EXPERIMENTAL DETERMINATION OF FERBERITE SOLUBILITY IN THE KCL-HCL-H2O SYSTEM AT 500° C AND 400-1000 BAR, AND THE KFS-MU-QTZ AND NI-NIO BUFFERS

Experiments have been performed in (0.7-8.9)mKCl- (0.01-0.1)m HCl solutions at 500^o C, pressures of 400, 500 and 1000 bar, fO₂ values corresponding to the Ni-NiO buffer (NNO) and acidity controlled by K-feldspar-muscovite-quartz assemblage (KMQ).  Under these conditions synthetic fine-grained crystalline ferberite (~10-30 µm) dissolves incongruently and is accompanied by precipitation of potassium tungstate bronze phases, K_xWO₃ (x=0.2-0.3).  Changes in the W/Fe mol ratios in the aqueous phase were also observed.  Aqueous W and Fe concentration values are a function of the total mKCl in the system and the initial mHCl.  Representative molal solubility values of W and Fe range from 0.002-0.05 in 0.7 mKCl to 0.02-0.15 in 8.9 mKCl.  An analysis of experimental results indicate monomer tungsten species are dominant in homogenous 0.7-3.0 mKCl solutions at 1000 bars whereas dimer complexes of tungsten are dominant in 4.0-8.9 mKCl and in the two-phase region at 400 and 500 bars.  The relationship between solubility and pressure is yet to be defined.  The data at 400 and 500 bar represent the apparent solubility values since two conjugate fluids of different density co-exist.  Estimated ferberite solubility (in wt. %) in the liquid (L) and vapor (V) phases are S_Ferb (500^oC, 500 bar)=0.21 (C_V=0.96 wt. %), 2.03 (C_L=34.59) and S_Ferb (500^oC, 400 bar)=0.5 (C_V =0.32), 2.57 (C_L=58.25), where C_L and C_V are KCl concentrations in liquid and vapor, respectively.  The results suggest a large bulk solubility value (ore-bearing capacity) in the dense, salt-rich component of the two-phase fluid.