Tellurides

The processing requirements and behavior of this class of gold ore is affected by the presence of gold tellurides. Tellurides are the only gold minerals with economic significance. There is a wide range of Au/Te and mixed metal tellurides. These minerals are dense (7.5-9.5) due to gold and silver content, which ranges from 12 to 44%.

There are two main geological environments in which Au-Ag tellurides occur: veins, fissures and breccia pipes in tertiary rocks and Precambrian rocks or metamorphosed volcanic lava. In the first group the occurrence veins are muggy and composed of quartz and carbonate minerals. Adjacent to the veins there is intense wall rock alteration due to the introduction of large amount of water, carbon dioxide and sulphur. Native tellurium is abundant and native gold is rare. Tellurides of metals other than gold are present in lesser amounts. Krennerite is dominant over calaverite.

In the second group wall rocks show some retrograde metamorphism, resulting in rocks of lower metamorphism grade, coupled with intense structural deformation. Native tellurium is rare, native gold is abundant and tellurides of mercury, copper and bismuth are generally present. The stable form of AuTe₂ is calaverite, not Krennerite, and the telluride minerals lack crystalline form.