Arsenopyrite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Iron arsenic sulphide
Chemical Formula: FeAsS
Method(s) of Verification: Dolgellau Gold-belt - semi-quantitative EDX analysis.

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: alpine type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: volcanogenic massive sulphides
Striated and iridescent arsenopyrite crystal (15 mm x 7 mm) from Clogau St. David's Mine, near Dolgellau. National Museum of Wales Collection (NMW 27.111.GR.286), ex G.J. Williams Collection. Photo M.P. Cooper, © National Museum of Wales.
Polished section showing arsenopyrite (white), cracked and veined by later chalcopyrite (yellow) and electrum (very pale yellow). The veins are up to 10 µm wide. NMW specimen J.W. Gilbey Collection, from the Ffridd-Goch Mine, © NMW.
Introduction: arsenopyrite is most frequently encountered in medium-to-high temperature hydrothermal veins and other ore deposits. It is commonly associated with pyrite, galena, chalcopyrite, sphalerite and cobaltite (medium-temperature assemblages) or with cassiterite and wolframite (high-temperature assemblages). Gold deposits, particularly those of the mesothermal gold-quartz type, often contain abundant arsenopyrite and the gold and arsenopyrite may in some cases be intimately associated. Arsenopyrite is readily distinguished from pyrite (with which it commonly occurs) by its tin-white colour: when struck with a geological hammer it emits sparks and produces a garlic-like odour.
Occurrence in Wales: arsenopyrite is an abundant mineral in some of the orefields of Wales. It is particularly common in the Dolgellau Gold-belt, in the outer zone of the Snowdonia Copper orefield, at Parys Mountain and at the Dolaucothi Gold Mine. Early references to 'mispickel' - an old miners's name for arsenopyrite, include this from Huddart (1904) regarding Clogau Mine: 'But little is found; this little is disliked at the battery as it hinders amalgamation'. While clearly a nuisance mineral in this instance as it hindered gold recovery, in the Bethesda area, conversely, arsenopyrite was formerly mined as a source of arsenic and the sublimation flues are still recognisable at one site (Dewey, 1920; Bick, 1982). At Dolaucothi Gold Mine, the amount of arsenopyrite and pyrite associated with the gold made ore-processing so difficult that in the 1930s the ore had to be shipped overseas for gold extraction; the outbreak of war therefore spelt the death-knell for the mine (Hall, 1993). Thus arsenopyrite is a mineral which has had a small but interesting role in Welsh metal mining history.

Key Localities:

  • Central Wales Orefield: Raybould (1974) described arsenopyrite inclusions in galena from many mines in this area. A review of the samples found these to be misidentifications of other inclusion-forming phases such as bournonite and ullmannite (Mason, 1994; 1998).
  • Dolaucothi Gold Mine, Pumpsaint, Carmarthenshire: arsenopyrite is abundantly present both within the quartz 'reefs' and in the associated pyritic shales at this mine. It has a porphyroblastic texture and may contain inclusions of gold between 15-30 µm across (Annels & Roberts, 1989).
  • Dolgellau Gold-belt, Gwynedd: arsenopyrite is widespread in the gold-mines and trials of this area, occurring both as an important component of the mesothermal gold-lodes and as a minor phase in later associated Alpine fissure-type veins (Gilbey, 1968; Naden, 1988; Mason et al., 2002). Notable localities include the following mines: Clogau (excellent euhedral crystals to 15 mm in cavities in Alpine fissure-type veins, wallrock impregnations and associated with electrum); Foel Ispri (massive, abundant); Cefn-Coch (euhedral crystals to 1 cm impregnating wallrock); Tyddyn Gwladys (skeletal porphyroblastic crystals); Ffridd-Goch (associated with electrum and chalcopyrite) and Prince Edward (massive, abundant). Arsenopyritization is an important aspect of the wallrock alteration process in the mesothermal gold-lodes of this area.
  • Parys Mountain, Anglesey: minor arsenopyrite forms zoned rhombs and laths associated with tetrahedrite-group and bismuth-bearing minerals (Pointon & Ixer, 1980).
  • Snowdonia, Gwynedd: In the Snowdonia copper orefield arsenopyrite is common at mines developed in Cambrian sedimentary rocks around the periphery of the Snowdon Caldera. Notable occurrences are at the following mines (from SE going clockwise): Moel Fleidiau, Pantywrach, Blaen-y-Pennant, Drws-y-Coed, Simdde Dyllaun, Llanberis, Tan-y-Garth and Cwm Ceunant (all described historically by Bick (1982) and noted by Bevins & Mason (1998)). Occurrences are either of massive arsenopyrite or as euhedral inclusions, visible in polished section, in later pyrrhotite.

References:

  1. Annels, A.E. & Roberts, D.E., 1989. Turbidite-hosted gold mineralisation at the Dolaucothi Gold Mines, Dyfed, Wales. Econonic Geology, 84, 1293-1314.
  2. 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.
  3. Bick, D.E., 1982. The Old Copper Mines of Snowdonia. Second Edition. Pound House, Newent.
  4. Dewey, H., 1920. Arsenic and antimony ores. Memoirs of the Geological Survey. Special Reports on the Mineral Resources of Great Britain, 15.
  5. Gilbey, J.W., 1968. The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration. Unpublished Ph.D thesis, University of London, UK.
  6. Hall, G.W., 1993. Metal Mines of Southern Wales. 2nd Ed. Griffin Pubs., Herefordshire.
  7. Huddart, L.H.L., 1904. St David's Gold Mine, N. Wales. Transactions of the Institution of Mining and Metallurgy, 14, 199-219.
  8. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. 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.