A famous mercury mine started from 1490. Produced 2.000 flasks per year of Hg (1984).
The first Hg impregnation of rock formations occurred during rifting in the Triassic period and was accompanied by the impregnation of rocks with barite, fluor, copper, manganese, strontium, and zinc. The Hg-bearing chlorine vapors came from deep beneath the crust. On their way to the surface, they likely dissolved the pyrite present in the carbonates, which introduced sulphur (S). The vapors also experienced a drop in pressure and temperature. These conditions were favorable for the precipitation of cinnabar (HgS). The impregnation process took a relatively long time and occurred under various conditions, depending on which forms of HgS were formed.
In the so-called Jeklenka ore, with its black steel-like hew, the cinnabar crystals are relatively small, and concentrations of Hg reach up to 78% in some sources. In the more reddish-colored Opekovka ore, the crystals are larger, but the Hg concentrations are lower. In some cases, the ore was deposited syngenetically with sedimentary layers, forming banded ore, Opekovka, Jeklenka, and Coral ores, while in others, it was deposited epigenetically, directly into the already-formed rock formation, creating impregnations or substitutions. Liquid Hg (Hg0(l)) is also present and thought to have originated from two sources: (I) the primary direct deposition of Hg during primary impregnation due to a lack of sulfur ions that would bind it to HgS or (II) subsequent remobilization of the primary HgS due to leaching by the pore water, which resulted from the tectonic fracturing in the area.

Mercury enrichment occurred in two main stages. In the first stage, mercury was concentrated in rocks of the Upper Paleozoic, Scythian, and Anisian ages. The second stage affected the Upper Ladinian Skonca beds and associated volcanic tuffs. The enriched sedimentary rocks included conglomerates, shales, sandstones, siltstones, dolomites, and limestones, reflecting a variety of lithologies capable of hosting mercury mineralization. It exhibits a lenticular shape ranging in areal extent from a few square meters up to 100 m2. Mercury mineralization is exceptionally rich, locally reaching contents of up to 78 wt.% Hg. Pyrite is abundant, ranging from 50 to 90 vol.% of the rock, occurring mostly as reniform and spheroidal diagenetic concretions, as well as in smaller euhedral grains, measuring from 0.5 to 10 mm in size. Cinnabar forms irregular grains and veinlets and locally replaces pyrite. The host rock is primarily sandstone that grades upward into bituminous shale and mudstone. The highest-grade ore within the Karoli orebody occurs in the Skonca layer, reaching a thickness of up to 40 m.

NOTE: This mine was once part of Italy and also part of Austria. Old labels may carry the region name Carniola (see there).

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References