Abstract
Microprobe and fluid inclusion analyses of hydrothermal ore deposits containing the subassemblage sphalerite+ tetrahedrite-tennantite ∼[(Cu, Ag)10(Fe, Zn)2(As,Sb)4S13] reveal that the Gibbs energies of the reciprocal reaction Cu10Zn2Sb4S13 + Cu10Fe2As4S13 = Cu10Fe2Sb4S13 + Cu10Zn2As4S13 and the Fe-Zn exchange reaction 1/2Cu10Fe2Sb4S13 + ZnS = 1/2Cu10Zn2Sb4S13 + FeS are within the uncertainties of the values established by Sack and Loucks (1985) and Raabe and Sack (1984), 2.59±0.14 and 2.07±0.07 kcal/gfw. However, this study suggests that the Fe-Zn exchange reaction between sphalerite and Sb and Ag-rich tetrahedrites does not obey the simple systematics suggested by Sack and Loucks (1985) wherein tetrahedrite is assumed to behave as an “ideal” reciprocal solution. Instead these studies show that the configurational Gibbs energy of this exchange reaction,RTln[(XFe/XZn)TET(XZnS/XFeS)SPH], corrected for sphalerite nonideality exhibits both a local maximum and minimum as a function of Ag/(Cu+Ag) ratio at a givenX FeS SPH and temperature. The local maximum forX FeS SPH ∼0.10 corresponds to the position of the cell edge maximum established for natural tetrahedrites by Riley (1974), Ag/(Ag+Cu)∼0.4. These studies and the results of structural refinements of Ag-bearing tetrahedrites suggest that in low silver tetrahedrites Ag is preferentially incorporated in trigonal-planar sites but that in tetrahedrites with intermediate and greater Ag/(Ag+Cu) ratio, Ag is preferentially incorporated in tetrahedral sites. A nonconvergent site ordering model for tetrahedrite is developed to quantify and extrapolate these predictions.
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