Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Copper sulphide
Chemical Formula: CuS2
Method(s) of Verification: all occurrences cited - XRD and/or ore petrological techniques
- Supergene: post-mining oxidation & weathering deposits
- Supergene: in situ natural oxidation & weathering deposits
- Hydrothermal: alpine type veins
Chalcocite, showing minor alteration to chrysocolla, with quartz from Alpine Fissure-type veins. Penrhyn Quarry. Specimen National Museum of Wales (NMW 98.16G.M.13), photo D.I. Green, © National Museum of Wales.
Introduction: chalcocite is typically, but not exclusively, formed in the supergene zones of copper-bearing ore deposits, along with covellite, malachite, cuprite and other secondary copper minerals. However, it may also occur in some deposits as a primary sulphide. It is a difficult mineral to identify in hand-specimen, as it belongs to a group of copper sulphides, including digenite and djurleite, which, superficially, all look rather similar. Analytical work is required in order to distinguish them.
Occurrence in Wales: chalcocite is present at a number of Welsh localities as a superficial alteration product of chalcopyrite, a paragenetic position it shares with covellite. At one locality, Penrhyn Quarry, it is present in considerable amounts and here it appears to be of primary origin. The localities detailed below are selected for being of paragenetic, specimen or historical significance.
- Benallt Mine, Llŷn, Gwynedd: thin chalcocite veinlets, with copper and cuprite, cutting banded chert, were found on the tips during the 1997 (Gwynedd) phase of MINESCAN (Bevins & Mason, 1998).
- Central Wales Orefield: superficial alteration of chalcopyrite to chalcocite and covellite is of frequent occurrence in this area, but more significant quantities of chalcocite are restricted in occurrence to a handful of localities, namely Llechweddhelyg, Eaglebrook (Nantycagl), Camdwrbach and Dolwen mines (Mason, 1994). At these localities it has occasionally been found in masses to 2-3 cm.
- Dinorwic Quarry, Llanberis, Gwynedd: minor chalcocite occurs in the neck-zones of boudinaged basalt dykes at this locality (Bevins & Mason, 1998), but the occurrence is overshadowed by the nearby similar, but much richer, Penrhyn Quarry.
- Dolgellau Gold-belt, Gwynedd: superficial alteration of chalcopyrite to chalcocite is frequent at the mines in this area.
- Dolyhir Quarry, Old Radnor, Powys: chalcocite was formerly common at this locality during the mid-1990s the areas in question have since been quarried away. It formed veins, up to 15 mm thick, and intergrowths with chalcopyrite and bornite, with frequently associated malachite and azurite. Specimens have been preserved in the National Museum of Wales collection.
- Great Orme Copper Mines, Llandudno, Gwynedd: Ixer & Davies (1996) record the presence of minor supergene chalcocite at this site.
- Llantrisant area, South Wales: Bowler & Kingston (1977) recorded chalcocite, occurring with digenite, as a replacement of galena from mineral veins in this district.
- Parys Mountain, Anglesey: Pointon & Ixer (1980) record chalcocite occurring with other alteration products of chalcopyrite in shallow borehole core.
- Penrhyn Quarry, Bethesda, Gwynedd: chalcocite occurs in some quantity in boudin neck-zones of basalt dykes cutting Cambrian slate at this locality. It is associated with a ferroan carbonate, and overprints an earlier Alpine Fissure-type assemblage of quartz, specular hematite and chlorite. This appears to represent a hypogene, as opposed to supergene, chalcocite occurrence. Masses of solid chalcocite to 10cm across have been observed. Alteration to chrysocolla is frequently observed (Bevins & Mason, 1998).
- Porth Ysgo, Aberdaron, Llŷn, Gwynedd: chalcocite was confirmed from a specimen identified by the Natural History Museum.
- Snowdonia, Gwynedd: Williams (1927) recorded chalcocite 'replacing the original groundmass' in so-called 'copper-dolerites' from Cwm Tregalen on Snowdon. Subsequent fieldwork (Colman & Appleby, 1991; Bevins & Mason, 1998) has failed to confirm the presence of chalcocite but has instead recorded the presence of specular hematite and magnetite in vesicles and veins within pillow-basalts. It is believed that Williams' chalcocite may have been a misidentification of one of the latter minerals.
- Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
- Bowler, C.M.L. & Kingston, G.A., 1971. Mineralisation in the Triassic rocks of the Llantrisant area, Glamorgan. 1970 Gregynog Mineral Exploitation Colloquium Report. University College Cardiff, 30-34.
- Colman, T.B. & Appleby, A.-K., 1991. Volcanogenic quartz-magnetite-hematite veins, Snowdon, North Wales. Mineralogical Magazine, 55, 257-262.
- Ixer, R.A. & Davies, J., 1996. Mineralisation at the Great Orme Copper Mines, Llandudno, North Wales. U.K. Journal of Mines and Minerals, 17, 7-14.
- Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
- 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.
- Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.