Luxemburg Field trip

Visiting three countries in two days / 11th and 12th of October 2014

The Mainland Europe branch of the OUGS spent the second week-end of October on a field-trip to Luxembourg and the surrounding area. We explored Tontelange quarry, near the town of Attert in Belgium, where the Luxemburg sandstone is quarried and a disused iron quarry near Differdange, in the south-west of Luxembourg. The field-trip also included a conducted tour of a former gemstone mine in Germany (see next report).

Day 1: Sedimentary deposits

On Saturday we were guided by Sylvain Dechamps, PhD student at Liège University, whose specialty is the dynamics of sedimentary formations during the Jurassic Period. He met us at a misty morning at Tontelange quarry.

fine-grained sandstone deposited in low energy environment
Figure1: Tontelange quarry / See clipbloard for scale

The climate in most of Europe during the early Jurassic was desert- like. It later became warm and sub-tropical, shallow seas formed and life and vegetation were abundant. There was no ice at the poles. The movement of the African Plate northwards created prominences of land that were exposed to erosion. The resulting sandstone deposits (200 km wide and 100 km long) are characterised by poor cementation, laminated from quartz grains and shows evidence of turbulence (figure 1). Accumulated quarry fragments of shells and the seductions of very course grain are still visible today.

Finely grained deposits that became tidal mud-flats were eventually covered by sand; evidence of the waves created by tidal currents was another trace fossil pointed-out by our guide. The sandstone formed during the Sinemurian, a period of about five million years in the early Jurassic, contained evidence of life forms that flourished on the sea floor; fossils seen by our group included gryphaea (an extinct predecessor of the oyster) and shells of what might have been inoceramus (an early form of bivalve).

Almost all iron originates from sedimentary deposits of which there are two major types. The first, and by far the most important, is banded iron formations (BIF); finely layered alternations of cherty silica and an iron mineral, generally magnetite, siderite or hematite.

In the past, a great deal of iron was mined from a second major type of chemically precipitated marine iron deposit that contained pinhead sized ooliths (small, rounded, accretionary masses of limonite formed by repeated deposition of thin layers of an iron mineral). These oolitic iron deposits have now been largely replaced in importance by BIF. In earlier times, however, they formed the foundation of the iron and steel industries in Western Europe (e. g. Luxembourg) and North America.

bioturbation in minettes (Differdange quarry)
Figure 2: Differdange quarry

European oolitic iron deposits, commonly called Minette-type deposits, contain ooliths of siderite, a siliceous iron mineral known as chamosite, and goethite. The deposits were formed in shallow, near-shore marine environments and, in addition to Luxembourg, are most extensively developed in England, the Lorraine area of France and Belgium.

Our guide pointed out the iron ooids and a bank of shell detritus visible on the cliff faces (figure 2). The Minette layer, it is now assumed, originated in a shallow coastal estuary as large rivers from the north deposited iron-rich sediment.

Text: Peter Whiteley / Photos: Gisela Lunkwitz

SOURCES: Sylvain Dechamps; “mineral deposit, iron deposits”, John J. Skinner, Encyclopaedia Britannica

More about day 1

The morning was dedicated to the Luxemburg Sandstone Formation – but we studied it in Belgian Lorraine! While we waited on this misty morning for our guide, Sylvain Dechamps from Liège University, in front of Tontelange quarry (just outside the village of Attert) a fossil hunter showed us some of the treasures he found in the quarry. They included some species we never saw there ourselves, including a closed calyx of a crinoid.

Luxemburg sandstones: schema of sandwave creation
Fig. 1: Genesis of the sandwaves in the Luxemburg sandstones
view over the lower level of Tontelange quarry
Figure 2: Tontelange quarry, 'path' to the exposure of stage C. In the background 'hills' of marl and sands of various grain sizes
Gryphaea
Figure 3: Tontelange quarry, Bed of Gryphaea

Our guide explained the local geology and its relationship to the Paris Basin using a neat set of pages containing maps, thin sections and other stuff - and we succeeded in getting a copy sent each after we were home again. The Luxemburg Sandstone Formation was deposited in the late Hettangian to early Sinemurian, and each of its stages is exposed in the quarry. Sylvain took us to all of them - and fortunately the mist lifted quickly so that we were able to actually see them.

We walked on the marly bottom of the quarry (stage A, see figure 1) to have a look at an exposure of stage B, a fine-grained sandstone deposited in a low energy environment with beautiful examples of wave actions. Only a few adventurous people followed Sylvain to the exposure of stage C because the hill was somewhat steep (fig. 2).

At site D, the exposure proved to have been the original sea floor deposited in a high-energy environment. It contained lots of fossils, most often Gryphaea (fig. 3). So, almost everybody did a little bit of fossil hunting. I even found a Gryphaea with both of its halves still attached to each other, though the smaller half is somewhat sunken down.

The afternoon's objective was to look at the ironbearing minettes, a type of lamprophyres deposited in the late stages of Early Jurassic and early stages of Middle Jurassic in Lorraine and southern Luxembourg. But since our destination for the afternoon, a closed-down iron mine just outside Differdange, was a way south from Tontelange quarry, we had lunch about halfway between these places at a restaurant in an outlet shopping centre.

Entrance to a closed-down mine
Fig. 4: Differdange quarry. Entrance of the close-down tunnel

Mining at Differdange quarry stopped more than thirty years ago and the area is now protected and includes a geologic trail with information boards – but sadly only in German. One about the cycles of the minette layers informs us that “... the Minette layers were formed in a shallow coastal area where large rivers from the northern mainland washed in their iron-rich load. The fine-grained deposits would in this case represent the intertidal mudflats overlain by sand waves formed by tidal currents. These iron-rich sand waves today form the inclined layers of the minette. At the cycle's end mudflats again come into being. The slowly seaward retreating shoreline is responsible for these constantly changing conditions. In the Differdange basin nine of these cycles can be distinguished which the miners named according to their respective prevalent colour 'grey layer', 'red layer', 'black layer' and so on” and “Such cycles start with clay deposits (called interburden), exhibit intense reworking by animals and are very fossiliferous. The iron content of theses layers is normally low [Minettes are low in iron anyway: their iron content is about 30%]. The border to the overlying thicker layer is pretty sharp. These deposits are more coarse and rich in iron ooids (pay ore-body). Inclined bedding is typical for these layers.”

minettes of high iron content
Fig. 5: Differdange quarry. Exposure of those minettes with the highest iron-content

Sylvain took us first to an obviously reddish layer by the closed entrance of a tunnel (fig. 4) where finegrained ironstones with lots of bioturbation and some fossils were exposed as well as the more coarse-grained ones with lots of small iron-ooids.

Our next stop was as small exposure of distinctly red ironstones with larger ooids (fig. 5). But Sylvain also took us along to the layers above the minette, muscovite-rich marls that higher up were followed by chalk.

By then the sun was near setting and this marvellous and this informative day was nearly over. It was time to return to Luxemburg city.

Gisela Lunkwitz

Day 2: all about gemstones

I was somewhat sceptical about the programme for Day 2 of the trip, I wasn’t sure whether the planned visits to the Precious Stone Mine in Steinkaulenberg and the Museum, both in Idar-Oberstein, would be worth the effort of driving some 120 km from Luxembourg, there and back. I could not have been more wrong, so it seems poetic justice that I was ‘selected’ to do the write-up for Day 2.

We had a fairly early start from the YH in Luxembourg, at 08:00 hrs; shortly afterwards, we picked up Peter, who had stayed over-night in the city centre. The journey was uneventful, despite the early morning mist, which had cleared by the time we reached the mine in Steinkaulenberg.

Precious Stone Mine Steinkaulenberg, Idar-Oberstein
In front of the mine at Steinkaulenberg, some of our group enjoying the sunshine

Having arrived at the mine, the 20 min. wait until our tour started gave us enough time to have a coffee in the sun. Our guide was a sixteen year old, very self-confident young lady who first took us to the shed to get our mandatory hard- hats. I can’t remember any part of the mine which we visited, where a hard-hat was essential. It was in the doorway of the shed, as far as I could see, the contact between my head and the top of the door frame resulted in no permanent damage to the door frame. After reciting the obligatory Do’s and Don’ts our guide proved to be very knowledgeable and she gave us a very professional tour, including answers to some tricky questions, outside of the normal script for the tour.

Layout of the Precious Stone Mine Steinkaulenberg
Inside the mine at Steinkaulenberg, our guide is giving us an overview over the mine

Idar-Oberstein is situated in the part of Rheinland-Pfalz, known as the Hunsrueck. During the Ordovician and Silurian, the Caledonian Orogeny affected the area and the original, Pre-Cambrian sediments were exposed to high temperatures and pressure at a depth of about 10 km, and were metamorphosed to Para-gneiss, which outcrops in some regions of the southern Hunsrueck.

During the Devonian, the Caledonian mountain range was eroded and sediments of up to 1000 m thick, consisting of iron-rich conglomerates, sandstones and slates, were deposited in Rhein basin. During the Permian, the whole region was uplifted, causing extensive volcanism, in particular in the Idar-Oberstein region. Gas bubbles, which formed in the molten andesitic lava, survived after cooling and solidification and they form the geodes where the semi-precious minerals could crystallise out. In particular, Idar-Oberstein is famous for its agate, amethyst and quartz crystals.

geodes of semi-precious minerals
Geodes in their 'natural environment'. Field of view ~1.5 m

The mine was originally worked by local peasants, during the agriculturally inactive winter months. Since their tools were very primitive, excavation was slow and usually carried out without artificial light. Nevertheless, the miners learnt by experience, how to follow the most promising lava seams, by observing the shape of the geodes and the direction in which they pointed. The geodes were generally flattened, with the thin-end pointing in the direction of the lava seam. Crystallisation within a geode is sometimes complete; in this case the geode is known locally as a ‘Mandeln’, or ‘almond’ in English. The geodes which were not completely filled were thrown away, as miners had no tools capable of working the amethyst and quartz spikes which partially filled the geodes. It is also the fully-filled geodes which contain the Idar-Oberstein agate. There was a good selection of minerals and artefacts made from the minerals, available in the shop.

During the guided tour of the mine, the weather had deteriorated, but the rain held off until we were having lunch and it conveniently stopped when lunch was over and we made our way to the ‘Edelstein Museum’ in the town centre. It is difficult to know how to do the museum justice, without vast quantities of photos, so I would sincerely recommend looking at the panorama viewer on the museum’s website.

This can be reached by accessing the main website under: www.edelsteinmuseum.de, then click on the tab ‘Museum’ and select the menu: ‘Ursprung and Gemology’, which contains the continuous panorama view of the displays. You can run the viewer full-screen and also, zoom in on any exhibit which may catch your eye.

agate layers worked in a cameo-like fashion
Image by Edelsteinmuseum, Idar-Oberstein

I was expecting endless show-cases of more or less precious minerals. In fact the exhibition is much more varied and aesthetically very well presented. It also contains reams of information about gem cutting and which cuts are applicable to which type of crystal. The ‘show piece’ is a crystal bowl, some 37 cm in diameter and 35 cm tall, cut from a single quartz crystal which originally weighed 630 kg! The work took 900 hours to complete. Impressive as this was, the group were fairly unanimous in holding the incredible craftsmanship shown in many of the smaller cameo-like exhibits on display, to be even more impressive. These usually consisted of two or more different agate layers, usually with a pastelcoloured background and a contrasting, darker foreground motive, as can be seen on the picture to the left. Again, the museum shop had a very large selection of minerals and artefacts made from them, at very reasonable prices.

The visit to the museum lasted around 2 hours, after which time I was fairly mind-blown; there is only a limited amount of visual information that my brain can process at any given time. After shuffling a few car seats, we said good bye to Elisabeth and Beatrice, who had a shorter journey home from Idar-Oberstein than if they had returned with the rest of the group to Luxembourg. On our way back, the rain which had been forecast, arrived with a vengeance. Luckily, the restaurant in the Luxembourg youth hostel was good by hostel standards; they also sold beer and wine, which made an excursion into the Old Town for dinner unnecessary.

Text: Mike Molloy / Photos: Marion Seitz

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