Although Au and As can be enriched up to the weight percent level in arsenian pyrite, there is little knowledge of their limiting concentrations and nature of incorporation. This study reports SIMS and EMPA analyses showing that As and Au contents of arsenian pyrites plot in a wedge-shaped zone with an upper compositional limit defined by the line CAu = 0.02 · CAs + 4 × 10-5 indicating a maximum Au/As molar ratio of ∼0.02. Arsenian pyrites with Au/As ratios plotting above this limit contain nanoparticles of native Au, as observed by HRTEM imaging/EDS analysis and SIMS depth profiling. In this case, a significant amount of the total Au is present in its elemental form. In arsenian pyrites with Au/As < 0.02, native Au nanoparticles were not observed by HRTEM, and all of the Au measured is inferred to be structurally bound in solid solution. The microanalytical results, coupled with previously published XANES-EXAFS spectroscopic measurements confirm that arsenian pyrite compositions plotting above this limit contains Au0, whereas arsenian pyrite compositions plotting below the limit contain Au+1. On the basis of these observations, the upper bound is interpreted to represent a solubility limit for solid solution of Au as a function of As in arsenian pyrite between ∼150°C and ∼250°C, the approximate conditions under which samples used in the study were deposited. The Au-As composition of arsenian pyrite relative to this limit can be used to predict the chemical state of Au as well as the saturation state of Au in the hydrothermal solution that deposited it. These observations confirm that the parent hydrothermal solutions for the giant Carlin-type deposits, where solid solution of Au is dominant in arsenian pyrite, were largely unsaturated with respect to Au0.
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