U–Pb zircon ages constrain the Miocene age of hydrothermal activity in the iron mining Hüttenberg-Waitschach district (Austria), faulting and landscape evolution in the Eastern Alps

Abstract U–Pb zircon dating usually yields precise timing of high-temperature magmatic and/or metamorphic events. Less common is the potential to date alteration events. Here, we report an unexpected example of a mainly Miocene hydrothermal alteration and ore mineralization event in the Austroalpine mega-unit of the Eastern Alps, which occurred closely related to the landscape surface formation, and which dates the latest hydrothermal stages in the iron mining Hüttenberg-Waitschach district. There, mining was in operation in the last 2500 years, and the famous, particularly strong Mn-rich Ferrum Noricum was mined there during the Roman Empire. Near to the latest exploration galleries, U–Pb zircon dating of an amphibolite from the Plankogel Complex, which hosts the iron ore, yields a Cambrian age (512.6 ± 1.8 Ma) for magmatic zircon crystallization. Magmatic zircons are overprinted by mostly concordant zircons during two alteration stages, according to cathodoluminescence textures and age, between 26 to 20 Ma and 19 to 15 Ma. These zircon rims are interpreted to relate to hydrothermal activity and ore precipitation along fault zones, which are a distant expression of the Görtschitz Valley fault zone and formation of the Miocene piedmont alluvial fan during extension in front of the rapidly uprising Saualpe Block. There, the base of the Miocene Waitschach Gravel is exposed ~ 100 m above the investigated amphibolite. As the base of the Miocene land surface is at an elevation of ca. 900 m, subsequent incision of the Görtschitz Valley along the Görtschitz Valley fault indicates minimum 140 m surface uplift since Early Miocene indicating a significant change from west-directed to south-directed drainage. Graphical abstract