Abstract

The sulfur isotope composition of sulfide ore minerals of the Drau Range varies considerably, down to centimeter-scale variations, both on a regional and a local scale. Broadly scattering δ34S values, ranging between +2 and –25 per mil with a mean around –17 per mil, are probably related to both local bacteriogenic and thermal reduction of Late Triassic marine sulfate. The 34S is typically more depleted in ore deposited in subtidal carbonate rocks than in the economic Zn and Pb ore, preferably deposited within intratidal carbonate host rock. The 34S/32S ratio increases on the order of FeS2 < PbS < ZnS, which reflects disequilibrium and low-temperature formation. The strong depletion of 34S in botryoidal sphalerite, ranging between –24 and –32 per mil, appears to be correlated with the crystal mode (wurtzite-sphalerite fibers) and the enrichment of the trace elements Fe, As, Tl, and Ge. Sphalerite generations of specific cathodoluminescense, color, and closely related trace element composition are correlated district-wide. A decrease of the trace elements Fe, As, Tl, and Ge in the sphalerite generations is coeval with a minor depletion of 34S in the range of –3 and –12 per mil for δ34S values in several hand specimens. Variations in δ34S values and trace element zonation on a regional to local scale indicates mixing of local sulfur sources with a regional metal-rich brine. However, distinct peaks in some ore deposits and more positive δ34S values of between –5 and –10 per mil are also compatible with derivation of the sulfur from basement rocks. The enrichment of the trace element Cu in orange-luminescing sphalerite negatively correlated with Fe, As, Tl, and Ge and, therefore, strongly suggests leaching of copper from Late Permian to Early Triassic continental red beds. Frequent evaporites in this stratigraphic section might have supplied sulfur of relatively heavy isotopic composition to the brine. Hence, some sulfur may also have been transported with a metal-rich brine.

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