Thermodynamic properties of tetrahedrite-tennantites; constraints on the interdependence of the Ag Cu, Fe Zn, Cu Fe, and As Sb exchange reactions

Experimental constraints, petrologic studies, and theoretical analysis suggest that, energetically, tetrahedrite-tennantite sulfosalts are remarkably well behaved multisite reciprocal solutions. Fe-Zn exchange experiments (500C) between tetrahedrite-tennantite and sphalerites yield values of 2.59+0.14 and 2.07+0.07 kcallgfw for the Gibbs energies of the reciprocal reaction CuroZnrSbnSr3 + CuroFerAsnSr3 : CuroFezSb+Srr * CutoZnrAsnSt. and Fe-Zn exchange reaction 1/2 CuroFerSb4sl3 + ZnS: ll2 CutoZnrSbnSl3 * FeS, respectively. These results, plus petrologic studies of tetrahedrite-tennantite + sphalerite assemblages, and preliminary experimental results at 435 and 365C 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)uCun(Fe,Zn), (As,Sb)nSt.): CuroZnrSbnSr, + AguCuoFerSbnS* CuroFerSbnSr. + AguCunZnrSbnSt3 and AguCunFerSboS* * CuroFerAsoSl3 : CuroFe2Sb4S13 + Ag5CuaFe2Sr. 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 PVZn-Cu-Ag deposits.