minerals of the deposit. The distribution of lead isotopic compositions from sulfides in the Huanggangliang Fe–Sn deposit exhibits a generally concentrated field, indicating that those sulfides have homogeneous lead isotopes and may derive from the same metallic source. The existence of such identical Pb isotopic compositions of sulfides and host granites suggests that granitic magmatism was probably largely responsible for the Huanggangliang mineralization.Zircon LA-ICP-MS U–Pb dating from the Huanggangliang granite exhibits a tight cluster on the Concordia plot and yields a weighted mean 206Pb/238U age of 139.96±0.87Ma, which is considered to be the best estimate of the crystallization age of the Huanggangliang granite. Results obtained from the molybdenite Re–Os isotopic analysis indicate Re–Os model ages of 133.6 to 141.5Ma and a weighted mean age of 135.1±2.3Ma. The molybdenite Re–Os dating yields an isochron age of 134.9±5.2Ma and an initial 187Os/188Os ratio of −0.22±0.35. The molybdenite Re–Os isochron age is largely consistent with the zircon U–Pb age, indicating that both granite intrusion and Fe–Sn mineralization were associated with the Mesozoic magmatism in NE China.Based on ore deposit geology, isotope geochemistry and geochronology of the Huanggangliang Fe–Sn deposit and regional geodynamic evolution, the formation of this deposit is believed to be related to lithospheric thinning, which was caused by delamination and subsequent upwelling of the asthenosphere under the intra-continental extension in NE China. The dynamic setting for the geological processes should be linked to the NNW subduction of the Izanagi plate beneath the Eurasian plate during the Mesozoic. This subduction triggered intensive magmatism and mineralization events in the Great Hinggan Range of NE China, making this district an important Cu–Mo–Sn–Fe–Ag–Pb–Zn metallogenic province.

Zircon U–Pb and molybdenite Re–Os geochronology, and whole-rock geochemistry of the Hashitu molybdenum deposit and host granitoids, Inner Mongolia, NE China

The Hashitu deposit is a newly-discovered Mo deposit in the southern part of the Great Hinggan Range, NE China. Molybdenum mineralization occurs as quartz-sulfide veins within the Hashitu granite-porphyry composite pluton. The sulfide assemblage in the veins is dominated by molybdenite, with minor amounts of galena, sphalerite, chalcopyrite, pyrite and marcasite. The associated gangue minerals are quartz, fluorite, calcite, sericite, chlorite and epidote. Whole-rock chemical compositions show that the Hashitu granites belong to the A2-type. The U–Pb ages of zircons from the Hashitu granite and porphyry units are 147±1Ma and 143±2Ma, respectively. The Re–Os isochron age of molybdenites from the deposit is 150±4Ma. The molybdenite Re–Os model ages vary from 144 to 150Ma, with a weighted mean of 147±1Ma. The results show that the ages of zircon crystallization and Mo mineralization are similar, mostly within analytical uncertainties, and that the host granite pluton is one of many late-Jurassic plutons in the Great Hinggan Range. The formation of the late-Jurassic granitic plutons in this region coincides with the subduction of the Pacific plate beneath the North China block which took place ∼2000km to the east at the time. The occurrence of abundant late-Jurassic granitoids with compositions similar to the Hashitu pluton in the Great Hinggan Range is a positive sign for more discoveries of Mo deposits in this region.

Depositional age of the early Paleoproterozoic Klipputs Member, Nelani Formation (Ghaap Group, Transvaal Supergroup, South Africa) and implications for low-level Re–Os geochronology and Paleoproterozoic global correlations

We present new Re–Os isotope data from the early Paleoproterozoic Klipputs Member (Nelani Formation, Ghaap Group, Transvaal Supergroup, South Africa) to further evaluate low-level Re–Os geochronology of organic-rich sedimentary rocks and improve global correlations of Paleoproterozoic sedimentary rocks. The Klipputs Member contains Re (0.3–0.8ppb), total Os (34–177ppt) and common Os (192Os; 10–66ppt) abundances that overlap upper crustal abundances of 0.2–2ppb, 30–50ppt, and 12–21ppt, respectively. Five samples yield a Model 1 Re–Os date of 2479±20Ma (±22Ma if the 187Re decay constant uncertainty is included, 2σ, Mean Square of Weighted Deviates [MSWD]=0.01) using the CrVI-H2SO4 digestion protocol. Dissolution of the same sample powders using inverse aqua regia yields a Model 3 Re–Os date of 2862±190Ma (2σ, MSWD=148). Comparison of Re–Os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital Os whereas the CrVI-H2SO4 medium did not. The Re–Os date of 2479±22Ma for the Klipputs Member is consistent with published geochronological ages from underlying and overlying stratigraphic units, suggesting the CrVI-H2SO4 protocol yields accurate Re–Os depositional ages from ORS containing very low levels of hydrogenous Re and hydrogenous+radiogenic Os. Hence, there is potential for widespread application of low-level Re–Os ORS geochronology, including sedimentary successions deposited before the Great Oxidation Event. The initial 187Os/188Os (0.12±0.01) from the Klipputs Member CrVI-H2SO4 regression indicates that dissolved Os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and/or cosmic dust rather than oxidative weathering of the radiogenic upper crust. In light of the Re–Os depositional age for the Klipputs Member, we propose that the stratigraphic units in the upper Hamersley Group above the Brockman Iron Formation (Western Australia) are younger than the Ghaap Group. Diamictite from the Doradale Formation in the Ghaap Group is constrained to be older than 2479±22Ma and thus may not be correlative with glacial deposits assigned to the oldest Paleoproterozoic glaciation.

Minor and trace element and Re–Os chemistry of the Upper Devonian Woodford Shale, Permian Basin, west Texas: Insights into metal abundance and basin processes

The trace and minor element and Re–Os geochemistry of the Upper Devonian Woodford Shale are analyzed in order to characterize elemental abundances, to identify associations among trace elements and to constrain paleoceanographic conditions and depositional processes. This organic-carbon-rich mudstone in the Permian Basin, west Texas, is a major source of hydrocarbons in the basin and is coeval with many other Upper Devonian shales in North America.The Woodford lacks enrichment in many trace metals. Only Mo, U, S and Se are significantly enriched. Other redox sensitive elements are depleted or similar to average shale composition, including Pb, Bi, Cr, Ti, Cu, Zn, Co, and V. Elements associated with granitic sources such as rare earths, Th, Ce, and TiO2 are also depleted relative to average shale; this appears to be related to be a source control. A strong basin reservoir effect is noted among several redox sensitive elements, including Mo, Cu and Ni, which largely accounts for the depletion. Dilution by biogenic silica had an additional effect on metal concentrations. Multivariate factors analysis identified associations between elements, including groupings of: rare earth elements; elements enriched in granitic crust; silica, varying antithetically with elements in carbonate minerals; organic carbon, Mo and U; V; phosphate; Fe and S. Noteworthy among the results are the different behavior of redox-sensitive elements, suggesting different precipitation mechanisms or varying dependence on reservoir effects.A strong redox effect is noted in the TOC/Ptot ratio at approximately the Frasnian–Famennian boundary, indicating an abrupt transition to an anoxic column boundary that coincides with a short-term significant fall in sea level. This suggests that anoxia was induced by isolation of the basin from the global ocean. However with the exception of the uppermost Famennian, initial 187Os/188Os values determined from Re–Os geochronology for the Permian Basin are similar to correlative sections of the Appalachian and Peace River Basins of North American and the Rhenohercynian basin of Europe. This indicates that although the Permian Basin became restricted during the upper Devonian and early Mississippian, ocean connectivity remained between regional and global basins.

Ages and Sources of Ore‐Related Porphyries at Yongping Cu–Mo Deposit in Jiangxi Province, Southeast China

AbstractWhole‐rock geochemistry, zircon U–Pb and molybdenite Re–Os geochronology, and Sr–Nd–Hf isotopes analyses were performed on ore‐related dacite porphyry and quartz porphyry at the Yongping Cu–Mo deposit in Southeast China. The geochemical results show that these porphyry stocks have similar REE patterns, and primitive mantle‐normalized spectra show LILE‐enrichment (Ba, Rb, K) and HFSE (Th, Nb, Ta, Ti) depletion. The zircon SHRIMP U–Pb geochronologic results show that the ore‐related porphyries were emplaced at 162–156 Ma. Hydrothermal muscovite of the quartz porphyry yields a plateau age of 162.1 ± 1.4 Ma (2σ). Two hydrothermal biotite samples of the dacite porphyry show plateau ages of 164 ± 1.3 and 163.8 ± 1.3 Ma. Two molybdenite samples from quartz+molybdenite veins contained in the quartz porphyry yield Re–Os ages of 156.7 ± 2.8 Ma and 155.7 ± 3.6 Ma. The ages of molybdenite coeval to zircon and biotite and muscovite ages of the porphyries within the errors suggest that the Mo mineralization was genetically related to the magmatic emplacement. The whole rocks Nd–Sr isotopic data obtained from both the dacite and quartz porphyries suggest partial melting of the Meso‐Proterozoic crust in contribution to the magma process. The zircon Hf isotopic data also indicate the crustal component is the dominated during the magma generation.

Re–Os geochronology, fluid inclusions and genesis of the 0.85 Ga Tumen molybdenite–fluorite deposit in Eastern Qinling, China: implications for pre‐Mesozoic Mo enrichment and tectonic setting

The East Qinling Molybdenum Belt (EQMB), central China, hosting tens of Mesozoic magmatic hydrothermal Mo deposits, is one of the largest molybdenum belts in the world. Recently, a new type of Mo mineralization characterized by molybdenite–fluorite veins was discovered with ongoing prospecting at the Tumen area in the Huaxiong block, representing the southernmost tectonic unit of the North China Craton. The molybdenite–fluorite veins occur in faults or ductile shear zones cross‐cutting the Neoproterozoic Luanchuan Group. At the Tumen deposit, CO2‐rich, aqueous and daughter mineral‐bearing fluid inclusions are observed in fluorite. These fluid inclusions resemble those reported from magmatic hydrothermal Mo systems formed in intracontinental tectonic settings and yield homogenization temperatures up to 450 °C, with salinities up to 39.8 wt.% NaCl equiv, suggesting that the mineralization resulted from a magmatic fluid system. Seven molybdenite analyses from the molybdenite–fluorite veins yield Re–Os isotope ages ranging from 845.8 ± 7.3 to 965.3 ± 7.2 Ma, with an isochron age of 847.4 ± 7.3 Ma (2σ, MSWD = 23), marking the timing of mineralization as Neoproterozoic. These ages are broadly comparable with the 844.3 ± 1.6 Ma age reported from the Shuangshan syenite located near the mining area, and the ca. 830 Ma gabbros occurring within the same tectonic unit, indicating that the deposit was possibly related to Neoproterozoic rifting at the southern margin of the North China Craton. The age data correspond to the transition from the culmination of the assembly of the Neoproterozoic supercontinent Rodinia to the beginning of its break‐up. Our results provide new insights into the regional tectonics and show that the Mo mineralization is not related to the much younger Mesozoic Yanshanian magmatism as considered in previous studies. Copyright © 2013 John Wiley & Sons, Ltd.