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

• Published in: Journal of geosciences (Prague) Vol. 63; no. 3; pp. 277 - 291
• Main Authors: Ferenc, Š., Biroň, A., Mikuš, T., Spišiak, J., Budzák, Š.
• Format: Journal Article
• Language: English
• Published: Prague Czech Geological Society 01.01.2018
• Subjects: Arsenates; Carbonates; Cations; Chemical composition; Gadolinium; Gangue; Geochemistry; Geological time; Geology; Hydrates; Lead; Metamorphism; Micas; Mineralization; Minerals; Organic chemistry; Oxidation; Palaeozoic; Phase transitions; Phosphates; Potassium; Quartz; Rock; Rocks; Silicates; Sulphates; Sulphides; Uranium; Uranium dioxide; Weathering; Slovakia; Gondwana
• ISSN: 1802-6222; 1803-1943
• DOI: 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: (Ca0.85K0.04Na0.01Fe0.02Zn0.02Ba0.01 Pb0.01)Σ0.96[(UO2)6O4(OH)5.84]·8H2O (becquerelite I) and (Ca0.36K0.27Na0.01Fe0.02Zn0.02Pb0.01Bi0.01)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: (K0.49Na0.02)Σ0.51(Pb1.20Fe0.05Zn0.04Ba0.03Sr0.02Al0.02)Σ1.36[(UO2)10O6(SiO4)0.05(PO4)0.02(OH)10.86]·11H2O. A leesite-like phase, with an average composition (K0.72Sr0.01Ba0.02Fe0.03Zn0.01Pb0.02Al0.02)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: (K0.62Na0.09)Σ0.71(Ca2.08Mg0.04Sr0.02Ba0.02Fe0.05Zn0.05Pb0.03)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 UIV minerals; (II) formation of uranyl carbonate- silicates (i.e., "calciolepersonnite") that replace uranyl-oxide hydroxy-hydrates, apparently indicating shiftto 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 (UIV) 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.