Enargite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Copper arsenic sulphide
Chemical Formula: Cu3AsS4
Method(s) of Verification: Dolyhir Quarry - XRD & optical identification (National Museum of Wales).

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: Mississippi Valley Type veins
  • Hydrothermal: mesothermal polymetallic veins
Blocky enargite crystals on fluorite from Halkyn, Flintshire. National Museum of Wales Collection (NMW 27.111.GR.192). Image T.F. Cotterell, © National Museum of Wales.
Introduction: enargite and related minerals occur in a variety of ore deposits, but the most important association is in high sulphidation gold lodes (the 'enargite-type' class of gold deposits), in which the mineralization has been precipitated from high temperature and highly acidic hydrothermal fluids, typically associated with high level intrusive and subvolcanic igneous activity. Enargite may also occur in porphyry-type copper deposits and in a relatively rare As-rich subclass of Mississippi Valley Type Pb-Cu-Zn-Ba vein deposits, as at Clevedon in Avon (Ixer et al., 1993) and at the only Welsh occurrence. Enargite is readily identified in polished section but care must be taken with crystalline or massive material to distinguish it from similar minerals such as the low-temperature dimorph, luzonite.
Occurrence in Wales: a previous report of enargite from Great Orme, Llandudno, Gwynedd (Bevins, 1994) is erroneous. However, the presence of enargite in polished sections of sulphide-rich material from Dolyhir Quarry was noted in the mid-1990s and subsequently small but well-formed crystals of the mineral have been collected from other exposures in the quarry.

Key Localities:

  • Dolyhir Quarry, Old Radnor, Powys: enargite was initially identified in polished sections taken from a Wenlock Limestone-hosted sulphide-rich vein. It occurs in a complexly-intergrown assemblage dominated by tennantite, galena and chalcopyrite. More recently, further fieldwork at various temporary exposures of vein mineralization within the quarry has revealed small (<1 mm) dark grey, equant blocky enargite crystals (N. Hubbard Collection) and anhedral grains occurring in calcite-baryte dominated veins hosted by the Precambrian strata in the lower part of the quarry.

References:

  1. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  2. Ixer, R.A., Pattrick, R.A.D. & Starkey, R.E., 1993. Lead-zinc-copper-arsenic-baryte mineralisation from Clevedon, near Bristol. Journal of the Russell Society, 5, 23-30