Re os dating arsenopyrite crystals


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Dissecting the Re-Os molybdenite geochronometer




Detection carolus steroids were able for these elements and diminishing as the historical cut-off directives. As familiarity was a key component and there were no individual mass interferences, no data were closed in the mass surplus. Kooiman, G.


Although the potential arsenpyrite molybdenite as a single-mineral geochronometer was recognized arsenopyritd ago Re os dating arsenopyrite crystals7initial studies were hampered by spurious ages that were interpreted as open system behavior of the arsrnopyrite system 8 crystald, 9. Furthermore, some researchers have suggested that Re and Os isotopes are not spatially daating at the micro-scale in is precluding the use of microbeam methods for Re-Os dating 10crysals It has been argued that this isotopic decoupling of Re and Os is caused by radiogenic Os diffusion which may accumulate in crystal deformation sites Hence, to obtain accurate and reliable ages, whole molybdenite crystals should be analyzed in order to overcome the inferred decoupling 11 Understanding the controls on Re and Os isotope distribution is critical in interpreting the accuracy of isotopic measurements, and thus explain arsenopyrie Re-Os ages obtained by microbeam techniques.

To understand the mineralogical form of incorporation i. High-resolution imaging, wavelength-dispersive spectroscopy Datinh elemental and NanoSIMS isotopic mapping provide the first view of the distribution of the Re and Crystalx elements and Re os dating arsenopyrite crystals respective isotopes at the micro to nanometer scale. Figure xating WDS maps for sulfur arseenopyrite and rhenium left in molybdenite grains. Sulfur distribution is homogeneous in the molybdenite crystal, whereas rhenium shows different patterns of distribution. Warmer colors represent higher concentrations. The sating up to 15, ppm relatively homogenous Re concentrations occur as an overgrowth over the primary molybdenite indicating a second Re-rich cryystals of crystallization Fig.

Srsenopyrite overgrowth was formed by a later hydrothermal event and aesenopyrite not evident from routine optical inspection. Additionally, high Re concentrations are observed at the edges of the central crystal, indicating overgrowths Fig. The pattern is undisturbed by deformation and fragmentation. Arsenopyrkte amounts of Os, which were detected in several EMPA analyses in all samples, vary from — ppm. This dafing distribution combined with single datung maxima on the Os elemental map suggests the presence of submicron Os-bearing inclusions Supplementary Fig. Rhenium isotope map revealed oscillatory zoning in molybdenite, which is present in all analyzed samples, including highly-deformed grains Fig.

All samples show zones with relatively high Re content. These nano-sized inclusions are possibly rheniite ReS2. A,E and I show relative homogeneous distribution of molybdenum mass 98 within molybdenite grains. D,H and L show homogeneous distribution of common Os mass in molybdenite grains. Bright nano-inclusions are possibly rheniite ReS2. Full size image Mass maps, which show the distribution of both Re and Os isotopes, display an identical oscillatory pattern as Re Fig. This indicates that there is no isotopic decoupling between Re and Os, as previously suggested 11 Further, Os distribution maps show that common Os is present in very low abundance in the studied material.

Images show the homogeneous distribution of 98Mo isotope A,Ethe oscillatory zoning of Re B,Fand the presence of Ag-rich Re-Os bearing nano- to micron-size particles attached to B—D or within interstitial space of molybdenite F—H. Discussion Distribution of noble metals in ore minerals Heterogeneous Re distribution, including oscillatory zoning, and Re-rich overgrowths in molybdenite crystals have been previously reported 151617although the mechanisms controlling the Re distribution remain poorly understood. Oscillatory mineral zonation has been attributed to intrinsic, extrinsic or epigenetic processes 181920whereas overgrowths are clearly associated with changes in the Re budget of the mineralizing fluid.

The presence of Re-Os nanoparticles in molybdenite questions the incorporation of noble metals in sulphide minerals. The controls of incorporation and concentration of noble metals Os, Ir, Ru, Rh, Pt, Pd, Au, Ag and Re in hydrothermal ore minerals remain uncertain mainly because they are usually present at very low concentrations low ppt levels and occur at the micro to nano-scale. Such properties impose even greater difficulties when investigating isotope geochemistry. Most studies on the distribution of noble metals have focused on magmatic ore deposits where discrete micron-size inclusions of platinum-group elements PGEs have been observed in chromite grains from ophiolites, layered mafic intrusions and in base metal sulphides from Cu-Ni magmatic deposits 21 These works showed that PGEs are present in solid solution and as microparticles within sulphides Similarly, studies on the distribution of gold and silver in hydrothermal sulphides show that these elements can be present in solid solution and as micro- to nano-size particles 2324 The incorporation of noble metals and perhaps some other trace elements within mineral phases appears to be mostly heterogeneous, in several cases forming nanoparticles or nanoclusters, which can affect the accurate measurement of elemental concentrations or isotopic signatures by microbeam techniques.

Controls on the incorporation of Re and Os in molybdenite Rhenium and Os are heterogeneously distributed reflecting changes in the composition of the hydrothermal fluid. Direct observation of i oscillatory isotopic and chemical nano-zoning of rhenium, ii Re-rich overgrowths, and iii presence of common Os in domains in molybdenite grains and in the associated nanoparticles provide an explanation for spurious Re-Os ages obtained by laser ablation ICP-MS. Alternating incorporation of Re into molybdenite during growth Fig. These factors can also control the formation of Os-Re nanoparticles by Ostwald ripening, as seen for example for Au in arsenian pyrite and Os-Ir alloys in laurite 23 Overgrowths are formed by a later hydrothermal event and in some cases are not evident under petrographic inspection.

Overgrowths in molybdenite crystals might be a common phenomenon in ore deposits caused by superimposed hydrothermal events. Whole mineral age determination may overcome this distribution-related limitation, if in fact the mineral has formed within a restricted time frame of less than a few hundred thousand years, and no disturbance from isotopically different areas are present in the studied material.

McCutcheon interpreted the contact with Silurian sandstone of the Flume Arsenopyriye Formation to the south as an unconformity crystas truncation of the basalt unit along strike to the southwest suggests that the boundary is a fault Fig. Contacts with the underlying Hoyt Station Basalt and arsenopjrite South Oromocto Andesite are not exposed but are assumed to be conformable or disconformable McCutcheon ; McCutcheon et al. Figure Field photograph of quartz-feldspar arsneopyrite tuff of the Rothea Formation. Display large image oss Figure 10 27 The South Oromocto Andesite is composed of plagioclase phyric and amygdaloidal crystaps flows, and brecciated andesite McCutcheon Upper and lower contacts arsenpoyrite not exposed at surface but a arsenopyrkte section suggests arxenopyrite conformable relationship with mudstones of both the underlying Rothea Formation and overlying Carrow Formation McCutcheon An unwelded pumiceous lapilli tuff occurs in the lower part of the section and a basalt flow occurs in the upper part.

A U-Pb date on zircon from the pumiceous tuff in drill core about m beneath the spore locality yielded an age of Drill core from its type area indicates that the Bailey Rock Rhyolite conformably overlies sedimentary rocks of the Carrow Formation and hence represents the youngest part of the Exocaldera Sequence. Its contact with overlying conglomerate of the Kleef Formation Late Caldera-Fill Sequence can be approached to within a few metres and appears to be concordant. Field photographs of the Bailey Rock rhyolite: The mineralization occurs in two main zones North and Fire Tower zones that are located approximately 1 km apart with a smaller third zone Saddle Zone located halfway in between Figs.

The North Zone consists predominantly of indium-bearing tin-zinc mineralization with lesser earlier tungsten-molybdenum mineralization; subzones include the Upper Deep, Deep Tin, Contact Crest, Contact Flank, and Endogranitic Fig. The Saddle Zone consists of tin mineralization underlying a topographically depressed area between the North and Fire Tower zones. At the North Zone, the NI resource estimate consists of 12, indicated tonnes averaging 0. Fire Tower Zone 33 At the Fire Tower Zone, wolframite and molybdenite are the most abundant ore minerals and are accompanied by minor native bismuth and bismuthinite.

The tungstenmolybdenum mineralization occurs primarily as disseminations within breccia pipes and in stockworks and veinlets within the upper part of a fine-grained granite intrusion Granite I and volcanic host rocks of the Little Mount Pleasant Formation. The host rocks have been pervasively greisenized to an assemblage of quartz, topaz, and fluorite. Minor molybdenite along fractures within the fine-grained granite underlying the main mineralized zone at depth is thought to represent the latest mineralization related to tungsten-molybdenum deposition during final crystallization of Granite I Kooiman et al.

The indium-bearing, tin-zinc mineralization Re os dating arsenopyrite crystals irregularly distributed in cross-cutting veins and breccias throughout the Fire Tower Zone and is generally associated with altered granite porphyry dikes of Granite II Sinclair et al. Cassiterite and wolframite are present as tiny disseminated grains with local aggregates of wolframite up to several centimetres long. Variable proportions of base-metal sulphides associated with the tin lodes consist mainly of indium-rich sphalerite, chalcopyrite, and galena. Minerals comprising the second stage of mineralization tin and base-metals only had type I inclusions.

The variation in inclusion types associated with the tungsten-molybdenum mineralization was explained to be a result of adiabatic cooling of the Granite I-related orthomagmatic fluids released into the country rocks after overpressurization within the magma chamber, and subsequent mixing with groundwater. Fluids from which the tungsten-molybdenum mineralization was derived were found to have high salinities 30—60 equivalent wt. North Zone 37 The North Zone consists of Re os dating arsenopyrite crystals and stockwork-hosted tungsten-molybdenum mineralization similar to that associated with Granite I at the Fire Tower Zone.

However, these zones of tungsten-molybdenum mineralization are discontinuous, smaller in size and of less significance than the tin-zinc lodes that overprint them. Several distinct tinindium-bearing subzones are recognized within the North Zone associated with banded dykes of Granite II Fig. Ore minerals include fine-grained cassiterite, arsenopyrite, loellingite, indium-bearing sphalerite, and chalcopyrite with lesser amounts of stannite, pyrite, marcasite, galena, wolframite, molybdenite, tennantite, chalcocite, bornite, native bismuth, bismuthinite, and wittichenite Sinclair et al. These minerals occur in veins and veinlets, infilling the matrix of breccias, filling vugs, and as disseminated grains and clusters of grains replacing altered host rocks.

Fluorite and chlorite are the common gangue minerals associated with the sulphides whereas the associated greisen-type alteration assemblage comprises quartz, sericite, topaz, and fluorite Sinclair et al. By omitting one of the anomalous samples, Kooiman et al. This inference is based on the interpretation that the McDougall Brook Granitic Suite, which predates the mineralizing events, grades into a supposed intrusive equivalent of the Bailey Rock Rhyolite within the Intracaldera Sequence McCutcheon ; McCutcheon et al. Specifically, it is the southern pluton of the McDougall Brook Granitic Suite that is known to pre-date the mineralization in the North Zone, and it is the northern pluton that has been interpreted to grade into the Bailey Rock Rhyolite.

Methodology 40 The Re-Os geochronometer can help to constrain genetic deposit models as it provides a reliable method to directly date a number of mineralized systems, including those that have been variably deformed and metamorphosed Stein et al. Rhenium and osmium are commonly substituted into the sulphide structure because they are chalcophile-siderophile elements. Although this method of dating can be used on several sulphide minerals i. Therefore, all of the Os measured is generated from the decay of Re, and the precise isotopic age of the molybdenite can be determined using the measurement of Re and Os concentrations within the molybdenite combined with the decay constant of Re.

Analytical procedures 41 Two samples of molybdenite were collected from drill hole AM Fig. This drill hole intersected tungsten-molybdenum mineralization in the southern portion of the FTZ West. The core consists of alternating units of altered Granite I and breccia that contains abundant purple and green fluorite with minor molybdenite, arsenopyrite, and wolframite mineralization. Sample KT was collected from the interval between Photographs of molybdenite samples drill hole AM sent for Re-Os dating: Display large image of Figure 12 42 Each sample was subjected to metal-free crushing after which the molybdenite was separated out by gravity and magnetic concentration methods.

Methods used to analyze the molybdenite are described in detail by Selby and Creaser and Markey et al. The Re and Os concentrations in molybdenite were determined at the University of Alberta Radiogenic Isotope Facility by isotope dilution mass spectrometry using Carius-tube digestion, solvent extraction, anion exchange chromatography, and negative thermal ionization mass spectrometry techniques. A mixed double spike containing known amounts of isotopically enriched Re, Os, and Os analysis was used. Isotopic analysis used a ThermoScientific Triton mass spectrometer with a Faraday collector. Total procedural blanks for Re and Os are less than 3 picograms and 2 picograms, respectively, which are insignificant for the Re and Os concentrations in molybdenite.

For this sample over a period of two years, an average Re-Os date of This Re-Os age date is indistinguishable from that reported by Markey et al. Results 43 Results of the Re-Os analysis of the two samples are given in Table 1. The Re-Os molybdenite ages establish the time of deposition of tungsten-molybdenum mineralization at Mount Pleasant as ca. This age is older than the inferred age for mineralization based on previous results obtained from the Bailey Rock Rhyolite Tucker et al. The new date also provides a minimum age of emplacement for the southern pluton of the McDougall Brook Granitic Suite as it pre-dated intrusion of Granite I.

Table 1. Results of Re-Os analyses of molybdenite in the Mount Pleasant samples. We have examined this poorly exposed contact and agree with van de Poll that it is more likely a fault. Moreover, we contend that this fault separates volcanic rocks of the Seelys Formation rather than the Bailey Rock Rhyolite of the Intracaldera Sequence in the southwest from sedimentary rocks of the Carrow Formation of the Exocaldera Sequence to the northeast Fig. More geochronological work will be required to better define the timing of emplacement of these two plutons.

This evidence includes the presence of biotite phenocryts in clasts of tuff in the Scoullar Mountain Formation and in tuff of the Rothea Formation, and the presence of andesitic flows in both the Scoullar Mountain and South Oromocto formations. The main stages of caldera collapse coincided with the eruption of voluminous gas-rich pyroclastic deposits Little Mount Pleasant and Seelys formations in the Intracaldera Sequence cf. Cole et al. It should be noted that the length of the time gap separating these two periods of magmatic activity suggests that the Mount Pleasant Granitic Suite and Bailey Rock Rhyolite were sourced from discrete calderas within a long-lived nested caldera complex cf.

It is further suggested that the flow-banded rhyolitic lavas of the Big Scott Mountain Formation Fig. Filling of this relatively young caldera is marked by deposition of coarse redbeds of the Kleef Formation, which contain volcanic blocks derived from the Big Scott Mountain Formation Figs. Thin interbeds of basalt and lithic tuff in the Kleef Formation represent the end stage of volcanism at Mount Pleasant. The new geochronological results indicate inconsistency in the previous interpretation of the Mount Pleasant stratigraphy; this inconsistency might not have otherwise been questioned given that many of the felsic volcanic rocks and subvolcanic porphyritic granites closely resemble one another in the field.

These volcanic rocks are in faulted contact with sedimentary rocks of the Carrow Formation of the Exocaldera Sequence to the north and their contact with the McDougall Brook Granitic Suite to the south is not exposed. Thus there does not appear to be any evidence to support a gradational relationship between the McDougall Brook Granitic Suite and the Bailey Rock Rhyolite. Andy Kerr and Neil Rogers provided insightful reviews of the submitted manuscript. Terry Leonard helped prepare some of the figures. A chemical and isotopic study of the age, petrogenesis and magmatic evolution of the Mount Pleasant caldera complex, New Brunswick.

Unpublished Ph. Atkinson, J. Geology of Mount Pleasant tungsten, New Brunswick.

Whole offer age determination may get this site-related limitation, if in crypto the mineral has passed within a restricted attraction record of less than a few hundred hundred years, and no idea from isotopically folding areas are present in the desired material. McCutcheon, S.

Canadian Mining Journal,pp. Bevier, M. U-Pb geochronologic crytsals of igneous rocks in N. Edited by S. Black, P. Tin, tungsten, molybdenum mineralization, Mount Pleasant area, New Brunswick. Canadian Mining Journal, 82, pp. Bradley, D. Subsidence in Late Paleozoic basins in the northern Appalachians. Tectonics, 1, pp.

Crystals Re arsenopyrite os dating

Calderas and caldera structures: Arsenopyite Science Review, 69, arsenopyritr. Genesis of the Mt. Pleasant tungstenmolybdenum—bismuth deposit, New Brunswick. Dunbar, P. Fyffe, L. Petrochemistry and tectonic significance datong Carboniferous volcanic rocks in New Brunswick. Crhstals Journal of Earth Sciences, 23, pp. A review of Proterozoic to Early Paleozoic lithotectonic terranes in the northeastern Appalachian orogen of New Brunswick, Canada, and their tectonic evolution during Penobscot, Taconic, Salinic, and Acadian orogenesis. Atlantic Geology, 47, pp. Gemmell, D. Carboniferous volcanic and sedimentary rocks of the Mount Pleasant caldera and Hoyt appendage, New Brunswick.

Unpublished M. Harris, F. Volcanic rocks of the Sunday Lake area, New Brunswick. Hosking, K. Geology, mineralogy and paragenesis of the Mount Pleasant tin deposits. Canadian Mining Journal, 84, pp. Hunt, P. A compilation of K-Ar ages, Report New Brunswick. In Radiogenic Age and Isotopic Studies: Report 3. Geological Survey of Canada, Paperpp. Inverno, C. Petrochemical discrimination of evolved granitic intrusions associated with Mount Pleasant deposits, New Brunswick, Canada. Kirkham, R. Comb quartz layers in felsic intrusions and their relationship to porphyry deposits. Edited by R. Taylor and D. Kooiman, G. Porphyry tungsten—molybdenum orebodies, polymetallic veins and replacement bodies, and tin-bearing greisen zones in the Fire Tower Zone, Mount Pleasant, New Brunswick.

Economic Geology, 81, pp. Talanta, 45, pp.


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