The goal of this contribution is to compile recent
knowledge on the geochemical signatures of the fluorite mineralization at Jilove and the magmatic- and crustal rock-types at the northern border of the Eger-Rift near Decin, Czech Republic.
The Jilove mineralization-type is a good example to study the role of mixing between magmatic and crustal sources in fluorine-rich hydrothermal fluids.
The vein deposit is located in Upper Turonian sandstones of the Saxonian-Bohemian Cretaceous Basin.The structures are partly unmineralized open systems showing pseudocarstic cavities.The fluorite veins reach lengths of about 800m. The structures are controlled by intersection of W-E and WSW-ENE striking faults at the northern flank of the Eger-Rift. Beneath the Cretaceous sediments, the rocks belonging to the Erzgebirge crystalline series - biotite-muscovite orthogneisses and migmatites of granodioritic composition. Eger-Rift derived volcanics are located in the southern surroundings of the deposit (olivine-nephlinites, tephrites, trachytes and phonolites).
The fluorite mineralization at Jilove shows a rhythmic banding in mm to cm range. Accessory minerals are barite in a first and barite, calcite and chalcopyrite in the last mineralizing stage. Fluid inclusion studies indicate temperatures from 120-155oC for deposition of fluorite and barite. From one sample of about 15 cm thickness the individual bands are sampled and analysed for REE and Sr- and Nd- isotopic composition.
The Y/Ho ratios determined for the fluorite bands vary from 25.5 to 12.4 and are generally lower than the chondritic value of 28. This is surprising because hydrothermal fluorites generally display Y/Ho ratios of 30 to 200 and positive YSN anomalies which is explained by Y/Ho fractionation during migration of the hydrothermal fluid (Bau and Dulski, 1995).
There is a slight negative correlation of Y/Ho and La/Ho for the analysed fluorites and the REYSN patterns show positive YSN anomalies indicating fractionation effects during migration of the mineralizing fluid. The low Y/Ho ratios cannot be produced by fractionation in the aqueous solution and, therefore, at least one of the sources of the brine must be characterized by lower than common crustal Y/Ho ratio.
For the first stages of deposition the Sr and Nd concentrations are inversely coupled and show a time dependent trend which indicates an open system with progressive change of the composition of the mineralizing fluid. Whereas the Sr isotopic composition of the fluorite for these early stages is nearly constant at ca. 0.7231 the eNd (70Ma) values decrease for increasing Nd concentrations from -0.5 to -3.9 suggesting a continuous admixture of fluids derived from a crustal source with low eNd and relatively high Nd/Sr ratio. For the younger stages of deposition the Sr isotopic compositions of the fluorites increase from 0.7249 to 0.7254 whereas the Sr and Nd concentrations as well as the eNd values scatter for the different bands of fluorite. It is concluded that at this stage of mineralization the fluid was added by a component derived from a different crustal source, characterized by 87Sr/86Sr > 0.7254, low eNd @ -10 and relative low leachable Nd/Sr-ratio.
In the eNd (70Ma) versus Sri diagram the two stages of fluorite mineralization follow different mixing lines of a mantle like source and a crustal component. The Nd and Sr isotopic compositions of the endmembers may be similar in both cases. However, because the Nd/Sr ratios picked up by the fluid from the endmembers appear to be different, probably, the crustal source has changed during the time of fluorite deposition.
Bau & Dulski, Contrib. Mineral. Petrol. 119, 213-223 (1995).