PROSPECTING SIDERITE WITH IP MEASUREMENTS – A FEASIBILITY STUDY
Norbert Klitzsch, Timo Houben, Marc Brunner, Rainer Kleinow, Ulrich Krueger, Peter Lokay · 2013
In the lignite mine Hambach, Germany, siderite (Fe[CO3]) occurs mainly in clays dispersed as well as concentrated. The latter known as clay iron ore appears either as layer or as an isolated nodule. Both cause mining and transport problems: They damage excavator buckets and conveyors due to their hardness and the sharpness of their fragments respectively. Therefore, the mining company (RWE Power AG) prospects siderite before mining. RWE builds a model of the lignite deposit and the overlaying sediments based on drilling and logging information. However, the applied logging methods (natural Gamma ray, resistivity, and density) are inadequate for prospecting siderite mainly due to its irregular spatial distribution. Hence, RWE was often unable to forecast the occurrence of siderite. The siderite prospection ability of the induced polarization (IP) method was investigated by field and laboratory measurements. In the laboratory, the frequency-dependent electrical properties of clay samples with different siderite contents were measured in the frequency range from 10 mHz to 40 kHz. While very high polarization effects (large phase shifts) were initially measured for dispersed and massive siderite in relatively dry samples, subsequent measurements on wet samples (corresponding to the in-situ condition) showed much lower polarization. Furthermore, the lab measurements revealed that, clay with dispersed siderite cannot be distinguished from siderite-free clay by its electrical properties in wet condition. Only the massive siderite (clay iron ore) sample stands out by a significantly higher imaginary conductivity. IP measurements were carried out on a 200 m long section in the lignite mine Hambach, Germany. After the measurements, the section has been excavated, so that the sediment and siderite distribution in the subsurface could be determined. The siderite distribution was surveyed in the embankment and compared to the inversion results of the IP measurement. We found the imaginary conductivity suitable for assessing the clay iron ore concentration (e.g. clay iron ore per m3) in the subsurface, even if the individual layers or nodules (for instance structures with a size of ≤ 1 m in ≥ 10 m depth) are not resolved by IP tomography.