HR: 0800h
AN: V41C-1408    [Abstracts]
TI: A Solution Model for Silver Solubility in Galena in the Pb$_{2}$S$_{2}$-AgSbS$_{2}$-AgBiS$_{2}$ System
AU: * Chutas, N I
EM: nichutas@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Kress, V C
EM: kress@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Ghiorso, M S
EM: ghiorso@geosci.uchicago.edu
AF: Department of Geophysical Sciences, The University of Chicago 5734 S. Ellis Ave., Chicago, IL 60637 United States
AU: Sack, R O
EM: sack@ess.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Sack, R O
EM: sack@ess.washington.edu
AF: OFM Research, 28430 NE 47 Pl, Redmond, WA 98053-8841 United States
AB: The extent of solubility of the Ag-bearing minerals miargyrite (AgSbS$_{2}$) and matildite (AgBiS$_{2}$) in galena has profound importance to models for ore genesis in Ag deposits. Sulfides are known to be reactive at temperatures of ore formation. Many of these sulfides likely do not preserve original compositions. Above $380\deg$C, all compositions in the Pb$_{2}$S$_{2}$ -AgSbS$_{2}$ -AgBiS$_{2}$ system are in the form of the $\alpha$-galena phase, which is the cubic (Fm-3m) structure of galena. X-ray data confirm that the high temperature structures of AgSbS$_{2}$, AgBiS$_{2}$, and ternary compositions are cubic, although the exact structures have not yet been determined and may not be quenchable. End-member compositions in this system are completely miscible at temperatures above $450\deg$C, even though the $\alpha$-galena structure cannot accommodate a high mole fraction of Ag at ambient temperatures. The solubility of Ag in galena is limited by a ternary miscibility gap, which is skewed toward the high Pb portion of the system. Reversal results from dry sinter experiments constrain the miscibility gap in the ternary system. A new asymmetric regular solution model for the ternary α-galena phase successfully reproduces observed coexisting compositions on the miscibility gap, which is predicted to close above about $450\deg$C. This solution model also satisfies data from previous workers on exchange equilibrium between Sb and Bi in the AgSbS$_{2}$-AgBiS$_{2}$ system. This thermodynamic model predicts that a low Ag/high Pb galena deposited at $400\deg$C may contain up to 8 mole$%$ Ag, and at $300\deg$C it may contain 3 mole$%$ Ag. For a deposit with molar Ag:Pb less than 1:30 and a temperature of formation at or above $300\deg$C, all of the Ag could have been accommodated originally in galena. This thermodynamic model can be used to evaluate original galena compositions in the context of ore grades and independent temperature of formation indicators. Results from this model may also have potential for improved ore genesis models and exploration tools.
DE: 3630 Experimental mineralogy and petrology
DE: 3665 Mineral occurrences and deposits
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting