David Selby | Durham University (original) (raw)

Papers by David Selby

Mineralium Deposita

The Qianhe gold deposit in the Xiong’ershan area is located along the southern margin of the Arch... more The Qianhe gold deposit in the Xiong’ershan area is located along the southern margin of the Archean-Paleoproterozoic North China Craton. The deposit consists of six orebodies that are hosted in Paleoproterozoic andesites to basaltic andesites and structurally controlled by roughly EW-trending faults. Individual orebodies comprise auriferous quartz veins and disseminated Au-bearing pyrite within hydrothermally altered rocks on both sides of, or close to, the veins. Ore-related hydrothermal alteration has produced various mixtures of K-feldspar, quartz, sericite, chlorite, epidote, carbonate, and sulfides. Pyrite is the most important ore mineral, associated with minor amounts of galena, sphalerite, and chalcopyrite. Other trace minerals include molybdenite, arsenopyrite, scheelite, rutile, xenotime, and parisite. Gold occurs mostly as native gold and electrum enclosed in pyrite or along microfractures of sulfides and quartz. Microthermometric measurements of primary inclusions in au...

Chemical Geology

Editor: K. Mezger Keywords: Columbite-(Mn) Zircon Pegmatite U–Pb dating LA-ICP-MS U–Pb isotopes a... more Editor: K. Mezger Keywords: Columbite-(Mn) Zircon Pegmatite U–Pb dating LA-ICP-MS U–Pb isotopes and trace elements of columbite-(Mn) and zircon from an early Cretaceous pegmatite dike in the Xiaoqinling district, North China Craton, were analyzed using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to better understand the suitability of these minerals for U–Pb geochronol-ogy when they contain unusually high U and Th. Columbite-(Mn) grains have high W, Ti, U, Th, and REE con-tents and yield concordant U–Pb age of 143 ± 1 Ma (2σ, n = 10) that is interpreted as the emplacement age of the pegmatite dike. In contrast, zircon grains from the same dike show three distinct U–Pb age populations. Nine out of seventeen zircon grains analyzed have textural features typical of magmatic zircons and yield a weighted mean 206 Pb/ 238 U age of 143 ± 1 Ma (2σ, n = 9), identical to that of columbite-(Mn) and thus con-strain the timing of pegmatitic magmatism. The second popul...

Proceedings of the National Academy of Sciences

After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitude... more After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and mor...

Interface focus, Jan 6, 2013

Thermochemical processing methods such as pyrolysis are of growing interest as a means of convert... more Thermochemical processing methods such as pyrolysis are of growing interest as a means of converting biomass into fuels and commodity chemicals in a sustainable manner. Macroalgae, or seaweed, represent a novel class of feedstock for pyrolysis that, owing to the nature of the environments in which they grow coupled with their biochemistry, naturally possess high metal contents. Although the impact of metals upon the pyrolysis of terrestrial biomass is well documented, their influence on the thermochemical conversion of marine-derived feeds is largely unknown. Furthermore, these effects are inherently difficult to study, owing to the heterogeneous character of natural seaweed samples. The work described in this paper uses copper(II) alginate, together with alginic acid and sodium alginate as model compounds for exploring the effects of metals upon macroalgae thermolysis. A thermogravimetric analysis-Fourier transform infrared spectroscopic study revealed that, unusually, Cu(2+) ions ...

Re-Os analyses of bitumen from the Polaris Mississippi Valley-type Zn-Pb deposit demonstrate the ... more Re-Os analyses of bitumen from the Polaris Mississippi Valley-type Zn-Pb deposit demonstrate the potential utility of the Re-Os system for establishing the absolute timing of hydrocarbon migration and generation. The Polaris deposit is hosted by carbonate rocks of the Upper Ordovician Thumb Mountain Formation, and sulphide mineralization during Late Devonian time is constrained by a Rb-Sr sphalerite date of 366 F 15 Ma and a paleomagnetic date. Bitumen occurs in open-space fillings and coatings, and is interpreted to have formed during the main and late stages of sulphide mineralization. Samples of bitumen were taken from a variety of locations from the main deposit at Polaris; Re and Os contents are~3-78 ppb and~55-1155 ppt, respectively. The 187 Re/ 188 Os ratios are very high, ranging from~300 to 1800, and the Os isotopic composition is moderately to highly radiogenic, with 187 Os/ 188 Os ratios ranging from~3.3 to 13.7. Calculated 187 Os/ 188 Os ratios at 366 Ma are variable, but an obvious grouping is defined by 11 of 16 analyses on a Re-Os isochron diagram, and these data yield a Re-Os date of 374F 9 Ma (MSWD = 12, Model 3), in agreement with Rb-Sr sphalerite and paleomagnetic dates for mineralization at Polaris. The combined dates suggest that the migration of fluids responsible for both Pb-Zn mineralization and bitumen formation at Polaris was broadly contemporaneous, and was coeval with the Late Devonian Ellesmerian Orogeny. Though considerable further research is required to understand the Re-Os isotopic systematics of migrated hydrocarbon systems, bitumen Re-Os geochronology may provide the possibility of establishing a more comprehensive understanding of MVT deposits, sedimentary basin and petroleum systems. D

The mean agedetermined for this standard in our laboratory is 219.7 0.8 Ma ( 0.36% 1SD, n = 5), s... more The mean agedetermined for this standard in our laboratory is 219.7 0.8 Ma ( 0.36% 1SD, n = 5), similar to thatreported by Markey et al. [1] of 221.0 1.0 Ma.Re-Os data for molybdenite from ribbon-textured veins yield two distinct ages, 148.3 0.05 (n = 1,duplicated sample) and 145.1 0.4 Ma ( 0.3% 1SD, n = 7) (Fig. 1). One

Precambrian Research, 2014

The Lewisian Gneiss Complex of NW Scotland represents the eastern margin of the North Atlantic Cr... more The Lewisian Gneiss Complex of NW Scotland represents the eastern margin of the North Atlantic Craton. It comprises mid-late Archaean tonalite-trondhjemite-granodiorite gneisses that were metamorphosed and deformed during the Late-Archaean and Palaeoproterozoic. A major swarm of mafic-ultramafic dykes, the Scourie Dyke Swarm, was intruded at ca. 2.4-2.3 Ga during a period of extension that can be correlated across the North Atlantic Craton. The majority of dykes are doleritic, with volumetrically minor picrite and olivine gabbro suites.

Spine, 1988

Internal disc disruption is a syndrome of traumatically induced low-back pain arising from the in... more Internal disc disruption is a syndrome of traumatically induced low-back pain arising from the intervertebral disc. The diagnosis is confirmed by abnormal discography with concordant pain reproduction at the affected level or levels. Thirty-four patients with internal disc disruption at one level were followed for an average of 29 months. Eighteen (53%) underwent anterior lumbar fusion at the L4-5 disc, 11 (32%) at the L5-S1 disc and the remainder at the L3-4 or L2-3 disc. Bank bone was used in all but seven patients for interbody fusion. Treatment was judged a success by the patient returning to work or normal activities and requiring either no medications or an antiinflammatory drug only. By the above criteria 25 patients (74%) had successful outcome of treatment. The average time to return to work or normal activities was 6.1 months. The overall union rate was 73% with an average time to union of approximately 12 months. Complications consisted of graft extrusion requiring revision and retrograde ejaculation. These occurred in one patient and were the only complications in the series. We concluded that disc excision and anterior interbody fusion is an effective treatment for internal disc disruption.

Palaeogeography, Palaeoclimatology, Palaeoecology, 2013

Gondwana Research, 2013

ABSTRACT The Lupa Goldfield, situated at the southwestern Tanzanian cratonic margin, comprises a ... more ABSTRACT The Lupa Goldfield, situated at the southwestern Tanzanian cratonic margin, comprises a network of auriferous quartz veins and greenschist fades mylonitic shear zones cutting a suite of Archaean-Palaeoproterozoic granitic-gabbroic intrusions. The existing geochronological database points to a protracted, but episodic 1.96-1.88 Ga magmatic history that is broadly coincident with the 2.1-1.8 Ga Ubendian Orogeny. Molybdenite, pyrite and chalcopyrite samples from mineralized quartz veins and mylonitic shear zones yield Re-Os model ages that range from 1.95 to 1.88 Ga, whereas ca. 1.88 Ga pyrite with gold bearing inclusions and sampled from the host mylonitic shear zone suggest that gold occurred relatively late in this hydrothermal history. The ca. 1.88 Ga gold event is recorded at all five of the studied prospects, whereas the relationship between gold and the disparately older 1.95 and 1.94 Ga Re-Os molybdenite ages is unclear. New U-Pb metamorphic titanite dating of a foliated Archaean granite sample (ca. 2.76 Ga) suggests that the onset of ductile deformation within the Lupa Goldfield occurred at ca. 1.92 Ga, and some ca. 40 Myr prior to auriferous and brittle-ductile mylonitic shear zones at ca. 1.88 Ga. Early ductile deformation is not associated with gold mineralization, but the ductile deformation fabrics and, in particular the development of rheologically weak chloritic folia, may have acted as zones of pre-existing weakness that localized strain and influenced the geometry of later auriferous mylonitic shear zones. The large age difference between U-Pb zircon and titanite ages for the Archaean granite sample is in contrast to new U-Pb titanite ages for the Saza Granodiorite (1930 +/- 3 Ma), which are only slightly outside of analytical uncertainty at the 2 sigma level with a previously reported U-Pb zircon age for the same sample (1935 +/- 1 Ma). These new age results, together with previously reported U-Pb and Re-Os ages, highlight the protracted magmatic, hydrothermal and structural evolution of the Lupa Goldfield (1.96-1.88 Ga). They are also consistent with other palaeo-convergent margins where orogenic gold style mineralization occurs relatively late in the orogen's tectono-thermal history. Crown Copyright

Geology, 2006

New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits i... more New Re-Os dates obtained from black shales overlying the Sturtian and Areyonga glacial deposits in southern and central Australia, respectively, challenge the prevailing consensus of three Neoproterozoic glaciations. The end of Sturtian glaciation in the Adelaide Rift Complex is constrained by a Re-Os date of 643.0 ؎ 2.4 Ma from the overlying Tindelpina Shale Member (basal Tapley Hill Formation). A Re-Os date of 657.2 ؎ 5.4 Ma for the basal Aralka Formation constrains the age of the underlying Areyonga glacial deposits in the Amadeus Basin. The Re-Os ages show that the Sturtian and Areyonga glacial deposits are younger than other radiometrically dated (ca. Ma) Neoproterozoic glacial intervals previously regarded as possible correlatives. Thus, the ''Sturtian'' ice age was markedly diachronous, and/or there was more than one ''Sturtian''-type glaciation. Some Neoproterozoic glacial deposits may represent the products of regional and diachronous glaciation associated with protracted breakup of the supercontinent Rodinia rather than ''snowball'' or ''slushball'' Earth ice ages.

Geology, 2002

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Earth and Planetary Science Letters, 2010

... 1; [Trompette, 1973], [Bertrand-Sarfati and Moussine-Pouchkine, 1985], [Bertrand-Sarfati and ... more ... 1; [Trompette, 1973], [Bertrand-Sarfati and Moussine-Pouchkine, 1985], [Bertrand-Sarfati and Moussine-Pouchkine, 1988], [Ahmed Benan and Deynoux, 1998] and [Deynoux et al., 2006]). ... S1 = mg of hydrocarbons (free and thermovapourisable) per gram of rock. ...

Chemical Geology, 2003

Rhenium and osmium in organic-rich sedimentary rocks are dominantly hydrogenous, but any nonhydro... more Rhenium and osmium in organic-rich sedimentary rocks are dominantly hydrogenous, but any nonhydrogenous component will influence the accuracy and precision of the Re -Os date obtained. To minimize the influence of any nonhydrogenous Re and Os, we evaluate analysis of isolated organic matter from the whole rock, together with whole rock analysis using a CrO 3 -H 2 SO 4 digestion medium instead of inverse aqua regia, for a black shale unit of the Exshaw Formation, Canada. This unit previously returned a whole rock Re -Os date of 358 F 10 Ma (Model 3) [Geochim. Cosmochim. Acta ] using inverse aqua regia dissolution. Organic matter isolated from the whole rock matrix using the HF -BF 3 technique [Org. Geochem. 20 (1993) 249] yields scattered data and a Re -Os date of 449 F 220 Ma (Model 3, MSWD = 616). The organic matter analyses show similar 187 Os/ 188 Os values, but significantly lower 187 Re/ 188 Os values in comparison to the whole rock analyses. We show that the Re -Os systematics of organic matter are altered during chemical isolation, and as such we suggest that the HF -BF 3 method should not be used for Re -Os analysis of organic matter. Whole rock Re -Os analysis using a CrO 3 -H 2 SO 4 digestion medium yields significantly better regression analysis compared with the inverse aqua regia method, and the Re -Os data identify two distinct initial 187 Os/ 188 Os values for the sample set. Separate regressions of these data yield precise dates [366.1 F 9.6, MSWD = 2.2 and 363.4 F 5.6 Ma, MSWD = 1.6 (Model 3)], which are indistinguishable from the age constraints for this formation (363.4 F 0.4 Ma, U -Pb monazite). Comparison of the Re -Os dates obtained from aqua regia and CrO 3 -H 2 SO 4 methods suggests that the former may contain nonhydrogenous Re and Os, whereas the CrO 3 -H 2 SO 4 method dominantly liberates hydrogenous Re -Os from organic matter, allowing for better stratigraphic age determinations and evaluation of the Os isotope composition of seawater. D

Chemical Geology, 2009

Presented is the first absolute age for the basal Albian from the Schwicheldt Ton Member, Gault F... more Presented is the first absolute age for the basal Albian from the Schwicheldt Ton Member, Gault Formation, Vöhrum, Germany. A 206Pb/238U age of 113.1±0.3 Ma is determined for chemically abraded zircon from a tuff horizon 65 cm above the Aptian/Albian boundary. The new U–Pb age, although within uncertainty of the GTS 2008 determination (112±1 Ma), is nominally older. The younger GTS 2008 basal Albian age is obtained from cyclostratigraphy using an 40Ar–39Ar age from the base Cenomanian. The nominal difference between the ...

Canadian Journal of Earth Sciences, 2003

New Re-Os molybdenite and U-Pb zircon dates from deposits of the Tombstone Gold Belt provide dire... more New Re-Os molybdenite and U-Pb zircon dates from deposits of the Tombstone Gold Belt provide direct timing constraints for sulphide mineralization and crystallization of spatially associated plutons. At Clear Creek Re-Os molybdenite dates (93.6 ± 0.3 to 92.4 ± 0.4 Ma) are in excellent agreement with the host intrusion U-Pb zircon age (92.3 ± 0.3 Ma). Consequently we propose the nominally younger existing 40 Ar/ 39 Ar hydrothermal mica ages (91.7 ± 0.4 and 90.0 ± 0.3 Ma) for Clear Creek are a result of slow cooling. At Dublin Gulch, the Re-Os molybdenite date (93.2 ± 0.3 Ma) for a late stage vein is nominally younger than the host intrusion U-Pb zircon age (94.0 ± 0.3 Ma), in agreement with the deposits paragenesis. The U-Pb zircon date (92.1 ± 0.2 Ma) for the Mactung pluton, which lies adjacent to the W-skarn, has a younger age than the Re-Os molybdenite dates (97.5 ± 0.5 Ma). Fluid inclusion geochemical data for ore-fluid indicate a magmatic origin for the W-skarn. However, the U-Pb and Re-Os age data suggest that the exposed Mactung stock is not the source of ore-fluid for the W-skarn and that the progenitor pluton for ore-fluid is unknown. The robust age constraints obtained for mineralization and magmatism in this study are in accord with the hypothesis that these deposits are intrusion-related ore systems. The results are also in agreement with previous studies demonstrating that the Re-Os molybdenite chronometer can provide accurate and precise dates for the timing of metallic mineralization with rigorous sampling protocol.

Earth and Planetary Science Letters, 2004

New Re -Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort P... more New Re -Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort Point Formation (Windermere Supergroup), a post-glacial Neoproterozoic marker horizon in western Canada. A Re -Os isochron date of 634 F 57 Ma (MSWD = 65, n = 5) was determined using the conventional inverse aqua regia digestion medium. However, dissolution of the same samples with a new CrO 3 -H 2 SO 4 dissolution technique [Chem. Geol. 200 (2003) 225] yielded a much more precise date of 607.8 F 4.7 Ma (MSWD = 1.2). Both dates are in agreement with existing U -Pb age constraints that bracket the Old Fort Point Formation between f 685 and f 570 Ma. The distinctive Re -Os systematics recorded by the two analytical protocols is explained by dissolution of a variably radiogenic, detrital Os component by the aqua regia method. In contrast, the CrO 3 -H 2 SO 4 technique minimizes this detrital component by selectively dissolving organic matter that is dominated by hydrogenous (seawater) Re and Os. The date of 607.8 F 4.7 Ma is thus interpreted as the depositional age for the upper Old Fort Point Formation providing a minimum age constraint for the timing of the second Windermere glaciation in western Canada. This ice age is correlative with the Marinoan ( f 620 -600 Ma) ice age and older than the f 580-Ma Gaskiers glaciation of northeastern North America. The new Re -Os age determined from the CrO 3 -H 2 SO 4 digestion technique thus provides further support to a growing body of evidence for a global Marinoan glacial episode. Such an interpretation would not be discernable from the imprecise Re -Os date obtained with the aqua regia protocol. These results also indicate the potential for Re -Os radiometric dating of black shales that was not previously recognized. Importantly, neither chlorite-grade metamorphism nor the low organic content (TOC < 1%) of the Old Fort Point Formation precluded the determination of a precise Re -Os depositional age using the CrO 3 -H 2 SO 4 analytical protocol. D