Starohory (proterozoikum)

site 4

Where is the site located?

GPS: 49° 54′ 52.25″ N, 15° 33′ 4.39″ E

The site lies in the northwestern part of the Iron Mountains National Geopark, right beneath the terminal part of the main ridge of the Iron Mountains.

What is the geological position of the site?

The site is located in the Bohemian Massif, in the marginal area of the Central Bohemian Region, where metamorphosed Proterozoic rocks are exposed. Rocks at this site are silent witnesses of the past orogenic processes and the presence of an old fracture in the Earth crust which enriched the rocks by spectacular minerals.

What happened at this site in the past?

– 545 million years

In the Proterozoic, this area was lying on the Southern Hemisphere. It was a part of a seafloor not far from the continent of Gondwana. Here, fine sediments were deposited in the sea, accompanied by submarine lava effusions.

– 350–300 million years

At the end of the Paleozoic, the rocks were subjected to another orogenic process – Variscan Orogeny. Again, this process changed their material composition and induced folding of individual rock bodies. The rocks were also affected by the heat in the proximity of an extensive magmatic body. The deep-reaching fracture in the Earth crust became an ascent path for hot fluids derived from metalliferous magma chambers.

– 25 million years

In the course of the Tertiary, this area was composed of hard metamorphic rocks. The rocks were lying in an unstable environment close to an old deep-reaching fault. As such, they were densely fragmented and jointed. The Alpine Orogeny in the SE part of Central Europe formed the Carpathians and the Alps. Related compressive stresses reactivated the movement of crustal blocks by as much as several hundred metres relative to one another. The sedimentary cover became soon eroded.

What does the site display today?

The area is composed of metamorphic rocks ranked within the Železné hory Crystalline Complex. They include rather weakly metamorphosed rocks of the Chvaletice Group and rocks of the Podhořany Crystalline Complex. A geological cross-section of the deposit shows a widely varied rock complex comprising orthogneisses, paragneisses, phyllites, mica-schists, amphibolites, slates and also lenticular bodies of marbles. The rock bodies are folded into one another. In addition, they are cut by a dense network of joints and faults which accompany the Železné hory Fault (Iron Mountains Fault).
Uranium mineralization concentrates to the joints, mostly having the character of veins hosted by fine-grained biotitic gneisses. The same type of mineralization is also developed at some other near sites in the Iron Mountains – e.g. at Březinka, lying to the northeast.

What was affected by man?

A uranium mine designated as the Licoměřice deposit was opened in 1964 and operated till 1986. Mining operations entailed the excavation of two shafts (the preserved Shaft 56 was 217.1 m deep). The mine consisted of four levels, a gallery (ca. 260 m long) and other horizontal passages which attained the length of 9 km. The deposit was exploited using the stepping method with a backfill of the excavated spaces. Besides, an extensive spoil tip originated next to the mine.
After the end of mining operations, the mine was closed, underground spaces were filled with water and the surface of the spoil tip was reclaimed and forested.
Radioactive minewaters started to be issued from the mine. This problem was solved by the construction of a decontamination plant, which was put into operation in 1991 and is now run by the company of Diamo Stráž pod Ralskem. The volumes of purified minewater equal around 100,000 m3 per year.

What was discovered?

The uranium mineralization was mainly concentrated to veins dominated by minerals of black colour: uraninite (uranium oxide) and coffinite (uranium-bearing silicate).
The most frequent accompanying minerals were pyrite (iron sulphide), sphalerite (zinc sulphide) and galena (lead sulphide). White calcite and layers of gold-glimmering sulphidic shales were common.
Nineteen veins and zones hosting uranium mineralization were disclosed by mining. In total, 383.3 tonnes of uranium have been exploited.

“The Iron Mountains – a geologically significant region” project of 2014

Two information panels were manufactured within the project of “The Iron Mountains – a geologically significant region”. They were erected at the southern margin of Licoměřice, near the terminal bus stop. A shaft of the former Licoměřice Mine can be seen some 300 m to the east, in the forested slope. The minewater purification plant lies some 300 m to the southwest, in the proximity of a stream.

Strongly folded tuffitic slates from Březinka (Daniel Smutek)