Author Archives: trugeo

The traditional annual field trip (October 3-5, 2014) of our chapter has mainly focused on stratiform iron mineralization in the Neoproterozoic metasediments of the Děsná unit and iron mineralization of the Lahn-Dill type in the Devonian volcanosedimentary cover in the Jeseníky Mts., northeastern Bohemian Massif.

The main part of the excursion was designed as a transsect across the metamorphic and deformation gradient imposed on the same iron-bearing precursor of the Lahn-Dill type. We have thus investigated transition from jaspilites to metamorphosed magnetite ores as well as their banded magnetite-quartz counterparts. This provided many interesting examples, based on a number of small historically mined occurrences.

The 13 students participated in this 3-days excursion, which was led by David Dolejs.

SGA News no. 36 page 17

The SGA Student Chapter Prague organized one of its field trips to visit sedimentary iron mineralization in the Paleozoic volcanosedimentary sequences of the Prague Basin (Barrandian) in central Europe.

The Barrandian is a classical area of Neoproterozoic to Devonian volcanosedimentary strata, affected by the Variscan orogeny. In the Middle Cambrian through Ordovician, the deposition of predominantly shallow marine conglomerates, graywackes, mudrocks and shales was accompanied by intermittent volcanic activity of calc-alkaline basalts, andesites, rhyolites and their subaqueous pyroclastic products. Several of these volcanic centers were source of energy and chemical components for the formation of perivolcanic or hanging-wall iron-oxide mineralization. The formation of ironstones continued to stratigraphic hanging wall as well as more distal sedimentary settings, and gradually evolved into oolitic iron deposits, which occur as several horizons In the Ordovician siliciclastics. The deposition of these siderite-, chamosite- or hematite-bearing oolitic iron ores coincides with their worldwide formation on shallow marine continental shelves at this time interval. Ironstones in the Barrandian represent classical and previously economic accumulations, which were exploited from prehistoric times until 1960’s. The field stops offered a rich opportunity to compare volcanic and epiclastic products and several mineralization styles, which recorded different influences of volcanic activity, hydrothermal and subaquatic alteration and redox gradients during iron precipitation and diagenesis.

SGA News No 36 page 16

The second field trip organised by the SGA Student Chapter Prague in 2013 was aimed at Slovakia and Hungary. Purpose of this excursion was to get in touch with different geological settings than the Bohemian massif. Major part of Slovakia and the northern part of Hungary (Tokaj Mountains) are situated in the mountain range of Western Carpathians. This system evolved during the Alpine orogeny as the northern branch of the Alpine-Himalayan fold and thrust system called the Alpine belt.

SGA News no. 35

The first field trip organised by the SGA Student Chapter Prague in 2013 lead to the famous Alpine deposits Habachtal and Knappenwand in Austria. Habachtal emerald deposit has been known since the Roman times. This locality provides the best emeralds in Europe and it is the only location where emeralds of gem quality occur. The mine has been active until these days, where the Steiner family has been irregularly mining the precious stones using just a simple technology. Access to the adit collar is rather difficult requiring good physical conditions. The emerald deposit is located at the tectonic contact of ortho-gneiss and basic to ultrabasic metamorphosed rocks: amphibolite,mica schist, serpentine and talc schist. The most important emerald bearing rocks are biotite-, talc- and actinolite schists occurring at the margins of serpentine bodies. The emeralds mostly appear in the form of automorphic hexagonal columns in a size up to 4–5 cm. The emeralds can be found in the Habach valley under the adit where they are transported by water.

SGA News no. 34 

Main focus was on the origin of ore deposits in both the Czech and German parts of the Krusne hory/Erzgebirge Mts. The main attraction of the field trip was an underground visit in the historical mines of Ehrenfriedersdorf and Pöhla and at the Zlatý Kopec near Boží Dar (Gotesgab). The common feature of all localities is a widespread cassiterite mineralization related to the hydrothermal fluid flow from late Variscan biotite to topaz granites (327–312 Ma). The tin deposits are located in the paleoroof of the Krusne hory/Erzgebirge batholith formed by Neoproterozoic and Early Paleozoic supracrustal sequences: phyllites, micaschists, paragneisses, amphibolites, and marbles that were altered and host stratiform or disseminated mineralization. Altered rocks are also intersected by numerous ore-bearing veins. The highest abundances of economic minerals are concentrated in skarns that formed by replacement of dolomites or surrounding metasediments and metavolcanics. Therefore skarns were target of historical mining as well as post-war exploration and became the principal target of our field trip.

SGA News no. 33