Modeling of the transport and deposition of tungsten in the scheelite-bearing calc-silicate gneisses of the Montagne Noire,France |
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| Authors: | François Gibert Bernard Moine Jacques Schott Jean-Louis Dandurand |
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| Institution: | (1) Laboratoires de Minéralogie et de Géochimie, UA 067 du CNRS, Université Paul Sabatier, 39, allées Jules Guesde, F-31000 Toulouse, France |
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| Abstract: | Scheelite-bearing calc-silicate gneisses (CSG) oceur in the Montagne Noire within a series of dominant micaschists. Detailed petrographical and mineralogical studies reveal three successive stages of metamorphism and hydrothermal alteration: (1) stage 1, a regional metamorphism at 550°C and 4.5 kb where no mineralization is formed; (2) stage 2a, a hydrothermal alteration at 500 to 450°C and 4 to 3 kb which is characterized by an intense sericitization of feldpars and deposition of Sn in Sn-bearing cale-silicates; and (3) stage 2b, a hydrothermal alteration characterized by the crystallization of idocrasegrossular in CSG with concomittant precipitation of scheelite. Tungsten was transported through the micaschist environment and deposited as scheelite only in the CSG of stage 2b at relatively low pressures. To characterize the mechanism of tungsten transport, tungsten speciation at high P-T and scheelite solubility in aqueous solations buffered by the CSG and by the micaschists assemblages were calculated. It was found that H2WO
0
4
, HWO
-
4
and WO
2-
4
are the dominant tungsten aqueous species in H2O–NaCl (one molal) solutions at 500°C and 2–4 kb. Calculations also indicate that scheelite deposition is controlled by decreasing pressure and increasing activity of aqueous calcium in this system. This is consistent with the petrographical and mineralogical observations. The consequences of the presence of volatiles (N2, CH4, CO2) in the regional fluids were examined by determining the effect of N2 on tungsten speciation and scheelite solubility. The addition of N2 (up to 10 mol%) to the mineralizing fluids results in a marked increase in H2WO
0
4
and HWO
-
4
concentrations relative to WO
2-
4
and in a large decrease of scheelite solubility. This mechanism favours scheelite precipitation and accounts for the commonly observed association of W (and Sn) deposits with graphitic series generating mixed volatiles fluids. |
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