Where is the vanadium in Jämtland?

In my last blog I wrote that the geology of Skåne and Jämtland is different because tectonic processes affected Jämtland much more than they did affect Skåne. This means that rocks in Jämtland are thrust and partly folded, whereas rocks in southern Sweden ‘only’ experienced some squeezing and dislocation. Alum shale deposits in Skåne are therefore found as more or less horizontal layers, whereas the situation is very different in Jämtland.

Information on Jämtland’s geology can be obtained from the work by Karis and Strömberg (1998) and from geological maps for the region, both of which can be downloaded from the website of the Geological Survey of Sweden (SGU). Geological maps show how rocks are distributed at the surface and are compiled by studying outcrops of rocks and rock successions in drill cores. By reading geological maps, it is possible to construct a geological transect, which shows how rocks are superimposed to each other. Depending on how much of the underground geology is actually know, such transects can be more or less speculative. Multiple drill holes in an area can therefore add considerable knowledge to making a geological transect more realistic.

I could not find a detailed geological map that covers the southern part of Lake Storsjö and the areas of ongoing geological exploration for Alum shale. Searching in SGU’s online tool kartgenerator did not help either: the area around Myrkviken shows up as a blank. This is a bit surprising given that as many as 28 deep drill holes have been made by SGU within the scope of their Alum shale project in the years 1977 to 1979 (Hedin 2015). Further searches on the internet did not reveal much more information. It thus seems that the results from the extensive survey by SGU in and around Myrviken have up to now only been published in bits and pieces, and not in their entirety. I should mention, however, that an unpublished report exists, but this report does not seem to be available.

To show a geological map for Myrviken, I therefore use Figure 2.2 from Hedin’s (2015) PhD thesis, which includes a very generalized geological map. The dark blue color indicates the occurrences of Cambrian Alum shale, which is prominent south of Myrviken. The map also displays the position of the many drill holes carried out by SGU (red dots).

Part of Figure 2.2. from Hedin (2015) showing a simplified geological map over the Storsjö area. The red dots on the map indicate the drill holes that have been made by SGU between the years 1977 and 1979. The occurrence of Alum shale is indicated by the dark blue color.

One can get a clearer view of where the actual SGU drill holes were located by searching in SGU’s online tool (https://apps.sgu.se/geolagret/ ). It is amazing to see that so many holes were drilled in such a restricted area. It must have cost a fortune to drill all of these and this leaves me wondering why no one has analysed these drill cores in greater details. I could only find the publication by Snäll (1988), who described the rock layers in some selected cores and analyzed the Alum shale for its maturity and elemental chemistry (cores from Klövsjö, Myrviken and Häggenås). But maybe there is much more and I just have not found it!?

By searching https://apps.sgu.se/geolagret/ one can find the position of the SGU drill holes placed on a topographic map. All of these were positioned close to Myrviken. The idea with this dense net of cores was probably to delineate the occurrence of the Alum shale and to better understand how different parts of the shale are stacked upon each other.

In an area where the rocks are heavily influenced by tectonic processes, it is however difficult to distinguish between rocks that are in place and rock packages that have been dislocated. In Jämtland, geologists therefore separate between autochthonous rocks and allochthonous rock packages. The Alum shale exists both as autochthonous and allochthonous layers and is thickest, of course, were several packages were once stacked upon each other. Disentangling these stacked upon packages is difficult (see Figure below) and the study of the SGU drill cores was an attempt to do so.

Figure 3 from Juhlin et al. (2016). Geological cross section through the Myrviken area boreholes based on an unpublished SGU report on Alum shales and shown
at a vertical exaggeration of 10 : 1.
Downloaded from: doi:10.5194/se-7-769-2016

But before I go into details regarding these drill cores, or into what has been published about them, I need to write a bit about the stratigraphy of the Precambrian to Silurian rocks in the region. Here I mean which of the rocks are oldest, which are youngest, where in this sequence can the Alum shale be found and what age do the different rock types have.

Stratigraphy of Precambrian to Silurian rocks in the Storsjö region of Jämtland. The shaded area means that rocks/layers from these periods are missing. Copied from the Bedrock map Af 207 of Östersund SO 19E.

When it comes to the oldest rocks in Jämtland, much of these seem to be missing or are only partly present. Oldest is the Precambrian basement and a quartzite of maybe early Cambrian age. The Alum shale formation is often termed Kläppeformation in Jämtland and belongs to the middle and upper Cambrian time. It is overlain by limestone and shale of Ordovician age. This is a bit different to Skåne, where the Alum shale encompasses the Cambrian and the lowermost Ordovician. And, as those of you who read my blog may remember, the focus of the current exploration in Skåne is on the lowermost Ordovician Alum shale or Dictyonema shale. It is this part of the Alum shale that contains high amounts of vanadium in Skåne. In a short paper from 1981, David Gee suggested that the high vanadium contents that are characteristic for the Dictyonema shale in Skåne and also in Norway could be a means of correlating rocks over larger distances. If so, then the part of the Alum shale with high vanadium content in Jämtland could be the equivalent to that in Skåne and Norway. But, here is the problem: according to the geological literature I have read, Alum shale of lower Ordovician age is not present in Jämtland … and the parts of the Myrviken Alum shale, which contain high amounts of vanadium, uranium, molybdenum, sulphur and organic carbon, are assigned to the upper Cambrian. So – are we dealing with the same shale in Skåne and in Jämtland, or are the target shales of different ages? Maybe a geologist who does research on Alum shale knows more?

The report by Snäll (1988) provides some hints regarding the geochemical composition of the Myrviken, Häggenås and Klövsjö Alum shale. As can be seen in the Table below, all samples from the Upper Cambrian from the three cores exhibit high uranium (U), vanadium (V) and molybdenum (Mb) contents, as well as high sulfur (S) and organic carbon (Corg) percentages. The values for U and V shown here are very similar to those for the Dictyonema shale in Skåne.

Uranium (U), vanadium (V), molybdenum (Mb), sulfur (S) and organic carbon (Corg) in selected samples from deep drill cores in the Storsjö area. Data according to Snäll (1988).

In Skåne, both U and V have been analyzed continuously along drill cores and thus provide a means to clearly differentiating zones with high and low values in the Alum shale. The bits and pieces of samples analyzed in Jämtland, however, do not provide a clear picture of where exactly in the stratigraphic succession of the Alum shale, high U and V values occur.

Nevertheless, as is the case for Skåne, it has long been known that the upper part of the Alum shale contains considerable amounts of uranium and vanadium. Enough drill cores obviously already existed in the late 1970s for the Storsjö area to assess the elemental composition of the Alum shale. Although the available drill cores do not seem to have been fully analyzed (or at least the results have not been published), the knowledge gained was nevertheless important information for Continental Precious, who entered the stage in the year 2005 and applied for exploration licenses for Hackåsen, Viken, Kämpdalen, Bölåsen and Åbbåsen to search for molybdenum.

References:

Hedin, P. 2015. Geophysical studies of the upper crust of the central Swedish Caledonides in relation to the COSC scientific drilling project. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1281. 87 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9320-2.

Gee, D. G. (1981): The Dictyonema-bearing phyllites at Nordaunevoll, eastern Trøndelag, Norway. Norsk Geologisk Tidsskrift, Vol. 61, pp. 93-95. Oslo 1981. ISSN 0029-196X.

Juhlin, C., Hedin, P., Gee, D. G., Lorenz, H., Kalscheuer, T., Yan, P. (2016): Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5km deep COSC-2 borehole, central Sweden. Solid Earth, 7, 769–787, 2016. http://www.solid-earth.net/7/769/2016/ doi:10.5194/se-7-769-2016

Snäll, S. (1988). Mineralogy and maturity of the Alum shales of south-central Jämtland. Sveriges Geologiska Undersökning, Serie C, nr. 818, 49 pp.