Geochemistry of Hydrothermal Molybdenite Inclusions: Linking Ore-Stage Pyrite with Accessory Minerals (original) (raw)
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Diffusion of osmium in pyrrhotite and pyrite: implications for closure of the Re–Os isotopic system
Earth and Planetary Science Letters, 2000
In order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization, we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite. Experiments consisted of either (1) isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered Os-bearing Fe-sulfide or (2)`relaxation' of initially high near-surface osmium concentrations produced in the latter experiments (pyrite only). Osmium penetration into samples was characterized by depth profiling using Rutherford backscattering spectroscopy (RBS) (pyrite) or electron microprobe analyses across sectioned run products (pyrrhotite). Results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces, with the extent of penetration uncorrelated with run duration. Images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near-surface occurred by solute uptake during step growth and not by volume diffusion. Prolonged (1000 h)`relaxation' experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for RBS, a maximum diffusion coefficient of 2.5U10 323 m 2 /s at 500³C was calculated. Experiments involving pyrrhotite over the temperature range of 950^1100³C showed extensive osmium uptake and osmium concentration gradients that conform with Fickian diffusion behavior. We found that pyrrhotite Fe/S could be varied by changes in the composition of the starting material and osmium source and over the range of Fe/S produced in experiments (molar Fe/S = 0.83^0.90), we observed no systematic variation in the osmium diffusion coefficient. Diffusion coefficients measured parallel to the a crystallographic axis were on average V1.4U higher than values measured parallel to c and regression of the c-axis data yielded the Arrhenius relation:
Re–Os isotopes of base metal porphyry deposits
This study uses Re-Os isotopes from sulfide minerals (molybdenite, pyrite, chalcopyrite, bomite, sphalerite, arsenopyrite, and galena) and magnetite from high (400-500°C) and low (OOO^C) temperature mineralization assemblages associated with the intrusion of igneous rocks to trace the source of Os and by inference the other metals. The source of Os is used as a proxy for the source of copper and gold in these systems. Concentrations of Re-Os for all sulfides except molybdenite in various deposit types vary from 0.005-2 ppb Os and 0.120-500 ppb Re. Initial '*^Os/***Os ratios derived from isochron plots ranges from 0.15-50. These data show interesting relationships among deposits of different styles within the district, region, and worldwide scales. On the district-deposit scale, Re-Os isotopes illustrate a complex behavior for the source of Os and by inference other transition and noble metals. The source of Os can be linked to one intrusive event, multiple intrusive events, and the sedimentary rocks into which the magmas intersect. On a regional scale, in the Andean Cordillera, Re-Os isotopes reveal a correspondence between the initial '*^Os/***Os ratio from sulfides of the ore deposit and amount of copper present in porphyry copper deposits. Eleven deposits sampled from difTerent crustal blocks and different mineralization ages form a trend in which deposits that contain large amounts of copper have less radiogenic "^Os/'^'Os initial ratios, suggesting a greater mantle component. Smaller deposits have more radiogenic Os suggesting greater crustal components. On a global scale, the significant observation for the deposits that form in arc environments is that all of the calculated initial '*^Os/'**Os ratios are greater then the Chapter 2-Re-Os molybdenite ages for mineralization and source information from low concentration sulfides for the iron oxide-rich Cu-Au ore at Candelaria, Chile
Geochimica et Cosmochimica Acta, 2004
We present a detailed study of Re-Os age determinations for eight natural molybdenite samples of like polytype (2H), spanning a range of age, natural grain size and deposit type. The focus of the study is to critically evaluate the effects of sampling, sample preparation and aliquant size on the accuracy and reproducibility of Re-Os ages for these molybdenite samples. We find that for some molybdenite samples, analysis of small sample aliquants (Ͻ20 mg) may not yield accurate or reproducible Re-Os ages, whereas analysis of larger aliquants from the same mineral separate do yield reproducible Re-Os dates. Such an observation is best explained if Re and 187 Os are internally decoupled within molybdenite grains. This finding is supported from spot analyses by laser ablation MC-ICP-MS analyses presented here and is consistent with previously published observations. The degree of decoupling between Re and 187 Os appears to increase both as a function of increasing grain size, and increasing age of molybdenite. From detailed dating of individual molybdenite mineral separates, we provide approximate minimum aliquant amounts required for reproducible Re-Os age dating, as a function of molybdenite age and grain size. Geologically younger, naturally finegrained molybdenite samples appear to show little Re and 187 Os decoupling, and reproducible ages can be determined from some samples with as little as 1 mg of aliquant. Geologically old, and coarse-grained molybdenite samples may require as much as 40 mg of aliquant from a much larger mineral separate to overcome Re and 187 Os decoupling. The mechanism(s) of Re and 187 Os decoupling within molybdenite is not constrained by this results of this study, but the observation that the degree of decoupling increases with grain size (distance) and age (time/geologic history) may suggest primary diffusive control. Assuming that Re and 187 Os decoupling in molybdenite results primarily from diffusion of 187 Os, apparent diffusion coefficients are calculated (D ϭ x 2 /t). Estimates of D for Os made in this way range from 2.8 ϫ 10 Ϫ26 to 2.1 ϫ 10 Ϫ21 m 2 /s, which are broadly similar to experimentally derived diffusion coefficients for Os in Fe-sulfide minerals and for Re in molybdenite at temperatures Ͻ500°C.
Variation of Mo isotopes from molybdenite in high-temperature hydrothermal ore deposits
Mineralium Deposita, 2010
Measurable molybdenum isotope fractionation in molybdenites from different ore deposits through time provides insights into ore genesis and a new technique to identify open-system behavior of Re-Os in molybdenites. Molybdenite samples from six porphyry copper deposits, one epithermal polymetallic vein deposit, four skarns, and three Fe-oxide Cu-Au deposits were analyzed. The δ 97 Mo‰ (where δ 97 Mo = [( 97 Mo/ 95 Mosample)/( 97 Mo/ 95 MoICPMS standard)]-1 x 1000 ) for all samples varied from 1.34±0.09‰ to −0.26±0.04‰. This is the largest molybdenum isotopic variation in molybdenite from high-temperature ore deposits recorded to date. δ 97 Mo‰ of molybdenite varies as a function of the deposit type and the rhenium and osmium concentrations of the samples. Isotope values for Mo also vary within the individual deposits. In general, molybdenites from porphyry copper deposits have the lightest values averaging 0.07±0.23‰ (1σ). Molybdenites from the other deposit types average 0.49±0.26‰ (1σ). The variations could be related to the fractionation of Mo into different mineral phases during the ore-forming processes. A comparison of the Mo isotope ratios and the Re-Os ages obtained from the same aliquot may possess a geochronological evaluation tool. Samples that yielded robust ages have different Mo isotopic compositions in comparison to samples that yielded geologically unreasonable ages. Another observed relationship between the Re-Os and Mo isotope data reveals a weak correspondence between Re concentration and Mo isotope composition. Molybdenites with higher concentrations of Re correspond to lighter Mo isotope values.
Geochimica et Cosmochimica Acta, 1993
Rhenium is concentrated mainly in molybdenite (MoSz) and occurs as a major cation in only a few rare minerals. This affinity makes molybdenite an ideal mineral for Re-0s geochronometry, but research on the behavior of Re in molybdenite is limited. Infrared microscope, XRD, back-scattered electron (BSE), and microprobe techniques have been used in this study to document Re behavior in molybdenite affected by hypogene and near-surface processes. In the hypogene environment, both 3R and 2H molybdenite can experience Re loss during hydrothermal alteration, which also causes increased infrared transparency ( IR) . Alteration at temperatures as low as -150°C can cause Re loss in the presence of advecting fluids, and will affect Re-0s dating if it occurs long after primary mineralization. Re loss in 3R and 2H molybdenite under supergene conditions does not increase IR transparency. Rhenium is not incorporated into supergene ferrimolybdite, but is enriched in K-Al-silicate intergrowths which may be illite( ?). These minute intergrowths are present in several samples and would not be detected in a simple optical examination. In the supergene environment elemental OS is stable, whereas Re is not. Rhenium in molybdenite may be removed by supergene fluids after some has decayed to '870s, causing erroneously old ages, or it may be adsorbed into the illite intergrowths, creating ages which are too young. In the weathering environment, Proterozoic molybdenites have altered to Re-enriched powellite, which can be detected using back-scattered electron imagery. Combined microprobe, XRD, BSE, and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating. These techniques are non-destructive and should be performed before any molybdenite is dated by the Re-0s system.
Trace elements and Sr-isotopes in some mantle-derived hydrous minerals and their significance
Geochimica et Cosmochimica Acta, 1978
New analyses of K, Rb, Sr and Ba contents and the 87Sr/86Sr ratios of eight amphiboles, one phlogopite, two diopsides, and one host alkalic basalt for an amphibole are reported: The samples are mostly inclusions in alkalic basalts and occur in association with peridotite inclusions. Two of the samples are from alpine-type peridotite bodies -one from the Etang de Lhers massif in the French Pyrenees and the other from the Finer0 massif in the Ivrea zone in northern-Italy.
Earth and Planetary Science Letters, 2000
Rhenium^osmium (Re^Os) isotope and elemental abundances have been obtained for primary mantle minerals, metasomatic phases, and a range of mantle rock types from xenoliths in recent volcanics in northern Tanzania. Re and Os abundances for sulphide and coexisting silicates in garnet lherzolites from Lashaine confirm that sulphide dominates the Os budget, but also show that Re is almost exclusively sited in silicate phases. Silicate minerals from two different samples yield 187 Re^1 88 Os ages of 15.4 þ 6.1 and 31.4 þ 6.3 Myr, respectively. Comparison with 232 Th^2 08 Pb (267.1 þ 4.4 Myr) 147 Sm^1 43 Nd (164 þ 18 Myr) and 87 Rb^8 7 Sr (in equilibrium at the present-day) ages for the same silicate minerals suggests differential closure between these isotope systems, and a closure temperature of v 670³C for the Re^Os system. Remarkably, sulphide inclusions were not affected by diffusional equilibration between the silicates, and preserve significantly older age information. Model calculations suggest that sulphide^silicate equilibration ceased some 200^300 Ma, and the Os isotope composition of the sulphide ( 187 Os/ 188 Os = 0.10432 þ 0.00013) suggests a minimum age of 3.4 Gyr. Most xenoliths possess Os isotope compositions that are less radiogenic than the present-day chondritic mantle indicating that they experienced Re-loss some time ago. Samples showing evidence for modal metasomatism have high Re concentrations and Re/Os ratios, but their relatively unradiogenic Os isotope compositions suggests that the metasomatism occurred recently, consistent with data for metasomatic vein minerals. In contrast, some dunites possess both high Re/Os ratios and radiogenic Os isotope compositions. These samples differ from those affected by modal metasomatism in having low Re and exceptionally low Os concentrations. These results provide quantitative constraints on the distribution of Re and Os amongst mantle minerals, highlight the potential of Re^Os isotope dating of sulphide inclusions for establishing the early history of mantle mineral assemblages, and demonstrate that mantle processes 0012-821X / 00 / $^see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 -8 2 1 X ( 0 0 ) 0 0 2 5 9 -4 Earth and Planetary Science Letters 183 www.elsevier.com/locate/epsl themselves (metasomatism and dunite formation) can significantly modify the Os isotope chemistry of mantle rocks. ß
Oxidative dissolution experiments were carried out on pyrite from multiple petrogenetic environments (hydro-thermal, sedimentary, and coal-related nodules) to investigate possible variations in the iron isotopic composition of pyrite and the products of pyrite dissolution. The experimental materials were leached under carefully controlled abiotic conditions, and a subset of leachates and starting materials from these experiments was analyzed for 56 Fe/ 54 Fe by multicollector ICP-MS. Bulk pyrite δ 56 Fe values (relative to IRMM-014) ranged from −0.1 to +1.3‰, with hydrothermal bulk pyrite values b+0.5‰ and those of coal and sedimentary nodular pyrite ≥+0.5‰, higher than most previously measured values for Phanerozoic sedimentary pyrite. We suggest that this reflects precipitation of coal pyrite from a high-δ 56 Fe continental source, such as Fe derived from dissolution of Fe(III) oxides. This interpretation is consistent with pyrite rare earth element (REE) patterns. This could allow differentiation of Fe contributed from coal-and shale-related pyrite at abandoned mine drainage sites. Leachates from oxidative dissolution of the pyrite at pH = 3 yielded, with few exceptions, δ 56 Fe values equal to or lower than those of the coexisting bulk pyrite, by up to about 1‰. These shifts are consistent in direction (but not magnitude) with equilibrium isotope fractionation predictions from theory, with possible second order effects from isotopic heterogeneity within individual natural pyrite samples.
Journal of Asian Earth Sciences, 2000
Magmatic pyrochlores from the Lueshe syenite±carbonatite complex from the northeastern part of Democratic Republic of Congo (ex-ZaõÈ re) are characterized by Ta/Nb ratios in an increasing order from pyroxenite, calcite-carbonatite (soÈ vite), silicate xenoliths (nodules) to syenite. Substitutions involving Nb, Ta, Ti and REE have been precisely described. Hydrothermal alteration of Lueshe pyrochlore involves the substitution of Na + +F À =VA+VY and Ca+O=VA+VY (VA=A-site vacancy and VY=Y-site vacancy). In calcite carbonatite, hydrothermal alteration of pyrochlore took place during and after the precipitation of ancylite-(Ce), strontianite, celestite, baryte and fayalite according to a¯uid composition of relatively low pH, a Na +, a Ca 2+ and a HF , and high a Sr 2+ and a LREE 3+. The supergene alteration is characterized by complete leaching of Na, Ca and F and partial incorporation of K, Ba, Sr and Ce resulting in the formation of kali-, bario-, strontio-and ceriopyrochlore respectively. The Na-poor pyrochlore may be an intermediate variety corresponding to an alteration stage between the hydrothermal and weathered pyrochlores. The IR spectroscopic study has indicated that the weathered pyrochlore is a hydrated variety containing two bands of OH vibration modes at 3413 and 1630 cm À1. During hydrothermal and supergene alterations, the cations at B-site remain relatively constant. The variable chemical compositions of the pyrochlores from the Lueshe complex represent geochemical memories of the dierent alteration conditions including the variation in the oxidation±reduction environment.