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

The north-eastern part of the Graz Paleozoic thrust sheet at Strassegg, Aus tria is characterised by metamorphic assemblages that reflect increasing te mperature conditions from 300 degreesC in hangingwall to around 550 degrees 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 an d is commonly boudinaged. There is a correlation between vein- and the host -rock mineralogy. Additionally the delta O-18 and delta C-13 signatures of vein minerals (quartz and/or carbonates) vary systematically with the isoto pic signature of the host rock and its minerals. Within individual units, d elta O-18 of vein quartz has a narrow range of values in contrast with a la rger variation in both with delta O-18 and delta C-13 of carbonates. At Str assegg, the greenschist unit hosts an As-Au mineralisation. Arsenopyrite, p yrite, galena and Pb-(Cu)-Sb-sulfosalts are the dominant sulfides. Fluids a ssociated with vein and ore formation are composed of H2O-CO2-NaCl. Formati on pressure and temperatures, estimated from fluid inclusion and arsenopyri te thermometry are about 400 degreesC and 4-6 kb, Sulfur isotopes on sulfid es and whole rock samples give a narrow range around 0 parts per thousand d elta S-34. In general, wall rock alteration is minor, although within the g reenschist unit small alteration zones (characterised by Fe-dolomite, plagi oclase and a halo of increased As-contents) occur around quartz-carbonate v eins. The data indicate that the vein material segregated from the host lit hologies by fluid circulation, which was largely limited to individual sedi mentary formations. We suggest that in this area, metamorphism, vein format ion and ore mobilisation were related to enhanced heat flow associated with Late Cretaceous extension. (C) 2001 Elsevier Science B.V. All rights reser ved.