Geology and Formation of the Ore Deposit
The surroundings of the Idria ore deposit is composed of four SW-verging nappes, with the mercury mineralization being emplaced in the lower part of the Trnovo nappe, in the Idria inner-thrust sheet. The geologic structure of the Idria-Cerkno mountain range was described by ČAR (2010). The mercury ore occurs in the Upper Carboniferous slaty claystones with lenses of quartz sandstone, Groden sandstones, in Upper Permian and Lower Triassic (predominantly Scythian) dolomites, slaty siltstones and oolitic limestone, in Anisian dolomites, Ladinian dolomitic conglomerate, and younger Ladinian Skonca beds consisting of shallow-water sedimentary and pyroclastic rocks deposited during the volcanic phase. The tectonic transformation of the ore deposit began during the mineralization (PLACER and ČAR 1977; PLACER 1982), whereas the Idria Fault has been active until the present day.