Petrological diversity of leucocratic rocks at the sodium-potassium feldspar deposit Krásno – Vysoký kámen

Abstract

The presented study aims to reevaluate the geological structure of the Krásno-Vysoký Kámen feldspar deposit and its relationship to the surrounding rocks using chemical and imaging methods. For this purpose, we have documented in detail all the preserved fragments of drill cores from the survey in the 1960s and 1970s years, and 21 exploratory boreholes of the KP series (Fig. 1) realized in 2021, including about 600 chemical XRF bulk-rocks analyzes of rocks from these drillings. The Krásno-Vysoký Kámen open pit is located about 1 km northwest of the Krásno village, western Bohemia. Subject of mining is a complex of leucocratic, feldspar-rich granitoids consisting of predominant medium-grained leucogranite with mainly subhorizontal intercalations of syenite, aplite-pegmatite and feldspar-rich metasomatites. Both the immediate bedrock and the exposed overburden of the leucogranite complex consist of biotite granites. The leucogranite complex forms a lenticular body, the lower boundary of which decreases from N and NW towards E and S (Fig. 5). Especially on the western side of the open pit, the contact of the two rocks is relatively steep, further to the east it flattens. Medium-grained leucocratic granite makes up most of the volume of the deposit. It has a magmatic texture (Fig. 2a) with euhedral, short columnar albite crystals, subhedral orthoclase grains, irregular late quartz grains and low mica content (Tab. 2); apatite and rare topaz and niobium rutile are also present. In the SW edge of the quarry, contact facies of the leucogranite with several layers of oriented crystallization of quartz is exposed. Locally, leucogranite changes to leucosyenite (Fig. 2b). The leucosyenite is still a medium-grained rock with a magmatic texture, but the quartz content decreases (<10%) and the mica is only accessory. Several flat veins of virtually mica-free aplite (Fig. 2c), from several dm to 5 m thick, are lined along the upper contact by a zone of oriented K-feldspar and quartz crystallization (i.e. stockscheider). Alkaline metasomatites (Fig. 2d) are medium to fine grained. Medium-grained varieties are macroscopically hardly distinguishable from igneous syenites; fine-grained varieties are similar to aplites. Feldspars in metasomatites already have a completely irregular worm-like shape, while the ratio of both feldspars fluctuates strongly. The underlying biotite granite is currently well exposed in the NW part of the quarry and was reached by wells KP4, KP5, KP6, and KP7. It is medium-grained granite with Li-biotite and topaz. The contact between this granite and the overlying leucocratic complex was interpreted as rapid transition in old boreholes (Pácal and Pavlů 1979), but sharp contact was found in the borehole KP4. The overlying biotite granite was exposed in the SW parts of the quarry and confirmed by 2 boreholes (KP2, KP3). Both types of biotite granite differ statistically in Fe, Na and K contents. The medians of the chemical composition of all rock types defined above are shown in Tab. 3, relationships between some elements in Fig. 3. While both types of biotite granites and aplite have a relatively homogeneous chemical composition, the composition of leucogranite, syenite and metasomatites is very variable, especially in terms of SiO2 and alkalis. When comparing both alkalis, a significantly greater variability of K2O contents (<1 to 9 wt. %) compared to Na2O (mostly 1-6 wt. %, but fewer samples with marginal values) is evident. Extreme samples containing almost only one of the feldspars can be found in the case of syenite, metasomatites and leucogranite. The total content of feldspars varies mainly between 40 and 70 wt. % in leucogranite, 70–90 wt. % in syenite, 50–70 wt. % in aplite and 50–90 wt. % in metasomatites. Significant vertical changes in the chemistry of macroscopically homogeneous leucocratic rocks are well documented in boreholes KP13 and KP14 (Fig. 4).