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 f<sub>S<sub>2sub>sub> 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 (Cu<sub>3sub>SbS<sub>4sub>) made it necessary to perform a theoretical approximation, which yielded ΔG<sup>298sup><sub>fsub> of −402.25 kJ mol<sup>−1sup>. This estimate was combined with thermochemical data of all other phases from the available literature. In the T-logf<sub>S<sub>2sub>sub> space, phase equilibria indicate that rising f<sub>S<sub>2sub>sub> 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 S<sub>2sub> condensation curve. Adjusting ΔG<sup>298sup><sub>fsub> of famatinite to 3–5 % more negative ΔG<sup>298sup><sub>fsub> values shifts the reactions towards reasonable conditions, and places the stability fields of the observed mineral assemblages within the T- logf<sub>S<sub>2sub>sub> 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-logf<sub>S<sub>2sub>sub> estimates. However, more experimental data on the thermochemical properties of famatinite are clearly needed.