The partition coefficients Kp=Cfluid/Cmelt of Pb, Zn, W and Mo between fluid and melt were measured at 1 and 5 kbars, 800-1000°C, and starting solutions of 1mNaCl+0.1mHCl using gold capsules, the Tuttle design apparatus and gas-media pressure vessels. Three starting haplogranitic melts had the following compositions (wt %): granodiorite-68.4SiO2, 17.9Al2O3, 5.2CaO, 4.0Na2O and 4.5K2O; granite-73SiO2, 16Al2O3, 2.5CaO, 3.4Na2O and 5.2K2O; leucogranite-76.4SiO2, 13.9Al2O2, 2CaO, 3.3Na2O and 4.5K2O. They were prepared using a standard gelling technique. The ore species were introduced into both the previously prepared silicate glass and starting solution. In one run, either Pb+Zn or W+Mo were introduced. Silicate melt was previously saturated H2O under corresponding pressure. The spectrophotometric analyses of the quench fluid for W and Mo and atomic-absorption analyses of fluid for Pb and Zn were performed. The metal content of the quench haplogranite glass in the margins was determined on the electron microprobe.
Our experiments show that at 1 kbar in the presence of the granodioritic melt the Kp of the Pb and Zn were close to 1. The highest Kp measured were those of base metals in the presence of the granitic melt (KpPbª3, KpZnª8). In the presence of the leucogranitic melt the Kp decreased, but these Kp values were higher than 1. The decrease of atomic ratio (Na+K+2Ca)/Al of quench melt composition correlates good with the increasing of the Kp of Pb and Zn. If this atomic ratio value is lower than 1 (granitic melt), the alkali components can be present in the melt only as "feldspathic" components. And extra alumina can be present as "andalusite" component. If this atomic ratio value is higher than 1 (granodioritic melt), along with "feldspathic" components, other alkali-bearing components can be present in this melt (f.e. Na4SiO4, Na2SiO3 and other). Namely they can keep the base metals in the melt (f.e. as (Na,K)4-x Znx(SiO4)). The partition coefficients of tungsten and molybdenum between chloride fluid and melt increase considerably with change of melt composition from the granodiorite to leucogranite (at 1 kbar from 0.05 to 0.5). That sort of influence of melt composition was stipulated by change of content of CaO and SiO2 in the melt, in this case the chemical composition of the coexistent fluid changes essentially. We suggest that CaO can keep W and Mo in the melt as "scheelite-powellite" components. With increasing pressure from 1 to 5 kbars the Kp increase in the presence of leucogranitic melt. On transition from the chloride solution to the fluoride solution there occurs a very sharp increase of Kp. With change of the melt composition from granodiorite to leucogranite in the presence of fluoride fluid the partition coefficients of W and Mo increase also.
This work was supported by Russian Foundation of Fundamental Researches grant No 93-05-9822 and by International Science Foundation grants No MUR000 and No MUR300.