Cobaltite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Cobalt arsenic sulphide
Chemical Formula: CoAsS
Method(s) of Verification: Dolgellau Gold-belt - EMPA (Gilbey, 1968).

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
Photomicrograph of modified cubic cobaltite crystals (white) to 0.5 mm across from Panorama Mine in the Dolgellau Gold-belt. The crystals are set in a quartz matrix (black).
Introduction: cobaltite occurs in a wide variety of high-to-low temperature ore deposits. In particular, it is known from the so-called 'five metals' association, where it occurs with nickel, silver, bismuth and arsenic-bearing minerals. It also occurs within polymetallic mesothermal lodes with pyrite and arsenopyrite. Cobaltite is not always easy to identify in hand specimen but the presence of Co and As is often alluded to by supergene coatings of pink erythrite. In polished section its tendency to form euhedral cubic or modified cubic crystals is distinctive, as is its bright white colour and the fact that it is isotropic.
Occurrence in Wales: cobaltite was first identified from Wales by Gilbey (1968) who described it from a number of mines in the Dolgellau Gold-belt. Raybould (1974) subsequently identified a mineral assemblage occurring at Hyddgen Mine in Central Wales as consisting of cobaltite overgrown by arsenopyrite: this has since been shown to consist of cobaltian pyrite overgrown by tucekite (Mason, 1994). Thus its Welsh occurrences are limited to the Dolgellau Gold-belt, although here it can be quite conspicuous.

Key Localities:

  • Dolgellau Gold-belt, Gwynedd: cobaltite is widespread within this mineral district but most evident in its SW part, at mines such as Panorama, Caegwian, Vigra, Nantgoch and Clogau. Further north and east it has been recorded at the Wnion, Cefn Coch, Cefndeuddwr, Tyddyn Bach, Ffridd-goch, Gwynfynydd and Afon Gain mines and trials. It is always early in the paragenesis and is associated with arsenopyrite and pyrite: however it is often overgrown by, and forms inclusions in, later base-metal sulphides such as pyrrhotite, sphalerite, chalcopyrite and galena (Mason et al., 2002).Where present in quantity it forms aggregates of minute (<1 mm) cubic to cubo-octahedral crystals.

There are no key localities for this specimen.

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. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  3. 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.
  4. 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.

There are no references for this specimen.