The spodumene deposit at ?Weinebene?, Koralpe, Austria

Abstract

The Weinebene spodumene deposit is located in Carinthia, about 270 km southwest of Vienna. It is hosted by the medium- to high-grade metamorphic rocks of the Koralpe. The spodumene-bearing pegmatites form unzoned, dikelike bodies in eclogitic amphibolites and kyanite-bearing micaschists with concordant foliations. Their lateorogenic emplacement is probably of Variscan age. A younger, probably Alpine metamorphic event affected the pegmatites; it caused extensive recrystallization of the micaschist-hosted pegmatites, but had only a minor impact on the amphibolite-hosted ones. The spodumene-bearing pegmatites have been traced over a length of 1.5 km along strike and to about 450 m down dip. Their average thickness is 2 m, with a maximum of 5.5 m. Emplacement of the pegmatites caused an alteration halo of several dm in the hosting amphibolites, characterized by biotitization and formation of holmquistite. An aplitic spodumene-free seam of about 10 cm symmetrically borders the pegmatites. No contact phenomena are observed along the micaschist-hosted dikes. Spodumene is the only lithium-bearing mineral. Its average content is 22 vol% (equivalent to 1.68 wt% Li2O) in the amphibolite-hosted pegmatites and 15 vol% (equivalent to 1.13 wt% Li2O) in the micaschist-hosted dikes. Such a significant difference is also shown by rubidium (1,100 ppm vs 880 ppm) and cesium (60 ppm vs 25 ppm). Beryllium and tin are the only other significant trace elements. They show average contents of about 100 ppm and 120 ppm, respectively, with maxima of 1,690 ppm beryllium and 1,500 ppm tin. No correlation is found between lithium, tin, and beryllium. Niobium and tantalum are very low. A Na/K ratio of 1.2 emphasizes the predominance of albite over microcline. There is no discernable correlation between spodumene and albite contents, or between lithium and any other alkalies. The average K/Rb ratio is 20, pointing to the high degree of fractionation. Spodumene contains 7.4% Li2O and 0.45% FeO. There is no difference in the spodumene chemistry between the amphibolite-hosted and the micaschist-hosted pegmatites. Strong structural and textural evidence, mineral zoning, bulk composition corresponding to thermal minima in relevant experimental systems, and trace-element signature support an igneous derivation of the pegmatites. However, no granitic intrusion is exposed in the area, and the pegmatites could have been displaced from their source by tectonic events.