Pyrite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Iron sulphide
Chemical Formula: FeS2
Method(s) of Verification: all cited occurrences identified in hand specimen or in polished section.

Chemical Group:

  • Sulphides

Geological Context:

  • Igneous
  • Hydrothermal: alpine type veins
  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: epithermal polymetallic veins & pipes
  • Hydrothermal: volcanogenic massive sulphides
  • Sedimentary
  • Hydrothermal: sedimentary exhalative deposits
  • Hydrothermal: porphyry-type mineralization
  • Hydrothermal: copper-dolomite
  • Metamorphic
Pyrite - a classic striated cubic crystal (10 x 12 mm) embedded in Cambrian slate from Penrhyn Quarry, Bethesda, Gwynedd. I. E. Jones Collection (1376A). © National Museum of Wales.
Pyrite in a very familiar mode of occurrence in Wales - as a replacement of a fossil (3 cm long graptolite, Glyptograptus sp.) from Cwmere Formation, Early Silurian age, Eaglebrook Mine. Specimen J.S. Mason, © J.S. Mason.
Zoned pyrite in polished section from Eaglebrook Mine. Cubic pyrite (pale yellow), overgrown by banded yellow to grey-brown nickeliferous pyrite (possibly grading into the species bravoite). Larger crystal 0.5 mm across. Specimen J.S. Mason, © J
Introduction: pyrite is the commonest of the sulphide minerals and is far more widespread than its orthorhombic dimorph, marcasite. It occurs in many geological settings, including sedimentary, igneous and metamorphic rocks of all ages and in a great range of hydrothermal mineral deposits. It is also abundant in many placer deposits. Because of its widespread occurrence, associated minerals are highly diverse. Pyrite is readily identified by its cubic crystal shape when well-formed. In polished section it often shows high relief against other sulphides due to its greater hardness: it polishes well when fresh and has a relatively bright appearance. The instability of pyrite (pyrite-decay) is well-known to curators of both mineral and fossil collections. Pyrite can react with air and moisture, the reaction creating an acidic solution of iron which then goes on to attack the remaining pyrite plus other more stable sulphides such as galena and sphalerite. However, not all pyrite specimens are prone to this effect. Relatively high-temperature pyrite, formed in mesothermal mineral veins or as porphyroblasts in metamorphic rocks, is often relatively stable. Conversely, it is the lower-temperature varieties, including the botryoidal form known as melnikovite-pyrite, that are most prone to decay. These occur in sedimentary and low-grade metamorphic rocks and in low-temperature hydrothermal assemblages. Often such varieties are intergrown with the even more unstable dimorph, marcasite, and in such cases decay may set in within days of sample collection. Once initiated, the process is extremely difficult to arrest: prevention of its initiation is therefore the key. Any sample suspected to be at risk from pyrite decay must be kept in as cold and dry an atmosphere as possible: minimising moisture minimises the risk of decay initiation and the lower the temperature, the slower any decay reaction will be because, as with many chemical reactions, heat speeds up the rate of chemical activity.
Occurrence in Wales: pyrite occurs throughout most of Wales as a component of igneous, sedimentary and metamorphic rocks. It commonly replaces fossils, especially in black shale sequences, such as those found in the Lower Palaeozoic strata of Central Wales where pyritized graptolites are locally common. It is a familiar sight as thin golden sheens on household coal and in the past these have been mistaken by the unwary for gold - 'fool's gold' is a common nickname for pyrite. Gold is soft and malleable - ie. it bends under pressure. Pyrite is hard and brittle - it breaks and crumbles. There are few hydrothermal ore deposits in Wales that do not contain pyrite but in some it is very common and was actually mined in its own right (for sulphuric acid manufacture) in some places such as the Cae Coch Mine near Trefriw in the Conwy valley. Only those localities where pyrite is of paragenetic or specimen significance are noted here.

Key Localities:

  • Cae Coch Mine, Trefriw, Gwynedd: this distinctive ore-deposit consists of laminated quartz and pyrite and has been likened to an exhalative, Kuroko-type deposit (Ball & Bland, 1985). Of particular interest are tubes of pyrite, 5-10 mm in diameter, infilled with quartz: these have been suggested to be fossilized 'black smokers', although Bottrell & Moreton (1992) have cited evidence to suggest that the Cae Coch deposit is of syn-diagenetic 'inhalative' orgin.
  • Central Wales Orefield: pyrite occurs throughout the 12-stage regional primary paragenetic sequence in this area, but is only present in significant amounts in late-stage mineralization where it is associated with quartz and marcasite (Mason, 1994; 1997). Of note in this late assemblage is the presence of zoned, nickeliferous pyrite as illustrated here; well-formed crystals, however, tend to be small and of little interest to collectors.
  • Dolaucothi Gold Mine, Pumpsaint, Carmarthenshire: pyrite is abundant at this historic gold-mine, where it forms aggregates of euhedral crystals to 10 mm in quartz and also forms thick beds in the host Lower Silurian dark mudstones. It is associated with common arsenopyrite and minor gold (Annels & Roberts, 1989).
  • Dolgellau Gold-belt, Gwynedd: pyrite is common as an early component of the mesothermal gold-lodes of this area and occurs, with arsenopyrite and cobaltite, both in quartz and as a wallrock impregnation (Mason et al., 2002). It also forms a major alteration halo surrounding the Coed-y-Brenin porphyry-copper deposit (Rice & Sharp, 1976) and is common in associated volcanogenic hydrothermal breccia-pipes, such as that at Glasdir (Allen & Easterbrook, 1978). A particularly distinctive form of reticulated pyrite also occurs in the area, forming aggregates of microcrystals lining vugs in otherwise massive pyrite: these are most commonly found at mines in the northern and eastern parts of the Gold-belt. A late-stage calcite-marcasite assemblage that occurs throughout the Gold-belt also contains pyrite in places, often as the highly unstable botryoidal form melnikovite-pyrite, which has occasionally been found in attractive specimens: these, however, have fallen victim to pyrite-decay very rapidly after collection (J.S. Mason, unpublished data).
  • Llanrwst Orefield, Gwynedd: some notable specimens of pyrite have been obtained from the lead-zinc mines in this area, particularly Parc Mine, which produced attractive crystalline pyrite coatings on quartz and calcite (National Museum of Wales Collection).
  • Lower Palaeozoic sedimentary rocks of North and Central Wales: well-formed porphyroblastic pyrite cubes are widespread, occurring in rocks ranging from sandstone to slate. Mostly small, they occasionally reach 1-2 cm in size and often reveal quartz and chlorite pressure-fringes formed during compressive deformation. Fine specimens have been found at some of the slate quarries, notably Penrhyn Quarry near Bethesda. Pyritized fossils, particularly graptolites, are of widespread occurrence as are framboids, which are small, raspberry-shaped (hence the name) aggregates of pyrite occurring scattered through the more pelagic facies; the origin of these remains open to debate.
  • Parys Mountain, Anglesey: pyrite is the most common sulphide occurring at this site and is present in at least four generations of differing habit (Pointon & Ixer, 1980). The pyritization of the wallrocks is responsible for the ochreous landscape in the area around the Great Opencast.
  • South Wales Coalfield: pyrite as thin sheens, lumps and nodules is common in the Coal Measures and the nodular forms were known to miners as 'coal brasses' (Adams, 1967). Excellent pyritohedra to 25 mm are known from Cwmgwili Colliery, Llanelli (National Museum of Wales specimens) while an impressive occurrence of large (1 m+ across) masses of pyrite, apparently replacing fossil tree-boles, was exposed during the late 1990s at the Bryn-Henllys opencast mine near Ystradgynlais (Bevins & Mason, 2000).

References:

  1. Adams, W., 1867. On the 'Coal Brasses' of the South Wales coal fields. Transactions of the South Wales Institute of Engineers, 5, 190-196.
  2. Allen, P.M. & Easterbrook, G.D., 1978. Mineralised breccia pipe and other intrusion breccias in the Harlech Dome, N. Wales. Transactions of the Institution of Mining and Metallurgy (Section B; Applied earth science), 87, B157-B161.
  3. Annels, A.E. & Roberts, D.E., 1989. Turbidite-hosted gold mineralisation at the Dolaucothi Gold Mines, Dyfed, Wales. Econonic Geology, 84, 1293-1314.
  4. 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.
  5. Bevins, R.E. & Mason, J.S., 2000. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Glamorgan and Gwent. National Museums & Galleries of Wales, Cardiff
  6. Bottrell, S.H. & Moreton, M.D.B., 1992. A reinterpretation of the genesis of the Cae Coch pyrite deposit, North Wales. Journal of the Geological Society, London, 149, 581-584.
  7. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  8. 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.
  9. 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.
  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. Rice, R. & Sharp, G.J., 1976. Copper mineralization in the forest of Coed-y-Brenin, North Wales. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied earth science), 85, B1-B13