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    François Farges


    François Farges


    This paper was published in a special issue of the journal entitled 'Rare-Element Geochemistry and Mineral Deposits.' The paper selected - one of four contributed to this special issue by Farges - summarizes XAFS spectroscopic data obtained revealing new molecular-level information relevant to the nature of molybdenum bonding in melts and the transport of chalcophile elements in late-stage magmas resulting in economic concentrations of such elements. In a series of novel and difficult experiments conducted over many years, the authors determined the local structure around molybdenum over a wide range of physical and chemical parameters using synthesized glasses. This work has shown that molybdate moieties are the dominant form of molybdenum in anhydrous melts down to very low oxygen fugacities, a finding not reported previously. Furthermore, this work shows that the addition of H2O and halogens has a limited affect on the local structure of molybdenum. In contrast, the addition of sulfur is shown to be significant, with the formation of thio-oxo-molybdate moieties. It is this latter complex that disconnects molybdenum within highly polymerized melts, such as those that host porphyry moly systems, thus making molybdenum mobile and, significantly, capable of being transported in a fluid phase. The results of these meticulous experiments reconcile some lingering issues concerning molybdenum. First, why the sulfur-bearing form, molybdenite, predominates in relatively oxidizing subvolcanic environments where it forms ore concentrations; and second, why large melt/fluid partition coefficients have long been noted in melt-H2O vapor experiments. Thus, the findings reported in this paper not only resolve the molecular-scale structural nature of molybdenum under a series of controlled experimental conditions, but importantly also advance our understanding of the geochemical controls of molybdenum mineralization in porphyry systems.


    © 2006 Mineralogical Association of CanadaLast update 2014-02-05