The mineralogy and occurrence of gold varies in a systematic manner in volcanic-hosted massive sulfide deposits of eastern Australia. In deposits where it is concentrated with zinc at or near the top of the massive sulfide deposit, gold occurs in auriferous arsenopyrite and pyrite in the center of the lens, and electrum of variable fineness (300-900) in the baritic top of the lens. The former occurrence results from precipitation of the gold into the lattice of arsenopyrite and coprecipitation of submicroscopic gold grains in arsenic-poor pyrite. The latter occurrence results from direct precipitation of electrum from thio complexes owing to the decrease in a (sub H 2 S) caused by dilution and oxidation as hydrothermal fluids mix with seawater.In deposits where gold is concentrated with copper at the base of the sulfide lens and in the stringer zones, the gold occurs as electrum of high fineness (750-1,000) or as gold tellurides that are commonly associated with chalcopyrite and/or pyrite. This occurrence is caused by the direct precipitation of gold from chloro complexes owing to an increase in pH or a decrease in temperature as the hydrothermal fluids move up through the sulfide body.Gold in auriferous pyrite is readily mobilized during deformation. As the pyrite fractures, gold migrates via solution-precipitation mechanisms to cracks and grain boundaries to form electrum by combination with silver, which is released from sulfide minerals in a similar manner. With increasing stress the electrum is released from pyrite to recrystallize with other easily remobilized minerals such as galena. During this process, electrum grain size increases.Thermodynamic modeling of the variation in electrum fineness shows good agreement with variations measured in massive sulfide ores. For gold transported as a thio complex, the predicted fineness of 190 to 990 for temperatures of 200 degrees to 300 degrees C and typical volcanogenic fluids matches the observed variation in deposits of the Zn-Au association. The very high finenesses observed in deposits of the Cu-Au association can be modeled assuming gold chloro complex transport and high Ag/Au ratios in the hydrothermal fluids.