Imaging Techniques (Microanalytical 2D Methods)

In the field of mineral resources, the Federal Institute for Geosciences and Natural Resources (BGR) employs several methods to obtain spectral spatial information. These methods complement each other, creating interconnections that significantly broaden the spectrum of information and unveil new possibilities for interpretation.
The transitional domain from the macroscopic to the microscopic is of particular interest, as it allows the visualization of relationships that remain obscured at the macroscopic level and are excessively isolated in the microscopic realm. Advanced spatially resolved spectroscopic techniques bridge this gap, applying the same methods across various scales, thereby integrating information into adjacent scale levels.
For capturing optical hyperspectral information, SPECIM provides a hyperspectral scanner capable of capturing spectral ranges from visible/near-infrared to shortwave and longwave infrared comprehensively (VNIR, SWIR, LWIR), from entire core boxes (65 cm X 120 cm) to details with a spatial resolution of 25 µm. This scanner facilitates the acquisition of mineralogical and mineral chemical information. The identical spectrometers are utilized for field measurements, both in soil and airborne measurements through remote sensing.
In addition, a LIBS scanner (Laser-Induced Breakdown Spectroscopy) and a LIBS microscope developed jointly by BGR and Laser Technik Berlin (LTB) are available. These instruments, applied to entire core meters at 50 µm or to sections at 60 µm intervals, utilize laser-induced plasma spectroscopy to reveal the distribution of nearly all elements of the periodic table. Particularly noteworthy are the capabilities for detecting light elements such as Li and Na, as well as heavy elements like rare earth elements, which are challenging to capture with other methods.
A higher level of detail is achieved through Energy Dispersive X-ray Fluorescence Analysis using the µXRF microscope M4 Tornado, a workhorse in the field. It achieves relatively large-scale overview information from specimens and drill cores (up to 20 cm x 16 cm, optionally down to 5 µm spatial resolution). These data can be visualized as element distribution maps and, when hyperspectrally interpreted, presented as mineral phase distributions. This aids in selecting specific areas for further analysis. Data acquisition and analysis are partially automated using Petrographic Analyst (PA), the evaluation software developed by BGR and NV5.
The Raman microscope Invia Qontor by Renishaw provides the highest spatial information level down to < 1 µm resolution in both 2D and 3D. The simultaneous focusing and measurement of the Raman signal, even on rough samples, enable the correlation of mineralogical, crystallographic, sometimes mineral chemical, and microtectonic information on a large scale. Additionally, the IR microscope from the field of mineralogy allows for the identification of Raman-inactive phases if needed.
The application of scanning electron microscopy (SEM) in combination with the Mineral Liberation Analyzer based on BSE (back scattered electrons) and EDX signals allow grain boundaries to be displayed more clearly and used to check other methods with lower spatial resolution.
On a microscopic level, cathodoluminescence microscopy is available for polished thin sections as an imaging technique.
Modern 2D scanning methods such as µ-EDXRF, LIBS, SEM-MLA, Raman, cathodoluminescence microscopy, and hyperspectral imaging provide highly spatially resolved chemical/mineralogical overview and detailed data relevant to many geological questions regarding exploration of ore deposits. Based on this information, optimized sampling of selected areas for more complex and highly precise microanalytical methods, such as microprobe, LA-ICP-MS, or isotopic geochemistry, can be conducted. This approach allows answering questions related to mineralogy, mineral chemistry, trace element chemistry and isotopic distribution, providing insights into ore potential, genesis, origin, and age of rocks.
The applications of various scanning methods are highly versatile, applicable not only to ore deposit exploration but also to almost all geological inquiries. These range from exploration of deposits and mineral residues focusing on the genesis and distribution of valuable/pollutant elements, mineral and rock characterization, examination of laminated/warved sediments, flood or tsunami deposits, identification of volcanic interlayers, heavy mineral layers for stratigraphic or climate-relevant questions, characterization of rocks such as granite, salt, or clay, to visualize element migration during metasomatism to weathering or soil structures. Investigations can be conducted using various methods on drill cores, rock slabs, hand specimens, lacquer peels, and smear slides, as well as on polished thin and thick sections.