Mechanisms and species of gold incorporation into crystals of cadmium, lead, and iron sulfides

The mechanisms and species of gold incorporation into crystals of simple sulfides are described. The concept of endocrypty in its modern treatment and the corresponding technique of experimental study were used. The technique is based on the principle of phase composition correlation and variation of parameters specifying the concentrations of active crystal defects. Gold species are determined with the method of statistical samples of analytical data for single crystals (SSADSC). The highest gold incorporation limits have been established for PbS and CdS under high sulfur fugacity (2.4⋅10−2 and 7⋅10−3 wt.%, respectively) at 500 °C and 1 kbar. It is shown that in this case, the main crystal defects are metal (Me) vacancies and the gold species is a donor-acceptor pair “Me vacancy– interstitial Au”. Under low sulfur fugacity, Au incorporation might be due to sulfur vacancies, and the gold species is a pair “Au in the Me position–S vacancy”; its concentrations are lower than those at high sulfur fugacity (≤10−3 wt.%). A specific feature of gold is a tendency to endocrypty. The estimated limiting contents of gold incorporated into sulfide crystals (FeS2, CdS, PbS) at 450–500 °C vary from n⋅10−4 to n⋅10−2 wt.%, which is nearly two orders of magnitude higher than the “true” isomorphous capacities of these crystals. For FeS, the incorporation limit of Au does not depend on defects (Fe vacancies). This phenomenon has not been explained yet. In general, the data obtained show that not only the crystallochemical features but also geochemical parameters (first of all, temperature and sulfur fugacity) of the ore formation medium are the main factors for gold concentration in sulfide minerals.