Discovery of native aluminum on Variscan metagranitoids in Upper Carinthia, Austria: natural or anthropogenic origin?

Abstract

Native aluminum was discovered in 2004 at Hochwurten, Goldberg mountain group, Upper Carinthia, Austria. The discovery site is close to the Wurtenkees (glacier), where the dominant rock (Central gneiss) was exposed during the retreat of the glacier. Here, also veins with gold-bearing ores associated with tellurides (tetradymite, tsumoite, joséite-A) and Ag–Pb–Bi-sulfosalts are exposed and were mined intermittently during the last centuries. Native aluminum occurs at some distance (several 100 m laterally and vertically) to the veins as an isolated crystalline aggregate measuring 10 × 3 mm, and a closely associated flake (200 µm length), which was used for most of the studies. Both occur on gneiss and are intensely intergrown with it. Two groups of inclusions in the aluminum were observed and studied by SEM and EPMA. Group one consists of Bi–Te–(S)–(O) minerals, such as tetradymite, Bi2Te2S, tsumoite, BiTe, tellurobismuthite, Bi_2Te_3, bismuth and a montanite-type secondary mineral. The other group contains a new phase with composition (Al,Si)_(19)Fe_4, rare grains of hollisterite (Al_3Fe), and abundant Zn,Mg-bearing metallic nano-phases. A transition zone is developed between native aluminum and albite, a constituent of the gneiss, and is characterized by strongly changing concentrations of aluminum, silicon, iron and sulfur. Micro-computed tomography studies reveal that the Al metal is clearly intermixed with the host rock and not just juxtaposed over it. To shed further light on the origin of the Al metal, SIMS Mg isotopic measurements were carried out on the Austrian aluminum and on selected synthetic alloys with similar composition. A thorough discussion about the possible naturalness of this native aluminum and its possible formation is presented. The paper aims at augmenting the rare, assured knowledge of native metal formation/alteration under crustal and subcrustal conditions