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American Mineralogist; October; v. 93; no. 10; p. 1630-1640; DOI: 10.2138/am.2008.2695
© 2008 Mineralogical Society of America
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A solution model for high-temperature PbS-AgSbS2-AgBiS2 galena

Nathan I. Chutas1,*, Victor C. Kress1, Mark S. Ghiorso2 and Richard O. Sack1,3

1 Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, U.S.A.
2 OFM Research-West, 7336 24th Avenue NE, Seattle, Washington 98115, U.S.A.
3 OFM Research Inc., 28430 NE 47th Place, Redmond, Washington 98053, U.S.A.

Significant quantities of silver can be accommodated in galena as a coupled substitution with Ag + Sb and/or Ag + Bi for 2 Pb atoms. The limitations of this solubility are quantified experimentally for the cubic (FmFormulam) {alpha}-galena phase. Above 441.7°C, the {alpha}-galena phase has complete solid solution across the Pb2S2-AgSbS2-AgBiS2 (galena-miargyrite-matildite) ternary. Dry sinter experiments at 400, 375, and 350°C extend the known miscibility gap on the Pb2S2-AgSbS2 binary into ternary compositions. Phase equilibrium and miscibility gap data and self-consistent thermodynamic data for other high-temperature phases in this system combined with data for other sulfides and sulfosalts are used to constrain an asymmetric regular solution model for the {alpha}-galena phase (Pb2,AgSb,AgBi) S2. Phase relations in this system suggest that simple retrograde re-equilibration during cooling can produce phase assemblages and textures commonly observed in Ag-bearing ore deposits. Bulk metal ratios of deposits in conjunction with the application of this thermodynamic model may be used to understand district Ag-distribution or as an exploration tool.

Key Words: Experimental petrology • Ag in galena • phase equilibria • thermodynamics • silver deposits






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