Formation and radiolytic alteration of uraniferous solid bitumen related to hydrothermal base-metal mineralization in the Bytíz deposit, Příbram district, Czech Republic

The Bytíz deposit is a part of the Příbram uranium and base-metal ore district. It is an example of a vein-type deposit with polyphase hydrothermal mineralization. Samples of uraniferous solid bitumen from Bytíz with U content up to 38 wt% are characterized petrologically, geochemically, and mineralogically using EPMA, Raman and infrared microspectroscopy. The bitumen-bearing samples consist of base-metal sulfides: galena, sphalerite, pyrite, chalcopyrite, and also minor amounts of tetrahedrite, stibnite, and acanthite, associated with Mn-bearing calcite, quartz and silicates (chlorite, muscovite). Solid bitumens were found in the form of small veins and droplets, and roundish to irregular accumulations, in association with uraninite and carbonate veins.

U-bearing minerals in the studied samples are represented by uraninite and more rarely by coffinite. Three generations of uraninite in association with solid bitumen were distinguished: 1. spherulites and large grains, filled with organic phase in the cracks; 2. as a part of complex textures inside areas with organic matter; in this case, the uraninite was assumed to have been remobilized; and 3. small inclusions in the latest calcite veins.

More than 80 vol% of the solid bitumen from the vein fillings appeared to be radiolytically altered. Radiolytic alteration results in changes in optical properties and in composition, and in the formation of various textures around uraninite grains: halos, and irregular textures from simple massive to flow, dendritic, and fractured to a very complex morphology. The random reflectance values of unaltered mineral-free bitumen range from 0.45% to 0.99%, while in the radiolytically altered bitumen the average reflectance values are higher, from approximately 1.72% to 3.44%.

The degree of graphitization of the organic matter was assessed by infrared micro-spectroscopy. Spectral maps show significant destruction changes of the aliphatic CH bonds and an increase in the content of oxygen functional groups in the vicinity of U minerals.

On the element distribution maps, obtained by EPMA, the distribution of S, U, Pb and other elements across solid bitumen in the vicinity of uraninite and coffinite has a very heterogeneous character. An elevated content of sulfur in bitumen was also found, as well as a clear interdependence between S and C. It is suggested that the presence of sulfur in solid bitumen may result in ‘clouding’ of the solid bitumen with tiny stibnite grains. The dark rims of the halos observed under the optical microscope may be due to an elevated U content at the rims around the uraninite.

Based on analysis of complex textural relationships of the solid bitumens with coexisting minerals, the formation of solid bitumen in association with uraninite is therefore assumed to relate to several stages of the influx of hydrothermal fluid. The temperature of the fluid, associated with bitumen formation was estimated to be up to 270 °C, according to chlorite thermometry.