The problem of mineral association and mineral paragenesis in ore deposits has been the subject of much discussion. The attempts at a rational explanation have, however, been largely unsuccessful since they were not based on sound physical-chemical principles. A new attempt is made in this paper, and it is shown that the temperature and the fugacity (f) of O, S and CO 2 in ore-forming fluids at the time deposition of the vein minerals largely control the mineralogy of ore deposits. fs 2 -fo 2 -T diagrams showing the stability fields of sulfides, oxides, and sulfates of nineteen metals are developed as well as fs 2 -fo 2 -fco 2 diagrams showing the stability of these compounds and of carbonates of the respective elements. The diagrams are based on recent compilations of thermochemical data; they are used in the second paper of this series to define the approximate composition of ore-forming fluids and the relationships between temperature, pressure, and mineralogy in hydrothermal ore deposits.