Uranium deposits Rozná and Olsi

Geology of the deposits

The Rozna deposit was discovered in 1956 by radiometric exploration and sampling of radon gas in soil. It is the largest deposit of uranium ore in the Moravian region and belongs to the most important deposits in the Czech Republic. Exploitation at the Rozna deposit started in 1957.

The uranium deposits Rozna (active mine) and Olsi (abandoned 1996) are localised in a formation of metamorphosed sedimentary - effusive rock of Precambrian age, nowadays mostly gneisses. Uranium mineralisation is bonded to graphitised zones of large and long-time developing faults, having the length of tens kilometres. The faults belong to the tectonic sutures of the Elbe Lineament Zone. For localisation see Fig. 1.

Fig. 1: Localisation of the Rozna (Dolni Rozinka) mine

The wallrocks are biotite- and hornblende gneisses with abundant intercalations of ortho- and paraamphibolites, quartzites and marbles. The spatial localization of ore zones is to a large extent controlled by the mineral composition of surrounding rocks and their physico-mechanical properties.

The rock complex of the Rozna deposit mostly consists of Moldanubian rocks represented by gneisses in different level of migmatisation and amphibolites.

Fig. 2 Cross-section of the Rozna deposit

1.                zones / veins

2.                biotite gneisses / migmatitised gneisses

3.                amphibolites / serpentines

4.                granites

The ore occurrences are bonded to the Ro~.ná - Ola.í anticline, about 12 - 15 km long. The saddle part of the anticline is formed by intensively metamorphosed gneisses, the wing parts by gneisses and amphibolites. The axial plane of the fold is overcast eastwards; due to it, both wings dip 45 - 65^o  westwards. The maximum width of the fold wings is 1,5 - 2,5 km. The deposit Ro~.ná is localised in the western wing of the anticline, deposit Ola.í - in the eastern wing (Fig. 1). The anticline wings are complicated by flexures, which influence the building of ore-bearing structures and control the location of individual ore knots of the deposit.

Dislocations of fault character form the main ore-bearing structures. Their morphology and spatial location is strongly connected with the fold fabric. The dislocations are both strike- and diagonal.

Strike (longitudinal) faults are developed in all deposits; the dip of these dislocations (45° - 70° to the west) mostly conforms to the dip of rock strips or transects them at an acute angle. Their spatial distribution is therefore influenced by the orientation of anticline wings. The uranium mineralisation is bonded to main strike dislocations and with them connected higher-level structures. Based on their origin they can be described as tectonic zones and linked veins. The thickness of zones is mostly several metres, exceptionally up to 25-30 metres. They reach up to 10 km in longitudinal direction. Zoned fillings consist mostly of broken surrounding rocks with a small amount of vein minerals (calcite, graphite, and pyrites).

The tectonic sutures bracket blocks of mylonitised, graphitised, pyritised, and chloritised rocks. In some cases, the dislocation zones are filled with aplite or carbonate dikes. Uranium mineralisation is spatially bonded to hydrothermally altered parts of chloritised rocks (see fig.13 - příloha).

Due to the presence of minerals of temporally different mineral assemblages, and the variety of textural ore types, the uranium ores are complex. The ores were formed during metasomatic processes, what is reflected by their disseminated or veiny-disseminated character. The following six stages of mineralisation (assemblages) were recognised in the Ro~.ná-Ola.í deposits:

1. graphite - pyrite (pre-Variscan)

2. quartz - sulphide  (pre-Variscan)

3. carbonate (siderite) - sulphide (late Variscan)

4. calcite - chlorite - uraninite (late Variscan, 270 + 15 Ma)

5. calcite - pyrite (late Variscan)

6. albite - chlorite - coffinite (USiO₄) (Kimmeridgian orogeny, 190 + 15 Ma)

(for age det. see Fig 7, p. 47)

The development of uranium minerals is closely connected with metasomatic chloritisation of biotite gneisses. Biotite and to some extent also potassium feldspars are replaced by chlorite, albite, hydromuscovite and sericite. Pitchblende and coffinite occur as massive aggregates, often with colloform structure. Coffinite usually replaces pitchblende along aggregate margins and fissures. In some parts, coffinite also replaces primary montroseite (uranyl vanadate). Pitchblende is often associated with selenides, which are bonded to carbonate veins, forming in them nests up to several tens of centimeters in size. Selenides are mostly represented by berzelianite (Cu_2-xSe) and umangite (Cu₃Se₂); les common are eskebornite (CuFeSe₂), clausthalite (PbSe), bukovite (Tl₂Cu_3+xFeSe_4-x - named after a nearby locality Bukov), klockmannite (CuSe), eucairite (CuAgSe), ferroselite (FeSe₂), crookesite (Cu₇(Tl,Ag)Se₄), and tyrrelite ((Cu,Co,Ni)₃Se₄).

The ores of Ro~.ná-Ola.í ore field developed in several stages; the changes connected with younger mineralisation processes overprinted the older ones, giving rise to complex zone of altered rocks.

The changes of original rocks along tectonic zones with graphite - pyrite mineralisation are graphitisation, pyritisation and silicification. The aureoles of altered rocks often range tens meters into the footwall as well as hangingwall rocks. Their spatial extent is greater than any of the ensuing ones. Graphite and pyrite replace mafic minerals, above all biotite.

Changes of wall rocks along carbonate (siderite) - sulphide veins of the pre-ore stage manifest themselves by intensive bleaching. The rocks gain pale yellow or light grey colour and the laminar texture turns to massive texture.

The hydrothermal changes consist in removal of Si, Fe, Na and influx of water, K, Ca, Mg, H₂CO₃. Their effects can be observed no more than 1 - 2 metres from the vein. The resulting low-T potassium metasomatism was not suitable for uranium precipitation, as it was responsible for unfavourably low reduction capacity and acidification of the rocks.

Changes of the original rocks along the veins of the calcite-chlorite-uraninite stage are visible only microscopically. They manifest themselves by intensive chloritisation and carbonatisation in the immediate vicinity of the vein. Chlorite makes pseudomorphs after biotite; biotite and plagioclase are replaced by carbonate. The chloritisation caused also increase in Fe and Mg and removal of K and Si.

The alteration of rocks connected with the youngest mineralisation stage (albite - chlorite - coffinite) is easily visible as reddening of the rocks. This alteration starts to occur 1,5 - 2 metres off the vein. There, biotite is replaced by muscovite, chlorite, and carbonate. The feldspar grains gain red tint, the acidity of plagioclase increases, and on the rims they are replaced by pure, transparent albite. Maximum intensity alterations revealed by total replacement of the gneisses by quartz, albite, fluorite, and montmorillonite. Original uranium minerals are replaced by coffinite, quartz, radioactive harmotome (rich in radium), clay minerals and uranophane. The composition of such rocks gives evidence of influx of sodium, calcium, and carbonic acid, and removal of potassium, iron, magnesium and silicium. They display positive correlation of sodium and uranium levels. The abundance of this sodium-metasomatism controlled mineralisation increases with depth from the surface. From the mining point of view this phenomenon is considered disadvantageous, as it indicates removal of uranium from main mineralised zones and its secondary re-dispersion.

Table 1: Basic facts on U mining in the region

The Rozna I mine

Mining takes place in one deep mine, with the dressing of mined ore at a chemical-processing unit, which is close to the mine.    Uranium content in mined ore is 0,1 - 0, 5 % (1 to 5 kilograms of uranium per metric ton of ore). The final product of the processing unit is uranium concentrate (NH₄)₂U₂O₇, called "yellow cake", which is reprocessed at other unit abroad.

The mine and the processing plant form a branch of GEAM and they are the part of the state enterprise of DIAMO. The current annual output of uranium from the Rozna deposit is used after reprocessing as a fuel for the nuclear power station Dukovany.

Table 2: Key facts on the uranium deposit Rozna

The deposit is exploited by several working shafts and main level crosscuts. The vertical distance of levels is 50 metres. In the levels there are longitudinal crosscuts in the footwall of the zones and short offsets running through the zones. The distance between these offsets is 50 metres. The stope dimensions are 50 metres both in longitudinal and vertical directions.

Single mined blocks are developed by block raises, which are raised from the bottom level to the upper level in the zone or in the footwall of the zone according to geological conditions.

The total depth of the mine is 1,200 m below the surface and the working depth at the moment is 900 - 1,000 metres.

The mining method

The methods used in underground mining depend on the geometry of the mineralized bodies and on the geomechanical characteristics of ore minerals and surrounding rocks. Overhand stoping with filling was a prevailing method in the early stages of mining of the Rozna deposit. Only the underhand stoping method (top slicing) is used at the present time.

The principle of the mining method:

After a slice is mined from a block raise to the limit of the mined block, blasting is used to cause the caving of the hanging wall on the artificial roof that was put down on the floor before that. The next slice is mined under the artificial roof, which separates broken ground and the working place. The maximum height of the slice is limited to 3 metres and its width depends on the thickness of ore mineralisation.

There is a mechanical venting system of at the workplaces. Mining by blasting is combined with wet hammer drilling. Timber supports (support by sets) are used to secure workplaces at stopes. Ore in stopes is transported with scrapers. At the main adits, the ore is loaded into mine cars which are hauled to the working shaft and hoisted to the surface.

Fig. 3: Ventilation and radiation protection

Ventilation in a deep mine enables fresh air supply into the underground, removal of hazardous gases off the mine and provide good microclimate at workplaces. The hazardous gases in a uranium mine involvee radon emerging from uranium ore, nitrogen oxides arising from blasting and fumes from diesel engines. The exhaust system of fan ventilation is used in the mine Rozna I.

The main exhaust fan VCD 31,5 M (runner diameter 3,15 metres) operates at the top of the ventilation shaft R6 in 1.1 MW power input. The fresh air enters the mine through several shafts (B2, Bl, Rl, R3, R7S) in the total volume of 260 cubic metres per second. The fresh air is distributed in the mine along crosscuts and airways close workplaces as a result of the depression of the main fan.

The ventilation of workplaces in mined blocks, during driving and raising, is pursued by ventilation pipes (diameter 0,4 or 0,5 metres) and pipe fans, which blow fresh M 55 air into workplaces.

Foul air goes through broken ground and along return airways to the upcast shaft R6 and up it to the surface. The mine ventilation is computer controlled.

Radiation safety

Uranium miners are exposed besides usual hazards of mining operations in a deep mine also to effects of radioactivity of uranium ore (radiation, dust, radon).

The radiation at all workplaces in the Rozna I mine is constantly monitored every day. Besides this monitoring, all miners wear individual ALGADE dosimeters, ensuring continuous individual monitoring during their stay in the mine.

If a miner receives a dose higher than radiation limit, he has to move to a mine workplace with lower radiation level or to a place on the surface. Miners in uranium mines in the Czech Republic may only work 2,100 work shifts in underground.

Environmental protection

The mining site environs are always affected by mining activities. The way of running of the mining activities at the Rozna deposit ensures that the damages to the natural environment are minimal. At the same time, appropriate steps are taken to eliminate the effects of mining and to restore the sites after finishing of mining back to the almost original state.

Mine water and surface water contaminated by uranium and radium represent the greatest danger to the environment during uranium ore mining.

Protection of the natural environment against contaminated water consists in cleaning all water from the mine surface (rain water, process water and sewage water) and mine water before it is discharged in the environment. The average annual volume of cleaned water is about 2 million cubic metres.

Fig 4a,b: Tailing impoundments at Ro~.ná mine

The cleaning operation in decontaminating plants is based on precipitation of uranium and radium salts, followed by their extraction with ion exchange resins. Radium is precipitated as radium-barite, which is disposed in the tailings pond, and uranium is processed to yield uranium concentrate ("yellow cake", ammonium or sodium diuranate).

Fig. 5: Water treatment flow sheet

The quality of all waters flowing around mining sites and discharge from the decontaminating plants is regularly monitored. Water samples are analysed for uranium, radium and other hazardous substances.

Restoration and safety operations are provided at those sites where mining activities have been finished. These operations consist in liquidation of buildings and plants, back-filling of shafts and raises going to the surface, and removal of contaminated soil, followed by biological reclamation.

Fig. 6: Waste rock dump Ola.í after technical and biological reclamation.

Ola.í - Drahonín mine flooding

The mine pits on ore deposit Rozna I has to be flooded after mine will be closed. Its closing is planned for year 2002. According to the water inflow into pits it will take about 6 to 10 years the water reach surface (or planed level below surface where it will be kept).

The geologists mining company GEAM assumed that the concentration of TDS and all dissolved species in mine water would gradually decrease during the mine flooding and by the time water reached surface there would be no need to clean or the otherwise process the mine water. It was planned to leak mine water directly to surface into river.

Recent experience from flooding the pit Olsi-Drahonin showed very different picture. The whole situation can be briefly described as follows:

Uranium mine Drahonin was closed in 1990. The regulated flooding of the mine continued till 1996 when mine waters reached the surface. During this period, the concentration of total dissolved species varied around 800 mg/l, sulphate ions around 600 mg/l, Fe 3 mg/l, Mn 0.2 mg/l, uranium 2 mg/l, and radium 500 mBq/l. After dilution with surface water, the mine water was discharged without any cleaning to the small creek Haduvka in amount about 10 l/s. The annual mean of Haduvka creek flow is about 25 l/s with higher values in early spring and in mid-autumn and nearly no water running in summer.

When mine waters reached the steady state after the mine flooding the concentration of TDS suddenly increased up to 3,000 mg/l, caused mainly by very high concentrations of dissolved sulphates (about 2,000 mg/l), iron (30 mg/l), manganese (8 mg/l), uranium (15 mg/l), and radium (2,300 mBq/l). It was therefore not possible to discharge mine waters to the Haduvka creek without cleaning. A decontamination station was built at the mine adit. The operation of the decontamination station effectively removed uranium and radium (below 0.1 mg/l and 300 mBq/l, respectively), the concentrations of other species remained almost unchanged. Discharging mine waters to Haduvka creek is connected with extensive precipitation of iron hydro-oxides, caused by fast oxidation of ferrous to ferric iron. Other changes were not visible.

Fig. 7a, b: Development of the chemistry of mine waters

Table 3a, b: Composition of mine water at the input to decontamination station. Water flow rate of is ≈ 15 l/s.

Composition of output water at the decontamination station.

Q . total volume of water processed per month in cubic meters.

To undertake effective actions, extensive sampling of waters and bottom sediments was performed each month along the Haduvka creek between the decontamination station and the place where Haduvka streams in Loucka river (3.5 km downstream). The values of pH, Eh, and temperature were measured in situ.

The water samples were analysed for all species present. The composition of bottom sediments was checked by complete chemical analysis. Sequential extraction procedure was used to follow the speciation of uranium, radium, iron, and manganese. The extracts were obtained by leaching the samples at pH = 8.2, pH = 5.0, pH = 3.0, pH = 2.0 in strong oxidising media, and by total decomposition of the residue. Mineral composition of bottom sediment was checked by X-ray diffraction.

The study gave very surprising results. Although the bottom sediment thickness is very small and seldom exceeds 2 cm and the creek is not steep, very strong and very fast interaction between waters and bottom sediment was found to exist. Along the 3.5 km of the stream, water loses more than half of its TDS, almost all iron, uranium, and radium. The pH values increase from about 6 to more than 8; similarly, the oxidising strength increases steeply. There is also a high increase in carbonate and clay minerals content in bottom sediments (from 0,X % at the beginning up to 15 % after 3.5 km). Due to precipitation of U, Ra, Mn and Fe, their mobile and exchangeable fraction in bottom sediments decreases downstreams. As a result, the portion of these elements bonded to stable minerals seemingly increases - from initial 60% to more than 90% at the influx of Hadůvka creek into the Loučka river.

General trends show strong seasonal dependance, but there is no direct correlation to the amount of precipitations. It seems more probable that the common pattern is directly influenced by variations of carbon dioxide concentration in the atmosphere.

Fig. 9 a,b: Changes in the Fe and Mn speciation in bottom sediments of the Haduvka creek

Because of this experience it is very important that we are able to predict the compositional changes of mine water at ore deposit Rozna I. The geological, hydrogeological and geochemical conditions on ore deposit Rozna I are very similar to that of the Olsi-Drahonin deposit. There is, however, a very important difference. Mine waters from Olsi-Drahonin deposit have influenced only a little creek Haduvka, and we are still seeking the proper explanation of what is going on there. The impact of similar pollution of bigger streams - rivers Nedvedicka and Loucka would be much severe; it could be caused both by direct surface outcharge and/or latent underground seepage of mine waters. The environmental and health risk connected with mine flooding is therefore very high. There is an urgent need to predict the future evolution of mine water composition. Such prediction would be used for decision about adopting appropriate measures.

In the file Model.xls in sheet "rock" you can find basic data about types of rocks, which represent typical wall rock, their mineral composition and average chemical composition of the rock. The mineral changes in veins and faults with uranium mineralization are given. These data describe the environment in which underground water gains its dissolved species. In sheet "water Rozna" is composition of typical mine water from ore deposit Rozna.

In first step the following has to be found by modelling:

1.What is speciation of dissolved species and if mine water is saturated or supersaturated to any mineral or compound. This water is not in contact with atmosphere so it has zero content of dissolved oxygen. Two alternatives can be checked as for dissolved CO₂: Mine water saturated with respect to the carbonates (calcite) and completely without contact with carbonate.

2.What will be changes in mine water when it will reach surface and become saturated with respect to oxygen and to carbon dioxide (pO₂ = 0,2 and log pCO₂ = -3.5).

In the second step, when mine water will equilibrate with respect to the wall rock, changes in chemical composition of mine water have to be found. Based on comparison with changes in composition of mine waters on ore deposit Olsi-Drahonin, the mine water on Rozna I deposit is not saturated and its TDS will increase highly but we still need to know how.

In the third step, changes which will saturated mine water undergo after it will be equilibrated with atmosphere have to be checked.

Results of such modelling can provide first very raw but reasonable estimate of future evolution of mine water composition and what we can expect. This can be first solid base for recommendations to the responsible officials.

Inspectional drawing of ore deposit Rozna with shaded area, which is endangered by upraised mine water after flooding the mine.

Fig. 10: Course participants preparing themselves for the excursion to the shaft