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Geochimica et Cosmochimica Acta 73 (2009) 3381–3392 www.elsevier.com/locate/gca
The partitioning of molybdenum(VI) between aqueous liquid and vapour at temperatures up to 370 °C Kirsten U. Rempel *, Anthony E. Williams-Jones, Artas A. Migdisov McGill University, 3450 University St., Montreal, Que., Canada H3A 2A7 Received 14 April 2008; accepted in revised form 9 March 2009; available online 18 March 2009
Abstract We have conducted experiments to evaluate the vapour–liquid fractionation of Mo(VI) in the system MoO3–NH3–H2O at 300–370 °C and saturated vapour pressure, using a two-chamber autoclave that allows separate trapping of the vapour and liquid. The measured total Mo concentrations in each phase were used to calculate a distribution coefficient, K V=L D , which increases as the density of the vapour approaches that of the liquid, and is greater than one for pH 6 4. Molybdenum speciation in the vapour is described by a single complex, MoO3H2O. By contrast, thermodynamic modeling of the distribution of Mo species in the liquid indicates that bimolybdate (HMoO4) is the dominant aqueous species at the conditions of our experiments, and that molybdate (MoO42) and molybdic acid (H2MoO40) are present in smaller quantities. As vapour–liquid fractionation occurs between neutral species, it is governed by the reaction H2MoO40(aq) = MoO3 H2O(g). Fractionation is therefore controlled by the concentration of H2MoO40 in the liquid, which increases with increasing temperature and decreasing pH. Owing to the pH dependence of K V=L D , it cannot be used to describe Mo fractionation in aqueous vapour–liquid systems with compositions different than those of this study. We have therefore calculated a composition-independent (Henry’s Law) constant, K V=L H , for each experimental point, using the measured total Mo concentration in the vapour and the modeled concentration of H2MoO40 in the liquid. This constant may be applied to aqueous vapour–liquid systems of known liquid composition to estimate the concentration of Mo in a vapour for which little chemical information is available, and thereby supplement the available fractionation data for natural porphyry-forming systems. The results of this study demonstrate that at conditions typical of natural porphyry ore-forming systems, a significant amount of molybdenum fractionates into the vapour over the liquid, and the vapour may transport quantities of Mo in excess of that in the liquid at pH conditions below those of the muscovite–microcline reaction boundary. Ó 2009 Elsevier Ltd. All rights reserved.
1. INTRODUCTION In magmatic-hydrothermal systems that form ore deposits, the magmatic aqueous fluid is frequently composed of a mixture of liquid and vapour, both of which may be important solvents for metals and other ore-forming components (e.g., Henley and McNabb, 1978; Cline and Bodnar, 1994). The potential for a combination of liquid- and vapourphase transport in such systems has led to considerable interest in the vapour–liquid fractionation of ore metals, which is critical to ore deposit modeling. To this end, a *
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[email protected] (K.U. Rempel).
0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.03.004
body of metal fractionation data has been collected by way of experiments conducted in hydrothermal autoclaves, as well as analyses of natural and synthetic fluid inclusions, geysers and hot springs (e.g., Smith et al., 1987; Heinrich et al., 1999; Ulrich et al., 1999; Audetat et al., 2000; Palmer et al., 1999, 2000; Shmulovich et al., 2002; Audetat and Pettke, 2003; Rusk et al., 2004; Pokrovski et al., 2002, 2005, 2008; Simon et al., 2005, 2006, 2008; Cauzid et al., 2007; Klemm et al., 2008; Nagaseki and Hayashi, 2008). These studies have led to an increased understanding of the fractionation of many elements, and suggest that metals such as Cu, Au and As may partition preferentially into the vapour over the liquid. However, very few data have been produced for the vapour–liquid distribution of molybdenum(VI),
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Table 1 Average Mo concentrations reported in the literature for coexisting liquid- and vapour-rich fluid inclusions from porphyry Mo deposits, determined from LA ICP-MS analyses. ID, L and V indicate intermediate density, liquid (brine) and vapour inclusions. Th is the homogenization temperature determined from microthermometric analyses of brine inclusions, and n is the number of measurements. Concentrations measured by Ulrich et al. (1999) were averaged by the authors, and n was not given. Th, °C
Deposit/pluton
Alumbrera, porphyry Cu–Mo Grasberg, porphyry Cu–Mo Questa, porphyry Mo Rito del Medio, barren pluton
550–650 >600 270–430 450
V=L
Mo, ppm ID (n)
L (n)
V (n)
– – 43 (10) 89 ± 37 (2)
70 ± 60 600 ± 120 204 (7) 760 ± 60 (1)
18 MX) grade water. The amount of solution added to the autoclave (20–30 mL) was sufficient to ensure saturation with liquid, as determined using the PVT properties of water (Kestin et al., 1984). The atmospheric-gas composition of the autoclave headspace provided an oxygen fugacity much greater than that required to oxidize Mo(IV) to Mo(VI) at the conditions of the experiments (1021 bar at 350 °C), ensuring that all molybdenum in the experimental system was present as Mo(VI). Therefore, all mentions of Mo in the following text refer to the latter oxidation state. The partitioning of electrolytes between aqueous liquid and vapour typically has very fast reaction kinetics (minutes to hours; Sourirajan and Kennedy, 1962; Bischoff et al., 1986; Pokrovski et al., 2005), so our experiments were run for durations of 20–40 h. In order to obtain the true Mo(VI) concentrations in the liquid and vapour phases, several corrections were applied to the measured concentrations in the samples retrieved from the autoclave. First, as described above, the masses of Mo from the acid solutions used to wash the autoclave walls were added to the masses of Mo in the liquid and vapour. A further correction to the vapour concentration was required because of partial loss of vapour in some experiments, caused by failure of the graphite o-rings around the autoclave valve when it was closed just before quenching. Owing to small temperature gradients within the valve, the molybdenum in the lost vapour was precipitated within the inner cell, as evidenced by higher concentrations of Mo in samples with greater vapour loss. Thus, the true concentration of Mo in the vapour could be calculated from the total mass of Mo and of vapour in the inner cell before the valve was closed. The latter quantity was calculated using the PVT properties of water (Steam Tables; Kestin et al., 1984) and the volume of the inner cell. To account for fluid lost from the outer cell, a similar correction was applied to the measured concentration of Mo in the liquid. Finally, although a component of vapour was present in the
headspace of the outer cell, the mass of vapour was insignificant compared to that of the liquid, and so no correction was needed to account for the contribution of the vapour to the apparent Mo concentration of the liquid (
