Pentlandite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Iron nickel sulphide
Chemical Formula: (Fe,Ni)9S8
Method(s) of Verification: these occurrences of pentlandite were identified by their optical properties.

Chemical Group:

  • Sulphides

Geological Context:

  • Igneous
  • Hydrothermal: mesothermal polymetallic veins
Introduction: pentlandite is observed most frequently among the opaques assemblage in basic and ultrabasic igneous rocks, where it may be present in sufficient quantities to consitute a nickel ore in the magmatic sulphide class of ore deposits. It is also present in hydrothermal veins, but is rarely found in quantity in this setting. Associated minerals are typically pyrrhotite and chalcopyrite, with other cobalt-nickel (Co-Ni) minerals in many cases.
Occurrence in Wales: records of pentlandite in Wales are few, although to date relatively little work has been done on the opaque assemblages present in the basic intrusive rocks. Their potential for hosting nickel-bearing magmatic sulphide deposits has, however, been investigated in one case: during the 1970s the mining company Noranda-Kerr undertook geochemical surveys over the outcrop of the layered Rhiw Instrusion on Llŷn. However, the results were not sufficiently encouraging to follow up the work with drilling. Two reports of pentlandite in the mesothermal lodes of the Dolgellau Gold-belt were made by Gilbey (1968) while Ixer (in Thorpe et al., 1991) noted its presence in intrusive rocks in the Preseli hills to the S of Cardigan in Pembrokeshire. A cobalt-rich variety of pentlandite, occurring in the Central Wales Orefield, is described on this site under cobalt pentlandite.

Key Localities:

  • Carn Menyn, Presely hills, Pembrokeshire: rare pentlandite exsolution flames in pyrrhotite were recorded by Ixer (in Thorpe et al., 1991) as part of the opaques assemblage from a dolerite intrusion.
  • Dolgellau Gold-belt, Gwynedd: Gilbey (1968) noted the rare occurrence of pentlandite as single, elongate lamellar inclusions in pyrrhotite, from the Clogau and Cesailgwm mines.

References:

  1. 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 Unpublished Ph.D thesis, University of London, UK.
  2. Thorpe, R.S., Williams-Thorpe, O., Jenkins, D.G. & Watson, J.S., 1991. The geological sources and transport of the bluestones of Stonehenge, Wiltshire, U.K. Proceedings of the Prehistoric Society, 57, 103-157.