Gold solubility in the common gold-bearing minerals: Experimental evaluation and application to pyrite

A method is proposed for determining gold solubility in the common gold-bea ring minerals (sulfides, etc.) using so-called "gold-assisting elements" (G AE). These elements increase gold solubility in the fluid phase, enabling m inerals formed from hydrothermal fluids to be saturated with gold. Two expe rimental approaches are discussed: (I) identifying the solid-solution limit of gold in a mineral structure by determining the maximum content of unifo rmly distributed gold constituent which remains unchanged with increasing g old content in the coexisting fluid phase, and (2) determining the gold dis tribution between the mineral under study and a reference mineral with a su fficiently high and well-defined solid-solution limit of gold and utilizing the phase composition correlation principle. Statistical treatment of anal ytical data for single crystals permits inference of the structurally bound gold constituent. Greenockite (alpha-CdS) incorporates a maximum of 50 +/- 7 ppm Au in solid solution at 500 degrees C and 1 kbar and this is used as a reference mineral to determine gold solubility in pyrite under the same conditions in the presence of As and Se as gold-assisting elements. The val ue obtained for gold solubility in pyrite (3 +/- 1 ppm Au) is in reasonable agreement with the results of ion-probe microanalysis of natural pyrites. The data suggest that monovalent gold substitutes for divalent iron, giving rise to an acceptor center compensated by a donor defect, either a sulphur vacancy or a hydrosulphide ion replacing S-2(2-).