WHAT MAKES A GOLD-TELLURIDE DEPOSIT?

Gold-telluride deposits are currently considered to result from low-temperature (<300ºC) hydrothermal processes, with the telluride-bearing parageneses recognized as a specific depositional event. The tellurides can be precipitated either at the same place or separately from native gold ore, depending upon precipitation mechanisms, local setting, etc. Contemporary viewpoints for telluride formation include a Te-rich source, generally magmatically-derived, transport of Te as aqueous/vapor species within hydrothermal fluid, and precipitation of ore minerals due to mechanisms such as boiling, mixing, temperature decrease, fluid-rock interaction etc. This model fits well with a shallow epithermal environment (<5 km). Indeed, most Au-telluride deposits are of such type, even though good examples of deposits formed in deeper (>5 km) settings, e.g., orogenic and intrusion-related types, are known.

In contrast, telluride-enriched deposits include a wider range of deposit types, e.g., Au-rich VMS deposits, porphyry Au(Cu) and Au skarns. In these, some proportion of the gold may occur as Au-(Ag)-tellurides, or as native gold/Au-minerals paragenetically tied with tellurides of other elements, notably bismuth. Bi-tellurides and altaite can also be considered as precious metal carriers given LA-ICPMS analyses on these minerals from various deposit types that show Au values of tens to hundreds of ppm.

The fact that Au(Ag)-tellurides are stable with native Te at oxidizing conditions (Py, Hem stability) implies that deposits formed under reducing conditions (reduced Au skarns, orogenic Au, intrusion-related) are excluded from the category of Au-telluride deposits. This is despite their enrichment in Bi-telluride species stable with native Bi (Po, Mt stability). An improved definition of the term ‘gold-telluride deposits' is thus necessary, encompassing the genetic connotation given by the presence of tellurides other than those of Au-Ag. Emphasizing this idea further, the role that melts from the Au-Bi-Te or Au-Pb-Te systems may play in scavenging Au from hydrothermal fluids, or concentrating Au during metamorphism, provides for new ways of considering Au-telluride deposits. Their formation may be dependant upon specific mechanisms of telluride enrichment rather than imposed by a Te-rich source.