Bournonite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Lead copper antimony sulphide
Chemical Formula: PbCuSbS3
Method(s) of Verification: Central Wales - XRD; Clogau - EMPA; Graig-Ddu - optical identification; St Elvis Mine & Parys Mountain - EMPA.

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: epithermal polymetallic veins & pipes
  • Hydrothermal: volcanogenic massive sulphides
Bournonite (dark grey) forming irregular inclusions (to ca. 50 µm) in galena (grey). A little ullmannite (bright white, unscratched) is also present. Typical inclusion assemblage snapshot from a sample of galena from Erglodd Mine, Central Wales.
Introduction: bournonite occurs in Pb-Cu dominated polymetallic vein and other ore deposits and is also found in complex lead-antimony (Pb-Sb) associations. In medium-temperature polymetallic veins, it is commonly found, along with a range of other minerals, as microscopic inclusions occurring in galena, in which case it is only discernable in polished section. Frequent associates are tetrahedrite, other Pb-Sb compounds and iron-nickel-cobalt sulpharsenides and sulphantimonides, such as ullmannite. In coarser-grained occurrences it is similar in appearance to tetrahedrite, but is a more silvery colour and may exhibit a poorly-developed cleavage.
Occurrence in Wales: the first records of bournonite in Wales resulted from petrological studies of the mineralization of Parys Mountain in the 1970s (Wheatley, 1971; Sivaprakash, 1977; Pointon & Ixer, 1980). In the 1980s, a research programme into the ore mineralogy of Central Wales turned up numerous new localities (Mason, 1994, 1997, 1998), and the Central Wales Orefield remains as its chief occurrence, both as inclusions in galena and as visible masses to several centimetres in quartz. In addition, further scattered localities have been identified in recent years in the Dolgellau Gold-belt, the Llangynog Orefield and in an isolated occurrence in Pembrokeshire.

Key Localities:

  • Bwlch Mine, Goginan, Ceredigion: formerly not uncommon in visible masses in breccia with galena and quartz, and also as microscopic inclusions in galena with tetrahedrite and ullmannite. Landscaping has resulted in these minerals now being relatively hard to find.
  • Bwlchrhennaid Mine, Goginan, Ceredigion: visible masses of bournonite in quartz to 10 mm, often altered to yellowish antimony oxides, used to be present at this site, in addition to inclusions in galena with tetrahedrite and ullmannite. The site has in recent years been landscaped, but similar material still occurs to a lesser extent at the nearby Pengraigddu and Ceunant mines.
  • Central Wales Orefield: bournonite occurs as part of the early (A1) polymetallic assemblage, forming inclusions in galena and larger masses to several cm in quartz cavities (Mason, 1994; 1997; 1998). Part-crystals have occasionally been found. Associated minerals are chalcopyrite. In addition to the mines listed bournonite inclusions in galena have been found at the following Central Wales mines: Alltycrib, Bronfloyd, Bwlch-glas, Erglodd, Esgairhir, Leri Valley, Llechweddhelyg, Lletty Evan-hen and Penpompren.
  • Clogau Mine, Bontddu, Gwynedd: 40-80 µm long isolated inclusions in quartz associated with gold and pyrite (Naden, 1988).
  • Cwmerfin Mine, Cwmerfin, Ceredigion: bournonite was formerly common at this site, forming visible masses in quartz, with tetrahedrite, galena and chalcopyrite. It also commonly occurred, along with tetrahedrite and ullmannite, as microscopic inclusions in galena. In recent years the site has been landscaped.
  • Cwmsymlog Mine, Cwmsymlog, Ceredigion: abundant as inclusions in galena with tetrahedrite and ullmannite, with occasional masses to 10 mm in quartz associated with galena and chalcopyrite.
  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: perhaps the most prolific Central Wales bournonite locality (in part because many others have been obliterated in recent years), the tips on the north-east facing slopes above the Erfin valley carry boulders of mineralized breccia in which bournonite occurs in quartz cavities in masses to 3 cm across. Part-crystals have very occasionally been observed. Associated minerals are tetrahedrite, chalcopyrite and galena, occasionally with a later calcite overgrowth. In addition, this assemblage occurs at other smaller mines on the course of the lode to the NE, namely Copper Level, Cerrigyrwyn and Gwaith-yr-Afon, although it is relatively weakly developed at these localities. Fine-grained, recrystallized "steel-ore" galena, rich in microscopic inclusions of bournonite, tetrahedrite and ullmannite is present at all of these mines.
  • Goginan Mine, Goginan, Ceredigion: prior to its landscaping in the 1980s, bournonite was present as masses to 10 mm with galena in brecciated mudstone.
  • Graigddu Mine, Llangynog, Powys: as inclusions in galena close to chalcopyrite grain boundaries, associated with tetrahedrite. In polished sections of ore samples collected by J.S. Mason during MINESCAN fieldwork in 1996.
  • Nantyrarian Mine, Goginan, Ceredigion: quartz on the small tips at this mine contains cavities filled with galena, chalcopyrite, bournonite, tetrahedrite and yellow-orange sphalerite.
  • Parys Mountain, Anglesey: as 5-30 µm grains associated with bismuth, bismuthinite and various bismuth sulphosalts (Wheatley, 1971; Sivaprakash, 1977; Pointon & Ixer, 1980).
  • South Darren Mine, Pen-bont Rhydybeddau, Ceredigion: bournonite was not uncommon at this site prior to its landscaping, forming masses to 20 mm in quartz, associated with tetrahedrite, chalcopyrite and galena. It was also abundant as microscopic inclusions in massive galena with tetrahedrite and ullmannite.
  • St Elvis Mine, Solva, Pembrokeshire: bournonite occurs here in intergrowths with tetrahedrite forming masses to 2-3 cm in quartz (Mason & Bevins, 2002).

There are no key localities for this specimen.

References:

  1. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  2. 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.
  3. 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.
  4. Mason, J.S. & Bevins, R.E., 2002. St Elvis Mine, Solva, Pembrokeshire: Another Elizabethan tetrahedrite occurrence? British Mining 71, 5-12, Northern Mines Research Society.
  5. Naden, J., 1988. Gold mineralisation in the Caledonides of the British Isles with reference to the Dolgellau Gold Belt and the Southern Uplands of Scotland. Unpublished Ph.D thesis, University of Aston, UK.
  6. Pointon, C.R. & Ixer, R.A., 1980. Parys Mountain mineral deposit, Anglesey, Wales: geology and ore mineralogy. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 89, B143-B155.
  7. Sivaprakash, C., 1977. Geochemistry of some sulphides and sulphosalts from Parys Mountain, Anglesey. Unpublished M.Phil. thesis, University of Aston in Birmingham.
  8. Wheatley, C.J.V., 1971. Economic geology of the Avoca mineralised belt, S.E. Ireland, and Parys Mountain, Anglesey. Unpublished Ph.D. thesis, Imperial College, University of London.

There are no references for this specimen.