On the limit and form of gold incorporation into hydrothermal pyrite

The method Is proposed to estimate the limit of gold incorporation into min erals by studying the gold behavior in the system <<mineral under investiga tion - reference mineral>>. The reference mineral is characterized by a suf ficiently high and well-defined limit of gold incorporation and by a unifor m Au distribution quantitatively conforming to isomorphous mixtures. A gold content variation required for constructing a correlation diagram of the p hase composition is achieved by using the so-called <<gold-guide>> elements . These elements increase the gold solubility in the fluid phase and enable the minerals formed from the fluid to be saturated with gold. A special statistical processing of analytical data for single crystals per mits the structurally bound gold to be substantiated. Greenokite (alpha-CdS ) is used as a reference mineral to determine the limit of gold incorporati on into pyrite under hydrothermal conditions at 500 degrees C and 100 MPa i n the presence of As or Se. The obtained value (3 +/-1 ppm Au) is in agreem ent with the results of ion-probe microanalysis of natural pyrites. The obt ained data show that the Au incorporation into pyrite is not connected with substitution of sulfur by arsenic, as usually supposed. The most probable incorporation mechanism is substitution of bivalent iron by univalent gold, giving rise to an acceptor center, which may be compensated by a donor def ect, such as a sulfur vacancy or hydrosulfide ion replacing sulfur dianion in the pyrite structure. The coefficient of the structural-gold distributio n between pyrite and hydrothermal fluid is low (about 0.06). Therefore, the obtained ion-probe data on the structural-gold contents in natural pyrites indicate sufficiently high gold contents in the ore-bearing fluids of gold deposits (3-23 ppm). Probably, such high concentrations are provided by co mplexes containing <<gold-guide>> elements, primarily As.