A total of 28.6 tons of phosphorite, along with approximately two tons of graptolite argillite and two tons of glauconitic sandstone, were brought to the surface from the Aru-Lõuna research area for study. The experiments conducted with these materials aim to determine the value of the mineral resources in Estonia's subsoil and assess whether further research is worthwhile, with the longer-term goal of potential mining and processing.

Initial tests on the phosphorite will take place soon at a Finnish company, which will likely use laboratories in Ireland for the analysis. "These tests are not large-scale industrial pilot experiments but rather routine geochemical analyses, which are standard practice for us and were also conducted in the preliminary stages of the study. These are small samples, and the results will provide precise information about the chemical composition of the phosphorite, including trace elements and other relevant data," said Johannes Vind, senior geologist at the Estonian Geological Survey.

The Estonian Geological Survey is currently in the second phase of its phosphorite and associated resource study, a three-year project costing €6.1 million, with reports due by the end of next year. However, delays in finding contractors for the technological tests have extended the timeline. According to Erki Peegel, an advisor at the Geological Survey, meeting the original deadline for submitting the final report to the government depends on securing agreements with the test contractors.

The first phase of the study took place from 2018 to 2022, focusing on historical data, supplemented by 37 drill holes in the Toolse, Rakvere and Aseri deposits in Virumaa. Based on the data collected, the decision was made to concentrate research efforts on the Aru-Lõuna study area, as it is already home to an active limestone quarry.

Since part of the limestone layer in the study area had previously been stripped away, discrepancies arose in information regarding the depth of the mineral resources. In the drilling area, phosphorite was found at a depth of 21.4 meters. The glauconitic sandstone containing potassium, extracted for study, was located at depths of 14.6 to 16.4 meters. Graptolite argillite, containing uranium, vanadium and small amounts of molybdenum, was found at depths of 18.5 to 19.9 meters.

"Approximately 4.5 meters of overburden, including loose soil, has been removed from the current drilling site. However, no deeper quarrying of limestone has occurred in this area, likely because the dolomitized limestone present here was not suitable as a resource. Just outside the drilling area, around 7.5 meters of rock have been removed. This means that, from the perspective of the original undisturbed surface, phosphorite begins at a depth of about 26 meters, whereas from the current surface, it starts at 21.5 meters. Based on data from three small-diameter drill holes, natural variations in depth are less than 0.3 meters. Across the broader Aru-Lõuna study area, the depth of phosphorite ranges from 15 to 35 meters, with phosphorite closer to the surface in the northern part of the area," explained senior geologist Johannes Vind.

Focus on rare earth metals

One of the goals of the study is to determine the amount of rare earth elements present in phosphorite. "According to previous research, the average total concentration of rare earth elements (REEs) in the phosphorite rock of the Toolse deposit is 0.036 percent, with individual locations showing total concentrations as high as 0.06 percent. However, the concentrations can also be significantly lower," said Johannes Vind, referencing studies published last year under the leadership of Lauri Joosu, project manager at the Estonian Geological Survey.

"This concentration does not meet the criteria for a mineable resource, and potential future utilization would require effective pre-concentration methods. For this reason, we consider phosphorus as the primary potentially valuable component in phosphorite, with other elements regarded as possible accompanying components. The average phosphorus pentoxide (P₂O₅) content in the Toolse deposit is about 9 percent, while in the Aru-Lõuna study area, it averages around 11 percent," Vind explained.

"In comparison to phosphorus content, the presence of rare earth elements in phosphorite has been studied far less extensively. Rare earth elements, listed as lanthanides on the periodic table, are nearly all present in phosphorite, with the exception of promethium. All isotopes of promethium are radioactive and decay quickly under natural conditions, which is why it does not appear in rocks like other lanthanides. The concentration of each rare earth element varies widely. From an industrial perspective, the most interesting are those used in magnets, such as praseodymium, neodymium and dysprosium, as well as a few others. While several other rare earth elements are also used in industry, the uneven distribution of lanthanides in the Earth's crust and ores results in certain lanthanides, such as cerium and lanthanum, being overproduced, leading to lower market prices," said Vind.

Advantage of Estonian phosphorite low cadmium and uranium content

According to Johannes Vind, virtually all chemical elements in the periodic table can be detected in soil, the Earth's crust or seawater. "What's noteworthy is which chemical elements are enriched in a specific type of rock, such as Estonian phosphorite, compared to the average composition of the Earth's crust. All ore-bearing rocks result from relatively unusual processes that redistribute certain chemical elements in the Earth's crust, and humans have found these concentrated elements useful. For example, one can imagine the hydrothermal vents or 'black smokers' of mid-ocean ridges, where elevated amounts of iron, zinc and copper sulfides precipitate. This is an example of ongoing ore formation. Over geological time, rocks around these 'black smokers' have been processed and redistributed by geological processes. Sometimes geologists discover these rocks, which then become useful for industrial purposes. Rocks formed by 'black smokers' can be found, for instance, in Finland, within more than a billion-year-old metamorphosed formations. Numerous reports and meta-analyses indicate that new mineral deposits are being discovered less frequently, and the concentration of valuable materials in these deposits is steadily decreasing. Common geological processes include sediment accumulation in river deltas, the deposition of lime muds forming limestone, mountain formation and more," said Vind.

"Both Estonian shell sandstone (phosphorite) and black shale (graptolite argillite) have formed as a result of unusual processes. For example, phosphorite originated from an exceptionally high concentration of brachiopods (shell-bearing mollusks) that once lived along ancient coastlines. This rock contains more phosphorus than the Earth's crust on average – more than 70 times higher. Among the elements associated with phosphorus, rare earth elements, or lanthanides, are particularly elevated in shell sandstone. Other accompanying elements, such as cadmium and uranium, which can be problematic in phosphorites elsewhere in the world, occur here at very low concentrations. The uranium content in our phosphorite is more than six times lower, and cadmium content is over one hundred times lower, than in the average global phosphorite," the researcher explained.

Graptolite argillite and glauconitic sandstone also of interest

After the formation of shell sandstone, a period followed when the present-day territory of Estonia was covered by deeper seas, where muds were deposited that are now known as graptolite argillite. "We simply call this rock (metal-rich) black shale. Black shale also formed under unusual conditions, where long-term oxygen depletion prevailed at certain depths of the water column. Under such oxygen-deprived conditions, certain chemical elements, like vanadium, molybdenum and uranium, precipitate out of seawater. In some ways, the modern-day analog of black shale is the Baltic Sea, where molybdenum and uranium also settle in its oxygen-depleted sediments.

The main components of this rock are silicon, aluminum and iron. In metal-rich black shale, the elements with elevated concentrations are vanadium, uranium and molybdenum. Additionally, there are somewhat higher levels of rhenium, silver, arsenic and occasionally zinc. Many of these components require greater environmental attention, which the Estonian Geological Survey is actively addressing," said the geologist.

Glauconitic sandstone, on the other hand, is primarily composed of aluminum, silicon and other widely distributed elements such as iron. "This rock has elevated levels of potassium, which can be up to 2.5 times higher than the average in the upper crust. However, the potassium is bound in the composition of the relatively stable mineral glauconite, making it difficult to release potassium for use in plant growth," noted Johannes Vind.

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