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Cristiana Ciobanu, University of Adelaide (Australia)
Nigel Cook, University of Oslo (Norway)
Allan Pring, South Australian Museum (Australia)
Joel Brugger, South Australian Museum (Australia)
Leonid Danushevskiy, University of Tasmania (Australia)
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Concentrations of invisible gold measured by LA-ICPMS in both bismuth-sulfosalts and -tellurides (25 deposits) are up to thousands of ppm, with mean values of 54 (n=70) and 25 ppm (n=223), respectively. This includes lillianite (Lil), cosalite (Cos), end-member bismuthinite derivatives (BDs) and tellurides from the tetradymite group and aleksite series. The latter two are mixed-layer compounds derived from the same 5-atoms wide unit that combines alternating single-atom layers of chalcogen and Bi (tetradymite archetype Bi2X3; X=Te,Se,S). Strong correlation factors are obtained between Au and elements that assist the 3 substitutional mechanisms for M+(Ag,Cu) in sulfosalts: R1: Ag++Bi3+=2Pb2+; R2: Cu++Pb2+=Bi3++v and R3: Cu++Ag+=Pb2++v. In those sulfosalts having an octahedral exchange site (e.g. Lil, Cos), there is also a correlation between Ag substitution and the noisiness of the Au signal across ablation intervals (presence of nanoparticles; AuNs). In Lil, AuNs (thousands of ppm Au) occur in half-substituted Ag-members along the R1 solid-solution line. In structures with vacancies adjacent to the octahedral sites (R3; Cos) Auss (<100 ppm) and AuNs (thousands of ppm) correlates with a 5-fold increase in Cu. In BDs, where Ag is present as a trace element (<1 wt%) and the exchange site is square-pyramidal, hundreds of ppm Au are measured only for specimens where the correlation factor for Ag-Te is highest (r≈1). Considering the oxidation state for elements in the substitution R2 for the square-pyramidal site, incorporation of Ag can only be in the adjacent vacancy, with Au substituting for M3+ (negative correlation with Sb) in the square-pyramidal site. The presence of Te, replacing S, indicates its role in assisting such substitutions by modifying the exchange site configuration. Values of bond valence parameters for Te with Ag relative to S to Ag (2.51 and 2.15, respectively) support this model. Hence, Auss saturation in this structure requires complex substitutions. An oxidation state of Au3+ instead of Au1+, as in the cases above, can be predicted. In mixed-layer tellurides the Bi(Pb) and X atoms form a distorted octahedron, comparable with the M-S polyhedron in sulfosalts but there are no available sites/strong bonds between the layer-units like in the latter. Instead, van der Waals gaps and metallic bonds occur at X-X and Bi-Bi contacts, respectively. The correlation trends are indicative for statistical incorporation of (Au, Ag) into octahedral sites where Pb and/or Sb replaces Bi. Higher Au concentrations are obtained for Bi2X3 relative to phases with Bi>X implying that the presence of van der Waals gaps is the main factor for Au incorporation. The presence of AuNs is inferred at variable Au contents (tens to hundreds of ppm calculated for intervals of stable Au signal) and Auss can reach high values (thousands of ppm) when compared to sulfosalts. This behavior can be attributed to the structural differences between the two groups.
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