Tennantite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Copper arsenic sulphide with highly variable amounts of iron, zinc and silver
Chemical Formula: (Cu,Fe,Zn,Ag)12As4S13
Method(s) of Verification: Dolyhir Quarry - XRD & EMPA; Capel Hermon - XRD; Great Orme - optical & EMPA; Parys Mountain - optical & EMPA

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: volcanogenic massive sulphides
  • Hydrothermal: porphyry-type mineralization
Grey sulpheret of copper, containing arsenic from North Wales. Reproduced from Sowerby (1811).
Introduction: tennantite occurs in a wide variety of hydrothermal ore deposits, but is perhaps best known from porphyry-type copper deposits, where it may constitute an important ore mineral in association with chalcopyrite and bornite. It also occurs in hydrothermal veins, where it may be associated with a wide range of sulphides, in stratabound deposits and in skarn assemblages. It is less likely to be found in high-temperature ore deposits. Tennantite is difficult to distinguish from tetrahedrite to the untrained eye: it is slightly darker in colour than the latter mineral and thin slivers may show red internal reflections, but analysis is the only surefire way of identifying it positively.
Occurrence in Wales: although there are quite a few early references to 'fahl-ore' (an old general name for minerals of the tetrahedrite-tennantite series), at the Cystanog Mine in Carmarthenshire, specific localities for tennantite have only been noted relatively recently. Firstly by Wheatly (1971) from Parys Mountain and secondly from the Great Orme mines (Ixer & Stanley, 1996). At both of these localities it is a relatively minor phase: however, in the 1990s, tennantite was found in large amounts in veins at Dolyhir Quarry in the Welsh Borders (Bevins & Mason, 1997) and in lesser but still significant quantities during the clearing of an exposure of the porphyry-copper deposit in the Coed Y Brenin forest near Dolgellau.

Key Localities:

  • Capel Hermon, Coed y Brenin, Gwynedd: tennantite is an important constituent of the 'ore-zone' of the porphyry-copper deposit, which is exposed in a recently-cleared forest road cutting on the W bank of the Afon Wen 500 m S of Capel Hermon. It occurs, with chalcopyrite and bornite, in disseminations and as granular to microcrystalline aggregates, exceptionally up to 25 mm across, in sheared and highly altered diorite with carbonates and minor quartz, but is largely altered to tyrolite and malachite.
  • Dolyhir Quarry, Old Radnor, Powys: several small veins carrying tennantite have been exposed at this working quarry in recent years. The most significant of these was a rib of almost solid sulphide up to 0.3 m in thickness, which occurred close to the top of a massive reef limestone (of Wenlock age) and just below a capping shale bed (Bevins & Mason, 1997). In this occurrence, now removed, massive tennantite was intimately intergrown with barite, galena and subordinate chalcopyrite with minor enargite, rammelsbergite and greenockite. The detailed petrography of this mineral assemblage is ongoing.
  • Great Orme Copper Mines, Llandudno, Gwynedd: tennantite occurs in trace amounts in rare siegenite-bearing veinstone, associated with siegenite, gersdorffite and sphalerite as inclusions in chalcopyrite and galena in the 2-30 µm size range (Ixer & Stanley, 1996).
  • Parys Mountain, Anglesey: both tennantite and tetrahedrite are present here, the former more so, but neither are common. Tennantite forms veinlets in sphalerite, isolated anhedral grains to 400 µm and thin (20 µm) rims between grains of chalcopyrite, galena and sphalerite (Wheatley, 1971; Sivaprakash, 1977; Pointon & Ixer, 1980).

References:

  1. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  2. Ixer, R.A. & Stanley, C.J., 1996. Siegenite-bearing assemblages found at the Great Orme Mine, Llandudno, N. Wales. Mineralogical Magazine, 60, 978-982.
  3. 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.
  4. Sivaprakash, C., 1977. Geochemistry of some sulphides and sulphosalts from Parys Mountain, Anglesey. Unpublished M.Phil. thesis, University of Aston in Birmingham.
  5. 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.