Technischer Bericht NTB 88-30

In the Krunkelbach uranium pilot-mine near Menzenschwand in the Black Forest massif (Southern Germany), U-ore veins, variably altered granite, waterbearing fractures and groundwater samples have been investigated mineralogically and geochemically and compared with rocks and groundwaters of the region. These investigations were performed in order to clarify the genesis and history of the deposit and to test the suitability of this natural accumulation of radioisotopes for the study of natural analogues of potential mobilization/retardation processes in radioactive waste repositories.

The Krunkelbach mine is situated in the higher part of the Black Forest which is a probable infiltration area for some of the deep groundwaters in the crystalline basement of Northern Switzerland. The study of groundwaters in the Black Forest can therefore give valuable information on the early evolution of the deep groundwaters in Northern Switzerland.

The host rock of the mineralizations is the Lower Carboniferous Bärhalde granite. This highly differentiated two-mica granite has affinities to tin granites. Its uranium content (14 ± 3 ppm) is quite high, while the thorium content is low (12 ± 5 ppm). Uranium in the unaltered granite occurs predominantly as magmatic uraninite. This rock-forming uraninite most probably was the source for the uranium concentrated in the ore deposit.

High-temperature (300 – 400°C) hydrothermal alterations in the granite led to the formation of greisens and of arsenopyrite-cassiterite veinlets and to the chloritization of biotite. Uranium was not mobilized during this alteration stage.

The uranium mineralization in the Krunkelbach deposit is bound to hydrothermal veins containing mainly quartz, baryte, fluorite, pyrite, pitchblende and hematite (so-called iron-barium-formation). The ore veins also contain high concentrations of Ba, Sr, Cu, Pb (radiogenic), W, Y, V, Co, As, Se, S and Li (bound in cookeite). The host rocks bordering the veins are highly altered; feldspars and biotite are illitized.

The genesis of the Krunkelbach deposit in the Upper Carboniferous is due to the interaction of ascending, reducing hydrothermal waters with near-surface, more oxidizing solutions of lower temperature. The uranium mineralization must have formed either due to mixing of both solutions or through the reaction of the oxidized solutions with sulfidic ore. From fluid-inclusion investigations, formation temperatures of 100 to 300°C were derived with salinities ranging from 0 to 7.9 eq% NaCl. The ascending fluid had a δ¹⁸O of +7 to +10 ‰ and a δD of -60 to -100 ‰. Strong fluctuations in temperature and fluid composition were caused by an episodic activity of the hydrothermal system triggered by tectonic events. There are indications that oxidation of the sulfidic ore during tectonically quiet phases was catalyzed by microorganisms.

Highly saline Na-Ca-Cl-brines (>20 eq% NaCI, 150 to 200°C, δ¹⁸O of 0 to +5 ‰, δD of ­40 to -52 ‰) overprinted the deposit at a later stage. This event can be correlated with a Pb-Ioss of pitchblendes in the Early Tertiary and with the intrusion of olivine-nephelinite dykes at the same time, probably related to the formation of the Rhine Graben.

K/Ar-dating of illites yielded ages of 120 to 208 Ma. These values are most probably mixing ages produced by a partial overprinting of the Hercynian illites by the Early Tertiary event and by interaction with Quarternary groundwaters. The K/Ar-ages show a positive correlation with δD-values. The formation age of the deposit cannot be deduced from the illite ages in this case.

During the oxidation of the deposit, iron oxyhydroxides, uranyI-mineraIs, the clay mineraIs kaolinite/halloysite and beidellite and the Ba-AI-phosphate gorceixite were formed. Dating of cogenetic secondary V-minerals indicates that the oxidation phase started at least 260 to 340 ka ago and continues to the present day. Kaolinite, beidellite and goethite have stable isotopic compositions indicating that the formation of these minerals took place in water significantly heavier than today due to a warmer climate at that stage.

The main U-carrier in the deposit is pitchblende (UO₂). This mineral was affected by both the Tertiary hydrothermal event and the low-temperature oxidation. The hydrothermal event caused a loss of radiogenic Pb, especially from fibrous pitchblende, combined with an uptake of Ca. During oxidation, pitchblende alteration starts with the development of micropores and the neoformation of uranyl phases. In contrast to the hydrothermal alteration, Pb is immobile during the oxidation. Pitchblende actually takes up lead during alteration.

In addition to pitchblende, illite-cookeite-aggregates are U-carriers in the unaltered ore. In the oxidized ore, U is bound to uranyl mineraIs, iron- and manganese-hydroxides and clay minerals.

The hydrogeology of the Krunkelbach mine is disturbed by the mining activities. The water pumped out of the mine (20 to 30 I/s) equals about 14 % of the precipitation in the surficial catchment area (3.7 km²) of the mine. Water temperatures and tritium contents indicate that the groundwaters in the mine represent mixtures of pre-1953 water with recent water. The stable isotopic composition of the groundwaters is in agreement with the measured mean annual temperature of 4 to 5°C. The N₂- and Ar-contents of the groundwaters indicate that infiltration takes place at an altitude of 1300 to 1500 m on the nearby Feldberg.

The most important waterbearing structures are hydrothermally silicified tectonic faults. This corresponds to the type of flow-paths most commonly encountered in the basement of Northern Switzerland. The most strongly waterbearing structure is the Krunkelbach fault, situated a few meters apart from, and parallel to, the largest U-ore body (vein 2).

The dominant dissolved species in the groundwater are Ca²⁺, Na⁺, HCO₃^- and SiO₂ making up an average 86 % of the totals. The main element chemistry of the groundwaters can essentially be explained as being due to plagioclase hydrolysis and calcite dissolution. Calcite dissolution must also be advocated to explain ¹³C- and ¹⁴C-values of the groundwaters. The groundwaters are oxidizing with O₂-contents of 8.2 to 9.4 ppm. The younger (tritium-rich) waters are generally less mineralized than the older waters (with Iower tritium content). Most groundwaters are not in contact with the uranium ore and show U-concentrations of 4 to 80 ppb (mean 18 ppb) and uniformIy Iow sulphate levels. Waters flowing in the ore have U-concentrations of several 1000 ppb and elevated sulphate Ievels.

Radium (²²⁶Ra) is not mobilized during the current ore-body oxidation due to coprecipitation with secondary baryte. Such young barytes have alpha-activities up to 4 times as high as pitchblende.

Saturation indices of common mineraIs calculated with the program PHREEQE are in agreement with the surface texture (corroded/uncorroded) of these mineraIs in the waterbearing fractures. Chalcedony and baryte are the only minerals which are saturated. Undersaturation of secondary uranium minerals even in the ore-zone waters may be due to the presence of freshly precipitated U-AI-Si-hydroxide as a U-solubility-limiting phase.

Enhanced solubility of baryte and possibly of Ra during an earlier stage of the ore-body-oxidation is evidenced by the presence of corrosion stages in secondary barytes, the prevalence of Ba-rich members of the autunite­family and the abundance of the Ba-AI-phosphate gorceixite. Enhanced solubility of baryte might be caused by the presence of thiosulphate as a product of pyrite-oxidation.

In the Krunkelbach mine groundwaters, clay minerals and amorphous Fe­oxyhydroxides (size > 0.2 μ) are carried in suspension in concentrations of (typically) 1 to 2 ppm. High Ti- and Mg-contents of these suspended clay materials indicate that they have been carried in suspension from Ti-, Mg­rich gneisses over a distance of 1 to 2 km.