ALBERT

Description: The interdisciplinary DeepEarthShape project focusses on the weathering zone with drillings and geophysical, geochemical and geobiological approaches. The weathering zone is the uppermost part of the Earth's crust where rocks and soils experience breakdown through the impact of air/gases, water and/or biological organisms. The full picture of weathering processes and our insight into the critical zone is still limited. Since some of the properties of the weathering zone seem to be linked with climate, a set of sites is studied within the framework of the DFG Special Priority Program 1803 belonging to different climate zones and thus experience different vegetation, precipitation and erosion. We utilised a combination of seismics and Radio-Magnetotelluric (RMT) measurements along ~200m long profiles at three study sites in Chile. For the RMT experiment, we used a horizontal magnetic dipole transmitter together with a nearby MT station. To develop the method into an applicable tool for this kind of inter-disciplinary studies, we show RMT results at different stages: Experimental layout tuned with synthetic simulations, RMT data processing using machine learning approaches. And finally, we will show and discuss 2D and 3D inversion results together with geophysical logging data and lab measurements. First results indicate that we can image precipitation and shallow fluid enhanced zones and provide crucial information for other disciplines. Minor faults and folds can be traced into the active weathering zone and linked with surface and borehole information.

Description: Fe-oxide deposits of the Lahn-Dill-type in the eastern Rhenish Massif comprise haematite and quartz with minor siderite, magnetite, and calcite. The deposits are located in the hanging wall of thick volcaniclastic rock sequences and mark the Middle to Late Devonian boundary. Varying ore types with accompanying footwall rocks were sampled from two formerly important ore deposits, the Fortuna mine (Lahn syncline) and the Briloner Eisenberg mine (East Sauerland anticline), in order to elucidate the interplay of processes leading to ore formation. Deposit geology, petrography, and whole-rock geochemistry suggest that the ores formed by iron mobilisation from deeply altered footwall volcaniclastic rocks, subsequent venting of a modified H2O-CO2-Fe-rich and H2S-poor fluid, and precipitation on the seafloor (sedimentary-type), or locally by metasomatic replacement of wall rocks (replacement-type). Petrographic analysis to the sub-micron scale revealed that the sedimentary-type ores most likely formed from a Fe-Si-rich gel and accompanying maturation. Early gel textures include the presence of spherules, aggregates, tubes, and filamentous stalks consisting of nanocrystalline haematite dispersed in a matrix of microcrystalline quartz. Local diagenetic Fe3+ reduction within the gel is indicated by siderite replacement of haematite. Replacement-type ores formed due to a two-step process including coprecipitation of (precursor) haematite and carbonates and subsequent metasomatic replacement by haematite. These ore-forming processes took place during a time when several restricted shallow marine basins in the north-eastern Rheic Ocean were influenced by extensive volcanism and associated hydrothermal fluid flux. Examples of similar volcanic-associated Fe-oxide occurrences of Silurian to Carboniferous age can be categorised as being of Lahn-Dill-type ores as well.