Geological control of trial excavation of Uranium ore in Žirovski vrh

Abstract

In 1981-1983, before the start of full production, trial mining of uranium ore was carried out at the Žirovski vrh mine to test the methods of geological and radiometric control as well as the technical methods of extraction. The trial excavation was done in block 1, in the extreme NW part of the mine in the upper fold of the double S structure. In this block, the ore deposits bodies were found only in the horizon of grey sandstone about 20–30 m thick. Geological monitoring of the excavation confirmed that the sandstones were deposited in sequences from a few dm to more than 2 m thick. The sequence usually graded bed started with coarse-grained sandstone, with pebbles of quartz and mudstone occasionally present at the base. Upwards, coarse-grained sandstone often passed into medium-grained, and more rarely into fine-grained sandstone and siltstone. The sandstones were characterized by the presence of organic fragments, which created an anoxic environment during diagenesis and that in turn enabled uranium to precipitate from the groundwater and concentrate in the host rock. Most often, ore bodies were found in coarse-grained sandstones, usually in several sedimentary sequences. As such, ore bodies appear in several sequences, usually under 1 m thick. In some places ore bodies can join together, exceeding 5 m. The length of the uninterrupted ore body in the longitudinal direction was up to 150 m, the width ranges from a few meters to more than 40 m. In the intermediate areas where the ore body is interrupted, we observed that the sandstones changed from dark grey and grey to light grey and greenish grey. These sandstones were usually barren or contained only small concentrations of uranium. In the longitudinal direction, such a transition took place between cross-sections P-35 and P-35a, where excavation was completed. Cross-sections P-36 and P-37 were also investigated inside the mine, where predominantly greenish-grey sandstone with only rare lenses of poor uranium concentrations appeared. Further to the west, two cross-sections 1.3 and 2.8 km from the mine area were also explored with surface boreholes. Grey and greenish-grey sandstones with very rare remains of organic matter and very rare traces of mineralisation were found in these boreholes, alternating with siltstones. Block 1 is therefore considered the boundary block on the NW side of the mine, intended for extraction.