Kerogen in these black shales is of algal origin and the content of total organic carbon is mostly between 10–25 wt% [
16]. The mineral matter of these black shales is dominated by clay minerals, illite–smectite and illite [
22,
27]. The high concentration of pyrite, which, together with kerogen, is thought to be the main carrier of some rare earth and other elements, is distinctive for black shale. The Alum shale and graptolite argillite form patches over extensive areas in the outskirts of the Baltica palaeocontinent [
16]—Baltoscandia and Fennoscandia. A possible spatial continuity of those complexes is the graphitic phyllites that are found in the tectonically disrupted allochthonous and autochthonous Caledonian complexes in central and northern Sweden and Norway [
28]. The metal-enriched phyllites exhibit similar geochemical signatures to the unmetamorphosed black shale of Baltoscandia [
28]. These geochemical similarities may imply that organic-rich muds might have accumulated over a wide geographic area and probably under quite different depositional conditions—from shallow marine settings to continental slope environments. The black shales of Fennoscandia (Alum shale) and the graptolite argillite (GA) of Estonia can thus be treated as metal ore and a twofold energy source (including U, Th and hydrocarbons). This means that these rocks have, apart from scientific value, a significant economic value as well.
3.1.1. Estonian Black Shale—Graptolite Argillite
If we compare the Fennoscandian sedimentary and metamorphosed black shales and the Estonian graptolite argillite (GA), the stratigraphic characteristics and geological position of the Estonian GA are very simple (
Figure 2). Early Ordovician, organic-rich marine metalliferous black shale–graptolite argillite lies beneath most of northern Estonia. It has been named “Dictyonema shale”, “Dictyonema argillite” or previously “alum shale”; however, the name “dictyonema” came from the benthonic root-bearing Dictyonema flabelliforme, which subsequently turned into planktonic nema-bearing Rhabdinopora flabelliformis [
29]. More recently the term graptolite argillite is used in Estonia, while “Dictyonema shale” is still used in Russian literature.
The Estonian graptolite argillite is commonly fine-grained, unmetamorphosed, horizontally-lying and undisturbed, organic-rich (8–20%) lithified clay (Türisalu Formation), where the layers are usually 0.3 to 6 m thick. The graptolite argillite belongs to the group of black shales of sapropelic origin [
2,
23,
24,
30]. The Estonian GA crops out in many places in Northern Estonia, especially in the klint area and in several narrow river valleys (
Figure 2). As the Estonian Lower Palaeozoic sedimentary section, as well as the Precambrian rocks, are inclined towards the south due to its geological position in the southern slope of Fennoscandian Shield [
31], at its southwest end the GA layers lie at a depth of more than 250 m.
The Estonian GA is commonly characterized by high concentrations of U (up to 1200 ppm), Mo (1000 ppm), V (can be over 1600 ppm), Ni and several other heavy metals and is rich in N, S and O [
2,
22,
23]. Examples of some major and trace element concentrations are shown in
Table 1. These high concentrations of certain metals may be potentially useful and hazardous at the same time. In the Soviet Union, the GA was mined for uranium production at Sillamäe (see below,
Section 3.3.). Between 1964 and 1991, approximately 73 million tons of graptolite argillite was mined and piled into waste heaps from a covering layer of phosphorite ore at Maardu, near Tallinn (see below,
Section 3.3.).
While the reserves of Estonian graptolite argillite surpass those of Estonian kukersite (proper oil shale), it is of too poor quality for energy production at present. The GA calorific value ranges from 4.2–6.7 MJ/kg [
22] and the Fischer Assay oil yield is 3–5% (for Estonian kukersite, it is about 30–47%, for example, [
1]). The moisture content of fresh GA ranges from 11.9% to 12.5%, while the average composition of the combustible part is: C—67.6%, H—7.6%, O—18.5%, N—3.6% and S—2.6% [
32]; however, considering that it is a low-grade oil source, its potential oil reserves are about 2.1 billion tons [
1].
The specific gravity (bulk density) of Estonian GA varies between 1800 and 2500 kg/m
3 [
30]. The pyrite content of GA is highly variable, ranging from 0.5% to 9.0%, averaging between 2.4% and 6%. Pyrite commonly forms fine-crystalline disseminations or thin interlayers and concretions of different sizes. The diameter of the pyrite concretions is usually 1.5–3 cm. It needs to be noted that some concretions are complex in a structure containing small crystals of sphalerite, galenite and/or calcite. A higher degree of sulfide mineralization within the GA may be associated with the occurrence of silt interbeds. These interbeds may contain a higher amount of other authigenic compounds such as phosphates (mainly apatite as biogenic detritus and nodules), carbonates (calcite and dolomite as cement and concretions), baryte and glauconite [
23]. The concentration of sulfur ranges between 2–6%, from which about 0.6–0.8% is composed of organic matter, ca. 0.3% is sulphatic and the remaining part is sulfidic sulfur [
33].
3.1.2. Estonian Graptolite Argillite Resources
Most of the geological information on Estonian GA is obtained from basement mapping and different exploration projects, which were conducted by the Geological Survey of Estonia and its predecessor institutions, starting from the 1950s. The huge amount of information on the lithology and geochemistry of the GA was collected during the exploration of Estonia’s phosphorite resources in the 1980s. The initial estimates of the total GA reserves in Estonia range from 60 [
30] to 70 billion tons [
1]; however, little is known about the previous calculation methods and the initial data (number of drill cores, etc.) that were used. Nevertheless, GA resource estimates were regularly reported in works dedicated to phosphorite exploration and its complex exploitation (
Table 2), because GA was considered as a possible useful material in phosphorite overburden. In those works, resource estimates mostly were calculated by the panels method (blocking) [
4].
The earliest partially preserved documentation of U resource estimates dates to 1944, when the information concerning radioactivity was checked and confirmed by geologists of the USSR’s North-Western Geological Administration. The largest concentrations were recorded in the middle part of the deposit within the northeast of Estonia and the western part of St. Petersburg (Leningrad at the time). Uranium concentrations varied from tens to a few hundreds of ppm-s (80–750 ppm U). In addition to uranium in the shale, elevated molybdenum and vanadium contents were also recorded.
Since 1945, the Baltic Geological Expedition identified 14 deposits of low-grade uranium ore—Sillamäe (5464 t U, 260 ppm average U), Toila (7000 t U, 250 ppm average U), Aseri, Saka and others in the present-day Russian territory. The deposits were explored by boreholes along networks of 250 m × 250 m and 125 m × 125 m. A total resource of 72,000 t U was reported in the Baltic region. Alongside uranium exploration, a black shale enrichment technology was developed to produce uranium concentrates from the rock.
As mentioned briefly, the efforts of estimating GA resources and U contained within were almost uniquely associated with phosphorite exploration. The highest resources of U were attributed to GA in the overburden of Aseri deposit phosphorite (
Table 2). Even though GA is wedging out in the territory of the Rakvere deposit, U resources were estimated in its northern parts where the higher U concentrations occur. A somewhat more independent GA exploration was finalized in 1989 when resources in western Estonia (highest shale thickness) were calculated with the focus on hydrocarbons, but also for U, V and Mo. The conditionally exploitable proportion of GA was evaluated based on a minimum resource thickness of 1.6 m, and a minimum calorific value of 1450 kcal/kg (~6.1 MJ) [
34].
Very little data have been collected in the last three decades; the modern GIS-based methods now allow us to obtain better estimates of the total resource and visualize metal distribution.
In recent research, the verified database of 468 drill cores (database of the Estonian Geological Survey and Estonian Land Board, [
3]) has been used as the initial data. The estimated area of the Estonian GA on the mainland and islands is 12,212.64 km
2, with a corresponding volume of 31,919,259,960 m
3 [
2]. For comparison, Estonian oil shale—kukersite—occupies an area of 2884 km
2, and its reserves (proven plus probable) are about 5 billion tons [
35]. To calculate the total mass of the GA, the value of the specific gravity (density) is required. It is known [
30] that the density of the graptolite argillite varies to a great degree, between 1800 and 2500 kg/m
3. So, assuming an average density of 1800 kg/m
3, the total mass of GA is about 57.45 billion tons, while in the case of 2500 kg/m
3 the mass is 79.80 billion tons. Assuming the average density to be 2100 kg/m
3, the total weight of GA is about 67 billion tons, which is in accordance with the earlier estimates of 60 to 70 billion tons.
It should be noted, however, that in the eastern part of the GA basin there are frequent silty interlayers, resulting in a lower specific gravity of about 1850 kg/m
3, which has also been the basis of deposit-specific calculations in
Table 2 [
4]. In the Toolse deposit, the average specific gravity of 1900 kg/m
3 was applied [
36]. For Western Estonia, the exploitable proportion of GA was based on a minimum resource thickness of 1.6 m, and a minimum calorific value of 1450 kcal/kg. The average specific gravity was 2080 kg/m
3, but variations were considered in the calculations [
34]. The distribution of several metals such as U, Zn, Mo and V in the GA has been modeled earlier [
2]. The initial data were selected from the databases of the Geological Survey and the Estonian Land Board. It needs to be mentioned that the metal concentration data represents the average metal concentrations of the GA drill core. There are differences in metal distributions—the central and western regions of eastern Estonia show the highest concentrations for V and Mo, whereas V is also high in the southern region of the Eastern and central Estonia. Uranium shows the highest contents in the eastern part of Estonia, while in Western Estonia the concentrations show medium values, while the lowest values are typical for central Estonia (
Figure 3).
Uranium distribution has not been modeled in the Estonian islands due to the small number of available analyses. It is also likely that the high concentrations in the southwest area may be an artefact of the model since there are only a few drill cores available but they show locally high contents of U. Based on these data, it can be concluded that the concentration of most of the metals (except Zn) is relatively low in central Estonia. It is also important to emphasize that the available geochemical data are relatively unevenly distributed across the area and the present geochemical generalization is informative but must be taken with caution. In addition, there are very little data on the southern margin of the GA area; however, we believe that due to its limited thickness of GA (less than 0.5 m), the calculated total elemental amounts have not much affected the calculations.
Concerning the shale standard values (PAAS and NASC), the Estonian GA is very rich in U and V. For instance, the average U concentration in the Saka section (267 ppm) is a hundred times higher than the corresponding values for NASC [
23]. In the case of V, there is a nine-fold difference between the concentrations in NASC and the average concentrations detected, for example, in the Saka section, in Eastern Estonia (1190 ppm; [
23]). In general, the U content of GA shows quite a strong positive correlation with the organic matter content, which most likely indicates early fixation via metal–organic complexes. At the same time, a correlation of P
2O
5 contents with other trace elements, such as U, was not detected. Nevertheless, it is well established that U is more enriched in the bottom of the black shale bed, at least in the western part of Estonia.
Recent ICP-MS geochemical data from 11 north-western Estonian drill cores were divided into upper and lower beds, which revealed a reverse trend with regard to U association with P
2O
5 and total organic carbon (TOC) [
38]. Namely, the correlation of U and TOC is weakly positive in the upper bed and reverses to weakly negative in the lower bed (see
Section 4, Discussion). The opposite was observed in the relation between P
2O
5 and U—their association becomes more relevant in the lower, U-enriched bed. Hierarchical clustering of dataset classifies U within the group of typical redox-sensitive elements such as Mo, Sb, V, Re as well as TOC, Ag, Pb and Te.
While average metal concentrations are very useful in dividing GA into “poor” and “rich” deposits, the total content of a certain metal depends on the thickness of the GA bed. To calculate the total amount of the metal on the square meters, the ESRI ArcGIS software was employed. As an example, the total tonnage of U in the Estonian GA is shown in
Figure 3B. The presented model is based on: (1) the element grid which shows the element distribution, in ppm; (2) an interpolated grid of the GA thickness, in meters; (3) assumption of the average density 2100 kg/m
3; (4) since the element and thickness grids were calculated with the cell size of 400 m × 400 m, the same cell size was used for the calculation of the total amount of element.
These calculations provide more realistic total amounts for the elements in the Estonian GA (not just based on an average concentration value in ppm). The calculated total tonnage of U is about 5.6656 million tons (6.6796 million tons as U3O8).
The calculated Zink tonnage is 16.5330 million tons (20.5802 million tons as ZnO) and Mo is 12.7616 million tons (19.1462 million tons as MoO3). The highest studied element amounts show a somewhat similar pattern—western Estonia has the highest potential, especially for U and Mo; however, there are also distinctions between those elements. For example, in central Estonia, where the enrichment is the lowest for most elements, except Zn.
Presently, the calculation for thorium and vanadium, based on the cell of 400 m × 400 m, was conducted (for Th, see
Figure 4). Vanadium concentrations (as drill core averages; 469 drill cores) commonly vary in between 190 and 1700 ppm, being in certain layers as high as 4500 ppm [
5]. The calculated vanadium tonnage is 47.7538 million tons.
Thorium concentrations are analyzed in less than 100 drill cores, where the average varies between less than 1 and 17 ppm. As there are very few measurements of Th concentrations in western and eastern Estonia, the distribution and tonnage calculations can be provided only for the central part of Estonia (
Figure 4). The concentrations of Th are higher in the southern part and the central part of the calculated area; however, this distribution model should be taken with care since the number of measurements in the southern part is very low. Depending on the area of the calculation, the Th tonnage is 213,000 to 254,300 tons (in
Figure 4, the tonnage of Th is 213 thousand tons).