Pyrrhotite

Crystal System: Monoclinic,Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron sulphide
Chemical Formula: Fe1-xS
Method(s) of Verification: all listed occurrences, identification by visual or optical means, often assisted by its magnetic properties.

Chemical Group:

  • Sulphides

Geological Context:

  • Igneous
  • Hydrothermal: alpine type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: volcanogenic massive sulphides
  • Sedimentary
  • Metamorphic
Pyrrhotite, polished section (right) showing alteration along cracks (darker, streaky areas), intergrown with chalcopyrite (yellow) with inclusions of sphalerite (grey) and quartz (black). Clogau Mine. Field of view 0.5 mm, © J.S. Mason.
Pyrrhotite, polished section, showing decay texture - alteration to marcasite & iron oxides. Sphalerite (grey) and holes (black) formed by plucking of the surface during polishing. Erglodd Mine. Field of view 0.5mm; © J.S. Mason.
Bronze coloured pyrrhotite with tarnish (sample 8 cm across) from a mesothermal polymetallic vein. Minor pyrite, chalcopyrite and galena are also present but massive pyrhhotite is dominant. Blaen-Y-Pennant Mine. Sample J.S. Mason, © J.S. Mason
Introduction: unusually for the mineral kingdom, both the monoclinic and hexagonal varieties of Fe1-xS are called pyrrhotite. This is a diversion from the norm: for example there are cubic and orthorhombic dimorphs of FeS2 and each is classified as a separate mineral species - pyrite and marcasite respectively. Hexagonal pyrrhotite is close to FeS while monoclinic forms have more sulphur (e.g. Fe7S8). To compound matters, 'pure' or stoichiometric FeS is another mineral species - troilite, which is hexagonal! It is a reasonable comment, therefore, that the name 'pyrrhotite' refers to a group of very closely related minerals which will no doubt be isolated and characterized as separate species at some point in the future. Pyrrhotite is frequently found as an accessory mineral in the opaques assemblage of basic igneous rocks and may be concentrated in large amounts with pentlandite, chalcopyrite and other minerals in magmatic sulphide deposits. It may also occur as disseminations in sedimentary rocks, particularly those that have been subjected to low-grade metamorphism. It is also an important component of many hydrothermal ore deposits, and particularly those formed at moderate temperatures and associated with igneous rocks, where it is typically associated with chalcopyrite, sphalerite, arsenopyrite and other sulphides. It may additionally occur in alpine fissure-type assemblages.
Occurrence in Wales: in Wales, pyrrhotite is widespread, occurring in disseminated form in igneous rocks such as the dolerites from the Preseli Hills in Pembrokeshire (Ixer, in Thorpe et al., 1991). It is also common as disseminations in certain sedimentary rocks, in particular the black shales of the Clogau Formation which outcrops in the Dolgellau Gold-belt in North Wales. Here, its magnetic properties have given rise to a significant aeromagnetic anomaly, reported by Allen & Jackson (1985). Within the same area it is common in mesothermal gold-bearing quartz lodes (Mason et al., 2002). Several other occurrences of pyrrhotite in hydrothermal vein systems are listed below. Its presence is often indicative of low sulphur activity in the hydrothermal fluids responsible for transporting and depositing the ore minerals. Pyrrhotite is the only common sulphide mineral which is magnetic, although the property varies from location to location. This helps with identification as does the distinctive bronze colour.

Key Localities:

  • Cae Coch Mine, Trefriw, Gwynedd: pyrrhotite occurs predominantly in numerous quartz-pyrite veins cutting basic volcanic rocks in the footwall of this bedded pyrite deposit (Ball & Bland, 1985).
  • Clogau Mine, Bontddu, Gwynedd: although a mine within the Dolgellau Gold-belt (see below), Clogau deserves a special mention as pyrrhotite also occurs there in late, syn-tectonic low-angle alpine fissure-type veins which cut the gold-lodes in places. Unlike the occurrences in the gold-lodes themselves, in this association the pyrrhotite is often euhedral, forming tabular pseudohexagonal crystals to a few millimetres across associated with arsenopyrite, chalcopyrite, chlorite and rutile in a quartz-carbonate matrix.
  • Dolgellau Gold-belt, Gwynedd: pyrrhotite occurs abundantly in the mesothermal gold-bearing quartz lodes of this metallogenic province. It is associated principally with chalcopyrite in a quartz and carbonate matrix and forms anhedral masses often several centimetres across. It post-dates arsenopyrite, pyrite, gold and tellurides but pre-dates the again widespread galena and sphalerite (Mason et al., 2002). Occurrences can be found at virtually every mine and trial in the area.
  • Dylife Mine, Penegoes, Powys: one of only two localities for pyrrhotite in the Central Wales Orefield. Pyrrhotite occurs on the tips opposite the Star Inn at Dylife but is very rare. The occurrence belongs to the late (A2) group of Mason (1994; 1997) and pyrrhotite is present as tabular crystals to 7 mm across in cavities in quartz, with galena in association. The occurrence was first documented by Jones & Moreton (1977).
  • Erglodd Mine, Talybont, Ceredigion: pyrrhotite is very rare in Central Wales and this is one of only two localities. It occurs very rarely associated with galena, sphalerite, chalcopyrite and siegenite as aggregates in quartz to ca. 1 cm in diameter. The assemblage belongs to the early or A1 group in the regional paragenesis (Mason, 1994; 1997).
  • Parys Mountain, Anglesey: the presence of pyrrhotite here was recorded by Pointon & Ixer (1980) as microscopic rounded inclusions associated with chalcopyrite in pyrite and as slightly larger aggregates replacing pyrite.
  • Snowdon, Gwynedd: Snowdon Caldera copper-veins: although rare in the central part of the caldera (Reedman et al., 1985), pyrrhotite occurs frequently in mines in the outer and marginal parts of the caldera, especially in the area from Bethesda-Llanberis-Nantlle-Cwm Pennant, where it is often associated with chalcopyrite and arsenopyrite. It is clearly later in formation than the arsenopyrite, euhedral crystals of which it commonly encloses. It is always massive and may have a granular, recrystallized texture due to later deformation.
  • South Wales Coalfield: pyrrhotite is an uncommon component of the septarian ironstone nodule-hosted sulphide assemblage, better known for its millerite specimens. Hexagonal platy crystals were recorded from Parc Colliery, Cwm Parc by Firth (1971), while more recently specimens have been found at tips derived from the Gelli and Ferndale collieries in the Rhondda Valley (I.E. Jones, unpublished data).
  • Tyllau Mwyn, Drws-y-nant, Gwynedd: pyrrhotite occurs as tabular crystals in calcite-dominated stilpnomelane-bearing veins cutting a metamorphosed bedded ironstone deposit at this locality high on the flanks of Aran Fawddwy (Matthews & Scoon, 1964).

There are no key localities for this specimen.

References:

  1. Allen, P.M. & Jackson, A.A., 1985. Geology of the country around Harlech. Memoirs of the British Geological Survey. Explanation of sheet 135, with part of 149, 112pp.
  2. Ball, T.K. & Bland, D.J., 1985. The Cae Coch volcanogenic massive sulphide deposit, Trefriw, North Wales. Journal of the Geological Society, London, 142, 889-898.
  3. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  4. Firth, J.N.M., 1971. The Mineralogy of the South Wales Coalfield. Unpublished Ph.D. thesis, University of Bristol.
  5. Jones, J.A. & Moreton, N.J.M., 1977. The Mines and Minerals of Mid-Wales 40pp.
  6. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  7. 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.
  8. Mason, J.S., Bevins, R.E. & Alderton, D.H.M., 2002. Ore Mineralogy of the mesothermal gold lodes of the Dolgellau Gold Belt, North Wales. Transactions of the Institution of Mining and Metallurgy (Section B, Applied earth science), 111, B203-B214.
  9. Matthews, D.W. & Scoon, J.H., 1964. Notes on a new occurrence of stilpnomelane from North Wales. Mineralogical Magazine, 33, 1032-1037.
  10. 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.
  11. Reedman, A.J., Colman, T.B., Campbell, S.D.G. & Howells, M.F., 1985. Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales. Journal of the Geological Society, London, 142, 875-888.
  12. Thorpe, R.S., Williams-Thorpe, O., Jenkins, D.G. & Watson, J.S., 1991. The geological sources and transport of the bluestones of Stonehenge, Wiltshire, U.K. Proceedings of the Prehistoric Society, 57, 103-157.

There are no references for this specimen.