Sulfur vacancies in fahlores from the Ag-Pb–Zn Mangazeyskoye ore deposit (Sakha, Russia).

Fahlores from the epithermal Mangazeyskoye Ag-Pb–Zn ore deposit (Sakha, Russia) typically occur in rhythmically zoned crystals. In contrast to typical (Ag,Cu)₁₀(Fe,Zn)₂Sb₄S₁₃ fahlores, up to about 2.6 of the 13 sulfur atoms in the formula unit may be vacant and many of them have Ag/(Ag + Cu) ratios substantially above those established for (Ag,Cu)₁₀(Fe,Zn)₂Sb₄S₁₃ fahlores coexisting with miargyrite, pyrargyrite, and sphalerite by experimental, field, and theoretical investigations. The charge balance inequalities consequent on the removal of sulfur are compensated by metallization of the nominally Ag⁺ and Cu⁺ ions in the octahedral metal clusters that are formed by the removal of S²⁻ from the octahedral site at the center and corners of the unit cell of fahlore (space group I 4 ¯ 3 m, Z = 2). The development of these vacancies leads to an effective ionic charge of [ (Ag , Cu) 6 6 - 2 ∗ vac + ] for these octahedral clusters, resulting in an intrinsic limit on sulfur vacancies of 3 per formula unit (pfu). The increases in the Ag/(Ag + Cu) ratio of some of the Mangazeyskoye fahlores relative to those defined for (Ag,Cu)₁₀(Fe,Zn)₂Sb₄S₁₃ fahlores are readily attributed to the lowering of the activities of Ag₁₀Zn₂Sb₄S₁₃ and Ag₁₀Fe₂Sb₄S₁₃ components in fahlore by sulfur vacancies. This results in expansion of the field of fahlore stability relative to that given by the fahlore breakdown reaction Ag₁₀Zn₂Sb₄S₁₃ (fahlore) = AgSbS₂ (miargyrite) + 3 Ag₃SbS₃ (pyrargyrite) + 2 ZnS (sphalerite) for (Ag,Cu)₁₀(Fe,Zn)₂Sb₄S₁₃ fahlores. A preliminary analysis demonstrates that the increased Ag/(Ag + Cu) ratios observed in the Mangazeyskoye fahlores relative to those defined for (Ag,Cu)₁₀(Fe,Zn)₂Sb₄S₁₃ fahlores are due to sulfur vacancies being almost exclusively in the octahedral sulfur site where the vacancy for sulfur substitution appears to be approximately ideal. However, it appears that a maximum in Ag/(Ag + Cu) ratios is reached around one pfu, and, to a first approximation, does not appear to increase with further increase in contents. This latter inference suggests that the for S (◻(S)_-1) substitution on tetrahedral sulfur sites may be highly nonideal. When properly calibrated for sulfur fugacity based on experimental, spectroscopic, and theoretical studies, reactions such as Ag 10 Zn 2 Sb 4 S 13 = Ag 10 Zn 2 Sb 4 S 12 □ OCT + 1 / 2 S 2 and Ag 10 Zn 2 Sb 4 S 11 , □ TET S = Ag 10 Zn 2 Sb 4 S 12 □ OCT will afford fahlore to serve as a petrogenetic indicator of sulfur fugacity in low-sulfidation environments. Until such calibrations are developed, the fahlore Ag/(Ag + Cu) ratios may nonetheless be used to define relative sulfur fugacities in rhythmically zoned fahlores from ore deposits such as Mangazeyskoye. [ABSTRACT FROM AUTHOR]