Statistical analysis and origin of the magnetic susceptibility of drill cuttings from the 9.1-km-deep KTB drill hole

Abstract

Magnetic susceptibility was measured at 2 m depth intervals on drill cuttings from the main drill hole of the German Deep Drilling Project KTB. Metamorphic rocks (metabasites and gneisses) were the rock types most frequently found down to a depth of 9101 m. Petrophysical (susceptibility, density), geochemical (element concentrations), lithological and petrological data (ore mineral concentrations, lithological components, alteration index) were used for a statistical analysis. The histograms of magnetic susceptibility show nearly log-normal distributions with two distinct peaks depending on the lithology. The most frequent susceptibility values are 0.266 x 10−3 SI for gneissic rocks and 0.847 x 10−3 SI for metabasic rocks (mainly amphibolites). The higher level of metabasite susceptibility is caused by higher contents of paramagnetic silicates such as hornblende. A theoretical paramagnetic susceptibility was calculated from the iron and manganese contents derived from X-ray fluorescence (XRF) measurements. The ferrimagnetic susceptibility was determined by subtracting the theoretical paramagnetic susceptibility from the measured susceptibility. Cross-plots of the ferrimagnetic susceptibility versus density are used to discriminate between samples with predominantly magnetite or pyrrhotite as the main ferrimagnetic mineral. Samples containing mostly pyrrhotite show susceptibilities not exceeding 6 x 10−3 SI, whereas the highest measured susceptibilities of 66.5 x 10−3 SI correspond to zones exclusively with magnetite. A factor analysis was applied to investigate the background factors representing the data variabilities. The factor analysis reduces 13 original variables from the complete depth section to five independent initial factors. These explain in total 66.2 per cent of the total data variance. The most significant factor, 1, correlates with metabasite content, density and paramagnetic susceptibility and it anticorrelates with gneiss content. The next significant factor, 2, correlates with ferrimagnetic susceptibility and magnetite content. Factor 3 correlates with the amount of cataclastic rocks, factor 4 with hornblende gneiss and factor 5 with pyrrhotite.