Die stratiforme Sulfidlagerstätte Walchen, Steiermark, Österreich: Geochemie und Genese

Zusammenfassung

Die Neubearbeitung derstratiformen Sulfidlagerstätte Walchen bei Öblarn, Steiermark, offen-bart eine eisenbetonte submarin-exhalative Metallkonzentration in einem vulkano-sedimentären Rahmen. Dieser liegt heute in Form metamorpher Gesteine der untersten Grünschieferfazies vor. Mineralchemische Untersuchungen zeigen, daß die ehemalige intensivste metamorphe Überprägung der Lagerstätte im Bereich von 450°–500°C oberhalb 4 kb stattgefunden hat. Metavulkanite, vertreten durch Grünschiefer, lassen alkalibasaltische Affinität erkennen. Die vulkano-tektonische Position der Lagerstätte ist in einem intrakontinentalen Riftsystem zu sehen.

Summary

The Walchen deposit consists of a 1–2 m thick layer of massive to disseminated pyrite-pyrrhotite-chalcopyrite ore with an average grade of 1% Cu. It has been mined intermittently in the past. The ore horizon occurs within sericitic and quartzitic phyllites which carry intercalations with significant carbonate, garnet and graphite contents. The succession is of lower Paleozoic age and forms part of the “Grauwackenzone”, a volcano-sedimentary unit which separates the Central Alps from the Northern Calcareous Alps.

The precursor rocks of the ore environment have been pelites and sandstones; greenschists occur in the hanging wall of the mineralization. Major and trace element analyses of greenschists reveal them as low-grade metamorphic equivalents of continental alkali basalts. Hydrothermal systems generated by volcanic activity were responsible for deposition of stratiform sulphides. The predominance of clastic sediments and the absence of stringer zones point towards a shallow depositional basin.

Microprobe analyses of garnets from the ore environment reveal an increase in spessartine contents from 7 mol% in phyllites to 20 mol% in the vicinity of the ore horizon. Similar data have been reported from other stratabound base metal deposits (Broken Hill, N.S.W.; Kreuzeck Mountains, Austria; Gamsberg, South Africa); the manganese concentrations represent “fossil” manganese haloes.

Pyrite carries minor Ni and Co (up to 0.03, respectively 0.3%), pyrrhotite averages 0.8% Ni and 0.1% Co, Fe-contents of sphalerite vary from 6.41–9.33%; Cd, In and Mn have not been recorded.

Garnet-biotite pairs suggest maximum metamorphic temperatures of 450°–500°C: pressures of 4 to 5 kb have been estimated. The Walchen deposit is interpreted as the product of submarine exhalative processes in an incipient rift. It was affected by prograde metamorphism during the Variscan orogeny, and by retrograde affects during a later event. Isochemical metamorphism resulted in the preservation of a primary manganese halo.