Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Štefan Ferenc, Adrián Biroň, Tomáš Mikuš, Ján Spišiak, Šimon Budzák

Initial replacement stage of primary uranium (U^IV) minerals by supergene alteration: association of uranyl-oxide hydroxy-hydrates and “calciolepersonnite” from the Krátka Dolina Valley (Gemerská Poloma, Gemeric Unit, Western Carpathians, Slovakia)

Journal of Geosciences, volume 63 (2018), issue 3, 277 - 291

DOI: http://doi.org/10.3190/jgeosci.268

Mineral association with uranyl-oxide hydroxy-hydrates and uranyl carbonate-silicates was found in J-1 quartz vein containing U-Au mineralization at Krátka Dolina Valley (Rožňava district, Slovakia). The vein penetrated Lower Palaeozoic graphitic phyllites and metalydites (Vlachovo Fm., Gemeric Unit).
Among minerals identified at the site, becquerelite I is characterized by the highest Ca content and its composition is close to the ideal . On the other hand, becquerelite II is characterized by increased K at the expense of Ca. The average chemical composition of both types of becquerelite can be expressed by the empirical formulae: (Ca_0.85K_0.04Na_0.01Fe_0.02Zn_0.02Ba_0.01Pb_0.01)_Σ0.96[(UO₂)₆O₄(OH)_5.84]·8H₂O (becquerelite I), and (Ca_0.36K_0.27Na_0.01Fe_0.02Zn_0.02Pb_0.01Bi_0.01)_Σ0.78[(UO₂)₆O₄(OH)_5.29]·8H₂O (becquerelite II). Vandendriesscheite with unusual chemical composition imitates a transition phase between gauthierite and vandendriesscheite. Negative correlation of K vs. Pb indicates that in the studied mineral phase lead is partially replaced by potassium (and other cations). An average chemical composition of the studied vandendriesscheite can be expressed as: (K_0.49Na_0.02)_Σ0.51(Pb_1.20Fe_0.05Zn_0.04Ba_0.03Sr_0.02Al_0.02)_Σ1.36[(UO₂)₁₀O₆(SiO₄)_0.05(PO₄)_0.02(OH)_10.86]·11H₂O. A leesite-like phase with an average composition (K_0.72Sr_0.01Ba_0.02Fe_0.03Zn_0.01Pb_0.02Al_0.02)_Σ0.83(H₂O)₂[(UO₂)₄O₂(SiO₄)_0.01(OH)_5,00]_Σ11,01·3H₂O, was found only rarely. An unnamed mineral phase, with chemical composition close to lepersonnite-(Gd), designated as “calciolepersonnite”, is younger at the studied site then uranyl-oxide hydroxy-hydrates. Compared to the ideal lepersonnite-(Gd) formula, there is a lower REE content at the cationic position, an increased Ca and there are also monovalent cations (especially K) entering the structure. An average “calciolepersonnite” chemical composition is: (K_0.62Na_0.09)_Σ0.71(Ca_2.08Mg_0.04Sr_0.02Ba_0.02Fe_0.05Zn_0.05Pb_0.03)_Σ2.30(Y + REE)_Σ0.92[(UO₂)_23.76{(SiO₄)_3.19(PO₄)_0.11(AsO₄)_0.02(SO₄)_0.02}_Σ3.34(CO₃)₈(OH)_26.37]·46.82H₂O.
In the supergene zone of J-1 vein at Gemerská Poloma, three stages of development can be defined: (I) formation of uranyl-oxide hydroxy-hydrates that directly, partially or completely, replace U^IV minerals; (II) formation of uranyl carbonate-silicates (i.e., “calciolepersonnite”) that replace uranyl-oxide hydroxy-hydrates, apparently indicating shift to relative acid environment (but still remaining alkaline to neutral pH) and (III) formation of uranyl phosphates/arsenates of the autunite group (“uranium micas”) that precipitated relatively far from accumulations of primary (U^IV) minerals (in cracks and cavities of the gangue, or in the surrounding non-mineralized rocks). Their origin documents the change of alkaline-neutral to acidic environment, due the more advanced weathering of vein sulphides. Given the absence of Y + REE in older uranyl-oxide hydroxy-hydrates that directly replace brannerite, most of these elements required for “calciolepersonnite” formation were probably released from the host rocks and not from the primary, hydrothermal uranium minerals.