Lithium content and mineralogical composition of fractured salt clay (Upper Permian)

Lithium is a trace component, which is frequently observed in salt deposits and salt solutions collected in salt mines, respectively (Mertineit & Schramm 2019). So far, no naturally formed Li-bearing salt mineral is known, thus, the origin of Li in salt deposits must be related to other sources, e.g. to detrital phyllosilicates (Braitsch 1971). Detailed investigations on the Li content, the occurrence within a mine and the mineralogical composition of specific stratigraphic layers enable the reconstruction of rock-fluid interaction and fluid migration pathways. This is important for the construction, design and dimensions for a repository for radioactive waste in rock salt.

To verify which minerals are Li-hosts, diapiric Upper Permian (Zechstein) samples from the uppermost Staßfurt-Formation and the lower Leine-Formation were investigated for their mineralogical-geochemical composition. The succession contains salt clays, anhydrite and carbonate rocks as these rocks reveal the highest Li content (up to 159 µg/g bulk rock). The samples were previously investigated using ICP-OES, ICP-MS, XRD, SEM and thin section microscopy. Beside typical salt minerals in varying amounts (halite, anhydrite, magnesite, sylvite, carnallite), most samples consist of quartz, illite-muscovite, chlorite (clinochlore) and biotite, all of them with a grain size of ≤100 µm, often <20 µm. Only few samples contain traces of kaolinite, koenenite, hydrotalcite, anatase and tourmaline.

Additionally, µXRF and imaging LIBS (Laser Induced Breakdown Spectroscopy) analyses were performed at the same specimen to obtain detailed information of the element distribution including Li on thick section scale (Nikonow et al. 2019).

The clay containing rocks are intensively deformed by boudinage and subsequent brittle fracturing. The fractures are oriented in different directions and are filled with halite and/or carnallite and single grains of anhydrite and magnesite. Relics of bedding are present, but the phyllosilicates do not show a pronounced shape-preferred orientation. Shear strain is indicated by a slight rotation of single rock fragments. The spatial distribution of Li shows that Li is enriched in certain areas. Li accumulations are observed in single silicate grains, which are unequally distributed in a very fine-grained clay matrix. Furthermore, Li is enriched at the fracture rims, often associated with seams of Fe-bearing phases and probably organic matter.

Depending on the mineralogical composition of the investigated rocks, the Li content varies significantly. Li probably originates from illite-muscovite and a Li-bearing variety of a tourmaline (elbaite). Li was mobilized during brine-host rock interaction and precipitated in fracture infill, probably at reducing geochemical conditions. However, due to the limited spatial resolution of most used methods compared to the very small grain size of the rocks, a distinct relation of Li content to a specific mineral phase requires further analysis.

 

Braitsch 1971. Springer-Verlag, https://doi.org/10.1007/978-3-642-65083-3

Mertineit & Schramm 2019. Minerals 9, 766; doi:10.3390/min9120766.

Nikonow et al. 2019. Mineralogy & Petrology 113, https://doi.org/10.1007/s00710-019-00657-z