Experimental constraints, petrologic studies, and theoretical analysis suggest that, energetically, tetrahedrite-tennantite sulfosalts are remarkably well behaved multisite reciprocal solutions. Fe–Zn exchange experiments (500°C) between tetrahedrite-tennantite and sphalerites yield values of 2.59±0.14 and 2.07±0.07 kcal/gfw for the Gibbs energies of the reciprocal reaction

Cu10Zn2Sb4S13+Cu10Fe2As4S13=Cu10Fe2Sb4S13+Cu10Zn2As4S13

and Fe–Zn exchange reaction

1/2Cu10Fe2Sb4S13+ZnS=1/2Cu10Zn2Sb4S13+FeS, respectively.

These results, plus petrologic studies of tetrahedrite-tennantite + sphalerite assemblages, and preliminary experimental results at 435 and 365°C suggest that the above parameters are insensitive to temperature and permit estimates for the Gibbs energies of the remaining two reciprocal reactions of “ideal” tetrahedrite-tennantite ((Ag,Cu)6Cu4(Fe,Zn)2 (As,Sb)4S13): Cu10Zn2Sb4S13 + Ag6Cu4Fe2Sb4S13 = Cu10Fe2Sb4S13 + Ag6Cu4Zn2Sb4S13 and Ag6Cu4Fe2Sb4S13 + Cu10Fe2As4S13 = Cu10Fe2Sb4S13 + Ag6Cu4Fe2S13 of 3.0±1.5 and 17±5 kcal/gfw, respectively.

These considerations suggest that tetrahedrite-tennantites are the “Cadillac” of reciprocal solutions and of petrogenetic indicators of hydrothermal mineralizing environments; they are the sulfide analog of amphiboles, the “Rolls Royce” of reciprocal solutions and petrogenetic indicators. In addition to providing a means for deducing aspects of the chemistry of many hydrothermal mineralizing fluids, our results afford an improved basis for understanding downstream chemical zoning in polymetallic base-metal sulfide and bonanza precious metal deposits. In particular they provide strong evidence that crystallochemical control coupled with As–Sb fractionation determines the distribution of silver in many zoned Pb–Zn–Cu–Ag deposits.

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