Massive sulphides within ophiolites

Pyrite, chalcopyrite, bornite and haematite. Aarja, Oman

Radiating aggregates of pyrite (light yellow-white, top left) have been extensively replaced by copper-iron sulphides, but locally are unreplaced (top left). The cores to most of the aggregates are now chalcopyrite (yellow, centre) and bornite (brown, centre top) with rare haematite (blue, centre left, top right) laths. Minor amounts of relict pyrite are visible within the copper-iron sulphides (centre right). Quartz (black) is the gangue.

Polished block, plane polarized light, x 80, air

Bornite, pyrite and chalcopyrite. Aarja, Oman

Radiating pyrite aggregates (light yellow-white, centre top) have been fractured and cemented by quartz (top centre). They have been extensively replaced by bornite (brown, centre), which locally is intergrown with minor amounts of chalcopyrite (yellow, centre left). In the cores of the original pyrite aggregates, where bornite replacement is complete, there is an almost total absence of relict pyrite. Bands of bornite (top left) between pyrite are probably fracture infilling rather than replacement. Quartz (grey) is the gangue.

Polished thin section, plane polarized light, x80, air

Bornite, pyrite and chalcopyrite. Aarja, Oman

Fine-grained radiating pyrite (yellow-white, bottom right) is extensively replaced by bornite (centre top) to give lower reflectance cores to the 'pyrite'. Coarser-grained bornite (brown, top right) is intergrown with chalcopyrite (yellow, centre) and cements pyrite aggregates. Black areas are polishing pits.

Polished block. plane polarized light. x 80, air

Sphalerite, chalcopyrite and pyrite. Aarja, Oman

Sphalerite (grey, left) encloses poorly polished spherical pyrite (pale yellow, top left) and pyrite framboids (centre). It is intergrown with chalcopyrite (yellow, centre right), which encloses small euhedral pyrite crystals (centre right). The majority of the pyrite is poorly polished, and poorly crystalline cores are surrounded by euhedral, well crystalline margins (bottom right). Dark grey areas are quartz, black areas are polishing pits.

Polished block, plane polarized light, x 80, air

Sphalerite, pyrite and chalcopyrite. Aarja, Oman

Sphalerite (grey, top left) forms botryoidal aggregates upon poorly crystalline pyrite (light yellow, top left). Euhedral sphalerite crystals (centre) have a hexagonal-looking morphology suggesting that they were initially wurtzite. Coarse discrete pyrite crystals are unzoned and euhedral (centre), whereas fine crystals within a northeast-oriented vein have lower reflectance cores. Chalcopyrite (yellow, bottom right) is intergrown with pyrite and rimmed by sphalerite. The sphalerite is free of chalcopyrite disease. Quartz is dark grey, black areas are polishing pits.

Polished block, plane polarized light. x 80, air

Sphalerite, chalcopyrite, pyrite and tennantite. Aarja, Oman

Sphalerite (grey), the main phase, encloses irregular chalcopyrite (yellow, top right) and fine chalcopyrite inclusions (centre) but is essentially free of chalcopyrite disease. Small euhedral pyrite crystals (pale yellow-white, centre) lie in a discontinuous veinlet cutting sphalerite. A triangular inclusion of tennantite (green-blue, centre right) is associated with chalcopyrite and two pyrite crystals. Quartz is dark grey, black areas are polishing pits.

Polished block. plane polarized light, x 80 air