Alteration of granitoids and crystalline rocks and uranium mineralisation in the Bor pluton area, Bohemian Massif, Czech Republic

Abstract

The Bor pluton belongs to the Variscan granitoid plutons of the Moldanubian zone of the central European Variscides. Along with similar granitoid plutons of the western part of the Bohemian Massif (Leuchtenberg, Babylon, Fichtelgebirge), it is close in its composition to low-F biotite granites of the Saxothuringian zone. This paper investigates the hydrothermal alteration of the Bor pluton in relation to uranium mineralisation. Uranium mineralisation of the Bor pluton is associated with shear zones occurring on the western margin of the pluton on its boundary with high-grade metasediments of the Moldanubian zone (Zadní Chodov) and with metasomatic mineralisation evolved in hydrothermally altered biotite granites of the Bor pluton (Vítkov II, Lhota). Uranium mineralisation in altered granites is accompanied by their intense hematitisation, albitisation, chloritisation and carbonatisation. Hydrothermal alterations of granites were accompanied by the enrichment in U, Na, P, Ti, Mg, Ca and depletion in Si and K. The altered high-grade metasediments occurring in shear-zones on the western boundary of the Bor pluton are enriched in U, Ca, Mg and P and carbonaceous matter. In the Bor pluton area with typical coffinite–uraninite association the presence of brannerite is significant for uranium mineralisation. The observed coffinite is enriched in Y (up to 4.3wt.% Y2O3) and Zr (up to 2.0wt.% ZrO2). Unaltered brannerite displays low Ca (0.9–4.5wt.% CaO), Al (0.0–0.5wt.% Al2O3), Fe (0.7–2.2wt.% FeO), Pb (0.0–1.4wt.% PbO) and Th (0.0–2.8wt.% ThO2) concentrations. The highly altered brannerite is depleted in U, enriched in Ti, Si and Al. The temperature of the ore stage was estimated using chlorite thermometry and ranged from 122°C to 258°C.