Delta97/95Mo variation in molybdenites : a link with mineralizing prossesses ?

Abstract : Molybdenum (Mo) isotopic compositions are usually studied to reconstruct (paleo-) redox conditions in ocean (e.g., Siebert et al., 2003). Some studies also focus on isotopic compositions of Mo in molybdenite in order to decipher a possible genetic link between isotopic variations and mineralizing processes. This is also the aim of our work based on a wide molybdenite databank. We propose here a comparison of δ97/95Mo of 101 molybdenite samples from different localities, different types of occurrences and different ages. This databank of 101 molybdenites, presently analyzed, represents various types of mineralization as samples from 5 skarns, 7 porphyry deposits, 9 pegmatites, 5 granites, 2 greisens, 7 perigranitic veins and 9 alpine-type fissure veins have been studied. Molybdenite sample ages vary from 5Ma to 2.7Ga. The Mo isotopic composition has been determined on molybdenites using a MC-ICP-MS Neptune after aqua regia dissolution and adjustment to [Mo] = 1µg.g-1. Mass bias was corrected by using Zr as dopant and standard-sample-standard bracketing. The δ97/95Mo ratios have been normalized to NIST 3134. The external reproducibility is 0.07‰ (2σ). The overall range of the δ97/95Mo ratio in the 101 molybdenite samples varies from -0.58 to 0.89‰. The δ97/95Mo ratio is higher for the molybdenites formed in alpine-type fissure veins, greisens and perigranitic veins than for the molybdenites issued from granites, pegmatites, porphyry deposits and skarns. The crystallization temperature can explain this difference as alpine-type fissure veins, greisens and perigranitic veins crystallize at lower temperatures than the other types of deposits. For some occurrences, the δ97/95Mo has been performed on several molybdenite samples and variability at occurrence scale has been observed. For example, in Azegour skarn (Morocco), the δ97/95Mo varies from -0.40 to 0.32‰ (n= 29) and in "Ravin de la Ruine" alpine-type fissure veins (France) from -0.08 to 0.77‰ (n=3). As noted by Hannah et al. (2007), no correlations appear between δ97/95Mo and age of the deposits. Further Re-Os dating, Pb and S isotopic compositions in particular deposits (e.g. Azegour skarn) will help to decipher the origins of these variations. References 1. C. Siebert, T.F. Nägler, F. von Blanckenburg and J.D. Kramers, "Molybdenum isotope records as a potential new proxy for paleoceanography", Earth and Planetary Science Letters, 211, pp.159-171, 2003. 2. J.L. Hannah, H.J. Stein, M.E. Wieser, J.R. de Laeter and M.D. Varner, "Molybdenum isotope variations in molybdenite: Vapor transport and Rayleigh fractionation of Mo", Geology, 35-8, pp.703-706, 2007.
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Noémie Breillat, Catherine Guerrot, Philippe Négrel, Eric Marcoux. Delta97/95Mo variation in molybdenites : a link with mineralizing prossesses ?. 10th Applied Isotope Geochemistry Conference, Sep 2013, Budapest, Hungary. ⟨hal-00835823⟩

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