Ullmannite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Nickel antimony sulphide
Chemical Formula: NiSbS
Method(s) of Verification: all J.S. Mason samples - XRD (University College of Wales, Aberystwyth); EMPA (British Geological Survey), this data showed near end-member, cobalt-free compositions but with minor substitution of arsenic

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: epithermal polymetallic veins & pipes
Intergrowth (2 mm across) of euhedral ullmannite crystals (bright white) perched on earlier chalcopyrite (yellow) and in turn overgrown by galena (grey-white). Ynystudor Mine in North Ceredigion. Image: J.S. Mason.
Coarse ullmannite crystals in quartz, from Dolclettwr Mine. Field of view 1 cm wide. National Museum of Wales Collection (NMW 86.100G.M.3). Photo M.P. Cooper, © National Museum of Wales.
Introduction: although not a common mineral globally, ullmannite is important locally in areas where base-metal (ie. Cu-Pb-Zn) sulphide veins carry enhanced nickel levels. Depending on the levels of Ni, Co, Sb, As and S present, a wide range of Co-Ni sulpharsenides/sulphantimonides and sulphides may accompany ullmannite together with common base-metal sulphides such as galena, chalcopyrite and sphalerite. Ullmannite belongs to the cobaltite group of minerals: with increasing cobalt content it passes into the species willyamite and with increasing arsenic content it passes into gersdorffite. Ullmannite closely resembles other cobaltite-group minerals and where present as microscopic inclusions requires chemical analysis to fully confirm its presence. Larger euhedral crystals bear a superficial resemblance to galena: however the superior hardness is diagnostic, as is the brighter lustre and the imperfect cubic cleavage.
Occurrence in Wales: ullmannite was first discovered in Wales by Ashton (1982) who described it in material collected from Goginan Mine during his PhD research. Subsequently a wider-ranging study of the ore mineralisation of Central Wales revealed ullmannite to be widespread in two distinct generations (Mason, 1994; 1997; 1998; Bevins & Mason, 1997). The mineral has yet to be confirmed from other areas of Wales, although some polished sections of galena from the Llangynog orefield reveal microscopic inclusions resembling ullmannite. The first generation of ullmannite in Central Wales is restricted in its occurrence to the main (A1-c) polymetallic mineral assemblage (Mason, 1994, 1997), which is dominated by galena. Ullmannite occurs, along with a considerable number of other minerals, as a microscopic (tens of microns in size) inclusion-forming phase in the galena, where it may be abundant. The distribution of ullmannite therefore follows the distribution of this galena generation, which principally occurs within the northern and western parts of the orefield. It is worth noting that the records of arsenopyrite in Central Wales galenas by Raybould (1974) were all references to this generation of ullmannite, as evidenced by a re-examination of his sections. The second ullmannite generation is present as much coarser crystals and some relatively rich specimens of ullmannite have been collected from occurrences of this distinctive assemblage, the A2-b, in the classification of Mason (1994, 1997). Ullmannite forms euhedral cubic crystals, generally 1-2 mm but up to 10 mm in size, sometimes nucleated upon chalcopyrite and often overgrown epitaxially by later galena. This sulphide assemblage occurs in a vuggy matrix of white to clear, often rather friable, crystalline quartz. Occurrences of this assemblage are scattered across the western parts of the orefield.

Key Localities:

  • Alltycrib Mine, Tal-y-bont, Ceredigion: inclusions in A1-c galena with bournonite and rare boulangerite.
  • Blaenceulan Mine, Talybont, Ceredigion: rare, occuring as crystals up to 1 mm in the A2-b assemblage with chalcopyrite and galena.
  • Brynyrarian Mine, Tre-Taliesin, Ceredigion: inclusions in A1-c galena with bournonite and electrum.
  • Bwlch Mine, Goginan, Ceredigion: inclusions in A1-c galena with bournonite.
  • Bwlch-glas Mine, Tal-y-bont, Ceredigion: inclusions in A1-c galena with bournonite and siegenite.
  • Bwlchrhennaid Mine, Goginan, Ceredigion: inclusions in A1-c galena with bournonite and rarely found as crystals to 2mm in the A2-b assemblage with chalcopyrite and galena.
  • Cefngwyn Mine, Bontgoch, Ceredigion: occasionally occurring as scattered crystals to 2mm with chalcopyrite and galena in quartz (A2-b assemblage).
  • Ceunant Mine, Cwmerfin, Ceredigion: frequent as inclusions in A1-c galena, with tetrahedrite and bournonite.
  • Cwmerfin Mine, Cwmerfin, Ceredigion: formerly abundant as inclusions in A1-c galena with tetrahedrite and bournonite. The site has now been landscaped.
  • Cwmsymlog Mine, Cwmsymlog, Ceredigion: Abundant as inclusions in A1-c galena with tetrahedrite and bournonite.
  • Cwmystwyth Mine, Cwmystwyth, Ceredigion: very rare as inclusions in A1-c galena.
  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: common as inclusions in A1-c galena with tetrahedrite and bournonite.
  • Dolclettwr Mine, Tre'r-ddol, Ceredigion: locally abundant as coarse crystals to 10mm in quartz, associated with galena and chalcopyrite. The A2-b assemblage is particularly well-developed at this locality, which has produced some relatively rich specimens in the past.
  • Eaglebrook (Nantycagl) Mine, Ceulanymaesmawr, Ceredigion: inclusions in A1-c galena with tucekite, siegenite and electrum.
  • Elgar Mine, Bontgoch, Ceredigion: locally abundant, forming small (2-3mm) crystals embedded in A2-b quartz with chalcopyrite and galena.
  • Erglodd Mine, Talybont, Ceredigion: abundant as inclusions in A1-c galena with bournonite
  • Esgairhir Mine, Tal-y-bont, Ceredigion: common as inclusions in A1-c galena with tetrahedrite, siegenite, millerite, tucekite, bournonite and boulangerite.
  • Esgairmwyn Mine, Ysbyty Ystwyth, Ceredigion: inclusions in A1-c galena.
  • Frongoch Mine, Devil's Bridge, Ceredigion: locally abundant in the A2-b assemblage as crystals to 2mm, often replaced pseudomorphously by bright yellow earthy bindheimite (Green et al., 1996).
  • Glogfawr Mine, Ysbyty Ystwyth, Ceredigion: uncommon as scattered small crystals in the A2-b assemblage with chalcopyrite and galena.
  • Goginan Mine, Goginan, Ceredigion: formerly abundant prior to the landscaping of the site, ullmannite occurred as rich specimens in which it was present at percentage levels in A2-b quartz veinstone. Crystals to 5mm, richly embedded in a quartz matrix with chalcopyrite and galena, use
  • Grogwynion Mine, Pontrhydygroes, Ceredigion: ullmannite is uncommon as a component of the A2-b assemblage, with chalcopyrite and galena in quartz on the deep adit dump.
  • Gwaithgoch Mine, Pontrhydygroes, Ceredigion: locally abundant in quartz-cemented breccia belonging to the A2-b assemblage, in which ullmannite occurs embedded in quartz with galena and chalcopyrite (Bevins & Mason, 1997). The small (1-2 mm) crystals are frequently replaced by bindheimite and and a mineral resembling bottinoite.
  • Hendrefelin Mine, Ysbyty Ystwyth, Ceredigion: locally common in A2-b quartz with chalcopyrite and galena. Some fine if small (2-4mm) crystals have been found. Alteration to bindheimite and less commonly to bottinoite is notable.
  • Leri Valley Mine, Talybont, Ceredigion: inclusions in A1-c galena with tetrahedrite and bournonite.
  • Level Newydd Mine, Goginan, Geredigion: rare as crystals to 1mm in the A2-b assemblage with chalcopyrite and galena.
  • Llawrcwmbach Mine, Bontgoch, Ceredigion: inclusions in A1-c galena with bournonite.
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: frequent as inclusions in A1-c galena with tetrahedrite and bournonite.
  • Llettyhen Mine, Bontgoch, Ceredigion: abundant as inclusions in A1-c galena with tetrahedrite and bournonite.
  • Mynyddgorddu Mine, Bontgoch, Ceredigion: frequent as scattered crystals to 2mm with chalcopyrite and galena in quartz (A2-b assemblage).
  • Pandy Mine, Tre'r-ddol, Ceredigion: common as a component of the A2-b assemblage in which it occurs as crystals to 5mm, on chalcopyrite and often epitaxially overgrown by galena, in a quartz matrix.
  • Pengraigddu Mine, Cwmerfin, Ceredigion: frequently occurring as inclusions, with tetrahedrite and bournonite, in A1-c galena.
  • Penycefn Mine, Bontgoch, Ceredigion: formerly common as small (1-3mm) crystals embedded in the A2-b assemblage quartz, with chalcopyrite and galena. The site is now mostly derelict.
  • Snowbrook Mine, Llanidloes, Powys: occurring as inclusions in A1-c galena.
  • South Darren Mine, Pen-bont Rhydybeddau, Ceredigion: formerly abundant as inclusions in A1-c galena with tetrahedrite and bournonite: the site is now largely derelict.
  • Ynystudor Mine, Tre'r-ddol, Ceredigion: frequent as a component of the A2-b assemblage in which it occurs as crystals (up to 5mm), on chalcopyrite and often epitaxially overgrown by galena, in a quartz matrix.

References:

  1. Ashton, J.H., 1981. Wallrock geochemistry and ore geology of certain mineralized veins in Wales. Unpublished Ph.D. thesis, University of Wales, Aberystwyth.
  2. 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.
  3. 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.
  4. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  5. Mason, J.S., 1998. Tucekite, a mineral new to Britain, and other rare ore minerals from the Central Wales Orefield. UK Journal of Mines and Minerals, 19, 30-36.
  6. Mason, J.S., 1997. Regional polyphase and polymetallic vein mineralisation in the Caledonides of the Central Wales Orefield. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), 106, B135-B144.
  7. Raybould, J.G., 1974. Ore textures, paragenesis and zoning in the lead-zinc veins of mid-Wales. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 83, B112-B119.