Wulfenite

Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Lead molybdate
Chemical Formula: PbMoO4
Method(s) of Verification: visual identification except:- Dylife Mine - XRD; East Glogfach Mine - XRD; Ty-Coch Mine - IR spectroscopy; Dolyhir Quarry - EMPA

Chemical Group:

  • Sulphites, Chromates, Molybdates & Tungstates

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: wulfenite is a supergene lead mineral, typically encountered in the oxidation zones of veins, stockworks, disseminated and stratabound ore deposits in which galena and other primary lead minerals occur. The molybdenum may be supplied by molybdenum-bearing primary minerals such as molybdenite but in many cases, in which primary molybdenum is not present in sufficient quantity, it may be supplied by pervasive leaching of the wallrocks hosting the mineralisation. The classic associate of wulfenite is pyromorphite, but it may occur in association with a wide range of supergene minerals, particularly carbonates, phosphates and silicates. Wulfenite is an extremely stable mineral that tends to occur in fairly evolved supergene assemblages, characteristically in association with pyromorphite and cerussite as in the majority of its Central Wales occurrences (Mason, 2004). The associated species help with identification but the key is its distinctive crystal morphology and colour.
Occurrence in Wales: the first record of wulfenite in Wales, from Treffgarne Rocks in Pembrokeshire, was made as long ago as 1866, by Spencer George Percival. However, this was an erroneous identification - the mineral proving instead to be brookite - and it was not until just over 100 years had passed that the first authenticated occurrence, in Central Wales, was described (Ryback & Saville, 1967). Since that time, growing interest in the minerals of Wales has led to a minor flood of new occurrences: a review by Rothwell & Mason (1992) listed 22 Welsh localities and in the years since the list has grown to 29, including some important material in specimen terms. The majority of the localities (22 out of 29) are in the Central Wales Orefield.

Key Localities:

  • Aberdaunant Mine, Llanidloes, Powys: listed by Rothwell & Mason (1992) on the basis of specimens obtained by S.A. Rust, wulfenite forms small (0.5mm) caramel-coloured bipyramidal crystals to 0.5 mm in association with pyromorphite and goethite. Due to the dull colour and small crystal size, these are easily overlooked.
  • Alltycrib Mine, Tal-y-bont, Ceredigion: again listed by Rothwell & Mason (1992) on the basis of specimens obtained by S.A. Rust, wulfenite was collected in situ in the late 1980s, but only in small quantity: the bipyramidal crystals found were associated with minor pyromorphite, were orange in colour and reached 0.5 mm in size.
  • Bwlch-glas Mine, Tal-y-bont, Ceredigion: this important locality was first described by Braithwaite (1982) and further details were provided by Mason (2004). Wulfenite was formerly common both on the tips and in situ underground, although twenty years of collector activity have since diminished the resource. The underground occurrence was along open joint-fractures in shattered siltstone and mudstone adjacent to pods of part-oxidised galena. Specimens reveal wulfenite crystals, exceptionally to 2 mm on edge, on pyromorphite (sometimes forming epimorphs after cerussite) and more rarely on corroded cerussite. Both tabular and bipyramidal crystals occur, plus blocky, equant examples, and the colour varies from yellow through orange to red-orange.
  • Bwlchrhennaid Mine, Goginan, Ceredigion: in a second Central Wales association - with hemimorphite - wulfenite was first reported from this site by Cooper (1987). Crystals, which are thinly tabular and orange in colour, stand proud from a matrix of brecciated mudstone coated in drusy white hemimorphite. Exceptionally, the crystals reach 1.5 mm on edge: however, <0.5 mm is more typical.
  • Cwm Orog Mine, Llangynog, Powys: listed by Rothwell & Mason (1992) on the basis of specimens collected by R. Belson, wulfenite occurs rarely as orange-yellow plates to 1 mm on baryte.
  • Cwmdarren Mine, Trefeirig, Ceredigion: a single specimen showing orange bipyramids to 0.5 mm on a film of yellow-green pyromorphite was noted by Rothwell & Mason (1992).
  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: listed by Rothwell & Mason (1992) on the basis of specimens obtained by S.A. Rust, wulfenite occurs rarely on the eastern tips at Darren and is associated with both cerussite and pyromorphite. The crystals, which are tabular to (rarely) equant, are yellow in colour and reach 0.75 mm.
  • Dolyhir Quarry, Old Radnor, Powys: microscopic elongated yellow crystals were collected from the oxidised zone of a galena-tennantite-chalcopyrite-baryte vein by N. Hubbard in 1995.
  • Dyfngwm Mine, Penegoes, Powys: some doubt exists with regard to the occurrence of wulfenite at this locality, as the workings in question are at least partly related to the outcrop workings of Dylife mine. It was listed from here by Jones & Moreton (1977).
  • Dylife Mine, Penegoes, Powys: wulfenite has been found in small quantities on the large dumps below the Star Inn (Jones & Moreton, 1977) and in more abundance around the lode outcrop workings on the hillside above. Here, yellowish tabular crystals, exceptionally to 1.5 mm, occur in association with microcrystalline pyromorphite and cerussite.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: Rothwell & Mason (1992) note a single specimen of brown tabular wulfenite associated with pyromorphite in a quartz cavity: this is believed to represent the only one known from this mine.
  • East Glogfach Mine, Ysbyty Ystwyth, Ceredigion: the first authenticated Welsh occurrence (Ryback & Saville, 1967), on tips from trial workings to the east of the much larger Glogfach mine. Wulfenite forms orange-yellow tablets or elongated bipyramids to 1mm on thin films of microcrystalline pyromorphite.
  • East Glogfawr Mine, Ysbyty Ystwyth, Ceredigion: listed on Mindat without further description.
  • Elgar Mine, Bontgoch, Ceredigion: this important locality was discovered in 1996 when the adit was temporarily opened by a team led by G.W. Hall (Bevins & Mason, 1996). Wulfenite was recovered on this occasion and in subsequent work over the following 12 months; a detailed report was presented by Mason (2004). Samples revealed tabular, transparent orange wulfenite crystals to 1 mm richly investing vivid yellow-green pyromorphite epimorphs after cerussite, the best of which are among the finest Welsh examples of the species found to date.
  • Esgair Fraith Mine, Tal-y-bont, Ceredigion: brownish bipyramids on microcrystalline pyromorphite were obtained and listed by Rothwell & Mason (1992).
  • Esgairhir Mine, Tal-y-bont, Ceredigion: corroded orange-brown bipyramids occur rarely, in association with a film of green pyromorphite (Rust & Mason, 1988).
  • Frongoch Mine, Devil's Bridge, Ceredigion: a small number of specimens have recently been found, both in the westernmost part of the sett (Green et al, 1996) and in 2003 in the central part of the site (J.S. Mason, unpublished data). The latter specimens show minute (0.25-0.5 mm) orange tabular wulfenite crystals which are of particular interest in that they exhibit hopper structures, a feature previously unknown from Central Wales.
  • Geufron Mine, Llanidloes, Powys: in an unusual association, tabular lemon-yellow wulfenite occurs partially embedded in malachite on a small number of specimens noted by Rothwell & Mason (1992) from the old workings on the lode outcrop.
  • Glogfawr Mine, Ysbyty Ystwyth, Ceredigion: crystals to 0.8mm with microcrystalline pyromorphite. Data listed (with image) on Mindat by S.A. Rust.
  • Henfwlch Mine, Ceulanymaesmawr, Ceredigion: extremely rare at this site, wulfenite was noted by Rothwell & Mason (1992) as an orange, modified octagonal tabular plate on cerussite associated with pyromorphite.
  • Llangynog Mine, Llangynog, Powys: crystals to 1 mm, listed on Mindat with no further details. Requires confirmation.
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: this important locality, described by Jones (1987), by Rothwell & Mason (1992) and Mason (2004) has in the past produced the largest Welsh wulfenite crystals known, some of which reached 5 mm on edge. These occur in voids in gossany quartz and iron oxides and also on open joints traversing massive lode-quartz; they are associated with pyromorphite but may occur alone. Predominantly, the yellow crystals are tabular in habit although Jones (1987) also recorded rare orange bipyramids.
  • Mynydd Gorddu Mine, Bontgoch, Ceredigion: brown to orange tabular crystals to 1 mm associated with pyromorphite coatings on mudstone were first recorded by Roe (1988): wulfenite is however uncommon at this site.
  • Nant-y-Car Mine, Rhyader, Powys: orange crystals to 1 mm with pyromorphite. Listed (with image) on Mindat by S.A. Rust.
  • Old Esgairlle Mine, Ponterwyd, Ceredigion: the second known Welsh occurrence of the hemimorphite-wulfenite association was discovered in 1996 at this site (Bevins & Mason, 1997). Lemon-yellow tabular wulfenite crystals to 0.25 mm protrude from thin hemimorphite coatings in cavities of oxidised vein-breccia.
  • Pandy Mine, Tre'r-ddol, Ceredigion: wulfenite was discovered in the upper level of this trial working in the 1980s and subsequently listed by Rothwell & Mason (1992) and by Mason (2004). It forms tabular crystals to 2mm, yellow-green to orange in colour, and also orange bipyramids, which occur on pyromorphite-coated galena, quartz and wallrock. Some rich specimens (by Welsh standards) were recovered shortly after its discovery.
  • Penybanc Mine, Tal-y-bont, Ceredigion: a single specimen showing a minute (0.25 mm) brownish bipyramid on green microcrystalline pyromorphite was noted by Rothwell & Mason (1992).
  • Rhiwnant Mine, Rhyader, Powys: listed without description on Mindat. Requires confirmation.
  • South Nant-y-car Mine, Rhyader, Powys: listed by Rothwell & Mason (1992) on the basis of specimens collected by S.A. Rust. Wulfenite occurs as tabular and bipyramidal crystals to 1.5 mm in association with microcrystalline yellow-green pyromorphite.
  • Tŷ Coch, near Porthcawl, South Wales: recorded by Braithwaite & Lamb (1987) as bevelled rectangular plates, yellow to white in colour and reaching 3mm in size, projecting from fracture surfaces in compact manganese ore with baryte and calcite lenticles.
  • Ystrad Einion Mine, Furnace, Ceredigion: noted by Rothwell & Mason (1992) and by Mason & Rust (1997), wulfenite is very rarely observed as yellow tabular crystals and brown bipyramids to 0.3 mm on filmy pyromorphite-coated mudstone.

References:

  1. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  2. Braithwaite, R.S.W., 1982b. Pyromorphite, wulfenite and other minerals from the Bwlch-Glas mine, Central Wales. Mineralogical Record, 13, 151-153.
  3. Braithwaite, R.S.W. & Lamb, R.P.H., 1986. Wulfenite from Ty Coch, Glamorgan (Powys), South Wales. Mineralogical Magazine, 50, 180-181.
  4. Cooper, M.P., 1987. New finds. British Micromount Society Newsletter, 20, 6-9.
  5. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  6. Jones, A.D., 1987. The minerals of Llechweddhelyg. U.K. Journal of Mines and Minerals, No. 3, 25-27.
  7. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  8. Mason, J.S., 2004. The development of supergene lead mineralisation in Central Wales. UK Journal of Mines and Minerals, 24, 35-46.
  9. Mason, J.S. & Rust, S.A., 1997. The Mineralogy of Ystrad Einion Mine, Dyfed, Wales. U.K. Journal of Mines and Minerals, 18, 33-36.
  10. Perceval, S.G., 1866a. Discovery of wulfenite, etc., in Pembrokeshire. Geological Magazine, 3, 377-378.
  11. Roe, D., 1988. New finds (Wulfenite from Mynydd Gorddu). British Micromount Newsletter, 22, 7.
  12. Rothwell, M. & Mason, J.S., 1992. Wulfenite in the British Isles. UK Journal of Mines & Minerals, 11, 30-41.
  13. Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.
  14. Ryback, G. & Saville, G., 1967. Wulfenite from Ysbyty Ystwyth, Cardiganshire. Mineralogical Magazine, 36, 458-459.