Finding lithological boundaries of Harzburgite and Ecker gneiss at Eckertal

The Harz mountains lie in the central part of Germany. They are made up of uplifted Paleozoic crust and belong to the Rhenohercynian zone of the European Variscides (Sano et al., 2002). The rock record spans approximately 500 million years starting from the Cambrian to the Permian which was then uplifted in the Cretaceous.

The oldest rock formation in the Harz Mountains is the Proterozoic Ecker Gneiss. Zircon dating revealed the age of the gneiss to be Silurian and Devonian and hence the protolith must be older than Devonian (Sano et al., 2002). The sequence also consists of granite intrusions like the Ocker, Brocken and Ramberg granites that were emplaced after the orogenic activities ceased; and the Harzburg gabbronorite complex.

The Harzburg gabbronorite was emplaced in the early Permian period- 293-297 million years ago (Baumann et al., 1991). This SSW-NNE trending ultramafic intrusion is composed of two oval-shaped bodies (Sohn et al., 1996) and is thought to be a layered mafic intrusion (Vinx, 1982). The intrusion is predominantly gabbronorite with local occurrences of mafic rocks like dunite, norite, diorite and quartz-diorite (Sano et al., 2002). It is also the type locality for a type of peridotite that is predominantly made up of olivine and orthopyroxene, called harzburgite.

Along its eastern margin, the Harzburg gabbronorite lies adjacent to the Ecker Gneiss. It is this contact that this study aimed to map.

The data used comprised of an integration of two geophysical methods- magnetic survey data and gamma-ray data collected during aerial surveys and during fieldwork.

Methodology

Study Area

The study area was selected from the 100k map to cover the area where the contact was situated. The coordinate reference system used is WGS 1984 UTM zone 32N as this covered Germany.

Harzburgite and Ecker Gneiss are two contrasting lithologies one being ultrabasic and the other felsic. Hence geophysical methods that were sensitive to this composition contrast were selected. Magnetic and Gamma-ray datasets were chosen. Table 1 & 2 show the values for magnetic susceptibility and Potassium (K), Thorium (Th) and Uranium (U) content for harzburgite (ultrabasic igneous) and gneiss.

Datasets

The datasets used in this study are described in this section. The gamma-ray and total magnetic intensity data were obtained from helicopter surveys carried out by BGR in 1983-84.

Gamma-ray

The gamma-ray data was obtained by a helicopter flown along fixed flight lines. The locations where measurements were made were plotted as points on a map. To digitize this map the intersections of the flight lines with the contours on the map were recorded as a table. Each point on a flight line was recorded as a point with X and Y coordinates and the corresponding element’s gamma-ray concentration. The table also contained information on the start and end point for each flight line. This was done for all the flight lines from west to east on the map, creating a database table stored in ASCII-xyz format. These points were then plotted in Geosoft wherein a raster was created by interpolating the values in between the flight lines. The concentrations of Potassium (40K) were reported in per mil (0/00) and those of equivalent Thorium (232Th) and Uranium (238U) in ppm. The concentration of Th and U are based on concentrations of their daughter isotopes after radioactive decay. Some of these isotopes along the decay chain are volatile and escape from the system and hence the concentrations are reported as ‘equivalent’ Th (eTh) and eU.

A ternary image was then prepared using these 3 element concentration maps with K in red, Th in green and U in blue. Field measurements using a gamma-ray spectrometer at two stops- one at a harzburgite outcrop and the other at the Ecker gneiss outcrop were used to set thresholds when comparing ternary image data so as to identify the contact.

Magnetic

The aeromagnetic data was digitized in a similar manner. The interpolated total magnetic intensity grid created in Geosoft was converted to reduced-to-pole values. The units for the grid were nanotesla.

Field data collected using a gradient magnetometer is plotted to corroborate the mapped contact. Line 154 of the aeromagnetic data is used for the current study as it is the closest to the field survey points and can be used in conjunction with them. Magnetic susceptibility values measured on field were also used. The unit for this dataset is 10^-3 SI units.

Method

The ternary image was used to map the contact. Harzburgite shows very low values of K and U (Table 1) and appears as dark grey or black pixels, while gneiss appears lighter owing to a higher concentrations of radioelements.

The data from aeromagnetic line 154 and the magnetometer readings was used to verify the mapped contact and make adjustments to the position of the boundary if needed. The field data- gamma-ray and magnetic susceptibility readings for the Harzburgite type locality and the Ecker Gneiss outcrop were then used to confirm the positions of the rock types on either side of the mapped contact.

Results

The Gamma-ray data taken from field measurements is shown in table 3. The instrument took 3 readings for each point and there was a repeat reading taken for the harzburgite outcrop. This data was plotted on a ternary diagram (Figure below) similar to the one shown in the legend for the ternary map.

The map (Figure 1 below) shows the location of the 2 points as yellow triangles. The values from the interpolated raster are much higher which could be due to the very course 50m resolution of this grid. K values for gneiss are higher than harzburgite and hence a map for areas with low K values was created to help distinguish the two lithologies (Figure 2 below).

The aeromagnetic data as well as magnetometer data taken on field were plotted. Line 54 was chosen for its proximity to the magnetometer data and the contact. The figure below shows the two datasets and the contact created from the ternary image. The field data had an attribute table for remarks and the presence of a felsic outcrop was noted.

The magnetic intensity also showed a change in values from positive to negative indicating the presence of a distinct contact, although since this dataset consists of reduced to pole values of total magnetic intensity, it can show the presence of an anomaly as a high and low but not the exact boundary. The remarks also stated a possible contact, but as no other datasets were available to support this, it was not considered. The field remarks were used to update the contact boundary to include the felsic outcrop on the gneissic side of the contact as shown in the figure below.

Conclusion

The contact between Harzburgite and Ecker gneiss was mapped using gamma-ray data compiled into a ternary image to demarcate the radioelement compositional contrast between the two lithologies. The use of magnetic datasets helped verify and modify this contact mapping. The use of both field data and aerial survey data enhanced the quality of the output as the aerial survey had a resolution of 50m and hence some outcrops could have been missed. The contact was mapped using datasets at a resolution of 50m which improved the resolution as compared to the 1:100,00 map.

References

Baumann, A., Grauert, B., Mecklenburg, S., & Vinx, R. (1991). Isotopic age determinations of crystalline rocks of the Upper Harz Mountains, Germany. Geologische Rundschau, 80(3), 669-690.

Dinh Chau, N., Dulinski, M., Jodlowski, P., Nowak, J., Rozanski, K., Sleziak, M., & Wachniew, P. (2011). Natural radioactivity in groundwater–a review. Isotopes in environmental and health studies, 47(4), 415-437.

Hunt, C. P., Moskowitz, B. M., & Banerjee, S. K. (1995). Magnetic properties of rocks and minerals. Rock physics and phase relations: A handbook of physical constants, 3, 189-204.

Mernagh, T. P., & Miezitis, Y. (2008). A review of the geochemical processes controlling the distribution of thorium in the Earth's crust and Australia's thorium resources (p. 48). Geoscience Australia.

Sano, S., Oberhänsli, R., Romer, R. L., & Vinx, R. (2002). Petrological, geochemical and isotopic constraints on the origin of the Harzburg intrusion, Germany. Journal of Petrology, 43(8), 1529-1549.

Sohn, W. (1956). Der Harzburger Gabbro. Geologisches Jahrbuch 72, 117–172.

Vinx, R. (1982). Das Harzburger Gabbromassiv, eine orogenetisch geprägte layered intrusion. Neues Jahrbuch für Mineralogie-Abhandlungen, 1-28.

Cover image: https://www.flickr.com/photos/dnishiohamane/