Spatial development of the grey part of the Val Gardena Formation on Žirovski vrh

Abstract

The Uranium Mine Žirovski vrh was actively being researched and excavations took place between 1960 and 1990. Ivan Mlakar (2000) divided the entire Val Gardena Formation into 6 members and named the oldest one the Brebovnica member. This is the only member with uranium ore mineralization. Previously, Tomaž Budkovič (1980) subdivided the gray part of the Val Gardena formation into ten horizons based on colour, grain size, and pebble associations. He did that by mapping of undercut P-10 and its continuation. In this article, we have shown the development of the Brebovnica member in the entire area of the ore deposit, both the mining area and its continuations to the SE and NW. We have modified Budkovič's original division by combining the upper part of the 3rd horizon with intraformational conglomerates and the former 4th sandy horizon into a single 5th conglomerate horizon. In his doctoral thesis (1995), Dragomir Skaberne divided the Brebovnica member into 2 megarhythms, each of which begins with conglomerates and ends with fine-grained sediments. Therefore, it is more appropriate that the 3rd horizon ends with fine-grained sediments and is at the same time the end of the 1st megarhythm, while the intraformational conglomerates, which were previously the upper part of the 3rd horizon, are now part of a single conglomerate horizon, which sometimes also contains sandstones.
The total thickness of the Brebovnica member is greatest in the central part of the mine area, where it reaches 410 meters and decreases towards the edges of the basin. Towards Goli vrh in the southeast, where the last uranium mineralization occurs, its thickness decreases to 50 meters. The thickness of individual horizons, except the second and fifth, decreases from tens of meters to only a few meters, and some horizons even wedge out. To the northwest from the former mining area there are no economically significant uranium ores. In that direction, the colorful conglomerates wedge out, and the nature of sandy horizons, which were the main carriers of uranium mineralization in the mining area, changes. Their colour changes from gray and dark gray to greenish gray, with fewer fragments of anthracitized organic matter that created reducing environments necessary for uranium precipitation. The colourful conglomerates contain important pebbles of acidic magmatic rocks with higher uranium contents compared to other rock groups. Their erosion enriched surface and groundwater with uranium already at the surface and additionally during percolation through the layers of colorful conglomerate. Uranium dissolved under oxidizing conditions, while under reducing conditions it precipitated in the binder of sandy grains. Of the entire 18 km length of the gray Bebovnica member as shown on the geological map, only 5.5 km show conditionally economic uranium mineralization, which approximately corresponds to the area where colorful conglomerates occur.