Molybdenite

Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Molybdenum sulphide
Chemical Formula: MoS2
Method(s) of Verification: Cae Coch - EMPA (Ball & Bland, 1985); all other occurrences identified either by visual examination of hand specimens or by ore petrography. Confidence is very high in all cases.

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: volcanogenic massive sulphides
  • Hydrothermal: porphyry-type mineralization
Molybdenite with quartz. Blaen-y-nant, Nant Ffrancon. National Museum of Wales Collection (NMW 27.111.GR.288), ex G.J. Williams Collection. Photo D.I. Green, © National Museum of Wales.
Introduction: molybdenite is found typically in mineral deposits formed with or near to igneous activity. It thus occurs in granitic rocks, including pegmatites; with porphyry-type ore deposits and in relatively high-temperature hydrothermal mineral veins in and around intrusive rocks. Tin and tungsten minerals, arsenopyrite, bismuthinite, pyrite and chalcopyrite are frequent associates.
Occurrence in Wales: the first report of molybdenite in Wales was as long ago as 1802, when W. Williams noted an occurrence above the Nant Ffrancon Pass in North Wales. Since then, and especially in the late 20th Century, a small number of additional occurrences, listed below, have been found. However, it is nowhere abundant. Molybdenite is a distinctive mineral due to its metallic bluish-grey color, extreme softness and tendency to cleave into thin sheets and as such is readily identified in hand specimen.

Key Localities:

  • Blaen-y-Nant, Nant Ffrancon, Gwynedd: thin veins of molybdenite are associated with intermediate intrusive rocks (Williams, 1802; Smith & Carruthers, 1925).
  • Cae Coch Mine, Trefriw, Gwynedd: as a very minor associate of the abundant pyrite in this massive exhalative sulphide deposit (Ball & Bland, 1985).
  • Clogau Mine, Bontddu, Gwynedd: small (10-20 ┬Ám) laths and clumps of crystals closely associated with galena (Naden, 1988).
  • Coed-y-Brenin, Dolgellau, Gwynedd: thin smears of molybdenite occur on joint surfaces and small blebs are present in microfractures and quartz-calcite veinlets within the Coed-y-Brenin porphyry copper deposit (Rice & Sharp, 1976). More recently observed as scaly aggregates to 5 mm associated with quartz, calcite and pyrite in mineralized breccia occurring as rare boulders in the alluvium of the nearby Afon Wen (J.S. Mason, unpublished data).
  • Ffestiniog (Tanygrisiau) Granite Quarry, Cefn Bychan, Blaenau Ffestiniog, Gwynedd: molybdenite has long been known from this locality and specimens are present in the National Museum of Wales Collection which were once within the G.J. Williams Collection, acquired by the National Museum of Wales in 1927. It occurs on joints in the microgranite and additionally in pipelike bodies where it is associated with allanite, pyrophyllite and quartz (Roberts, 1979).

References:

  1. 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.
  2. Dewey, H. & Eastwood, T., 1925. Copper ores of the Midlands, Wales, the Lake District and the Isle of Man. Memoirs of the Geological Survey. Special Report on the Mineral Resources of Great Britain, 30.
  3. 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.
  4. 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
  5. Roberts, B., 1979. The geology of Snowdonia and Llyn: an outline and field guide. Adam Hilger, Bristol.
  6. Williams, W., 1802. Observations of the Snowdon Mountains. S. Collingwood, Oxford.