Skip to main content

Hydrothermal processes related to submarine iron ore formation: Insights from Devonian Lahn-Dill-type ores

Numerous studies on submarine iron-rich sediments (BIF, ironstones, etc.) focus on environmental changes and ocean chemistry, however, many questions related to underlying hydrothermal processes remain unresolved. As proximal chemical sediments, Lahn-Dill-type iron ores can provide insight into mechanisms of hydrothermal seafloor alteration related metal scavenging, and subsequent Fe-mineral deposition.

Lahn-Dill-type iron ores formed during the Middle/Upper Devonian within the Rhenohercynian back-arc basin associated with intraplate alkali basaltic volcanism. Ores formed on top of volcanogenic successions proximal to centres of volcanic activity. Typically, they occur as hematite-(siderite-)quartz ores reaching up to 60 wt.% Fe. We sampled a 5 m profile within the Fortuna mine in the eastern Rhenish Massif (Lahn syncline, Germany) and conducted a detailed petrographic study and whole rock ICP-MS as well as in-situ LA-ICP-MS geochemical analyses.

Iron is commonly mobile under reducing and acidic conditions. However, geochemistry of Lahn-Dill-type iron ores indicates that hydrothermal fluids may have been able to mobilise HFSE pointing at possible alkaline fluids. This is suggested by positive correlations of Fe with certain HFSE including Zr and HREE. Upon venting into seawater, iron likely precipitated as oxyhydroxides that preferably scavenged Si, W, Mo, Pb and V complexes from seawater by sorption. Subsequent deposition on the seafloor as a Si-Fe-rich gel is indicated by crescent-shaped shrinking cracks. Today, mineral assemblages within ore are characterized by fused hematite mats, hematite dispersed in quartz and/or siderite and microcrystalline quartz that may either be interpreted as diagenetic dissolution-precipitation processes, cyclic changes in primary fluid composition, or potentially both.


Leanne Schmitt1,5, Thomas Kirnbauer1, Thomas Angerer2, Dennis Kraemer3, Sabine Klein4,5
1Technische Hochschule Georg Agricola, Germany; 2Universität Innsbruck, Austria; 3Jacobs University Bremen, Germany; 4Deutsches Bergbau Museum Bochum, Germany; 5Ruhr-Universität Bochum, Germany