Monomineralic Fe-Zn tetrahedrite-tennantite (fahlore) veins from the historic silver mines of Brixlegg in the Inn Valley (North Tyrol, Austria) show replacement textures containing a newly formed mineral assemblage (enargite/luzonite-famatinite + chalcostibite + pyrite + sphalerite ± stibnite), resulting from fahlore breakdown. In order to deduce the T-log fS2 conditions of this reaction, appropriate mineral equilibria in the Cu-Fe-Sb-S and Cu-Zn-Sb-S systems were calculated using available thermochemical data.
The lack of thermochemical properties of famatinite (Cu3SbS4) made it necessary to perform a theoretical approximation, which yielded ΔG298f of −402.25 kJ mol−1. This estimate was combined with thermochemical data of all other phases from the available literature. In the T-logfS2 space, phase equilibria indicate that rising fS2 and/or dropping T lead to the breakdown of Fe-Zn tetrahedrite-tennantite. However, uncertainties stemming from the theoretically estimated thermochemical properties of famatinite place the calculated reactions near or above the S2 condensation curve. Adjusting ΔG298f of famatinite to 3–5 % more negative ΔG298f values shifts the reactions towards reasonable conditions, and places the stability fields of the observed mineral assemblages within the T- logfS2 window estimated by independent methods.
This study therefore indicates that combination of existing thermochemical data and detailed mineralogical and petrological investigations on natural enargite/luzonite-famatinite-bearing assemblages may lead to meaningful T-logfS2 estimates. However, more experimental data on the thermochemical properties of famatinite are clearly needed.