Reactions involving famatinite and Fe-Zn tetrahedrite: thermochemical evaluation of phase relations in the Cu-Fe-Sb-S and Cu-Zn-Sb-S end-member systems

Monomineralic Fe-Zn tetrahedtite-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(S2) 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 Delta G(f)(298) of -402.25 kJ mol(-1). This estimate was combined with thermochemical data of all other phases from the available literature. In the T-logf(S2) space, phase equilibria indicate that rising f(S2) 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-2 condensation curve. Adjusting Delta G(f)(298) of famatinite to 3-5 % more negative Delta G(f)(298) values shifts the reactions towards reasonable conditions, and places the stability fields of the observed mineral assemblages within the T- logf(S2) 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(S2) estimates. However, more experimental data on the thermochemical properties of famatinite are clearly needed.