ON LIMIT CONCENTRATION AND MANNER OF INCORPORATION OF GOLD IN HYDROTHERMAL PYRITE

The paper reports on a method to estimate the limit concentration of lattice-bound gold incorporated into minerals on the basis of its behavior in the system “study mineral–reference mineral”: The reference mineral is characterized by a sufficiently high and reliably determined limit concentration and uniform distribution of gold, with a coefficient corresponding to isotropic mixtures. Variability of gold content in the system, required for the diagram of phase equilibrium and evaluation of limit miscibility, was achieved using so-called “gold-guide elements”. They increase gold solubility in the fluid phase and facilitate Au saturation of the crystallized minerals. Special statistical processing of the analytical results on a great number of single crystals from coexisting minerals permits separation of lattice gold. Greenokite (α-CdS) was used as a reference mineral to determine the limit Au concentration in pyrite synthesized hydrothermally at 500 °C and 1 kbar, and in the presence of Asor Se. The obtained value (3±1 ppm Au) agrees with the maximum contents of invisible gold in natural pyrites determined by ion-microprobe analysis. Experimental data show that gold incorporation into pyrite does not require simultaneous S → As substitution, as generally agreed. A more probable mechanism of incorporation is substitution of bivalent iron by monovalent gold; the resulting acceptor center is associated with a donor defect, such as sulfur vacancy or a hydrosulfide ion substituting a sulfur dianion in pyrite. The true distribution coefficient of lattice gold in pyrite and hydrothermal fluid is very small (∼0.06), and, consequently, the lattice gold contents detected by ion-microprobe must indicate its rather high concentrations in the ore-forming fluids of gold deposits (∼3–23 ppm). These concentrations may be due to complexes including gold-guide elements, first of all arsenic.