ABSTRACT

Oxygen isotopic compositions of various silicate minerals of sedimentary and igneous origins have been investigated. The determined δ¹⁸O values of these hydrothermal silicates vary between -3 and +30‰ most of them correlate well with the crystallization temperature. The δ values of hydrothermal solutions (δ_W) from which quartz had been deposited have been calculated. The values thus calculated for samples which crystallized at temperatures above 300°C show a samll variation about a mean value of +8‰ and decrease systematically with temperature below 300°C. An ore fluid that existed above 300°C is interpreted as having equilibrated with a large reservoir of silicate rocks at high temperatures on the basis of ¹⁸O content. The variations of δ_W with temperature for samples which crystallized below 300°C are attributed to the mixing of the ore fluid with ¹⁸O-poor meteoric water. This view supports the cooling mechanism of hydrothermal ore fluids, which was proposed by BARTON and TOULMIN (1961). Hydrothermal solutions of ore deposits of shallow, so-called “subvolcanic” origin, however contain large proportions of meteoric water. The isotopic composition of the initial hydrothermal solution can be measured directly only in fluid inclusions in minerals deposited above 300°C and with no oxygen atoms. When ore deposits are classified, the separation of the group of deposits as “volcanic” or “subvolcanic” is helpful.