Abstract

Partition ratios of zinc and lead between synthetic granitic melts and aqueous phase were determined at 800 degrees C and under P (sub H 2 O) = 3.5 + or - 0.25 kb. Two starting glasses in the system SiO 2 -Al 2 O 3 -CaO-Na 2 O were prepared. Their bulk chemical compositions are similar, i.e., SiO 2 content of 75.65 percent in glass 1 and of 74.68 percent in glass 2. Glass 1 is prepared to be slightly alkaline with 1.89 percent normative wollastonite and glass 2 is slightly aluminous with 2.42 percent normative corundum. The partition ratio of sodium between aqueous fluid and melt, K PNa , can be expressed by the following equation over the whole range of chloride concentrations in the aqueous solution m vCl (mole/kg), regardless of the normative composition of the melt: K PNa = 0.468 X m vCl .However, the partition ratio of zinc, K PZn , for the aluminous melt is more than two orders of magnitude larger than that for the alkaline melt. K PZn varies with the square of the chloride concentration in the aqueous phase and is satisfied by the following two equations: K PZn = 9.40 X (m vCl ) 2 (aluminous melt 2) and K PZn = 0.0833 X (m vCl ) 2 (alkaline melt 1).A similar contrast was also observed in the partition ratios of lead between aqueous phase and melts: K PPb = 2.26 X (m vCl ) 2 (aluminous melt 2) and K PPb = 0.0474 X (m vCl ) 2 (alkaline melt 1).The log of the partition ratio shows an excellent inversely proportional relation with the pH (at 25 degrees C) of the aqueous solution which was equilibrated with granitic melts and quenched. A slope of -2 is obtained for alkaline melt and is reasonably explained by the equation, PbO melt + 2H (super +) vapor + 2Cl (super -) vapor = PbCl (sub 2 vapor ) + H 2 O. The slope for the aluminous melt, -1/2, may be explained only by complex reactions involving silicate ligands in the melt.The results indicate that only aluminous granite is capable of producing hydrothermal fluids which would contain a significant amount of ore metals. In other words, aluminous granite can be "ore-bearing" by postmagmatic processes. This is likely to happen when a granitie magma which originally was slightly alkaline changed its composition to slightly aluminous during diapiric uplift by assimilation of pelitic sediments. If this compositional evolution happened, then granitic magma would tend to store up base metals during the early stages of its evolution and release them very efficiently in the later stages to form a hydrothermal ore solution.

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