Tellurides of Au, Ag, Bi, Pb and other elements are common trace minerals in gold deposits that span the magmatic-hydrothermal spectrum, as well as those formed in metamorphic terranes. The association between tellurides and gold is long recognized, and is most evident from the fact that a small number of deposits, mostly but not exclusively of epithermal type, contain exploitable Au-(Ag)-tellurides. These are the true Au(Ag)-telluride deposits in which a significant proportion of the precious metals are carried by the tellurides themselves. Although the majority of such deposits are epithermal, Au-(Ag) tellurides are also known from many Archaean and Proterozoic ‘orogenic’ gold deposits, the giant Golden Mile deposit, W.A. being a prime example.
A Bi-Te enriched signature is common to many orogenic gold systems. Bi-tellurides may also be prominent and commonly paragenetically associated with gold in skarn deposits, with Au-Bi, Au-Te or Bi-Te associations being prominent in many cases. This implies a genetic link and necessitates a consideration of the role that Bi-tellurides may have played in ore genesis.
The observed distribution of tellurides in the main types of Au-deposit spanning the magmatic-hydrothermal and metamorphic spectrum allows the identification of Au-telluride and Au-Bi-telluride types of deposit. By emphasizing this distinction, the genetic links between Bi-tellurides and Au are recognised as important as those between Au-tellurides and gold in deposit formation.
Insights into mechanisms responsible for telluride deposition have been examined in relatively few instances and no general model currently exists. A general scheme for telluride deposition via gas condensation is attractive to explain features of mineralising systems that experienced sustained boiling, but tectonically-driven hydrothermal systems could be as efficient in making a Au-telluride deposit. Depending on temperatures, scavenging of Au by Bi(Te) melts, or partial melting of a pre-existing ore, may offer alternatives to generate telluride-rich gold ores. Pressure variation, e.g., by throttling, appears to impact on telluride deposition and distribution in any given deposit.
Bismuth chalcogenides of the tetradymite group [BixXy (X=Te,Se,S)] are a large group of minerals and unnamed compounds, commonly, but not exclusively found in gold deposits of various types. The minerals are rhombohedral or trigonal layer structures, which can be readily interpreted in terms of distinct stacking sequences between two layer types: 2-atom ‘BiBi’ and 5-atom ‘XBiXBiX’ layers. Distinct isoseries exist with specific Bi:X ratios, i.e., 2:3, 3:4, 1:1, 4:3 and 7:3. High resolution transmission electron microscopy (HRTEM) has been employed to structurally characterise both a range of known minerals, as well as several unnamed phases within known or new isoseries. HRTEM studies identify lattice-scale stacking disorder which may explain ‘deviant’ stoichiometry observed in some natural samples.
As well as reviewing the mineralogy of the group, the talk examines the paragenetic implications of tetradymite group minerals in different types of gold deposits, how their speciation reflects the formation conditions of those deposits, and how their presence may, in some cases, reflect the involvement of Bi-Te melts that scavenge gold, explaining the commonly observed Au-Bi-Te association. LA-ICPMS measurements of Bi-chalcogenides from various types of Au deposits indicate that they may contain gold concentrations up to hundreds of ppm.
Nigel J. Cook, Natural History Museum, University of Oslo, Norway