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Pohorje tonalite

Of all our igneous rocks, the most widened and famous one is Pohorje tonalite, from which the main part of Pohorje is bulit of. In the past, common name for it was Pohorje granite, today the name tonalite is more commonly used. The name tonalite originates from the classic deposit of tonalite in the area of Passo Tonale in Adamell massif in northern Italy. Detailed studies have shown that the composition of Pohorje tonalite differs slightly from the classic tonalite. For the Pohorje plutonic rock, the most appropriate name is granodiorite.

Geochronological research according to the method K-Ar has shown that the rock came into existence approximately 16 million years ago in the era of Miocene. Pohorje granodiorite is grey with a slight blue shade. The homogeneity of the rock is broken off with white aplite and pegmatite streaks that are of different width and arrangement. The white crystals of Na-Ca feldspars (plagioclase) are prevalent (50-60%). Flintstone of slight grey shade is next (20-30%). What remains are biotite grains (10-15%), K-feldspar (orthoclase) (5-10%) and hornblende (1%). In addition to the main minerals, there are also traces of: apatite, zircon, pyrite, titanite and epidot. Mineral grains’ size is up to some milimeters, but usually they are smaller. The structure of the rock is medium-grain, hipidiomorph. The texture is oriented and is mainly caused by parallel-oriented biotite minerals and partly flintstone. The oriented texture has impact on production, particularly on the manufacture of cut products.

Tonalite can be distinguished from granite by the fact that granite contains more alkali feldspars and less plagioclase, which means more K-feldspar (orthoclase) and Na-feldspar (albite) and less Ca-feldspar (anorthite).

Aplite and pegmatite streaks are of the greatest importance for the visual appearance of the rock. They are mostly composed of feldspar and flintstone. If the minerals are fine-grained, these are aplite streaks. If the minerals are coarse-grained, the streaks are pegmatite. In addition to feldspar and flintstone, other minerals can be found: mica, pyrite, garnet and others. The contact between the streak and the rock is visible everywhere. As a rule, the contact is lithificated, therefore is does not present the impairment of the rock. 

The history of extraction

In the 19th century, the farmer Cezlak started extracting the stone. In 1891, with Windischgrätz’s permission, he cleared the slope and began cutting the stone. In 1905, the quarry was taken over by the Windischgrätz family and they made the first geological researches by 1910. During this time, they built a narrow-gauge railway up to Oplotnica (a few kilometres away) and a power station on direct current. The Windischgrätz family managed the quarry until 1919 when it was bought by the German called Erlich.

Due to the economic crisis, the quarry did not operate in the years from 1930 to 1935. After the year 1935 the quarry was reopened, but the intensive production didn’t begin until 1938. In 1941, the Germans nationalized the quarry and handed it over to the Austrian company SS GRATZ.

After World War II the quarry was nationalized. Granite industry Oplotnica became the new manager of the quarry. In 1949, the quarry employed 550 workers. They mostly produced blocks for paving the roads. 

In 1962 the quarry began connecting with other local quarries, which operated under the common name INGMAG until 1976. 

In 1984, the workers voted for joining the company Mineral Ljubljana. Mineral company had already opened limestone quarries Drenov Grič, Lesno Brdo and Podpeč. First year after the merging, the leadership began modernizing the quarry. The first major innovation in the extraction was introduced in 1985 by the method of perforating drill. In 1993, the quarry introduced a new method of extracting tonalite in the wall with a diamond wire. A 25-ton heavy hoist was placed in the quarry.