002.1.Idrija

Idrija Mercury Heritage Management Centre, Bazoviška 2, SI-5280 Idrija

Public

Republic of Slovenia

4644 Idrija ore deposit

Ore deposit

The Idrija ore deposit is the world’s second largest mercury deposit. It is world known and professionally important due to the way of its creation in the Middle Triassic rift valley, especially rich sedimentary ores, which represent globally unique specimens, geochemical and mineralogical composition and in view of the complex tectonic transformation to the current state. Mineralisation in the ore deposit appears in three different forms: syngenetic and epigenetic cinnabar mineralisation and epigenetic mineralisation with native mercury in economical amounts. In the ore deposit, 156 ore bodies were found, of which 141 were mineralised with crystals of cinnabar and in the other 15 native mercury prevails. In the ore deposit, 25 different minerals were also found.

The Idrija ore deposit runs underneath the southern and the eastern part of Idrija town. It runs in a northwest-southeast direction, it is 1500 m long, 400 to 600 m wide and 450 m deep.

Cinnabar ore, native mercury

Approximately 238 million years ago, in the Mid-Triassic tectonic stage, which took place in middle and upper Anisian and Ladinian.

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The Idrija ore deposit was created in the Mid-Triassic tectonic trench. At the time of its creation, it contained 250 to 300,000 tons of mercury. It is hydrothermal by origin, thus mineralisation occurred in two stages. In the first stage, the hydrothermal solutions at delivery faults mineralised older rocks, which created epigenetic cinnabar ores. In the second phase large amounts of mercury flew into the rift. Skonca beds and rich sedimentary ores called syngenetic layered ore, all typical for Idrija ore deposit, deposited in the then wetlands. In the thrust stage of tertiary tectonics (according to Eocene), a large subsided fold occurred with mercury mineralisation in the central part. Therefore, the bottom part of the ore deposit got the subsided syncline form (i.e. concave part of the folded Earth’s layers, whose central part contains the youngest rocks), and the upper part got an inverse position. In later tectonic stages, several faults and slips intersected the ore deposit. The Idrija ore deposit was discovered in 1490. Commercial excavation was implemented until its termination in 1977, and a permanent closure of the mine was adopted 1988.

The Idrija ore deposit is being preserved, secured and presented as important natural heritage of Idrija with exceptional universal values. It has had the status of a geological natural value of national importance since 2004. Within the natural value Idrija ore deposit, 14 geological profiles of exceptional scientific and research significance has been evaluated, which represents an important part of the European natural heritage. The Geological Collection of the Idrija Municipal Museum and the Mining Geological Collection are also a part of the natural value ex-situ. A part of the latter is displayed in the Anthony’s Main Road and is on display for visitors of the mining museum. Around 500 different publications were published on the Idrija ore deposit and the Mine, which represent a large and important scientific achievement of European importance.

The Idrija ore deposit is located under the town of Idrija and under the Smukov grič and Poljančev grič hills, south and east of the town. Mineralised rocks stretch from the surface to a depth of 450 m.

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Performing regular maintenance services by the programme, including status control and preservation, security and presentation of the heritage.

- Official Gazette of the Republic of Slovenia no. 111/2004 of 14/10/2004; 4623. Rules on the designation and protection of natural values, p. 13173–13175; https://www.uradni-list.si/glasilo-uradni-list-rs/vsebina/51517. See: Annex 1: Seznam naravnih vrednot in njihova razvrstitev na vrednote državnega in lokalnega pomena;
- Čar, Jože. Kotna tektonsko-erozijska diskordanca v rudiščnem delu idrijske srednjetriasne tektonske
zgradbe. Geologija, 31‒32, 1988/89, p. 267‒284;
- Čar, Jože. Geološka zgradba idrijsko–cerkljanskega hribovja. Tolmač h geološki karti idrijsko – cerkljanskega hribovja med Stopnikom in Rovtami 1:25 000. Geological Survey of Slovenia, 2010, p. 1– 127;
- Čar, Jože. Ladinijske plasti skonca idrijskega rudišča (Z Slovenija). Geologija, 56/2, 2013, p. 151–174;
- Drovenik, Matija, Čar, Jože, Strmole, Dragica. Langobardske kaolinitne usedline v idrijskem
rudišču. Geologija, 18, 1975, p. 107–155.
- Lavrič, Jošt Valentin. Isotopic, organic and inorganic geochemistry of the Idrija mercury deposit,
Slovenia: constraints on the formation of the Hg-PAH association: These de doctorat. Université de
Lausanne, Faculté des géosciences et de l'environnement, 2004;
- Mlakar, Ivan. Primerjava spodnje in zgornje zgradbe idrijskega rudišča. Geologija, 10, 1967, p. 5– 72;
- Drovenik, Matija, Mlakar, Ivan. Strukturne in genetske posebnosti idrijskega rudišča. Geologija, 14,
1971, p. 67‒126;
- Placer, Ladislav. Tektonski razvoj idrijskega rudišča. Geologija, 25/1, 1982, p. 7–94;
- Čar, Jože, Placer, Ladislav. Srednjetriadna zgradba idrijskega ozemlja. Geologija, 20, 1977, p. 141–
166.

Mestni muzej Idrija

Miha Kosmač

27. 2. 2018

Never