Os Dating Research Articles

Zircon U–Pb and Molybdenite Re–Os Dating of the Dongleiwan Skarn Cu Polymetallic Deposit in Jiangxi Province, Eastern China

Zircon U–Pb and Molybdenite Re–Os Dating of the Dongleiwan Skarn Cu Polymetallic Deposit in Jiangxi Province, Eastern China

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.

Palaeoproterozoic Chibaisong mafic intrusion and mineralization (Northeast China), the oldest Cu–Ni sulphide deposit in China: evidence from Re–Os dating of pyrrhotite

The Chibaisong magmatic Cu–Ni sulphide deposit is located in Tonghua City, Jilin Province, in the eastern part of the northern margin of the North China Craton. The geological characteristics of the deposit have been investigated, and pyrrhotite Re–Os isotope dating has been utilized to constrain the age. Five pyrrhotite samples separated from the Chibaisong Cu–Ni sulphide deposit yielded a Re–Os isotopic isochron age of 2237 ± 62 Ma (mean squared weighted deviation = 1.13, n = 5), indicating that the only Palaeoproterozoic magmatic Cu–Ni sulphide deposit in China is the Chibaisong Cu–Ni sulphide deposit. The geodynamic setting during ore formation was related to the Liaoning–Jilin Palaeoproterozoic rift split. The Re–Os isotope analyses showed an initial 187Os/188Os ratio of 0.778 ± 0.033, and (187Os/188Os)i and γOs(t) values ranged from 0.7531 to 0.8013 (average 0.7734) and from 574 to 617 (average 592), respectively, indicating that abundant crustal material (5–10%) was mixed with the Cu–Ni sulphide ore system during magma ascent and ore formation.

Evidence of fluid inclusions for two stages of fluid boiling in the formation of the giant Shapinggou porphyry Mo deposit, Dabie Orogen, Central China

The Shapinggou porphyry Mo deposit, one of the largest Mo deposits in Asia, is located in the Dabie Orogen, Central China. Hydrothermal alteration and mineralization at Shapinggou can be divided into four stages, i.e., stage 1 ore-barren quartz veins with intense silicification, followed by stage 2 quartz-molybdenite veins associated with potassic alteration, stage 3 quartz-polymetallic sulfide veins related to phyllic alteration, and stage 4 ore-barren quartz±calcite±pyrite veins with weak propylitization. Hydrothermal quartz mainly contains three types of fluid inclusions, namely, two-phase liquid-rich (type I), two- or three-phase gas-rich CO2-bearing (type II) and halite-bearing (type III) inclusions. The last two types of fluid inclusions are absent in stages 1 and 4. Type I inclusions in the silicic zone (stage 1) display homogenization temperatures of 340 to 550°C, with salinities of 7.9–16.9wt.% NaCl equivalent. Type II and coexisting type III inclusions in the potassic zone (stage 2), which hosts the main Mo orebodies, have homogenization temperatures of 240–440°C and 240–450°C, with salinities of 34.1–50.9 and 0.1–7.4wt.% NaCl equivalent, respectively. Type II and coexisting type III inclusions in the phyllic zone (stage 3) display homogenization temperatures of 250–345°C and 220–315°C, with salinities of 0.2–6.5 and 32.9–39.3wt.% NaCl equivalent, respectively. Type I inclusions in the propylitization zone (stage 4) display homogenization temperatures of 170 to 330°C, with salinities lower than 6.5wt.% NaCl equivalent. The abundant CO2-rich and coexisting halite-bearing fluid inclusion assemblages in the potassic and phyllic zones highlight the significance of intensive fluid boiling of a NaCl–CO2–H2O system in deep environments (up to 2.3kbar) for giant porphyry Mo mineralization. Hydrogen and oxygen isotopic compositions indicate that ore-fluids were gradually evolved from magmatic to meteoric in origin. Sulfur and lead isotopes suggest that the ore-forming materials at Shapinggou are magmatic in origin. Re–Os dating of molybdenite gives a well-defined 187Re/187Os isochron with an age of 112.7±1.8Ma, suggesting a post-collisional setting.

Metallogenic events and tectonic setting of the Duobaoshan ore field in Heilongjiang Province, NE China

The Duobaoshan ore field, a major center of metal production in Northeast China, is located in the northeast of the Xing’an–Mongolia Orogenic Belt (the eastern part of the Central Asian Orogenic Belt) and within the northern Greater Xing’an Mountains. Several types of ore deposits are mined in the Duobaoshan region, including the Duobaoshan and Tongshan porphyry copper–molybdenum deposits, the Sankuanggou skarn iron–copper deposit, and the Zhengguang epithermal gold deposit. Zircon grains from the Tongshan granodiorite and porphyritic granite yield laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb weighted mean ages of 475.9±0.8Ma and 230.9±0.9Ma to 240.7±0.8Ma, respectively. The Re–Os isochron age of molybdenites from the Tongshan deposit is 473±4Ma. Because both field observations and petrographic analysis identified disseminated chalcopyrite, pyrite, and malachite in the porphyritic granite, the isotope dating indicates that the Tongshan deposit underwent at least two magmatic–mineralization events, during the Ordovician and the Triassic. Zircon grains from the metallogenic granodiorite of the Sankuanggou skarn deposit yield an age of 176.1±0.3Ma, and Re–Os dating of molybdenite gives an age of 173±6Ma, indicating a Jurassic event. Based on previous research and the new geochemical analysis presented in this study, it is inferred that the magmatism and mineralization of the Sankuanggou deposit were associated with the subduction of the Paleo-Pacific Plate. The Duobaoshan region has therefore experienced at least three major magmatic and mineralization events, during the Ordovician (470–480Ma), the Triassic (230–240Ma), and the Early Jurassic (170–180Ma).

In-situ LA-ICP-MS trace elemental analyses of magnetite and Re–Os dating of pyrite: The Tianhu hydrothermally remobilized sedimentary Fe deposit, NW China

The Tianhu Fe deposit (>104Mt at 42% TFe) in the Eastern Tianshan (NW China) is hosted in the schist, quartzite, marble, and amphibolite of the Neoproterozoic Tianhu Group. The deposit consists of disseminated, banded and massive ores. Metallic minerals are dominantly magnetite and pyrite, with minor titanite, pyrrhotite, chalcopyrite, and sphalerite. Gangue minerals include dolomite with minor forsterite, diopside, apatite, biotite, chlorite, tourmaline, tremolite, talc, calcite, and magnesite. Pyrite separates from ores have 10.7 to 54.7ppb Re and 0.033 to 0.175ppb common Os. Those from the massive ores have a model 1 isochron age of 535±36Ma (2σ), in agreement with the isochron age (528±18Ma) of pyrite from the banded ores by regression of seven Re–Os analyses. The Re–Os age of ~530Ma reflects the timing of a hydrothermal event that remobilized the Tianhu deposit. Magnetite has Mg, Al, Ti, V, Mn, Zn, and Ga contents ranging from ~5 to 3500ppm and Cr, Co, Ni, and Sn contents ranging from ~1 to 200ppm. Most magnetite grains have Ca+Al+Mn and Ti+V contents similar to those of the banded iron formation (BIF). Some grains have elevated Ti and V contents, indicating that that magnetite was formed by sedimentary process and overprinted by hydrothermal activity. Pyrite has δ34SCDT values from −9.23 to 10.96‰, indicating that the sulfur was reduced from the marine sulfates either by bacterial or thermochemical processes. Pyrite has relatively high Co (~346 to 3274ppm) but low Ni (~5.6 to 35.4ppm) with Co/Ni ratios ranging from ~10 to 270, indicating remobilization from a volcanic–hydrothermal fluid. Therefore, the Tianhu Fe deposit was originally a sedimentary type deposit but was overprinted by a hydrothermal event related to volcanic activity.

Late Cretaceous metallogeny in the Zhongdian area: Constraints from Re–Os dating of molybdenite and pyrrhotite from the Hongshan Cu deposit, Yunnan, China

The Zhongdian area in Yunnan, southwestern China, located at the southern end of the Yidun volcano-magmatic arc that was formed during the Triassic westward subduction of the Gaze-Litang Ocean, hosts numerous Triassic large porphyry and skarn deposits. The arc suffered Jurassic to Cretaceous arc-continental orogenic collision and Cenozoic intracontinental strike-slip shearing. The Hongshan Cu (–Mo–Pb–Zn) deposit is potentially a large deposit and contains two ore types: 1) predominant layered skarn Cu–(Pb–Zn) ores along marble-hornfels contacts; and 2) minor crosscutting vein-type Cu–Mo mineralization. Previous research forwards a two-stage genetic model without sufficient dating evidence, supposing the skarn mineralization is related to the Triassic calc-alkalic intrusions and the vein-type mineralization related to Cretaceous quartz monzonite porphyries. Re–Os dating of molybdenite from vein-type ores and quartz monzonite porphyries and that of pyrrhotite from skarn ores are presented here to constrain the mineralization age and rebuild the genetic model. Analyses of eight molybdenite samples yield an isochron age of 79.7±3.1Ma (MSWD=9.2) for the vein-type mineralization and a model age of 81.9±1.1Ma for the quartz monzonite porphyries. Isotope data on seven pyrrhotite samples from the skarn ores yield an isochron age of 79±16Ma z(MSWD=8.4). The Re–Os ages for the two ore types are concordant within analytical errors, indicating that the Hongshan deposit was formed in the Late Cretaceous. Elevated Re contents in molybdenite (13.65 to 63.91μg/g) and extremely radiogenic initial 187Os/188Os ratios in pyrrhotite (0.7673 to 0.8184; weighted average 0.796±0.038), together with elevated γOs values in pyrrhotite (507 to 547; average 528) imply a significant crustal component in the ore-forming materials that was likely derived from a lower crustal reservoir. Combined with the tectonic evolution of the Zhongdian area and geochemical characteristics of corresponding intrusions, the ages of mineralization obtained in this study indicate that the Hongshan deposit was formed in a post-collision setting with a genetic relationship to the emplacement of the quartz monzonite porphyry. These results provide significant new information for the study and exploration of the Late Cretaceous metallogeny in the Zhongdian area.

Genesis of Si-rich carbonatites in Huanglongpu Mo deposit, Lesser Qinling orogen, China and significance for Mo mineralization

The Huanglongpu carbonatite-related Mo ore field is located in the Lesser Qinling Orogenic belt in the southern margin of the North China block. The ore field is composed of six deposits, Yuantou, Wengongling, Dashigou, Shijiawan, Taoyuan and Erdaohe, all of which are genetically related to carbonatite dykes except for the Shijiawan deposit which is associated with a granitic porphyry. The Yuantou carbonatite dykes intruded into Archean gneiss and other carbonatites emplaced into Mesoproterozoic volcanic and sediment rocks. The carbonatites are mainly composed of calcite and variable amounts of quartz and K-feldspar and minor molybdenite. Re–Os dating of molybdenite from the Yuantou carbonatite yields a weighted average age of 225.0±7.6Ma, consistent with the molybdenite age (221Ma) from the Dashigou deposit. The rocks are characterized by high heavy REE (HREE) contents and consistent flat REE distribution patterns with La/Ybcn ~1. Quartz in the carbonatites from Yuantou and Dashigou deposits shows consistent O isotopes (8.1–10.2‰) similar to the associated calcite (7.2–9.5‰). The quartz and associated K-feldspar contain lower Zr, Hf and higher HREE abundances and negligible Eu anomaly relative to those from the granite porphyry in Shijiawan. Both minerals are primary products in the carbonatitic liquid, and not captured from the wall-rocks or crustal-derived silicate magmas, or a hydrothermal origin. Thus, the Huanglongpu carbonatitic liquids were enriched in Si and Mo, which may be produced by intensely fractional crystallization of non-silicate minerals.

Re–Os andU–Pb Geochronology of theDawanMo–Zn–FeDeposit inNorthernTaihangMountains,China

AbstractThe DawanMo–Zn–Fe deposit located in theNorthernTaihangMountains in the middle of theNorthChinaCraton (NCC) contains largeMo‐dominant deposits. The mineralization of theDawanMo–Zn–Fe deposit is associated with theMesozoicWanganzhen granitoid complex and is mainly hosted withinArchean metamorphic rocks andProterozoic–Paleozoic dolomites. Rhyolite porphyry and quartz monzonite both occur in the ore field and potassic alteration, strong silicic–phyllic alteration, and propylitic alteration occur from the center of the rhyolite porphyry outward. TheMo mineralization is spacially related to silicic and potassic alteration. TheFe orebody is mainly found in serpentinized skarn in the external contact zone between the quartz monzonite and dolomite. Six samples of molybdenite were collected forRe–Os dating. Results show that theRe–Os model ages range from 136.2 Ma to 138.1 Ma with an isochron age of 138 ± 2 Ma (MSWD= 1.2). U–Pb zircon ages determined by laser ablation inductively coupled plasma mass spectrometry yield crystallization ages of 141.2 ± 0.7 (MSWD= 0.38) and 130.7 ± 0.6 Ma (MSWD= 0.73) for the rhyolite porphyry and quartz monzonite, respectively. The ore‐bearing rhyolite porphyry shows higherK2O/Na2Oratios, ranging from 58.0 to 68.7 (wt%), than those of quartz monzonite. All of the rock samples are classified in the shoshonitic series and characterized by enrichment in large ion lithophile elements; depletion inMg,Fe,Ta,Ni,P,andY; enrichment in light rare earth elements with high (La/Yb)nratios. Geochronology results indicate that skarn‐typeFe mineralization associated with quartz monzonite (130.7 ± 0.6 Ma) formed eight million years later thanMo andZn mineralization (138 ± 2 Ma) in theDawan deposit. FromRe concentrations in molybdenite and previously presentedPb andSisotope data, we conclude that the ore‐forming material of the deposit was derived from a crust‐mantle mixed source. The porphyry‐skarn typeCu–Mo–Zn mineralization around the Wanganzhen complex is related to the primary magmatic activity, and the skarn‐typeFe mineralization is formed at the late period magmatism. The DawanMo–Zn–Fe porphyry‐skarn ores are related to the magmatism that was associated with lithospheric thinning in theNCC.

Structural characteristics and Re–Os dating of quartz-pyrite veins in the Lewisian Gneiss Complex, NW Scotland: Evidence of an Early Paleoproterozoic hydrothermal regime during terrane amalgamation

In the Archaean basement rocks of the Assynt and Gruinard terranes of the mainland Lewisian Complex in NW Scotland, a regional suite of quartz-pyrite veins cross-cut regional Palaeoproterozoic (Badcallian, ca. 2700Ma; Inverian, ca. 2480Ma) fabrics and associated Scourie dykes. The quartz veins are overprinted by amphibolite-greenschist facies Laxfordian deformation fabrics (ca. 1760Ma) and later brittle faults. The hydrothermal mineral veins comprise a multimodal system of tensile/hybrid hydraulic fractures which are inferred to have formed during a regional phase of NW-SE extension. The almost orthogonal orientation of the quartz veins (NE-SW) to the Scourie dykes (NW-SE) are incompatible and must result from distinct paleostress regimes suggesting they are related to different tectonic events. This hypothesis is supported by Rhenium–Osmium dating of pyrite that yields an age of 2249±77Ma, placing the vein-hosted mineralisation event after the oldest published dates for the Scourie Dykes (2420Ma), but before the youngest ages (1990Ma). Sulphur isotope analysis suggests that the sulphur associated with the pyrite is isotopically indistinguishable from primitive mantle. The presence of the ca. 2250Ma quartz-pyrite veins in both the Assynt and Gruinard terranes confirms that these crustal units were amalgamated during or prior to Inverian deformation. The absence of the veins in the Rhiconich Terrane is consistent with the suggestion that it was not finally amalgamated to the Assynt Terrane until the Laxfordian.

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.

Results of pilot Re–Os dating of sulfides from the Sukhoi Log and Olympiada orogenic gold deposits, Russia

The pilot study with Re–Os dating of sulfides from Sukhoi Log and Olympiada gold deposits revealed early Paleozoic ages of the auriferous sulfides from the two largest orogenic gold systems in the Neoproterozoic orogens of the Baikalides framing the Siberian craton. The age-dating results indicate that gold mineralization is therefore epigenetic. The formation of the dated orogenic gold deposits is synchronous with some regional metamorphic events in the Baikalides, at least in case of the Sukhoi Log deposit. The metamorphic events occurred in the rear parts of the early Paleozoic magmatic arcs, where coeval subduction-related magmatism produced porphyry copper–(molybdenum) mineralization.

Shapinggou: the largest Climax-type porphyry Mo deposit in China

Shapinggou porphyry Mo deposit is the largest Climax-type Mo deposit in China and probably also in the world, with total proven Mo reserves of over 2.2 million tonnes at an average grade of 0.17%. It is located in the western Dabie Mountains, along the eastern extension of the East Qinling Mo mineralization belt. Similar to the majority of Mo deposits in the Qinling Mo belt, it is located north of the Triassic suture between the north and south China blocks. The orebody is mainly hosted in Cretaceous high-K granitic porphyry and explosive breccia, with potassic, silicification, and sericite-pyrite alterations. Fluorite is common in Shapinggou, indicating high-F content. The porphyry is closely associated with a large quartz syenite intrusion. Re–Os dating of molybdenite yielded an isochron age of 111.1 ± 1.2 Ma for the mineralization. Zircon U–Pb ages are 111.7 ± 0.8 Ma and 111.9 ± 0.6 Ma for the granitic porphyry and quartz syenite, respectively. Shapinggou is similar to the well-known Climax and Henderson Mo deposits in terms of geochemical characteristics and alterations, etc. We propose that Mo-rich ore-forming materials accumulated underneath the Shapinggou region during the convergence of the North and South China blocks, whereas the final mineralization was triggered by asthenosphere upwelling induced by Pacific Plate subduction.

Metallogenesis at the Carris W–Mo–Sn deposit (Gerês, Portugal): Constraints from fluid inclusions, mineral geochemistry, Re–Os and He–Ar isotopes

The Carris orebody consists of two partially exploited W–Mo–Sn quartz veins formed during successive shear stages and multipulse fluid fillings. They cut the Variscan post-D3 Gerês I-type granite. The most important ore minerals are wolframite, scheelite, molybdenite and cassiterite. There are two generations of wolframite. The earlier generation of wolframite is rare and has the highest WO4Mn content (91 mol%) and the most common wolframite contains 26–57 mol% WO4Mn. Re–Os dating of molybdenite from the ore quartz veins and surrounding granite yields ages of 279 ± 1.2 Ma and 280.3 ± 1.2 Ma, respectively which are in very good agreement with the previous ID-TIMS U–Pb zircon age for the Carris granite (280 ± 5 Ma). 3He/4He ratio of pyrite ranging between 0.73 and 2.71 Ra (1 Ra = 1.39 × 10− 6) and high 3He/36Ar (0.8–5 × 10− 3) indicate a mixture of a crustal radiogenic helium fluid with a mantle derived-fluid. The fluid inclusion studies on quartz intergrown with wolframite and scheelite, beryl and fluorite reveal that two distinct fluid types were involved in the genesis of this deposit. The first was a low to medium salinity aqueous carbonic fluid (CO2 between 4 and 14 mol%) with less than 1.95 mol% N2, which was only found in quartz associated with wolframite. The other was a low salinity aqueous fluid found in all the four minerals. The homogenization temperatures indicate minimum entrapment temperatures of 226–310 °C (average 280 °C) for the H2O–CO2–N2–NaCl fluid and average temperatures of 266 °C for scheelite and 242 °C, 190 °C and 160 °C for the last generations of beryl, fluorite and quartz, respectively. It was estimated that wolframite was deposited ~ 7 km depth, assuming a lithostatic pressure, probably due to strong pressure fluctuation caused by seismic events triggered by brittle tectonics during the exhumation event. Precipitation of scheelite and sulphides took place later, at the same depth, but under a hydrostatic or suprahydrostatic pressure regime, and probably caused by mixing between the magmatic–hydrothermal fluid and meteoric waters that deeply penetrated the basement during post-Variscan decompression.

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.

Age and origin of Pan-African granites and associated U–Mo mineralization at Ekomédion, southwestern Cameroon

Various granites in the Pan-African Neoproterozoic Fold Belt of Cameroon were investigated in an area around Ekomédion (southwestern Cameroon) in order to set constraints on their genesis and age. The studied granites were likely emplaced in relation with the Central Cameroon Shear Zone (CCSZ). Hence dating the granites provides an age for the dextral transcurrent tectonics event responsible for the CCSZ. The interest in these granitic rocks is spurred by noticeable Mo and U mineralization in a pegmatite that is hosted by two-mica granite. The studied granites comprise post-collisional syn-D4 two-mica granite, alkali-feldspar granite, biotite-rich granite and porphyritic granite. Based on lithogeochemical data, the granites can be characterized as ferroan, peraluminous and high-K calc-alkaline. The biotite-rich and alkali feldspar granites show I-type characteristics, whereas the porphyritic and two-mica granites reveal S-type affinity. All of them are marked by strong light rare earth element fractionation and a strong negative Eu anomaly, and they are strongly depleted in Ba, Nb, P, Sr and Ti, pointing to a crustal origin. Crust assimilation is also evident from zircon xenocrysts in the two-mica granite with upper intercept U–Pb ages of 2051±44Ma and 1550±24Ma. U–Pb dating of zircon, monazite and xenotime grains/domains yielded indistinguishable ages for both the two-mica granite (578±4Ma) and the porphyritic granite (579±3Ma). An identical age (578±11Ma) was obtained by Re–Os dating of molybdenite that occurs together with uraninite in pegmatite within the two-mica granite, thus clearly attesting to a late-magmatic origin of the ore. The studied granites have many similarities, both in composition and age, with granites in Nigeria and northeastern Brazil, supporting the continuation of a large Pan-African/Braziliano magmatic province from west-central Africa to northeastern Brazil.

Zircon U–Pb and molybdenite Re–Os dating of the Chalukou porphyry Mo deposit in the northern Great Xing’an Range, China and its geological significance

The newly discovered Chalukou giant porphyry Mo deposit, located in the northern Great Xing’an Range, is the biggest Mo deposit in northeast China. The Chalukou Mo deposit occurs in an intermediate-acid complex and Jurassic volcano-sedimentary rocks, of which granite porphyry, quartz porphyry, and fine-grained granite are closely associated with Mo mineralization. However, the ages of the igneous rocks and Mo mineralization are poorly constrained. In this paper, we report precise in situ LA-ICP-MS zircon U–Pb dates for the monzogranite, granite porphyry, quartz porphyry, fine grained granite, rhyolite porphyry, diorite porphyry, and andesite porphyry in the Chalukou deposit, corresponding with ages of 162±2Ma, 149±5Ma, 148±2Ma, 148±1Ma, 137±3Ma, 133±2Ma, and 132±2Ma, respectively. Analyses of six molybdenite samples yielded a Re–Os isochron age of 148±1Ma. These data indicate that the sequence of the magmatic activity in the Chalukou deposit ranges from Jurassic volcano-sedimentary rocks and monzogranite, through late Jurassic granite porphyry, quartz porphyry, and fine-grained granite, to early Cretaceous rhyolite porphyry, diorite porphyry, and andesite porphyry. The Chalukou porphyry Mo deposit was formed in the late Jurassic, and occurred in a transitional tectonic setting from compression to extension caused by subduction of the Paleo-Pacific oceanic plate.

Re–Os dating of sulphide inclusions zonally distributed in single Yakutian diamonds: Evidence for multiple episodes of Proterozoic formation and protracted timescales of diamond growth

The timing of diamond formation in the Siberian lithospheric mantle was investigated by Re–Os isotope dating of sulphide inclusions from eclogitic and lherzolitic diamonds from the Mir, 23rd Party Congress and Udachnaya kimberlite pipes in Yakutia. The diamonds contained multiple sulphide inclusions distributed over their core-to-rim zones. Cathodoluminescence, carbon isotope and nitrogen aggregation studies demonstrate that the diamonds are zoned and that the distinct zones are associated with different diamond growth episodes. There are coherent relationships between carbon isotope composition, nitrogen concentration and aggregation state of the diamond hosts, and major and trace element compositions, Re–Os compositions and initial Os isotope ratios of the included sulphides. This suggests that the different diamond and sulphide populations formed at different times from fluids/melts with different chemical compositions. Based on the Re–Os isochron ages and the nitrogen aggregation states we conclude that the sulphides are co-genetic with their diamond hosts.The ages obtained for the different diamond populations demonstrate two major periods of Yakutian diamond formation for both eclogitic and lherzolitic suites, i.e., ∼2.1–1.8Ga (Mir, 23rd Party Congress and Udachnaya) and ∼1.0–0.9Ga (Mir and 23rd Party Congress). These two episodes correspond to major tectono-magmatic events: collision between the different terranes of the Siberian Craton during the formation of the Palaeoproterozoic supercontinent ∼2.0–1.8Ga and accretion leading to formation of the supercontinent Rodinia ∼1.1Ga.Importantly, Re–Os isotope dating demonstrates that some single diamonds have protracted timescales of growth with different ages associated with core and rim zones. Zoned single eclogitic crystals from Mir and 23rd Party Congress in the current study crystallised over intervals that were irresolvable (<200Ma) up to ∼1Ga apart.The very radiogenic initial Os isotope ratios of the eclogitic and lherzolitic sulphides (187Os/188Os=0.14–2.22) imply incorporation of radiogenic Os from subducted oceanic lithosphere, a conclusion that is also supported by the anomalously light carbon isotope compositions of some diamonds. Subducted lithosphere is therefore considered a significant component in Proterozoic diamond formation beneath the Siberian Craton.

U–Pb and Re–Os Ages of the Huaheitan Molybdenum Deposit, NW China

AbstractThe Huaheitan molybdenum deposit in the Beishan area of northwest China consists of quartz‐sulfide veins. Orebodies occur in the contact zone of the Huaniushan granite. LA‐ICPMS U–Pb zircon dating constrains the crystallization of the granite at 225.6 ± 2.2 Ma (2σ, MSWD = 4.5). Re–Os dating of five molybdenite samples yield model ages ranging from 223.2 ± 3.5 Ma to 228.6 ± 3.4 Ma, with an average of 225.2 ± 2.4 Ma. The U–Pb and Re–Os ages are identical within the error, suggesting that the granite and related Huaheitan molybdenum deposit formed in the Late Triassic. Our new data, combined with published geochronological results from the other molybdenum deposits in this region, imply that intensive magmatism and Mo mineralization occurred during 240 Ma to 220 Ma throughout the Beishan area.

Re–Os and U–Pb geochronology of porphyry Mo deposits in central Jilin Province: Mo ore-forming stages in northeast China

Central Jilin Province lies along the eastern edge of the Xing–Meng orogenic belt of northeast China. At least 10 Mo deposits have been discovered in this area, making it the second-richest concentration of Mo resources in China. To better understand the formation and distribution of porphyry Mo deposits in the area, we investigated the geological characteristics of the deposits and applied zircon U–Pb and molybdenite Re–Os isotope dating to constrain the age of mineralization. Our new geochronological data show the following: the Jidetun Mo deposit yields molybdenite Re–Os model ages of 164.6–167.1 Ma, an isochron age of 168 ± 2.5 Ma, and a weighted mean model age of 165.9 ± 1.2 Ma; the Houdaomu Mo deposit yields molybdenite Re–Os model ages of 167.4–167.7 Ma, an isochron age of 168 ± 13 Ma, and a weighted mean model age of 167.5 ± 1.2 Ma; and the Chang’anpu Mo deposit yields a zircon U–Pb age for granodiorite porphyry of 166.9 ± 1.5 Ma (N = 16). These new age data, combined with existing molybdenite Re–Os dates, show that intense porphyry Mo mineralization was coeval with magmatism during the Middle Jurassic (167.8 ± 0.4 Ma, r > 0.999). The geotectonic mechanisms responsible for Mo mineralization were probably related to subduction of the Palaeo-Pacific plate beneath the Eurasian continent. Combining published molybdenite Re–Os and zircon U–Pb ages for northeast China, the Mo deposits are shown to have been formed during multiple events coinciding with periods of magmatic activity. We identified three phases of mineralization, two of which had several stages: the Caledonian (485–480 Ma); the Indosinian comprising the Early–Middle Triassic (248–236 Ma) and Late Triassic (226–208 Ma) stages; and the Yanshanian phase comprising the Early–Middle Jurassic (202–165 Ma), Late Jurassic–early Early Cretaceous (154–129 Ma), and Early Cretaceous (114–111 Ma) stages. Although Mo deposits formed during each phase/stage, most of the mineralization occurred during the Early–Middle Jurassic.