Evolution of veins and sub-economic ore at Strassegg, Paleozoic of Graz, Eastern Alps, Austria: evidence for local fluid transport during metamorphism

Abstract

The north-eastern part of the Graz Paleozoic thrust sheet at Strassegg, Austria is characterised by metamorphic assemblages that reflect increasing temperature conditions from 300°C in hangingwall to around 550°C in footwall units. The main quartz and/or carbonate vein generation cuts the schistosity at moderate to low angles is folded with the schistosity and is commonly boudinaged. There is a correlation between vein- and the host-rock mineralogy. Additionally the δ 18O and δ 13C signatures of vein minerals (quartz and/or carbonates) vary systematically with the isotopic signature of the host rock and its minerals. Within individual units, δ 18O of vein quartz has a narrow range of values in contrast with a larger variation in both with δ 18O and δ 13C of carbonates. At Strassegg, the greenschist unit hosts an As–Au mineralisation. Arsenopyrite, pyrite, galena and Pb–(Cu)–Sb–sulfosalts are the dominant sulfides. Fluids associated with vein and ore formation are composed of H 2O–CO 2–NaCl. Formation pressure and temperatures, estimated from fluid inclusion and arsenopyrite thermometry are about 400°C and 4–6 kb. Sulfur isotopes on sulfides and whole rock samples give a narrow range around 0‰ δ 34S. In general, wall rock alteration is minor, although within the greenschist unit small alteration zones (characterised by Fe–dolomite, plagioclase and a halo of increased As-contents) occur around quartz–carbonate veins. The data indicate that the vein material segregated from the host lithologies by fluid circulation, which was largely limited to individual sedimentary formations. We suggest that in this area, metamorphism, vein formation and ore mobilisation were related to enhanced heat flow associated with Late Cretaceous extension.