Oscillatory Zoning in Tennantite-(Fe) at the Darasun Gold Deposit (Eastern Transbaikal Region, Russia)

Abstract

Oscillatory zoning in tennantite-(Fe) crystals found in carbonate–sulfide aggregates at the Darasun gold deposit has been studied. Similar elements have been distinguished in zoned crystals: core, “identical oscillatory rhythm,” rhythms with crystallographic and wavy oscillatory zoning, and rim with poorly defined fine oscillatory zoning. Oscillatory zoning is due to considerable variations in the contents of semimetals (As and Sb) and formally divalent metals (Fe and Zn). Variations in As and Sb are coupled with changes in Fe and Zn, respectively. Both As and Fe contents increase in zoned crystals from the center to the edge: Sb/(Sb + As) ratio varies from 0.44 to 0.03, while Fe/(Fe + Zn) ratio varies from 0.38 to 0.75. The evolution of the composition of zoned tennantite-(Fe) crystals is similar to that of the fahlore at the deposit and to that in pseudomorphic rhythmically zoned tennantite aggregates. A negative relationship between Sb/(Sb + As) and Fe/(Fe + Zn) ratios has been revealed: it is stronger in the internal rhythms than in the external. The oscillatory zoning formed under conditions close to local equilibrium under the influence of external factors at the early crystal growth stages. The composition of zones reflects (random) fluctuations of the external parameters caused by variations in physicochemical conditions (T, fS2, fO2, and pH) and fluid composition, by fluid boiling or mixing of fluids from different sources. Subsequently, changes in the crystal morphology, irregular fluctuations in crystal compositions, and various trends in the behavior of components were caused by the appearance of a concentration gradient at the mineral–fluid boundary and kinetic phenomena on the mineral growth surface. The zoning developed under the influence of “internal” mechanisms under conditions far from equilibrium due to the interrelation between the growth of zones enriched in one component, resulting from the concentration gradient at the crystal–fluid boundary, and the incorporation of another component, in particular, as a result of the self-organization process uninfluenced by external factors. It has been concluded that the oscillatory zoning in fahlore is caused by the difference in solubility of the end-members of the tennantite–tetrahedrite solid solution due to changes in the metal and semimetal migration conditions with a decrease in temperature and fluid salinity. Hence, the oscillatory zoning in tennantite-(Fe) crystals at the Darasun deposit was related to self-organization processes complicated by random fluctuations (noise) of the external parameters.