Understanding gold-(silver)-telluride-(selenide) mineral deposits

as part of the epithermal deposit spectrum in his textbook "Mineral deposits".Telluride-enrichment is, however, observed in a far wider range of deposit types, e.g., Au-rich VMS deposits, porphyry Au(Cu) and Au skarns.In at least some of these, a proportion of the gold occurs as Au-(Ag)-tellurides, or as native gold/Au-minerals paragenetically tied with tellurides of other elements, notably bismuth.Despite their enrichment in Te, gold deposits formed under reducing conditions are generally not acknowledged as Au-telluride deposits, because the telluride-rich character is largely expressed through an abundance of Bi-telluride species, commonly stable together with native bismuth.Gold-Bi compounds, such as maldonite and/or jonassonite, contribute to the mineralogical balance of gold ore, instead of Au(Ag)-tellurides (Ciobanu et al., 2005).A modified definition of the term 'gold-telluride deposits' was thus proposed (Cook and Ciobanu, 2005) to encompass the genetic connotation given by the presence of tellurides other than those of Au-Ag.In a preface to a special issue of Mineralogy and Petrology, Ciobanu et al. (2006a) posed a series of questions concerning the 'how?' and 'why?' of gold-telluride deposit formation.They questioned some of the established thinking about how these deposits were understood, not because the ideas are wrong (on the contrary!), but to encourage debate, and look beyond the confines of what was known and which fitted well to certain epithermal deposits.Likewise, Cook and Ciobanu (2005) attempted to build a framework for the role of other types of hydrothermal systems in the generation of telluridebearing deposits, raising certain taboos about classification, alternative settings and deposition mechanisms, the role of tellurides in orogenic gold systems and skarns, and what studies of trace mineralogy could contribute to the broader ore genesis perspective?They stressed the need to expand thermodynamic databases and modelling of oreforming systems (Afifi et al., 1988; Zhang and Spry, 1994a;Simon et al., 1997) to cover a broader range of formation conditions, e.g., pH/eH variation vs. activity/fugacity of various aqueous/gas species in a hydrothermal system.Within a given deposit, telluride-rich ores can be precipitated either at the same site or separately from native gold ore, depending upon precipitation mechanisms and local setting.Contemporary perspectives (e.g., Cooke and McPhail, 2001) include a Te-rich source, generally magmatically-derived, transport of Te as aqueous/vapour species within hydrothermal fluid, and precipitation of Au-Ag-tellurides due to multistage boiling.In an epithermal-porphyry environment (<5 km), these vapours are transported at the upper part of the veins forming a lowgrade Au-Te cap on top of the main Au mineralisation underneath.Whereas this model fits some telluride-bearing Au deposits (e.g., Gold-(silver)-telluride (selenide) ores occur as epithermal orogenic and intrusion related deposits.Although Te and Se are chalcophile elements and share geochemical affinity with Au, formation of selenides and other elements Ag-Au require acidic or reducing environments.The thermodynamic stability conditions for Au and Agtellurides and native tellurium indicate an epithermal environment.Analysis of mineral paragenensis, textures and compositional variation in tellurides/selenides suggest petrogenetic processes involving interaction with fluids leading to Au scavenging and entrapment in tellurides, changes in chemistry/rates of fluid infiltration and attaining equilibrium in a given assemblage.Jensen and Barton, 2000) there are other examples of epithermal deposits for which this model is not suitable.Where can the model of Cooke and McPhail (2001) be applied?-does it always work? -can zonation always be expected?Good examples also exist of telluride-bearing Au deposits formed in deeper (>5 km) settings, e.g., intrusion-related and orogenic gold deposits.For example, the giant Golden Mile deposit, Kalgoorlie (W.A.; Shackleton et al., 2003), which at 1,500 tonnes contained Au is the largest single lode gold system in the world.Whereas phase separation is commonly attributed to boiling in epithermal systems, this is associated with a catastrophic drop in fluid pressure in orogenic Au systems (Mikucki, 1998).An association between alkaline magmatism and telluride-bearing epithermal mineralisation has often been assumed (e.g., Richards, 1995); many classic epithermal Au-Te systems (Cripple Creek, Emperor, Porgera, Ladolam and the Montana Au-Ag telluride belt) are associated with alkaline magmatism.Some such systems grade downwards into porphyry-type Cu-(Au) or Mo-(Au) deposits.Commenting on the association of tellurides with alkaline magmatism, Jensen and Barton (2000) emphasised that melting of Te-rich ocean floor sediments may be a key source of mantle-sourced alkaline magmas in subduction settings.Cook and Ciobanu (2005) and Ciobanu et al. (2006a) questioned whether the link to alkaline magmatism may have been overstated.Examples of telluride-enriched epithermal mineralisation in calc-alkaline volcanic rocks include the Baguio District, Philippines (Cooke and McPhail, 2001) and numerous Cretaceous-Quaternary deposits in Japan.The Golden Quadrilateral (Romania) and the Kurama Belt (Tien Shan, Uzbekistan), which both received attention during IGCP-486, are also associated with calc-alkaline volcanic rocks.The present contribution summarises the main results of IGCP-486.