Sulphides associated with serpentinized dunite. Unst, Shetland Islands, Britain

Dunites have suffered extensive hydrothermal alteration with the formation of serpentine minerals, chlorites, talc and magnesium-rich carbonates. The alteration processes have altered the chromite to ferritchromit and have precipitated magnetite and sulphides, arsenides and antimonides of nickel, iron and copper. They also remobilized the platinum group metals.

Major Minerals:

Magnetite, heazlewoodite, pentlandite

Minor Minerals:

Chromite, ferritchromit, nickel arsenides, breithauptite

Trace Minerals:

Include millerite, godlevskite, chalcocite, bornite, native copper, native silver, Pd-Cu antimonides, irarsite, geversite, sperrylite, Pt-Fe alloys, molybdenite


Characteristic intergrowths include magnetite-pentlandite and heazlewoodite ± nickel arsenide minerals and breithauptite. Magnetite forms rims around and lies along the cleavage planes of pentlandite. Pentlandite alters along grain boundaries to heazlewoodite which carries crystallographically oriented NiAs blebs and alters to millerite. Crystallographically oriented bornite-chalcocite intergrowths occur in relict olivine and in ferritchromit rims around chromite. Nickel-bearing sulphides are replaced by copper sulphides and native copper within silicates. The Pd-Cu antimonides are zoned and are associated with breithauptite


Pritchard and Neary, 1981; Pritchard et al., 1986; 1987