Data on telluride-bearing mineral assemblages from various types of ore deposits are compiled and interpreted in terms of the prevailing fugacities of Te 2 , S 2 , and the temperature during deposition. In the majority of hydrothermal deposits, the appearance of tellurides after initial deposition of sulfides reflects an increase in the f (sub Te 2 ) /f (sub S 2 ) ratio, most likely due to input of H 2 Te from a magmatic source. In some vein deposits such as Boulder County (Colorado), initial deposition of native tellurium is followed by deposition of minerals with progressively lower tellurium content, indicating a decrease in f (sub Te 2 ) over the course of telluride deposition. This trend may reflect local isolation of early formed minerals from the hydrothermal fluids and/or progressive depletion of tellurium at the source. Ore minerals in other deposits, such as Mahd Adh Dhahab (Saudi Arabia), reveal systematic temporal or spatial variations in mineralogy that are attributed to introduction of a tellurium-rich fluid into a preexisting sulfide assemblage, and subsequent buffering of f (sub Te 2 ) and f (sub S 2 ) by reactions such as 2PbS + Te 2 = 2PbTe + S 2 . This buffering may exert considerable control on vein minerals, such as the presence and relative abundance of gold and calaverite, and the presence or absence of native tellurium.