Multi-parameter petrophysical characterization of Variscan greisen rocks from the Southern Bohemian Batholith (Austria) and the Eastern Erzgebirge Volcano-Plutonic Complex (Germany)

Abstract

Understanding the variation and correlation between physical properties of ore bodies of interest is vital for every exploration project. Therefore comprehensive petrophysical laboratory measurements on representative sample material from two dispersemineralized greisen bodies of the Southern Bohemian Batholith (Austria) and the Eastern Erzgebirge Vulcano-Plutonic Complex (Germany), respectively, are presented in this study. As many greisen bodies host mineral deposits of economic interest, the aim of the study was to identify petrophysical properties which are suitable for distinguishing greisen bodies from the surrounding rocks. The gathered physical information (density, porosity, velocity, magnetic susceptibility, gamma spectroscopy, electrical resistivity, spectral induced polarization (SIP)) indicates that a differentiation of mineralized greisen zones of interest from the surrounding rocks is only possible based on a combined investigation of several parameters. Elevated grain and bulk densities (mean: 3 2.75 g/cm and 2.67 g/cm , respectively) are indicative of the Erzgebirge low-porosity (mostly < 3%) mineralized greisen rocks in comparison to the lower densities of the surrounding Teplice rhyolite and albite granite. Bulk density, however, is critically influenced by porosity and is therefore not suitable to distinguish the Austrian greisen rocks from the surrounding two-mica granites, 3 despite the greisens' comparably high grain density (mean: 2.74 g/cm ). Their higher porosity (mean: 5.7%) also results in lower elastic wave velocities (mostly < 2900 m/s) and lower electrical resistivities (mostly < 2100 Ohmm) than the surrounding rocks. The electrical resistivities and elastic wave velocities of the German greisen samples are, in contrast, rather variable with no distinct level above or below the neighboring rocks. Magnetic susceptibility also delivers a good contrast between the greisens and their surrounding rocks as it is highly responsive to elevated mica contents or the presence of oxidic ore minerals. If quartz content predominates or oxidic ores are absent, however, there is no contrast in the magnetic susceptibility of the greisens and the surrounding rocks. With regard to natural gamma radiation, the greisens are characterized by generally low absolute readings as well as low U counts, while K and Th counts seem to be dependent on mica type. Results from SIP measurements suggest that especially phase information can be used to adequately differentiate greisen zones associated with Li-mineralization from country rocks in the Erzgebirge. In Austrian greisen rocks, however, the characteristics of the SIP spectra are less distinct. A crossplot of the real vs. the imaginary part of complex conductivity, however, is suitable for identification of rocks with high mica content. Based on these results a combined investigation of density, magnetic susceptibility, and spectral gamma seems to be most promising for identification of greisen bodies. und dem Auftreten von oxidischen Erzmineralen ko...

Ambient seismic noise analysis of LARGE-N data for mineral exploration in the Central Erzgebirge, Germany

“…Variations in resistivity are instead attributed to differences in the degree of fracturing, alteration and mineralization. There is no reference to greisen in this report, but in the nearby Altenberg area, the greisen characteristically exhibits higher specific electrical resistivities (∼ 3250 versus ∼ 1485 m) than their granite host rocks (Müller-Huber and Börner, 2017).…”

“…The geophysical method used for the delineation of mineral deposits needs to be adapted to the geological setting and type of mineralization. In this case, tin and lithium-bearing greisen and vein deposits similar to those occurring in the Geyer-Ehrenfriedersdorf area have been documented to differ from the surrounding host rocks in terms of their seismic velocities and electrical conductivities (Müller-Huber and Börner, 2017). However, the region is densely populated and contains environmentally sensitive areas, placing additional constraints on the chosen mineral exploration techniques.…”