The Uranium Deposit of Žirovski Vrh
Veselin Jokanovič, Staniša Radoševič, and Milan Ristič SUMMARY
The Žirovski Vrh uranium deposit is situated about 30 km west of Ljubljana. The ore bearing ročk is the grey Groden sandstone of the Middle Permian age. Its basement is represented by dark Permo-Carboni-feroius shale, whereas it is overlain by Middle Permian red Groden sandstone and Upper Permian limestone and dolomite.
The thickness of the grey sandstone is about 300 meters. The sequence of the red series is between 300 and 500 meters thick.
The grey sandstone has a variable grain size, from the very fine up to coarse ones. It is composed of sharp-edged quartz grains, K-feldspars and felsic plagioclases, and contains, as a rule, fragments and phenocrysts of volcanic quartz and feldspars, the fragments of usually recrystallized quartz-porphyry and keratophyre, the quartzite fragments and, less fre-quently, those of chert. The diabase fragments were also observed. The muscovite is frequent, and very frequent chlorite.
The cement of the uranium-bearing rocks is composed of quartz-clay-sericitic material, replaced partially by calcite. The calcite occurs in veinlets or it is dispersed thrcvughout the ročk. In addition to calcite, there are also small quantities of dolomite, ankerite and cerussite.
The uranium ore is confined to the zones in the grey series that contain the dispersed organic matter and anthracite aggregates.
As the most common uranium mineral pitchblende occurs, disseminated in the sandstone cement, or rarely associated with galena, pyrite, chalco-pyrite, and anthracite.
The mineralized zones contain also sphalerite, tetrahedrite, chalcocite, bornite, realgar, marcasite, arsenopyrite, covellite, ilmenite, chrome-spinel, and pyrrhotite. Among the secondary uranium minerals the presence of autunite, torbernite, metatorbemite, dumontite, the minerals of the pho-sphuranylite-renardite series, gummites, arsenolite and beta-uranotile has been established. Limonite, malachite and azurite are also present.
The strata-bound uranium ore constitutes numerous unregular lenses of highly variable dimensions. The surfaces of the ore bodies are of the order of several hundreds to thousand square meters. Their thickness vary from 0,5 to 1,5 meters, attaining in most favourable cases several
meters. The uranium contents varies most frequently in the interval of 0,05 to 0,23 °/o of U3Os. The ore itself is controlled by the stratification. The rocks, however, mostly do- not show a well marked bedding. The most developed structure of the Groden beds is the cleavage oriented at right angle to the bedding planeš,
DISCUSSION
Mittempergher: Nelle arenarie sono presenti in grande quantita detriti di porfidi e di keratofiri. La mia domanda e questa: queste formazioni affio-rano* nell'area di Zirovski vrh? Le arenarie di Zirovski vrh sono delle arcosi poco elaborate contenenti plagioclasi e ciuarzo non arrotondato', etc. Le rocce di origine vulcanica dovrebbero quindi affiorare a breve distanza, nelle vicinanze di Zirovski vrh.
Jokanovič: A Zirovski vrh et dans les environs il n'ya pas de roches volcaniques. Mais aux environs de Cerkno, a peu pres a 15 km du Zirovski vrh, affleurent les quartz-porphyres, ou 1'uranium a ete constate au cours de la prospection.
Mittempergher: La quantita di carbonati presente e tale da determinare un eccessivo consumo di acido' nella fase deli'«ore dressing» del minerario uranifero?
Ristič: Si j'ai bien compris, vous avez demande combien il y a de matiere carbonatee dans les gres. II y en a en moyenne 5,4 °/o.
Mittempergher: Cest beaucoup pour les gres.
Ristič: Cest vrai, mais elle varie. II n'y a pas de correlation entre la matiere carbonatee et le contenu de l'uranium. La matiere carbonatee est anterieure a la mineralisation, puis elle s'est remobilisee pendant le metamorphisme.
Amstutz: Je n'ai pas tres bien compris ou vous mettez la limite de ces lentilles. D'apres ce que j'ai compris, il y a la mineralisation plus ou moins comme un nuage. Je voudrais vous demander ou vous mettez la limite de votre ligne: est-ce en pourcentage, ou bien quel est le critere pour mettre la ligne qui limite la lentille?
Jokanovič: Cest une interpretation qui est assez complexe. Tous les niveaux et toutes les teneurs sont etablis sur la base de la radiometrie.
Petrascheck: Eine Frage bezieht sich auf die Dimensionen der erzfiih-renden Linsen, da wir einerseits auf den Bildern gesehen haben, daB sie etwa 10 m betragen, auf der anderen Seite war aber die Rede von einigen 1000 m2 Flache. Die zweite Frage ist, ob die Fazies der Grodener Schichten in Zirovski vrh vergleichbar mit jenen von Eisenkappel ist.
Jokanovič: En ce qui concerne la dimension des lentilles, c'est tres different. Les dimensions en direction sont assez elevees et elles peuvent atteindre jusqu'a 100 m. En pendage, les dimensions sont a peu pres de 20 a 30 m.
Petrascheck: Une petite interruption. Vous determinez peut-etre les dimensions par une manchette de plomb sur le tube du scintillometre? De quelle fagon determinez vous les limites du minerai radioactif?
Jokanovič: Cela depend de l'epaisseur et du terrain. Et il s'agit d'un calcul assez complique. Les lentilles sont notamment quelquefois tres minces et la mineralisation n'est pas visible. Dans la galerie vous ne pouvez presque rien voir. Nous avons aussi des problemes concernant le calcul des reserves.
Petrascheck: Und die zweite Frage, bitte, ist die Schichtfolge von Eisenkappel identisch mit der von Zirovski vrh?
Grad: The uranium-bearing beds of Žirovski Vrh and the beds of Eisenkappel are not identical. There are differences in age, lithological development and type of mineralization.
In spite of unsufficient research of the Eisenkappel terrain it has been ascertained that there Th is more abundant than U. Occurrences have been observed in the dark grey shale, quartz sandstone and conglomerate, more rarely in limestone and dolomitized limestone. The occurrences are in places certainly bound to the proximity of the organic matter. These sediments are of the Upper Carboniferous to Middle Permian age. Further investigation would be necessary to determine their ages more exactly. The Groden beds at Eisenkappel do not contain any uranium as in the čase of Zirovski Vrh.
Socolescu: N'y a-t-il pas un enrichissement en niobium et en zirconium?
Jokanovič: Non. Je pense que seulement en yttrium.