Residual kaolin deposits are operated on a worldwide basis. The majority of them is derived from chemical weathering of felsic rocks during the Cenozoic. This is true for the kaolin deposits on the western edge of the Bohemian Massif. Here this type of deposit provides industrial minerals for the ceramic industry in SE Germany. This raw material formed under tropical climatic conditions during the Miocene and Pliocene across a vast peneplain. Only within the Naab-Wondreb Depression, however, were economic accumulations preserved from erosion. Here near Tirschenreuth, kaolin has been mined since its discovery in 1830. The semi-consolidated regolith is composed of quartz and opaline material, alkaline feldspar, different types of kaolinite, muscovite-illite, a variegated spectrum of 14 脜 phyllosilicates, Fe-Mn oxide-hydroxides and minor heavy minerals, mainly containing Ti. A precise determination of the mineralogical composition of the ceramic raw material has so far solely relied on laboratory techniques, e.g., XRD, XRF, and spectroscopic methods. To the contrary, this study takes a different approach, capturing digital data in the field to determine its mineralogical composition. Gamma spectrometry (K, U, Th), measuring the susceptibility of and analyzing short-wave infrared spectra, allows for an in-situ quality control of the run-of-mine raw material. It further assists in mapping the monotonous lithology and enables the authors to constrain the physico-chemical regime throughout formation of the kaolin deposit. The benefit for
technical mineralogy and applied economic geology, and the strong points of capturing digital data in the field, is the easy and cost-efficient handling of the various methods; methods that were applied in this way for the first time. The in-door calibration and test measurements as well as the data acquisition during the field survey are presented and the data are correlated with the enhancing or deteriorating impacts of the inferred minerals on the firing behavior of the ceramic product.
Collecting digital data can also help constrain the physico-chemical regime during formation of the argillaceous deposit and improve further exploration through fine-tuning the genetic model of residual kaolin accumulation. The late Carboniferous granites around Tirschenreuth underwent strong supergene alteration in the pH range of 2.5-7 at log a SiO2(aq) 鈭?#xA0;2.5 to 鈭?#xA0;3.5, which resulted in a thick kaolinitic regolith. Illite formed under slightly increased pH, between 4 and 7, attests to a change from tropical to more temperate humid conditions in the area. The chlorite-group phyllosilicates and nontronitic members of the smectite group came into existence when the pH increased to a pH interval of 7 to 11. This considerable change in the pH of the meteoric solutions occurred at the end of the Neogene between 4.55 Ma and 3.99 Ma.
The different field methods compensate for each others' weak points. Neither method is successful on its own, but the current configuration can easily be expanded to include additional methods as there are neutron- and micro-resistivity and air- and space-borne approaches to cover more swiftly a larger exploration or study area.
