High radioactivity mineral nature of the Azov rare-earth deposit of the Eastern Azov Sea Region of the Ukrainian shield
Omelchenko A. A.
*Ukrainian State Research and Design Institute of Mining Geology, Rock Mechanics and Mine Surveying (UkrNIMI) of the National Academy of Sciences of Ukraine (NANU), Donetsk, Ukraine
As a result of systematic prospecting and geological exploration works conducted by the Priazovskaya Integrated Geological Party (KGP) belonging to the Public Company (KP) Yuzhgeologia a number of deposits and mineralizations of increased radioactivity have been explored in the Eastern Azov Sea Region. At present, the most interesting one is a rare-metal ‑ rare-earth deposit (Sheremet, 2007). It is not being operated as yet. Before starting development of this deposit, it is necessary to research into the trends for provision of environmental safety.
Connection between compositions of radioactive and rare-earth elements is known for a long time. In particular, references gave a positive curvilinear dependence between the amount of rare-earth elements (REE) (ranged 0 – 8.0 %) and thorium.
The Priazovskaya Integrated Geological Party during exploration of the Azov deposit has made comparison of natural gamma-ray logs of the scale 1:200 (geological map of the scale 1:200) for six (6) ore holes (Nos. 3, 7, 45, 49, 52 and 68) with data of core sampling for zirconium oxide and amount of REE (Sheremet, 2007).
As a result of this analysis it became clear that distribution of the amounts of REE in vertical section of ore-bearing zones correlates well, with rare exception, with the shape of natural gamma-ray logs. Maximum gamma activity, which achieves 500 µR/h in the above-mentioned holes, corresponds to the maximum values of the amounts of REE for core, and, vice versa, decreased gamma activity corresponds to the minimum compositions of REE (Fig. 1). Sometimes to obtain such correspondence it is necessary to offset from vertical either a gamma-ray or REE distribution log (Fig. 1, hole No. 52).
Fig. 1 Comparison of gamma-ray logs and the results of core sampling for the amount of REE.
Fig. 1 shows the most expressive examples of the comparison of gamma-ray logs and distributions of the amounts of REE for ore-bearing zones in the mantle of waste (hole No. 3), in crystalline rocks (hole No. 52) and in both cases (hole No. 45). We can seen that in the mantle of waste opened by hole No. 45 no full correspondence between both logs is available: in the range 34-43 m there is no adequate increased composition of the amount of REE to increased gamma activity. Probably, this is due to either washing out of ore mineralization or due to any other reasons. Large angularity of natural gamma-ray logs for ore-bearing zones reflects complexity of their structure due to interleaving intervals of peridot-amphibolic alkali-feldspar syenites, which are enriched with zircons and rare-earth minerals and are rich zirconium-rare-earth ores and quarts-containing barren syenites.
By the selected intervals, we have constructed the relation of the amounts of REE and gamma activity of ore intervals. Sampling of 35 pairs includes gamma activity range of 30-40 to 500 µR/h, and the amount of REE ranges 0.2 -0.3 to 10 %. Sampling is interesting in that it allows determining the value of gamma activity that corresponds to the minimum amount of REE equal to 0.4 %, which presents prospecting interest. The value of this gamma activity equals 35-40 µR/h, background gamma activity being 12-15 µR/h.
Judging from the results of mineralogical analysis, the main radioactive rare-earth minerals at the deposit are zircon, orthite, britholite, and bastnaesite, which are noted for different relation of uranium and thorium compositions. Average content of radioactive rare-earth minerals at the deposit are as follows: zircon ‑ 19 kg per ton of rock, orthite – 6.4 kg per ton of rock, and britholite – 8.8 kg per ton of rock. Average content of rare-earths in these minerals are (in %): in zircon – 1.56, in orthite – 29.0. and in britholite – 61.65.
Scatter of points in the relation of the amounts of REE and gamma activity is apparently associated with different uranium-thorium relation in rare-earth ores resulting from different relation of light and heavy lanthanides – REE mainly of cerium and yttrium groups. Finally, this is determined by composition of rare-earth minerals. Perhaps, in some cases increase in gamma activity exists owing to zircons at their abnormally high concentration.
Sheremet Ye.M., Strekozov S.N., Krivdik S.G. et al. Prediction of mineralization of rare elements of the Ukrainian shield. Donetsk: Veber (Donetsk Branch), 2007. 220 p. (in Russian).