Mercury ore deposit: world natural heritage

The Idrija ore deposit is the second largest mercury deposit in the world. The mercury is extremely pure and frequently appears in the form of drops as native Hg.

The mercury ore deposit is, by its origin and geological composition, extremely complex, and for this reason designated as natural heritage of international importance. A geologically rich and interesting part of the ore deposit has been preserved in the area of Anthony’s Main Road.

The ore deposit was formed 230 million years ago in a period of massive tectonic movements. Flowing out from the Earth’s mantle along the deep faults were hot solutions rich in mercury, from which the mineral cinnabar (HgS) crystallised during cooling and replaced various rocks (epigenetic mineralisation). Part of the mercury-rich hydrothermal solutions flowed into marshes where sedimentary ores formed (syngenetic mineralisation).

The Skonca beds are, due to their exceptional richness and enormous quantities of mercury, the most important ore-bearing horizon of the Idrija ore deposit. Concentrated in these beds was a massive quantity of mercury in the form of various cinnabar ores (HgS) and native mercury. Most important were the sedimentary (syngenetic) cinnabar ores that were given mining names – jeklenka (steel ore), opekovka (brick ore), jetrenka (liver ore), koralna ruda (coral ore) and various other bedded ores, with an average content of 5 to 60%, sometimes even up to 78% of mercury. The mineralised Skonca beds were excavated until the termination of works in the Idrija ore deposit in 1977. It is estimated that as many as 30 to 40% of mercury was obtained from sedimentary ores.

Geological exploration of the ore deposit and its surroundings was begun in the mid 18th century and has continued to the present. After 1960, Slovenian geologists investigated the unusual origin of the Hg ore deposit in a deep tectonic fault trough, and explained its highly complex present-day structure, which represents a major scientific achievement of European importance.