Pyrite crystals in the copper-sulphide veins at Nistru Mine (Románia)

Abstract

Cubic pyrite crystals are present in the deeper copper-sulphide rich portion of the epithermal veins
of the Nistru Mine (Misztbánya). At these deeper levels of the mine, the mineral assemblage includes
adularia, magnetite, molybdenite and there are higher Sn, Bi and W values. The presence of Ni and
detectable Co concentrations in the pyrite crystals indicate higher temperatures, the proximity of
magma, and alkaline-neutral pH conditions. Fluid inclusion formation temperatures of 250-275 °C
confirm that there were higher temperatures this vein environment. Breakage of crystals of various
mineral phases demonstrates ongoing tectonic activity subsequent to mineralization, and the corrosion
of these crystals shows that pH conditions changed with subsequent hydrothermal activity. At
intermediate levels in the vein system crystals of pyrite commonly have cubic octahedral forms, or
combinations of cubic, octahedral and dodecahedral forms. These are coupled with detectable arsenic
values in the pyrite. Here the fluid temperatures were lower and pH conditions different than at deeper
levels. At the highest levels of the vein system can be found silver-rich galena, bournonite, marcasite,
scalenohedral calcite, higher Sb values and Se, Te and TI contents can be detected in pyrite. Furthermore
the predominance of the dodecahedral form of pyrite indicate that the final and coolest epithermal fluids
were responsible for mineralization. Sparkly trapezohedral faces appear on pyrite only in this portion of
the veins. At the various depths at which copper-sulphide ore bodies occur throughout the mine, the
mineral assemblage and the distribution of trace elements suggest that with variable pH and pressure
conditions, pyrite crystal morphology follows a zonal pattern.