The Genesis of the Gold-Arsenopyrite Mineralization in Straßegg in Relation to the Pb-Zn Mineralization of Arzberg in the Graz Paleozoic and the Pyrrhotite Mineralisation/Naintsch

The Upper Austroalpine Graz Paleozoic is classically interpreted as a nappe complex of low-grade metamorphosed sediments and volcanics of Middle Silurian to Carboniferous age. The internal structure includes three major nappe systems with similar lithological assemblages but different degree of metamorphism: (1) the lower nappe system comprises the Schöckl Nappe, dominated by greenschist-facies volcanic and siliciclastic rocks of Late Silurian to Early Devonian age and Middle Devonian carbonate platform sediments. (2) The intermediate nappe system, consisting of Laufnitzdorf and Kalkschiefer Nappe, contains pelagic limestones, siliciclastics and volcanoclastic rocks of Silurian to Late Devonian age. (3) The upper nappe system, including Rannach and Hochlantsch Nappe, is composed of Silurian to Early Devonian volcanoclastics followed by Early to Middle Devonian siliciclastics and fossil-rich carbonates. Finally, the stratigraphic sequence terminates with shallow marine carbonates and carbonate-free shales of Westphalian age. The lower nappe system of the Graz Paleozoic contains numerous ore deposits. The studied ore deposits contain comparable lithologies but are different concerning the time and mechanism of ore precipitation. 1) An electrum-arsenopyrite mineralisation at Strasseggis is hosted in quartz veins within the metavolcanics. This ore deposit is structurally controlled during quartz vein precipitation caused by Late Cretaceous extensional teconics. 2) At Naintsch area a former sub-economic sulphide ore deposit is hosted within metavolcanics and black schists. Mineralisation is not associated with quartz veins and defines therefore a primary, metasedimentary, stratiform sequence of Upper Silurian /Lower Devonian age. 3) Pb-Zn-baryt mineralisation at Arzberg is distributed into a primary sedimentary ore sequence of Silurian/Devonian age and a second structurally controlled mineralization hosted within quartz veins during Late Cretaceous extensional tectonics. Three P-T-deformational paths at the study areas are correlated with mineralogical and petrological studies. They are related to distinct stages of Late Cretaceous extensional tectonics. The chemistries of the possible mineralizing fluids extracted from quartz extensional gashes vary from fluid mixing of CO2-H2O and H2O-NaCl at Strassegg to subordinate CO 2 -N2 -H2 O and H2 O-NaCl-CaCl2 MgCl 2 at Arzberg and to dominant H2O-NaCl-CaCl2-MgCl2 at Naintsch. P-T-conditions of the secondary mineralisation are as follows: T = 450-500C and P = 3-5 kbar at Strassegg; T = 300-350C and ca. P = 2 kbar at Arzberg. Based on the studied ore deposits the geodynamic evolution of the Graz Paleozoic reflects a first stage mineralisation that is correlated with the formation of a passive continental margin. Rifting is documented by intracontinental volcanism of alkaline affinity in the Silurian time. Mineralisation appears now as stratiform, alternated and folded metasedimentary sequence that is nearly untouched during the Variscan and Permian orogenic events. The absence of compressive Variscan structures and the continuous sedimentary facies distribution up to Westfal, supports the assumptions that Upper Austroalpine units (e.g. Graz Paleozoic, Gurktaler Nappe System), were holding a passive upper plate position until the Alpine thrust tectonics occurred. Related structures are mainly preserved within the central parts of the Graz Paleozoic. Subsequent extensional tectonics generated second stage of mineralisation due to quartz vein precipitation and local fluid flow.