Home – Home – Jornals – Russian Geology and Geophysics 2015 number 12

The relationships and chemical compositions of silver sulfoselenides in the ores of the Rogovik gold-silver deposit (northeastern Russia) were studied to refine the low-temperature field of the Ag ₂S-Ag ₂Se phase diagram and identify contradictions between natural and experimental data. Two types of relationships between the phases of the system Ag ₂S-Ag ₂Se have been recognized using optical and scanning electron microscopy: (1) Se-acanthite and S-naumannite occur as monomineral microinclusions or fill cracks in the grains or the interstices of other minerals, and acanthite (free of impurities) forms rims on Fe-sphalerite; (2) Se-acanthite forms rims on S-naumannite. Electron probe microanalysis of silver sulfoselenides from the Rogovik ores revealed 0-7.9 wt.% Se in acanthite and 0-3.2 wt.% S in naumannite, which corresponds to the acanthite series Ag ₂S-Ag ₂S _0.76Se _0.24 and naumannite series Ag ₂S _0.28Se _0.72-Ag ₂Se. The composition ranges of the studied acanthite and naumannite series are wider than those of natural silver sulfoselenides from the Guanajuato (Mexico), Silver City (USA), Salida (Indonesia), and other deposits (Ag ₂S- Ag ₂S _0.85Se _0.15 and Ag ₂S _0.12Se _0.88-Ag ₂Se, respectively) but are significantly narrower than the composition ranges of the synthetic samples: Ag ₂S-Ag ₂S _0.4Se _0.6 and Ag ₂S _0.3Se _0.7-Ag ₂Se. The presence of intergrowths of two phases of the Ag ₂S-Ag ₂Se series in the form of Se-acanthite rims on S-naumannite in the Rogovik ores and the absence of three-phase intergrowths of silver sulfoselenides Ag ₂S _1-x Se _x from this and other deposits do not confirm the assumption on the existence of the third solid solution. The results of earlier studies of natural solid solutions Ag ₂(S,Se) show the existence of two solid solutions (of the acanthite and naumannite series) in the system Ag ₂S-Ag ₂Se and confirm the experimental data. It is necessary to carry out a detailed examination of natural silver sulfoselenides falling in the interval from Ag ₂S _0.4Se _0.6 to Ag ₂S _0.3Se _0.7 in order to identify the limits of two-phase immiscibility.