Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Uranium oxide
Chemical Formula: UO2
Method(s) of Verification: all localities listed - EPMS (Parnell, 1988)
- Oxides & Hydroxides
- Hydrothermal: copper-dolomite
- Hydrothermal: Mississippi Valley Type veins
Introduction: uraninite occurs typically in hydrothermal veins where it is commonly associated with cobalt, nickel and bismuth-bearing minerals. It is found in hydrocarbons which are present in some low-temperature mineral veins and is also found with other uranium minerals as impregnations in red-bed sandstones, which are an important economic source of uranium. Uraninite is a black, heavy mineral which tends to occur in massive form or, as in the Welsh occurrences, as fine disseminations. It is therefore not readily identified in hand specimen, although its radioactivity readily confirms it to be a uranium-bearing species.
Occurrence in Wales: occurrences of uraninite in Wales are limited to hydrocarbon associations in copper-dolomite and Mississippi Valley-type (MVT) deposits across the north and north-east of the country. The presence of uraninite in hydrocarbon from North Wales was first described by Parnell (1988), although the hydrocarbons occurring at Great Orme, Llandudno, have been known to be enriched in uranium (and therefore radioactive) for some time (Sylvester-Bradley & King, 1963; Parnell, 1983). Similar occurrences, this time in the MVT veins of the Halkyn-Minera Orefield of north-east Wales, were described by Eakin (1989) and by Eakin & Gize (1992). During survey work for the Minescan project in 1998, further occurrences of radioactive hydrocarbon were noted from the Halkyn district (Bevins & Mason, 1999); although their mineralogy has not been investigated in detail, they too are probably uraninite-bearing.
- Great Orme Copper Mines, Llandudno, Gwynedd: present in hydrocarbon, particularly at the Ty Gwyn Mine, where it forms regular arrays of inclusions (1-10 µm) in association with annabergite, gersdorffite and cobaltian niccolite (Parnell, 1988).
- Halkyn-Minera district, NE Wales: first recorded by Parnell (1988) from unspecified mines in this area. Radioactive (uraninite-bearing) hydrocarbon masses, several centimetres across, in calcite, was noted at Pant Quarry on Halkyn Mountain in 1998 (Bevins & Mason, 1999): the mineralized zone was subsequently buried under a haulage road.
- Llandulas, Clwyd: uraniferous bitumen, probably containing uraninite, was described from this area by Eakin & Gize (1992).
- Bevins, R.E. & Mason, J.S., 1999. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Clwyd. National Museums & Galleries of Wales, Cardiff.
- Eakin, P.A. & Gize, A.P., 1992. Reflected-light microscopy of uraniferous bitumens. Mineralogical Magazine, 56, 85-99.
- Parnell, J., 1988. Mineralogy of uraniferous hydrocarbons in Carboniferous-hosted mineral deposits, Great Britain. Uranium, 4, 197-218.
- Parnell, J., 1983. The distribution of hydrocarbon minerals in the Welsh Borderlands and adjacent areas. Geological Journal, 18, 129-139.
- Sylvester-Bradley, P.C. & King, R.J., 1963. Evidence for abiogenic hydrocarbons. Nature, 198, 728-731.