Tellurides from Sunrise Dam gold deposit, Yilgarn Craton, Western Australia: a new occurrence of nagyágite

The complex Pb-Sb-Au tellurosulfide nagyágite is found together with eight tellurides (hessite, petzite, calaverite, altaite, tellurantimony (and Bi-bearing tellurantimony), melonite, tetradymite and an unnamed Au(Ag)-As-telluride) in sulfide-sulfosalt assemblages from late, high-grade veins (D4) and post-D4 veinlets in the world-class orogenic gold deposit at Sunrise Dam, Eastern Goldfields Province of the Archaean Yilgarn Craton, Western Australia. The composition of nagyágite at Sunrise Dam conforms to ideal stoichiometry, with negligible As content and Au/(Au+Te) ratio of 0.325 [i.e., (Pb4.84Sb1.10 As0.05)5.99S5.99(Au0.98 Te2.03)3.01]. The diverse mineralogy of the post-D4 veinlets, relative to the host veins, is attributed to small-scale reaction fronts established along zones of replacement at the polished section scale. The association of telluride assemblages and native gold is interpreted in terms of remobilization of ore components (including Ag, Sb, Te and Au) from the pre-existing assemblages, and their redeposition during subsequent tectonic events. The presence of nagyágite and Au-Ag tellurides in veins, in quantities that may be significant in economic terms, as well as the character of their breakdown products, have implications for ore processing and gold recovery, as well as for the genetic interpretation of the deposit. The strong structural control upon formation of the telluride-bearing assemblages at Sunrise Dam and the ability of these minerals to reflect changes in the local environment, contradicts the current view that these tellurides have a magmatic affiliation.