Solubility of gold in common gold-concentrating minerals

The paper is a summary of the authors' and published data on the occurrence of Au in common gold-concentrating minerals (pyrite, arsenopyrite, pyrrhotite, chalcopyrite, bornite, galena, sphalerite, and magnetite). The solubility of gold in minerals is evaluated through identification of the limiting element incorporation into the real crystal. The distribution of gold between coexisting minerals is considered. Obtaining reliable data on the gold solubility involves discrimination of the structural form of the element and correct separation of Au forms between the surface and the volume, which is not always possible because of the small size and low quality of crystals (defects and highly developed internal surfaces). It is also necessary to have a phase (individual or nonautonomous) limiting the incorporation of Au or to compare the mineral under study (within the framework of the principle of phase composition correlation) with a reference mineral with a reliably established structural form of Au. The most reliable and consistent estimates for the hydrothermal parameters (450-500 degrees C, 1 kbar) are as follows (mu g/g): sphalerite - 0.7, highly ferrous sphalerite - 5, magnetite - 1, pyrite - 3, manganese and copper-containing pyrite - 10, pyrrhotite - 21, chalcopyrite - 110, bornite - 140, and galena - 240. The highest solubility of gold (up to 30,000 mu g/g) is established in arsenopyrite, but it is likely to be a metastable miscibility caused by the nonstationary conditions of crystal growth or by the crystal growth at the expense of the surficial nonautonomous phase. The same factors can cause supersaturation of pyrite with Au admixture at low temperatures. The dual behavior of Au in pyrrhotite and magnetite is for a different reason: Under reducing conditions, these minerals can contain a submicroscopic elemental form of Au indistinguishable from the structural one. We consider the forms of Au occurrence and the relationship between the solubility of gold and its metallic bonds in minerals.