Coupled dissolution–precipitation reactions of tennantite-tetrahedrite minerals in the Darasun gold deposit (Eastern Transbaikalia, Russia)

Heterogeneous rhythmic-zonal aggregates of tennantite-IV replaced partly or completely early homogeneous Zn-tetrahedrite-I and euhedral (Fe-Zn)-tennantite-I crystal were found in ores of the Darasun gold deposit. Different replacement stages of fahlore were observed. It initiates at grain boundaries and is terminated by a complete transformation into pseudomorphic newly formed (Zn-Fe)-tennantite-IV aggregates rimed with Zn-tetrahedrite-IV. These aggregates associated intimately with bournonite and galena and their deposition initiated the pseudomorph formation. EMPA revealed that (Fe-Zn)-tetrahedrite richer in As relative to Zn-tetrahedrite-I was deposited at initial stage. Tennantite with wide variation in Sb/(Sb + As) and Fe/(Fe + Zn) ratios predominantes in heterogenous zonal aggregates of (Fe-Zn)-tetrahedrite-tennantite-IV. A negative correlation between the Sb/(Sb + As) and Fe/(Fe + Zn) was found in these minerals. In each site at the contact between Zn-tetrahedrite-I and newly formed (Fe-Zn)-tetrahedrite-tennantite-IV a miscibility gap between As and Sb a sharp drop in the Sb/(Sb + As) ratio and an increase in Fe/(Fe + Zn) ratio occur. Sharp saw-shape boundaries between Zn-tetrahedrite-I and tennantite-IV and voids in newly formed aggregates are considered to be evidence for couple dissolution-precipitation reactions. The dissolution was initiated due disequilibrium between Zn-tetrahedrite-I and an undersaturated fluid resulted from deposition of galena and bournonite. Precipitation of tetrahedrite-tennantite-IV occurred under oscillation in Sb/(Sb + As) and Fe/(Fe + Zn) ratios due to the metal and semimetal contents in the fluid. Crystallization temperature of zonal-heterogenous tennantite-IV aggregates was calculated by sphalerite-fahlore geothermometer which shows (134–161) ± 20 °С. Instability of early Zn-tetrahedrite-I resulted from fluid cooling, decreasing in fluid salinity, changing in tetrahedrite and tennantite solubility due to an evolution of migration conditions of semimetals.