| Abstract |
Crustal abundance of tungsten lies between 1 and 2 ppm (Krauskopf. 1970), which is obviously lower than those of copper. lead. and zinc (e.g •• Turekian and Wedephol. 1961). Most of the analytical data on tungsten in granitic rocks show that the concentrations do not show any significant differences between those related to tungsten mineralization and those Unrelated (e.g .• Ivanova. 1963; Ivanova and Butsuzova, 1968; Shepherd et al., 1976). Newberry and Swanson (1986) stated that the bulk enrichment of tungsten does not appear to be prerequisite for the formation of scheelite skarn. Einaudi et al. (1981) summarized and characterized the physico-chemical conditions for the formation of tungsten-. copper-, and lead and zinc skarn deposits In the world. The differences point to the formation of tungsten skarn at deeper environment (1 to 3 kilobars) and higher temperature (600° to 400°C) than that of the base-metal skarn. As the pressure and temperatureof hydrothermal solution have a significant effect on mineral solubilities as well as the salt concentration. the difference can be considered as the cause for the enrichment of tungstencompared with the base metals in the deposits. Are there any other explanations for the localization and relative enrichment of tungsten compared with the other ore metals? Exact evaluations of tUngsten concentrations in the ore-forming solutions have not yet been carried out. There exists a possibility that the hydrothermal solutions for tungsten deposits are enriched in tungsten compared with the other ore metals. Judging from the previousanalytical works on rock chemistry, the process for the enrichment of tungsten compared with the other ore metals In hydrothermal solutions should be conSidered If we stand on this point of view. "Volatil1 ty of tungsten" was discussed based on the facts that the boiling points of tungsten halides and tungsten oxyhalides are lower than those of base-metal chlorides (e.g., Lindgren. 1933). "Volatility of tungsten" is one of the possible causes for the enrichment of tungsten compared with base metals if we consider the concentrations in the vapor phase. As "volatility of tungsten" was discussed based on the behavior of tungsten in oxygen or halogen present anhydrous system at 1 bar, the validity should be examined in hydrous system at eleVated pressures. Shibue (1988a.b) showed some of the phYSico-chemical conditions for the formation of major tungsten deposits in Japan. Temperatures of the most Intense mineralization range from 550° to 400°C. pressures are around 1 kilobar. and the salinities in the hydrothermal solutions are less than 10 NaCI eq. wtt. Considering the formation conditions of these deposits. experiments on the behavior of tungsten should be performed above and near the critical point (221 bar, 374°C) of H20 (Keenan et al .• 1969). It Is a convenient method that the weight of the product formed from the vaporized species Is measured for purpOSe of the examination of the volatility. This method does not give the exact partition coefficient of the considered species between vapor and liquid phases. However. volatilities of various species can be compared by this convenient method. choosing appropriate captures for the vaporized speCies. This paper aims to show the volatility of tungsten in hydrous system at elevated temperatures and pressures. and to present a tentative model for the enrichment of tungsten in hydrothermal solutions for the deposits related to granitic activities. |