Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Distribution: Rare
Chemical Composition: Silver copper antimony sulphide
Chemical Formula: (Ag,Cu)16Sb2S11
Method(s) of Verification: Tyddyn Gwladys Mine - appears to be by EMPA (Gilbey, 1968).

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
Introduction: polybasite occurs with other silver sulphosalts and base-metal sulphides in low to medium-temperature hydrothermal mineral veins.
Occurrence in Wales: Gilbey (1968) identified polybasite, associated with pyrargyrite, from Tyddyn Gwladys Mine in the Dolgellau Gold-belt. The locality is historically known for its high-grade silver ores (Hall, 1990). Mason et al. (2002) suggested the possibility that the Ag-Au-Sb-Pb rich mineralization occurring at this mine might be a variant of the high-grade Au-Ag-Bi-Te-Pb mineralization that occurs very locally in gold-bearing oreshoots in the area.

Key Localities:

  • Tyddyn Gwladys Mine, Ganllwyd, Dolgellau, Gwynedd: identified in polished sections of sulphide ores by Gilbey (1968), occurring as part of an Ag-rich inclusion assemblage occurring locally in galena and including pyrargyrite, argentian tetrahedrite and electrum. Polybasite was stated to be much rarer than pyrargyrite.


  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 Unpublished Ph.D thesis, University of London, UK.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. Hall, G.W., 1990. The Gold Mines of Merioneth. 2nd ed. 99pp. Griffin Publications, Kington.
  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.